Euro Forecaster 2015

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The European

Forecaster

Newsletter of the WGCEF N° 20

September 2015


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Cover:

“Medicane” on 7/11/14,source METEO FRANCE

Introduction

Draft Minutes of the 20th Annual Meeting of WGCEF

Infra-Synoptic High Impact Events,

…..the Forecaster’s Obsessive Fear

Saint Nicolas Storm 5 December 2013

Social Media at Deutscher Wetterdienst (DWD)

Bow Echoes: Conceptual Schemes and European Relevance

Simulation Training

A tool for forecasters and staff

A Global Forecast Quality Score for Administrative Purposes

Severe Freezing Rain in Slovenia

Synoptic analysis of the Catastrophic Floods in SE Europe, May 2014

Representatives of the WGCEF

Printed by Météo-France

Editor Will Lang - Met OfficeLayout Kirsi Hindstrom - Basic weather Services

Published by Météo-FranceCOM/CGN/PPN - Trappes

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C r é d i t M e t e o - F r a n c e

Contents


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Dear Colleagues,

Welcome to the 20th edition of “The European Forecaster”, a newsletter which has always sought to offer

something a little different to academic journals or trade publications, with its focus firmly on the forecast-ing profession itself. Once more the WGCEF kindly thanks Meteo France for their support in publishing this

edition. We hope it will find a home in forecast offices across Europe, and that our forecasting colleagues

find plenty to interest them within its pages in some of the all-too-brief quieter moments during their opera-

tional duties!

As we all know, the modern operational meteorologist is multi-skilled – not just a scientist but an expert

decision-maker and communicator who can offer excellent service to our “customers”. We are involved with

the latest developments in technology and making use of new sources of information. And we understand

that our participation in training, strategic and business matters, and research can be as beneficial as our

traditional operational roles. I am very pleased, therefore, to see that the articles in this edition are equally

wide-ranging in scope. As ever, we have some excellent case-studies, here concerning a freezing rain event,the “Saint Nicolas” storm, and the catastrophic Balkan floods of May 2014. We have a summary of current

thinking around “bow echo” convective phenomena, and presentation of a sophisticated new verification

scheme. There are also articles outlining simulation training for forecasters, and our increasing reliance on

social media.

Looking back on the last twelve months, it is encouraging to see the WGCEF gaining greater influence and

recognition within the European meteorological circles, largely through our involvement in EUMETNET, but

also through the dedication of our members. Increasingly our advice and experience is being sought, and

the cross-border networks we have formed have allowed us to share knowledge, data and discussions

around warnings to the mutual benefit of our organisations and nations, and for Europe as a whole. I am

sure that in the coming years, the modest efforts and resources of the WGCEF will continue to clearly

demonstrate the merits of collaboration in operational meteorology and in continued development of our

forecasting capabilities and skills.

The European Forecaster 5

I ntroduction

Will LangChair, WGCEF

May 2015


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Draft Minutes of the 20th Annual Meeting

of the Working Group on Co-operation Between

European Forecasters (WGCEF)Friday 3rd October 2014, WMO Building, Geneva, Switzerland

1. Will opened the meeting and welcomed the

participants, with each member then introducing themselves.

2. The agenda was agreed, and actions from the last

meeting (newsletter production, arrangement of the

next meeting) were judged complete aside from the

standing actions to look to increase membership of

the group and promote our work.

3. Pierre Eckert, Meteo Suisse Head of Regional

Centre, West Switzerland then warmly welcomed the

WG to Geneva. He gave an overview of MeteoSuisse’s structure, responsibilites and operations.

He then made a plea for more forecaster involve-

ment into science and NWP activies, particularly

within European working groups. He recognised that

WGCEF involvement in EUMETNET was a positive

step towards this goal, but urged group membersand their colleagues to do more to engage with other

groups.

4. Will then gave his Chairman’s address. Looking

back on another year of eventful weather across

Europe, he noted that forecasters are now increas-

ingly influencial outside the meteorological commu-

nity and routinely involved in big decisions within

our governments. So the importance of expressing

ourselves correctly and making sure our advice is

understood continues to grow. He then noted thatGeneva was the home of CERN, and noted some

parallels and differences between meteorology and

particle physics. In particular, the discovery of the

Higgs Boson had achieved great public awareness,

List of Participants

Members: Will Lang (UK, Chair),Andre-Charles Letestu (Switzerland, Host),

Klaus Baehnke (Germany), Dick Blaauboer(EUMETNET FP), Christian Csekits (Austria),

Jos Diepeveen (Netherlands), Karen-HelenDoublet (Norway), Tessy Eiffener(Luxembourg), Alessandro Fuccello (Italy),Bruno Gillet-Chaulet (France), Tim Hewson(ECWMF), Cecilia Karlsson (Finland), Piotr

Manczak (Poland), Janez Markosek (Slovenia), Jean Nemeghaire (Belgium), LolaOlmeda (Spain), Taimi Paljak (Estonia) Antii Pelkonnen (Finland), Chryssoula Petrou(Greece), Vida Raliene (Lithuania), Natasa Strelec-Mahovic (Croatia)

Observers and Guests: Daniel Cattani (Switzerland), Pierre Eckert (Switzerland),Knut-Helge Midtboe (Norway), Marcel van Schaik (Netherlands),

Apologies

Evelyn Cusack (Ireland, Vice-Chair), Bernard Roulet (France), Knut-Jacob Simonsen(Denmark)



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despite the complexity of ideas involved.

Meteorology is a more mature and well-understood

science, with very tangible outcomes, and we should

be able to gain similar understanding of, and

support for, our work.

5. There was a discussion around the latest (19 th )

issue of our Newsletter, ‘The European Forecaster’,

led by Bruno. We agreed that this issue had been

the best, and certainly the biggest, yet, and passed

on many thanks to Bernard and the publishing team

and management at Meteo France for their support.

Bruno announced that Meteo France would be able

to produce the next three issues of the newsletter.

This offer was very gratefully accepted by the group.

It was also acknowledged that there should be someplanning to find alternative arrangements for the

publishing of the 23rd and subsequent newsletters.

Tim stated that, as a new member, he thought the

Newsletter could have wider distribution and more

publicity, to which the group agreed.

6. Andre-Charles gave an update on our website

www.euroforecaster.org and we thanked him for hiscontinued maintenance of the site. It was agreed

that the site would remain autonomous from EUMET-

NET sites, but that we would work to strengthen the

mutual links between WGCEF and EUMETNET sites.

Will mentioned that a WGCEF Linkedin page had

been created, and that he would look to expand

membership of this in the next year.

7. Group members then gave short summaries of

developments at their respective NMSs in the last

year. These summaries are outlines in Annex A

below.

8. Dick gave an update on EUMETNET activites, espe-

cially within the Forecasting Programme. Heexplained the structure of the working groups and

expert teams and outlined further potential for

WCGEF involvement in wider activities.

9. Christian described our strong involvement in,

and his leadership of, a EUMETNET Task Team to

investigate options for collaboration in the naming

of European windstorms. The Team had produced a

number of recommendations for the EUMETNET STAC

based on substantial input from WGCEF members. A

lively discussion followed, demonstrating that there

are both pros and cons for not just the collaborative

aspects, but whether storms should be named at all.

There was a collective feeling, however, that NMSs

needed to assert their authority on this issue, else

others will name storms for us and perhaps confuse

our severe weather messaging.

10. Members of the Group then gave 10-15 minute

presentations on a wide range of relevant topics,

including case studies of recent weather, new

systems and tools for forecasters, and use of social

media. The content of these presentations is avail-

able on our website, at www.euroforecaster.

org/gpeasy/gpEasy_CMS/ Presentations_2014

11. The meeting concluded with a decision on the

location and date of the next meeting. Jean

Nemeghaire offered to host the 2015 meeting in

Belgium, which would appropriately commemorate

his retirement from both our group and from RMIB.This offer was unanimously accepted, and the 21st

WGCEF is therefore scheduled for early October 2015

in Brussels.

12. Will declared the meeting to have been success-

fully completed, and the group adjourned to the

Meteo Suisse office for refreshments.

Actions

All: Promote, and increase circulation of, The

European Forecaster.All: Send presentations to Andre-Charles for the

website

All: Send articles for next newsletter to Will between

January and March 2015

All: Discuss possible topics for next meeting via

email

Will, Evelyn and Jean: Organise next year’s meeting

Will, Bernard: Produce next issue of The European

Forecaster

Christian: Report back on Storm-Naming task at next

meeting.

Annex A: Summaryof NMS Developments

Andre-Charles (Switzerland)• A new radar has been installed in Vallee

• A new web platform has been designed to warn

the public of dangerous conditions, including pollen

forecasts.

Klaus (Germany)• DWD is installing a new Cray HPC

• The DWD global model will have 13km resolution

and 90 vertical levels. Within this is nested a

European model, and a 2.2km resolution model

covering Germany.



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• Some stations are now closed overnight, with an

increasing reliance on auto observations.

Christian (Austria)• No dramatic changes at ZAMG in the last year, and

no big reductions in numbers of staff, though some

merging of departments is planned.

• Automatic observations are being used more

frequently, especially at night.

• They are running at 2.5km version of AROME

• They are unifying their forecast production

systems.

Natasa (Croatia)• The office in Rijeka has been closed due to staff

shortages, so in addition to the HQ in Zagreb, thereis only one regional office, in Split. This has caused

some issues due to some locations now being

served by offices further away and with different

dialects.

• ALADIN is run 4x per day, with a version of ALARO

running at 2km resolution.

• 2 new forecasters have been recruited at the

Zagreb office.

Tim (ECWMF)• The move to the new ECMWF website is 90%

complete, and feedback is very much encouraged.

• The website includes a facility to submit and

discuss case studies, along with known problems

with the models.

• The global model will move to 8-10km resolution

next year.

• In the medium term, the next computer upgrade

will also see a move to a new building.

Alessandro (Italy)• A project to reorganise Met activities is underway.

Currently warnings are issued at Regional level,though a new procedure for severe weather warn-

ings is being devised.

• New systems are being developed, and there is a

culture of real-time feedback between operations

and developers.

Taimi (Estonia)• The weather service continues to operate six

manned observation stations.

• The HIRLAM model is being developed, moving to

a new computer and also using HARMONIE.

• Since June there has been a new webpage withbetter visualisation of warnings.

Bruno (France)• ARPEGE is moving to 7km resolution with 105

layers, and a new DA system.

• The new PCMT convection scheme has been

delayed.

• The domain of AROME is being widened, incorpo-

rating an hourly DA cycle.

• Staff reductions continue.

• New Exec Director is seeking a reorganisation of

forecasting, with a focus on higher quality prediction

processes which require less human input.

Lola (Spain)• The ongoing reorganisation of the service results in

reduction of personnel and greater use of automated

products, based on the Global Forecast Editor (GFE)approach.

• Closer links with media are being developed.

• Work is underway to improve marine forecasts.

• A renewal of the HPC is expected early in 2015.

Chryssoula (Greece)The service has financial difficulties, and when fore-

casters retire they are not being replaced.

Observing stations are being automated, especially

at night, with only the international airports retain-

ing manual observations.

A colour-coded warning system has been developed,

aligned with MeteoAlarm.

The COSMO model is used. Performance can fluctu-

ate, particularly for wind forecasts between the

islands.

Karen (Norway)• They are cooperating with Sweden to run local

AROME model.

• There is increased cooperation between forecasters

and R&D, with forecasters now expected to take part

in project work.• There is emphasis on the development of marine

services, especially for the Arctic.

Vida (Lithuania)• No significant staff reductions this year.

• A new workstation for aviation has been devel-

oped, and general forecasting is moving onto IBL

workstations.

• They are moving into a new, better equipped, fore-

cast room.

There has been a decision to join MeteoAlarm.


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Janez (Slovenia)• No staff changes in the last year.

• There is further automation of the monitoring

network.

• A new radar is operational in western Slovenia

• Forecasters use 3 hr’ly ALADIN at 5km along with

EC models.

• The Met Service has a developing Twitter presence.

Antii (Finland)• FMI celebrated 90 years of aviation weather

services this year.

• A reorganisation is underway, which should allow

greater cooperation between operations and

research.

Piotr (Poland)• There will be further automation of the observing

network, and this process is becoming faster, with

staff reductions likely.

• A new provider for aviation weather services is to

be selected later this year.

• During the last storm season, a special team

supported the regional centres.

Jos (Netherlands)• The KNMI reorganisation continues, with a new

financial model and a flatter management structure.• There should be better integration between

research and development.

• Staff are used flexibly, depending on the weather

situation.

Tessy (Luxembourg)• There are 22 staff at MeteoLux, but 3 new forecast-

ers will increase that number to 25.

• Its funding model has changed, with half provided

by the airport and now half by the state.

• A new warnings system has been developed withstronger emphasis on collaboration with civil protec-

tion and on publication of impact-based alerts

which push information towards the media and

public.

• Flood warnings are now available on Meteoalarm,

in partnership with the hydrological agency.

Jean (Belgium)• There is a new dual polarisation radar at

Maastricht, and 3 new lidars for aerosol measure-

ment.

• There is discussion of longer range forecasts, eg upto 2 weeks.

UK (Will)• Meteorologists are being trained in a ‘5 Facet

Model’ of modern forecasting which includes

emphasis on customer needs, ‘soft’ skills and

involvement in development.

• UKMO makes extensive and two-way use of social

media.

• A 24/7 Space Weather centre has recently become

operational



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Infra-Synoptic High Impact Events,

…..the Forecaster’s Obsessive Fear.

Bruno Gillet-Chaulet, Meteo France

Introduction

Forecast errors are inherent elements of the fore-

caster’s job. Some are inevitable. Some are unim-

portant. But some are fraught with consequences

and make a deep impression. Nowadays the “synop-

tic” scale is widely under control. Numerical weather

prediction models have become remarkably power-ful, for the short range at least. Synoptic elements

then often take all our attention; the “mesoscale” is

sometimes put aside and remains a challenge.

During the last few years, as a forecaster, I have

been witness to several situations where violent

small scale phenomena had a national impact. In

some cases, these events were not anticipated and

called into question the role of the forecasters. In

this short article, a recent situation is described and

issues raised by the events are discussed.

Scenario:

The situation took place in the summer of 2014. On

Thursday, 3d of July, around midday, a cut-off low

was situated over Iberia (Figure 1). At lower levels, a

“barometric marsh” was spreading over a large part

of Western Europe with warm air (Figure 2). It was

mostly dry over France at that time except over the

southern regions. Instability developed over Spain in

response to diurnal heating and the presence of cold

air in the mid and upper levels. Showers, thunder-

storms and lightning were observed. Because of

southerly flow, clouds and rain were crossing over

the Pyrenees and were reaching the south western

part of France in the evening (Figure 3). This was not

expected - a sunny day had been forecast, an error

that sometimes happens, fortunately in this case

without damaging repercussions!

For the next day, models gave the following

scenario: The cut-off low was predicted to turn to a

dynamic short wave trough, moving quickly north-

eastwards to the Alpine regions (Figure 4). At lower levels, pressure was forecast to decrease, flow

speeding up with continual warm advection

Figure 1: Thursday 3 July, 12 UTC, Geopotential heightand temper-

ature at 500hPa, ARPEGE analysis.

Figure 2:Thursday 3 July, 12 UTC, Mean sea levelpressure and wet-bulb potential temperature at 850hPa, ARPEGE analysis.

Figure 3: Thursday 3 July, 12 UTC, MSGimage andlightning strikes.


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(increasing of wet bulb potential temperature).

Moisture was brought from the Mediterranean Sea

with south-easterly winds (Figure 5).

The graphical product below (PRESYG, Figure 6)summarizes these conditions and highlights the key

synoptic elements: dynamic forcing with an active

PV (Potential Vorticity) anomaly; strong upper level

jet-streaks with an area of divergence (left exit, right

entrance); low level convergence with strong wester-

ly/south-easterly winds on both sides of this area.

Finally, the vertical profile shows thermal instability

with significant values of CAPE (Convective Available

Potential Energy) and vertical wind shear (Figure 7).

Models were in good agreement, and uncertaintywas very low. Logically, a decision was made to

issue an “orange” warning for violent thunder-

storms accompanied by hail, heavy rainfall, and

severe wind gusts. Several “Vigilance” watch maps

Figure 4: ARPEGE forecast for 12 UTC, Friday 4 July, base time

Thursday 3 July, 12 UTC, Geopotential height and temperatureat 500 hPa.

Figure 5: ARPEGEforecast for 12 UTC, Friday 4 July, base time

Thursday 3 July, 12 UTC, Mean sea level pressure, 10m wind barbsandwet-bulb potential temperature at 850hPa.

Figure 6: Conceptual view for Friday 4 July, mean sea level

pressure, base time Thursday 3 July, 12 UTC.

Figure 7:ARPEGE forecast for12 UTC,Friday 4 July, base time

Thursday 3 July, 12 UTC, vertical profileover South-Eastof France.

were published for this situation. The first one was

issued on Thursday, 4 p.m. The second one

confirmed the warning with the threat extended to

the east on Friday, 6 a.m. (Figures 8, 9).

Figure8: “Vigilance” watch mapbase timeThursday3 July, 4 p.m.


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Right Decision:

This was the right decision to make. Over a wide

part of eastern and south-eastern regions of

France, thunderstorms were numerous and some-times violent (Figures 10, 11). Various risk criteria

were met: wind gusts more than 100 km/h, hail-

stones with a 2 cm diameter, heavy rainfall with

accumulated precipitation above 40 mm within an

hour. The thunderstorms caused little damage but

occurred in numerous places. Local fire brigades

had to step in because of fallen trees and branch-

es, roof tiles blown down, flooding, and one house

destroyed by fire after lightning strike. In short, this

inventory indicates that the warning was complete-

ly justified thanks to an accurate synoptic forecast. Figure 9:“Vigilance” watch map base time Friday 4 July, 6 a.m.

Figure10:24 h rainfall accumulationbetween Friday4 July,

00UTC andSaturday5 July, 00UTC.

Figure11:Lightningstrikesbetween Friday4 July, 00 UTC

and Saturday 5 July, 00UTC.

Figure 12:Time seriesof level of the“Nive” near“Cambo-les-

Bains” with previous records (blue straight lines).

One can add that the forecaster's job was not so

difficult taking into account the relevant synoptic

elements.

However:

The reader with a sharp eye will have noticed that

heavy rainfall also occurred in the south-western

part of France (Figure 10), which became the hot

issue of the time! Effectively, during the night, a

strong stationary convective system gave excessive

precipitation over a quite wide area. More than 100

mm were measured by a rain gauge in the village of

“Bustince” (Pays Basque). Radar observations gave

hourly intensity above 50 mm. Because the soils

Figure 13:Headlines of Newspaper “Sud-Ouest”.Note thewords

“controversy”,“surprise”,“victims fulminate”, “catastrophic toll”.


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had already been saturated after a rainy spring,

there was a destructive flash flood from the river

“Nive”(Figure 12). The previous record, dating from

1915, was largely beaten. One person was killed,

swept away by an actual wave. Several villages

suffered a lot of damage along the riverside.

People and the media quickly complained because

the event had not been anticipated (Figure 13). As

a result, Meteo France forecasting services were

put under strain.

Late Signal:

At this stage, it must be said with strength and

honesty that this episode couldn’t have beenpredicted the day before. Looking carefully at the

accumulated precipitation charts from different

models available at that time (Figures 14, 15), no

signal emerged to bring to light an exceptional

rainy event on the South-West of France. Rain was

predicted, in the form of showers or thunder-

storms, but the amount of precipitation remained

under severe thresholds over this area.

Additionally, gaze was turned to the South-East

side of the Country where deep convection was

expected because of the synoptic context previous-ly described. Some squall line features appeared in

the mesoscale rain fields focusing eye on the

threat in this direction.

The right indication was only given at the end of

the night by the fine-mesh model “AROME” (Figure

16), whereas the event had just started. This latest

forecast proved to be perfectly accurate with a

Figure 14:ARPEGE 12 h rainfall accumulation between Thursday 3

July, 18 UTCand Friday4 July, 6 UTC, forecast basetime Thursday 3 July, 6 UTC.

Figure15:AROME 12 h rainfall accumulation between Thursday 3

July, 18 UTCand Friday4 July, 6 UTC, forecast base timeThursday3 July, 6 UTC.temperature at 500hPa, ARPEGE analysis.

Figure16:AROME12 h rainfallaccumulation between Friday4

July, 00 UTCand 12 UTC, forecast base timeFriday 4 July, 00 UTC.

Figure17:“Vigilance” watch mapbase timeFriday4 July, 7 a.m.

strong clear signal of heavy rainfall over the Pays

Basque. Consequently, the warning for rain was

issued on Friday, 7 a.m. (Figure 17), was updated

twice during the day, including flood risk, and

ended at 4 p.m. Unfortunately, this came a little

too late to be really helpful for the population and

civil protection services…


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Conclusion:

To be able to detect when and where models move

away from reality and to be then in a position tocorrect the forecast is the primary task of the fore-

caster. Nevertheless, a diagnostic sometimes is not

obvious. Furthermore, these unexpected situations

correspond to those the forecaster is the least

prepared for, by definition! Luckily, these condi-

t ions seem to remain rare. As far as I am

concerned, in the past 15 years, I can remember 4

or 5 circumstances similar to the one described in

this article. And no doubt that the improvements of

numerical weather prediction will make them less

and less frequent in the future. However, they are

important because they can have detrimentalconsequences for the forecast team by creating

conflicts between forecasters themselves and

between forecasters and management.

These situations point up some psychological

aspects:

Attention is often focused on a specific area where

troubles are expected. Our mind is less attentive to

what happens elsewhere.

These cases can occur most often at the worst

times, such as at the end of a night shift, when

tiredness is maximal. The ability to assimilate fresh

information, to react in a proper way, and the moti-

vation to trigger a new warning production cycle

are minimal. The paradox is that a forecaster who is

not aware of the situation (open-minded) could be

more reactive!

The point of view presented in this article is not

intended to make forecasters paranoid, neverthe-

less, writing this paper, the following quote came

back to my memory:

“…il en ressort que celui qui doit prévoir le temps,

s’il le fait avec conscience et application, ne peut

plus avoir une vie tranquille et court un grand

danger de voir craquer ses nerfs et devenir fou.” *,

by Buys Ballots (President of InternationalMeteorological Organization), from the speech at

the first International Meteorological World

Conference ( 1873, Vienna, Austria ).

* “…as a result, he who is in charge of predicting

the weather, if he works conscientiously and care-fully, can’t lead a quiet life and is exposed to

serious danger to mentally snap and go mad.”


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The Saint Nicolas Storm

Synoptic setting

An active low near Iceland moved

eastwards and reached Norway on

Thursday 5 December (figure 5 and

6). This low had a pressure core of

965 hPa. The Most Notable and

important feature of this low pres-

sure system was an active cold

front that moved southeastwards

over the North Sea. This front

reached the Netherlands in the


Introduction

An active low pressure system moved east across the

northern part of the North Sea during 5 and 6 December

2013. Because of this system, a weather alarm was

issued for very strong gusts along our coast and further

inland.This storm also caused water levels to rise along

our coast because of the strong northwesterly winds atthe western flankof the aforementioned low.

In this article I would like to give an overview of the

development of the low pressure area, the model fore-

casts and the consequences that this storm had for

our country. This will be done from the perspective of a

marine forecaster and therefore water level forecasts

will be mentioned as well. This is also the reason that

this article will start with a short description of the

vulnerability of The Netherlands to high water levels.

This case is called the ‘Saint Nicolas’ storm in theNetherlands because of a Dutch national celebra-

tion that takes place on the 5th of December.

Vulnerability

The Netherlands is, as the name indicates, a very

low lying country. About half of the country is situat-

ed around or below sea level (figure 1). The area

that is below sea level comprises the western part of

the country. That is also the area where most people

live (figure 2); important cities like Amsterdam, The

Hague and Rotterdam are situated in this area

Saint Nicolas Storm 5 December 2013Marcel Van Schaik, KNMI

The Netherlandsis situated on thesouthern edge of the

NorthSea (figure 3). It is not hard to imagine that water

levels along the Dutch coast can become critical when

strong northwesterly winds push North Sea water

towards the coast. Along our coast are several large

dune areas that are able to withstand high water levels.

Along other parts of the coast the Dutch had to build

several water barriers to improve the coastal defence.The ‘Hondsbossche Zeewering’ is one of these exam-

ples (figure 4 a). Other famous barriers include the

Maeslant barrier and theOosterschelde barrier (figure 4

b and c). These two barriers are movable barriers and

can be closedif necessary.

Figure 4 abc

Figure 3

c)

b)

a)

Figures1 and2


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afternoon. The cold front left the country early in the

evening. The wind veered to a northwesterly direc-

tion directly behind the cold front which caused

high water levels along the coast.

Average wind speeds reached 9-10 Bft (figure 6).

Highest wind speeds were measured just before

passage of the cold front. The strongest gusts were

measured during the passage of the cold front

(figure 7).

The passage of this front was accompanied by a

narrow line of (thundery) showers (figure 8). These

showers were responsible for the strongest gusts

Model Output

Meteorologists at KNMI make use of the following

models; ECMWF, Hirlam and the newest and mostdetailed one, Harmonie. All models had a more or

less similar forecast; strong gusts were predicted by

all models and the timing of the cold front passage

was also quite similar. In the end Harmonie turned

out to be the most accurate model; the representa-

tion and prediction of thundershowers along the

frontal zone was very accurate (figure 9 and 10) and

also the forecast wind speed and wind gusts were

described accurately (figure 11 and 12).

Figure5

Figure8

Figures6and7


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Figures9 and 10

Figure 11and12


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Water levels

As mentioned above, water levels started to rise

after the passage of the cold front. The synoptic

charts show a northwesterly flow over a large part of

the North Sea. This caused water levels to rise along

the Dutch coast. ECMWF did calculate the storm

depression in an early stage. That is why water level

calculations showed a peak around the 5 th of

December as well. An example of this water levelcalculation can be seen in figure 13. Water levels

are calculated by a water level model (WAQUA) and

all ECMWF members are used as input for this water

level model. As a consequence 52 water level

members are represented. The colored dashed lines

in this graph represent several important water level

thresholds. A video capture of a 3D animation

(figure 14) gives a representation of water levels

along the Dutch coast.

Water related consequencesof the storm

Near-record high water levels were reached in thenortheastern part of the country. Also in other parts

of the country very high water levels were reached;

the moveable barrier in the Oosterschelde had to be

closed (figure 15). The last time that this barrier was

closed was in 2007. Some low lying houses were

partly flooded.

Figure14

Figure 15

Figure13


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Social Media at Deutscher Wetterdienst (DWD)Klaus Bähnke, Deutscher Wetterdienst/Offenbach

Introduction

Social media complement and increasingly deter-

mine the public discourse. By March 2010, 30

million Germans were already members of internet

communities. Maintaining contact with friends and

families, the exchange of common interests and

experiences, and the opportunity to find newfriends are the main reasons for the membership of

users in these communities. Seven social media

sites were among the twenty most visited websites

in Germany (YouTube, Facebook, Wikipedia,

LinkedIn, Twitter, Blogger, Xing). The VZ network

alone had almost 17 millon users, and Facebook

had over 11 million in Germany. Although Twitter

has so far only 350,000 German-speaking active

accounts, it continues to receive high media atten-

tion. Among Twitter users an above average number

of opinion leaders were present.

All this, along with the pressure to respond in crisis

situations, urged the German Weather Service (DWD)

to enter the field of modern social media in 2011.

Top Story of the Day

In the broadest sense, the first step

of our social media is the newslet-

ter "Top Story of the Day" (Fig. 1),

which was launched by DWD in

1994 and is published by the media group of the

central forecasting office. This is published daily and

is text-based. It contains interesting meteorological

topics or explanations, mostly influenced by current

events. Its targets are journalists, press agencies and

the interested public. The newsletter is distributed

every day by email to about 1800 customers. Since it

has also been placed on the website of the GermanWeather Service, it has achieved significantly higher

distribution and readership.

Facebook

With the appearance of modern means of communi-

cation such as smartphones, the advent of new

media phenomena such as "shitstorms" as well as

severe weather events, the German Weather Service

decided to play a bigger part in the social media

sector.

Thus DWD started its Facebook page (Fig. 2) on the

web in March 2011.

Figure 1: Example for“TopStoryof the Day” Figure 2: DWDon Facebook


20/47


The aim is to position the German Weather Service

in social media, to attract the press to its web

pages, to interact with the public and have a better

means of reaction during crises.

The Facebook page is monitored daily and supplied

with interesting topics from the media department

of the central forecasting office during normal office

hours (0700-1700). During a crisis event the Public

Relations department of the Meteorological Service

monitors activities on Facebook continuously and

can respond immediately. By the end of 2014 the

website had about 69 000 followers.

TwitterAnother important social media tool is Twitter.

Twitter is an online social networking service that

enables users to send and read short 140-charac-

ter messages. The German Weather Service has

participated in this service since 2011. The aim is

to position the public weather service in social

media, but here 2 separate sectors are served:

firstly the press and secondly other special users

such as the agricultural sector. This is done by

means of Twitter_press (Fig. 3) as well as by

Twitter_agrar (Fig. 4). Twitter_press is served by the

PR department and has about 4700 followers while

Twitter_agrar is supplied by the agrometeorological

department and has about 220 followers.

YouTube

YouTube is a ver y impo rta nt soci al pl at form.

YouTube is a video portal of Google. Users can view

and upload free of charge video clips on this portal.

With the installation of a professional televisionstudio in the headquarters of the German Weather

Service in Offenbach (Fig. 5) it was possible to

Figure3: Twitter_presse

Figure4: Twitter_agar

Figure5: TV-studio

at DWD-Headquarters

Figure 5:

DWD on YouTube

produce video clips that were used for YouTube

(Fig. 6) from March 2011 on. The goal was position

DWD in social medias and also to be able to

communicate professionally with the public in

case of severe weather events and crisis situations

as well as in-house with its own employees.

Regularly, interesting meteorological topics andstories about DWD are produced and uploaded to

YouTube. Every day a moderated weather forecast

clip is produced by the media department of the

central forecasting office. However, for legal


21/47


reasons it is open only to employees on the

intranet. Only in the case of severe weather will it be

opened to the public. Hereupon the clip will be

referenced on DWD’s warning page, on Facebook

and Twitter. The YouTube clips in a severe weather

event are then regularly linked by press agencies or

used on television.

Regularly the DWD YouTube page has 5000- 9000

followers daily, during severe weather events even

reaching 30000-40000 followers.

Flickr

Flickr is a commercial web-service portal that allows

users to upload digital and digitized pictures with

comments and notes to the site and make it avail-

able to other users.

DWD’s Flickr-site (Fig. 7) is supplied with interesting

weather pictures by the PR department and has

about 69 000 followers. The aim is also to position

the DWD in social media.

Summary

The DWD is widely represented in the field of social

media. This had already started with a daily

newsletter with interesting, current meteorological

topics in 1994, but later reinforced with Facebook,

Twitter and Flickr at the beginning of 2011. With the

installation of a professional television studio in the

headquarters of DWD, it was possible to produce

video clips for Youtube – demonstrating the idea

that 'a picture or a clip says a thousand words'.

The activities focus on:

• Positioning DWD in social media

• Interaction with the public

• Customer service

• Crisis communication

References

• Leitfaden Social Media, BITKOM 2010,

Bundesverband Informationswirtschaft,• Telekommunikation und neue Medien

e. V., Albrechtstraße 10 A, 10117 Berlin-Mitte

• www.facebook.com/DeutscherWetterdienst

• www.twitter.com/dwd_presse

• https://twitter.com/dwd_agrar

• www.youtube.com/DWDderWetterdienst

• www.flickr.com/deutscherwetterdienst

Figure 7: DWD onFlickr


22/47


Bow Echoes :

Conceptual Schemes and European RelevanceLaurent Goulet, Meteo France

Introduction

Convective systems are traditionally classified

according to three categories: single cells, multi-

cells and supercells (Weisman and Klemp, 1982).

These features can be discriminated in various

ways, such as by their complexity, the environment

in which they develop, or their consequences. For example, amongst the three categories, supercells

are probably the most complex.

However, convective reality is more complex than

prescribed by this classification. For instance, the

multicell class encompasses convective systems

whose effects can be very different. For example,

squall lines, which belong to this category, are very

mobile systems providing potentially strong gusts.

On the other hand, multicells can be also station-

ary, as regularly observed in Mediterranean regions,

instead giving significant rain accumulation.

Moreover, in some cases, classification into three

categories appears coarse. Indeed, intermediate

kinds of convective organization can occur. They

possess attr ibutes from different classes.

Sometimes one could define a new category.

This is probably the case regarding bow echoes.

In fact, in terms of consequences, scale, structure

and mechanism, bow echoes could be summarized

as an intermediate system between a supercell and

squall line. This question will be developed later in

this paper.

A convective system may be named a “bow echo” if

its rain pattern seen from radar shows a bow-

shaped envelope. The case observed on September

12th, 2004, in southeastern France, matches this

criterion well (Figure 2b).

Nolan (1959) was first in noticing a link between

this particular feature and dangerous manifesta-

tions, such as tornadoes.

But an important step was made by Fujita (1978)

who provided pioneering work regarding bow

echoes. In particular, he proposed a simple

conceptual model which described the typical life

cycle of a bow echo (Figure 1) and its three dimen-

sional dynamic structure. In the Fujita framework,line-end vortices, the Rear Inflow Jet (RIJ),

and the strongest winds at the bow’s apex

were described.

Since then, knowledge about bow echoes

has much progressed. In part icular,

researchers have focused on two facets.

The first topic concerns the “optimal” envi-

ronment favouring such a phenomenon

(Johns and Hirt, 1987; Evans and Doswell,

2001). The second one is about mecha-nisms which are responsible for the

strongest winds. In this respect, great atten-

tion has been paid to mesoscale vortices

related to bow echoes’ life cycle (Weisman,

1993; Atkins et al, 2005; Wakimoto et al,

2006).

Bow echoes were “invented”, then mainly studied,

in the USA. However, the bow echo concept also

seems relevant in a European context. Indeed, in

recent years, several articles concerning this topicand mentioning cases observed in Europe have

been published: Schmid et al (2000), Gatzen

(2004) and Punkka et al (2006) investigated Swiss,

German and Finnish cases respectively.

Figure 1 : COMET illustrationof typical bow echo life cycle,as firstdescribed

byFujita (1978). One cannotice three main steps :the Echo(A),Bow Echo(B andand C), and CommaEcho(D and E) phases.


23/47


which an equil ibrium has to exist between

baroclinic vorticity (associated to density current),

and environmental vorticity (related to vertical wind

shear).

Moreover, we give some insights about environ-

mental conditions favouring such a convective orga-

nization. A short review will be proposed.

To conclude this article, we outline some checks

linked to bow echoes observational footprints,

giving some precious information about the convec-

tive system. Also, some perspectives will be provid-

ed regarding forecast and monitoring.

Typical Life Cycleand Key Features

A bow echo is at first characterized by a bow-

shaped pattern as observed by a radar network

(Figures 2b and 3). Its size is of the order of tens to

150 km (compared to 100 to 1000 km for a squall

line). Its lifetime varies from tens of minutes to

several hours.

Before the development of the ‘bow’, the convective

context is very varied. Convection can be very orga-nized such as in supercells or squall lines (Figure

2a), respectively in 15 and 40% of cases (Klimowski

et al, 2004). But in a slight majority of cases (45%),

convection appears only weakly organized.

Typically, bow echo formation starts from a cluster

of more or less independent cells. Then these

merge, leading to a flat system (Figure 1, step “A”).

Afterwards, the reflectivity pattern starts to bow.

Concavity rapidly amplifies (Figure 1 – phases B

and C, Figure 2b). In some cases, reflectivities showa spearhead shaped pattern (Figure 3). This evolu-

tion is related to a strong acceleration of midlevel

flow at the rear of the convective system: the so

called Rear Inflow Jet (RIJ; Fujita, 1978).

The Rear Inflow Jet is focused around the centre of

the line, helping its deformation and subsequent

bow pattern evolution. The RIJ is an important facet

of a bow echo. It is at the heart of its dynamics. As

the RIJ goes down to the ground, it leads to strong

acceleration of the surface wind with substantialstraight-line damaging effects (sometimes from F0

to F1 level). One reliable radar signature of the RIJ is

the Rear Inflow Notch (“RIN”), a channel of weak

radar echo (Figure 3; Przybylinski, 1995).

Figure2 :

Radar reflectivities

from theFrenchradarnetwork

ARAMIS,at 2245utc(a),0015 utc (b),0100 utc(c).This sequence

shows typicallife cycle ofabowecho froma linear squallline(a)to acommashapeecho (c). Thebowecho

phase seems tobe consecutiveto merging of thesquall line

withthe“convective

systemnumber 2”, off thePyrénéesOrientales.

The goals of this paper are multiple. First of all, our

purpose is to provide the main structural and

temporal characteristics of bow echoes: spatial

organization, typical life cycle, internal dynamics,

consequences in terms of hazards, and their

various footprints in observations.

Secondly, we focus our attention on the mecha-

nisms responsible for this unique kind of convec-

tion. In particular, we place emphasis on building

mesovortices, RIJ dynamics, and the “RKW”

(Rotunno, Klemp and Weisman, 1988) paradigm, in


24/47


Rear Inflow Jet dynamic is partly connected to

midlevel (from 3 to 7 km above ground level) line-

end mesovortices: the bookend vortices. These can

strengthen the RIJ, provided they are sufficiently

close to each other. For a bow echo which is north-

south oriented, the northern and southern vortices

are respectively cyclonic and anticyclonic. In the last

part of the bow echo life cycle, the cyclonic vortex

generally prevails. Convective organization then

resembles a ‘comma’ pattern (Figure 1 – phases D

and E, Figure 2c). In general, the size of vortices is of

the order of tens of km.

not negligible. The tornadoes are mainly F0 to F2.

But stronger intensities (F3 to F4) have already been

observed.

Some studies suspect a link between midlevel

vortices, in particular the northern cyclonic one,

and tornado genesis (Funk et al, 1999). In this

context, the influence of midlevel eddies may

extend toward ground level.

Many tornadoes seem also to be generated by low

level meso- γ-scale vortices, in addition to midlevel

ones. These typically form in the 0.3

km above ground level layer, along

the leading edge of the convective

system (Atkins et al, 2005). Suchvortices whose size is of the order of

several km are not systematically

organized into couplets. They can

appear solitary. It may be possible

to discern between the tornadic and

the non-tornadic vortices. Thus,

tornadic vortices are stronger, long-

lived (duration longer than 1 hour)

and deeper (Atkins et al, 2005).

Low level vortices could also beimplicated in straight line wind

damage, in association with the RIJ,

by modulation of pressure gradient

(Wakimoto et al, 2006).

The density current or cold pool is

another important component of bow

echoes. Several studies revealed

that it is deeper than earlier thinking

suggested; it typically approaches

3-5 km in depth, with thermal deficit

from 6 to 8°C (Bryan et al, 2004).This is not surprising, as the cold

pool plays a fundamental role regard-

ing gusts occurrence and more

generally in the life cycle of the

storm. Recently, Adams-Selin et al

(2010 and 2013), have proposed

mechanisms in which a cold pool

contributes to bowing development by the intermedi-

ate mechanism of a so-called mesohigh surge.

To conclude, bow echo structure is sometimes rela-

tively complex. One or several embedded bowing

segments may develop inside a larger scale system,

which can also be a bow echo or a squall line.

Bowing segments generally have their own bookend

Figure 3:(a)Typicalhorizontal pattern of a bow echo.Onecan notice: a) bookend

vortices(“MV”)at theendsof theconvectiveline ; b) theRearInflowJet (greenarrow),amid-tropospheric current comingfrom therear ofa bow echo ; c) lowlevelmeso vortex whicharecloseto theconvectivesystem boundary ; d) therear inflow notch,a channelof weak reflectivityin thethe stratiform region ; e) theapex, thesummit of theconcavepattern.(b)Mediterranean case ofthe17 august, 2004. Some keyfeatureshavebeen reported.

Such mesovortices may strongly determine the

hydrometeors’ distribution, and therefore the reflec-

tivities. Thus a rolling up of high reflectivities may

reveal the existence of an eddy (Figure 2c).

In the USA, around 20% of the total number of

tornadoes may be induced by bow echoes and

squall lines (Tessendorf and Trapp, 2000). This is


25/47


mesovortices, and their own RIJ, which represents

typically a local strengthening of the system scale

RIJ.

If a bow echo or larger scale system containing bow

echo(es) is sufficiently intense, one can call it a

derecho.

Mechanisms

a. The “RKW” Paradigm

The cold pool is very prominent inside bow echoes.

Its boundaries are a site of strong baroclinic hori-

zontal vorticity. In the “RKW” theory (Rotunno,

Klemp and Weisman, 1988), this baroclinic vorticity

has to be more or less balanced by the environmen-

tal one (associated to wind shear). According to this

condition, ascending motions are more upright and

stronger. More severe and more durable storms are

favoured.

b. Mesovortices Genesis

One important bow echo attribute is its mesovor-

tices. These have two types: 1) the midlevel bookend

vortices, which appear at the ends of the convectiveline; 2) the low level vortices which develop along

the leading edge of the storm system.

Mechanisms proposed in various studies are gener-

ally based on the tilting of horizontal crosswise

vorticity (Figure 4). Horizontal vorticity is most of the

time baroclinically generated along the cold pool

boundary. However, some of the genesis of the

midlevel bookend vortices, could alternatively, at

the beginning, be the result of environmental wind

shear.

Concerning low level vortices, some uncertainties

exist. Moreover, mechanisms have to be found for

both solitary structures and couplet ones. Tilting of

crosswise vorticity is not compatible with solitary

vortices. Indeed, it necessarily generates couplets.

Thus, tilting must imply streamwise vorticity, that is

the component of the vorticity parallel to the storm

relative flow.

As vertical vorticity has been created, stretching byascending motions amplifies the whirling motion.

Moreover, stretching of the planetary vorticity

(terrestrial rotation effect) explains why cyclonic

circulations prevail finally, in particular in the north-

ern bookend vortex (Weisman and Davis, 1998;

Weisman and Trapp, 2003).

c. Dynamics of the Rear Inflow Jet (RIJ)

The RIJ is an important component of bow echoes.

At first, it is partly responsible for the strongest

winds, in particular as it is associated with low level

meso- γ scale vortices (Wakimoto et al, 2006).

Secondly, it is a very active piece in the complex

puzzle of bow echo dynamics.

Figure 4: Main mechanismrelative to bookendmesovortices

genesis. Thisone is based on upwardtiltingof baroclinichorizontalvorticity. Baroclinic horizontal vorticity appearsalong boundariesof the densitycurrent.

Bookend vortices genesis : primary mechanism

Figure5: Complementary explanationsof theRear Inflow Jet.

Lafore and Moncrieff (1989)’s explanation(a) is based on the xistence of a pressure gradient from front to rear of theconvective

system.Weisman(1993)proposean alternativemechanism,implying generatedbaroclinically vorticity, in relation to cold poolandthestratiformpart(asassociated latent heating).

(b) RIJ : vorticity paradigm (from Weisman, 1993)

(a) RIJ : the pressure gradient paradigm (from Lafore and Moncrieff, 1989)


26/47


The RIJ has received various explanations, which are

generally complementary. Historically, Lafore and

Moncrieff (1989) were the first to formulate an inter-

pretation. According to the authors, the RIJ results

from a midlevel pressure gradient between the rear

and the front of the precipitating system stratiform

part (Figure 5a). In particular, the RIJ develops from

a midlevel meso low at the beginning of the strati-

form part.

Weisman (1993) offers an alternative explanation in

terms of baroclinic vortices. As the stratiform part

forms, the cold pool intensifies as it spatially

extends. Thus a baroclinically generated vortices

couplet is set up at the rear of the stratiform part,

driving the RIJ (Figure 5b). One of the vortices isclose to the cold pool boundary, while the other is

linked to the buoyant airmass around the back limit

of the stratiform area. Weisman (1993) also propos-

es that bookend vortices can accelerate the RIJ

when they are sufficiently close to each other.

d. The Mesohigh Surge (Figure 6)

Recently, Adams-Selin et al (2010, 2013) put

forward a new proposal. They observed an intrigu-

ing phenomenon they called a mesohigh surge. It

corresponds to a sudden and local pressureincrease ahead of the convective line, just before a

bowing phase. The pressure surge implies winds

rotation perpendicular to the system orientation.

This could help in creation of a bow echo.

The mesohigh surge may be the result of a gravity

wave, and could be promoted by the following

causal chain: 1) Strengthening of the RIJ, 2)

Subsequent intensifying of evaporative cooling and

downdrafts, 3) The cold pool becomes sharper and

triggers a gravity wave which propagates ahead of

the line.

e. Bowing Development : Concluding Remarks

A bow echo seems to be the result of two kinds of

processes:

1- The RIJ may have a direct effect, putting out of

shape the convective envelope. This could be partic-

ularly true as the RIJ is focused by bookend vortices.

2- As the RIJ intensifies, evaporative cooling increas-

es under the stratiform part, stimulating the cold

pool, and thus leading to a gravity wave. This one

produces a mesohigh surge ahead of the system,with rotation of winds. This could help promote a

bowing phase.

European Relevance

Bow echoes were first identified in the USA (Nolan,

1959; Fujita, 1978). In Europe, the first studies

concerning bow echoes date backto the mid-nineties

(eg, Ramis et al, 1997). If interest remains less than

in USA, various studies show that no European coun-

try is immune to this phenomenon (Figure 7). Indeed,bow echoes have been observed in Scandinavia

(Punkka et al, 2006), in Central Europe (Tuschy,

2009; Gatzen, 2004; Schmid et al, 2000), in Great

Britain (Clark, 2007; Clark et al, 2014), and in

Southern Europe (Ramis et al, 1997). Moreover, one

can identify typical “American” features - mesovor-

tices, tornadoes and damaging winds and the RIJ.

Consequently, European bow echoes seem similar to

their American counterparts.

Figure6: “Meso highsurge”. Oneintriguingcompanion phenomenomof bowingsegmentapparition (Adams-Seilinet al,2010).This

consists in a sharp pressure increaseahead of thelinear convective system,just beforea bowingstep. Meso high surge implieswinds rotation.This onecould help bowecho development.


27/47

MeteorologicalIngredients (from Johns and Hirt, 1987;Evans and Doswell,2001; Burke andSchultze, 2004; Cohenet al, 2007…)

Bow echoes can emerge all year

round. Of course, they are more

frequently observed during the

warm season from May to

August. However, this already

shows that bow echoes can

develop in various environ-

ments.

Typical ly, one observes a

dichotomy between warm

season cases and cold season

ones (table 1).

Generally, cold season bow

echoes are driven by both

strong synoptic-scale high level

forcing and fast mean flow,c ompe ns at in g l ow C AP E

(Convective Available Potential

E ne rg y) a nd l ow D CA PE

(Downdraft Convective Available

Potential Energy) respectively.

In summer, the situation is

reversed: the CAPE/DCAPE is

here the determining factor. The

DCAPE, and especially the CAPE

must be elevated. For instance,

CAPE has to approach 2500

J/kg. One explanation is that

CAPE and upper level forcing

both control convective ascend-

ing movement, while DCAPE

and mean flow both regulate wind gust magnitude.

During warm season, if the value CAPE/DCAPE is

fundamental, it is nonetheless not discriminating

(table 2). In other words, high values of CAPE/DCAPE

are necessary, but not sufficient. Other more discrim-

inating ingredients have to make their contribution

First of all, the mid-troposphere has to be very

unstable, that is characterized by a sharp vertical

gradient of the temperature: at least -7.3°C/km.

Furthermore, intense bow echoes require fast flow

from mid to upper level (typically 35 to 40 knots in

the [4-8km] layer). This increases the probability of

strong gusts via vertical transfer of momentum in

downdrafts.

Mid to upper flow also affects the speed of theconvective system. Indeed strong flow involves a

fast system, thus a vigorous one: the density

current progresses more rapidly and develops more

convergence with anterior warm air.


Figure 7: Several European bowechoeexamples:

(a) inSpain(20ms-1 ), (b) in England, (c) in Switzerland (44ms-1 ), (d) in Germany(42ms-1 ), (e) inFinland(51ms-1 ) and (f) in France (42 ms-1 ).

(a) The 22 July 1995 at 1511 UTC, in Switzerland.From Schmid et al (2000)

(b) The 28 May 2009 at 1531 UTC, in Germany.From Tuschy (2009)

(c) The 5 July 2002 at 1545 UTC, in Finland.From Punkka et al (2006)

(d) The 19 July 2013 at 0330 UTC, in SoutheasternFrance. From DIRSE/PREVI (2015)

(e) Finland (f) France


28/47


Fast system speed is in fact particularly favoured

here. In effect, the angle between deep wind shear

and mean wind is generally weak ([0-4 km] as [0-6

km] layer). In other words, the propagative compo-

nent and the advective one of the motion of the

system as a whole add up. By virtue of the “RKW”

theory, the direction of the wind shear provides

more or less the direction of the propagative

component. More precisely, ascending movements,thus new cells, are promoted close to the downs-

hear boundary of the cold pool.

According to Cohen et al (2007), a convective

system may be even accelerated as low level warm

advection and axis of maximal instability take place

in front of it, more or less aligned with mean flow.

The authors recall also that an elongated zone of

intensified instability ahead is crucial for longevity

of such a very mobile storm.

Besides, wind shear is here also a discriminating

parameter, more particularly “deep” shear.

Shear is not only required at the cold pool level

(up to 3 to 5 kilometers), but also at mid to upper

level. The reasons for this are not yet clear, but

the main idea is that lifting may be reinvigorated

somewhere over the cold cool (for instance,

Coniglio et al, 2006).

Parameter

CAPE

DCAPE

Mean flow

(0-6 km)

Deep shear

(0-6 km)

Hight level forcing

Warm period

MUST BE HIGHT(2500 J/kg)

HIGHTER

WEAKER

(20/30 kt)

WEAKER

(20/30 kt)

CAN BE WEAK

Cold season

LOW

LOW

MUST BE STRONG

(45/55 kt)

MUST BE STRONG

(45/55 kt)

MUST BE STRONG

Table 1: Synthesis of some studies (Johns and Hirt, 1987; Johns,1993; Evans and Doswell , 2001; Burke and Schultze, 2004;

Cohen et al, 2007 etc…) describing “climatological” ingredientsrelative to bow echo emergence. Seasonal dichotomy is hereemphasized.

Parameter

CAPE

DCAPE

(2-6 km) Lapse rate

Mean flow (0-6 km)

Mean flow (6-10 km)

Mean flow (4-8 km)Angle between (MCS motion,

shear, mean wind)

MCS SPEED

Tropospheric Shear (0-10 km)

Deep Shear (0-6 km)

Low Level shear (0-2 km)

Discriminator

NO

NO

YES

-

YES

YES

YES

YES

YES

YES

YES

DERECHO(Strong Bow Echoes)

Warm Season

HIGH

HIGH

≤ -7.3°C/km

20/30 KT

FAST (45 KT)

FAST (35/40 KT)

VERY WEAK

FAST (40 kt)

STRONG (40/50 kt)

STRONG (30/40 kt)

SMODERATE (20/30 kt)

Table 2: Synthesis of Cohen et al (2007)’s study regarding warm season derechos (strong bow echoes). Several parameters arereviewed according to their discriminating character.

Figure 8: Illustration of the positive influences of anenvironmental midlevel jet (around 700 to 600 hPa). A jet

permits more rapid building of a cold pool whose baroclinic vorticity can be balanced by stronger low level shear (RKW

paradigm, 1988). Moreover environmental jet favorstriggering of the meso-scale RIJ.

Finally, beyond the shear problem, the vertical

distribution of wind may have also some important

implications for bow echo organization. An ideal

wind profile could be characterized by a jet at

midlevel (Figure 8; see also Punkka et al, 2006).

Indeed, a jet (or even a small increase of the wind at

upper level) helps a more rapid building of a cold

pool1 whose horizontal baroclinic vorticity can be

balanced by stronger low level shear (RKW para-

digm), and be tilted for quicker building of bookend

vortices.

Bow echoes may be promoted via a sharper cold

pool and anticipated meso-scale RIJ triggering,

favoring an environmental jet and earlier bookend

vortices.

1- A cold pool is favoured because: 1) Stratiform development isanticipated and 2) Dry air is injected by the rear of the system.Convective system organization is such that perfect decouplingexists between cold pool building and the warm conveyor.


29/47


Concluding Remarksand Bow Echo Monitoring

A bow echo is a very specific mode of organizedconvection. It is more or less intermediary between

the supercell and squall line modes, having the

attributes of both but with some specificities. It is

similar to squall line linear organization with strong

straight line winds. However its scale is smaller,

going from tens to 150 km. On the other hand, bow

echoes and supercells share potential for develop-

ment of vortices and tornadoes. Moreover, bow

echo meso-vortices have mechanisms which look

like those often prevailing within supercells.

As with a squall line and a supercell, a bow echo is a

very dangerous kind of storm, generally associated to

very strong gusts, typically more than 25/30 ms-1.

Despite large improvements in forecasting, predic-

tion of such storm remains a true challenge. Today,

the ingredients for bow echo formation are better

known. They strongly depend on season: high CAPE

(especially)/DCAPE during the warm season, while

high level forcing and strong mean flow dominate

along the cold season. During the warm season, the

factor CAPE/DCAPE is nonetheless not discriminat-ing. Discriminating parameters are rather midlevel

vertical gradient of temperature, deep shear, mean

wind at mid and upper levels, and orientation

between mean flow and shear. More precisely,

warm season bow echoes’ environment is more

sheared, faster, with an alignment between wind

shear and mean flow. In this environment, fast

convective systems are promoted, having more

potential to be severe.

Moreover, to be long-lasting, a bow echo must have

at its disposal a great deal of fuel - warm air, over a

large area ahead of it.

One notices also the existence of an “ideal” vertical

wind profile, characterized by a jet at midlevel. The

advantages of such a profile for bow echoes are that

the cold pool, bookend vortices and rear inflow jet

are clearly promoted.

Numerical forecasting has made great progress.

Finest mesh (< 5 km) models have now the capacity

to explicitly forecast convective systems like bow

echoes, sometimes with great realism (see Figure

9). This new generation of models greatly helps

forecasters.

And observation systems permit an efficient moni-

toring of convective situations in real time. At first,

radar imagery and monitoring of convective merging

can provide some anticipation regarding bow echo

formation. Indeed, Klimowski et al. (2003) observe

that bow echoes are preceded by thunderstorm

mergers roughly 40–50 percent of the time (see

also figure 2)!

Furthermore, radar observation can give valuable

clues regarding severity of bow echoes: 1) The Rear Inflow Notch may provide indication of a descend-

ing RIJ (and risk of very strong gusts, Figure 10a); 2)

Rolling-up at the extremities of a bow echo may

reveal book-end vortices (Figure 10a), implying

acceleration of the RIJ and risk of tornadoes; 3)

Existence of an important stratiform part may

suggest building of a very sharp cold pool, also an

index of severity (risk of strong winds as tornadoes,

Figure 10b).

Figure 9: Simulated refelectivity

produced by (a) AROME,on July

19th, 2011over Southern France;(b)WRF-ARW onMay8th, 2009overMissouri (USA).

(a) AROME reflectivity from the 19 July, 2011 over southernFrance

(b) WRF-ARW reflectivity from the 8 May, 2009 overMissouri (USA)


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Ground level observations can help complete a

characterization of bow echoes. Thus, a sharp

decrease (increase) in temperature (pressure), or a

strong wind gust, may both suggest a prominent

cold pool and a severe bow echo (Figures 10c and

10d) and can alert forecasters about this dangerous

phenomenon.

In the relatively near future, French forecasters willmake more use of real time Doppler Radar data.

These will permit direct access to dynamic attribut-

es and the dangers of bow echoes: RIJ, book-end

vortices, maybe some “large” meso- γ scale low

level vortices, and of course nearby surface wind. A

better characterization should give better anticipa-

tion of these events, more relevant alerts, and finer

monitoring of a convective situation.

Figure 10:Radar imagery is a powerful tool for monitoringbow echoes. In (a),the radar imagery provides clues of midlevel meso-scalevortex and Rear Inflow Jet (notice the Rear Inflow Notch). In(b), thisone

shows an importantstratiform partwhich canbe theclue of an sharpcold pool. Of course, groundlevelobservationnetwork cangive also

valuable information, concerningfor instance thecold pool andgustsoccurrence (c and d): bow echo case of December 10th, 2000,over theNord-Pas de Calais region.

(a) Bow Echocase : Nightfrom 6 till 7september

2005 (southernVar)

(b) Bow Echocase : Nightfrom 6 till 7september2005(TyrrhenianSea)

(c) Bow Echocase :10 December2000, temporalevolutionof 2 mTemperature,Mean SeaLevel Pressureand 10 m Gust.

(d) Bow Echocase : 10December2000 at 14local hour(over Nord-Pasde CalaisRegion)

References

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Adams-Selin, R., and R.H. Johnson, 2013 :Examination of gravity waves associated with the 13March 2003 bow echo, Weath. Forecast., vol. 28,3735-3756,

Atkins, N.T., C. Bouchard, R.W. Przybylinski, R.J. Trapp,and G. Schmocker, 2005: Damaging surface windmechanisms within the 10 June 2003 Saint Louis BowEcho during BAMEX, Month. Weath. Rev., vol. 133,2275-2296.

Bryan, G., D. Ahijevych, C. Davis, M. Weisman, andR. Przybylinski, 2004: An assessment of convectivesystem structure, cold pool properties, and environ-mental shear using observations from BAMEX.Preprints, 22nd Conf. on Severe Local Storms, Hyannis,MA, Amer. Meteor. Soc., 4.2.

Burke, P.C., and D.M. Schultz : A 4-yr climatology of

cold-season bow echoes over the continental unitedstates, Weather and Forecasting, vol. 19, 1061-1074.

Clark, M., 2007: The southern England tornadoes of 20 December 2006, Tornadoes and storm researchorganization, sumo.


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Clark, M., K. A. Browning, C. J. Morcrette, A. M. Blyth,R. M. Forbes, B. Brooks and F. Perry, 2014 : The evolu-tion of an MCS over southern England. Part 1:Observations, Quat. Journ. of Roy. Meteor., vol. 140,

439-467.

Cohen, A. E., M.C. Coniglio, S.F. Corfidi, and S.J.Corfidi, 2007 : Discriminating of mesoscale convec-tive system environment using sounding observa-tions, Weather and Forecasting, vol. 12, 1045-1062.

Coniglio, M.C., L.J. Wicker, and D.J. Stensrud, 2006 :Effect of upper-level shear on the structure and main-tenance of strong, quasi-linear mesoscale convectivesystem, J. Atmos. Sci., vol. 63, 1231-1252.

Evans, J.S., and C.A. Doswell, 2001: Examination of derecho environments using proximity soundings,Weather and Forecasting, vol. 16, 329-342.

Fujita, T.T., 1978: Manual of downburst identificationfor project Nimrod. Satellite and MesometeorologyResearch Paper 156, Dept. Of Geophysical Sciences,Universityof Chicago, 104 pp.

Funk, T.W., K.E. Darmofal, J.D. Kirkpatrick, V.L. DeWald,R.W. Przybylinski, G.K. Schmocker et Y-J Lin, 1999 :Storm reflectivity and mesocyclone evolution associ-ated with the 15 April 1994 squall line over Kentuckyand southern Indiana, Weath. and Forecasting, vol.14, 976-993

Gatzen, C., 2004: A derecho in Europe: Berlin, 10 July

2002, Weather and Forecasting, vol. 19, 639-645. Johns, R.H., and W.D. Hirt, 1987: Derechos : wide-spread convectively induced windsorms, Weather and Forecasting, vol. 2, 32-49.

Klimowski, B.A., M.J. Bunkers, M.R. Hjelmfelt and J.N.Covert, 2003: Severe convective windstorms over thenorthern high plains of the United States, Weather and Forecasting, vol. 18, 502-519.

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Wakimoto, R.M., H.V. Murphey, A. Nester, D.P. Jorgensen, and N. Atkins, 2006 : High winds generat-ed by bow echoes. Part I : overview of the Omaha bowecho 5 july 2003 storm during BAMEX, Month. Weath.Rev., vol. 134, 2793-2812.

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Simulation Training

A tool for forecasters and staff Jos Diepeveen, Heleen ter Pelkwijk, Frans Debie, John Kambeel, KNMI

Introduction

Simulation is extensively used for educational

purposes, most frequently by way of adaptive hyper-

media. It is often used in the training of civilian and

military personnel. This usually occurs when it is

prohibitively expensive or simply too dangerous to

allow trainees to use the real equipment in the realworld. In such situations they spend time learning

valuable lessons in a "safe" virtual environment yet

living a lifelike experience (or at least this is the

goal). Often the advantage is to permit mistakes

during training for a safety-critical system.

Simulation training can also be useful in the case of

forecasting the weather.

The Concept

The learning process can be dividedinto three phases:

1) The Education phase: gaining

knowledge

2) The Training phase: transforming

knowledge into skills

3) The Practice phase: applying and

reinforcing skills in near real-time

situations under a lot of pressure

Obviously, the third stage is a

phase in which simulation training could be helpful. At KNMI a wish

had risen to examine all three

stages of the learning process.

Previously, education had tended

to get stuck in phase 1 and 2. This

article will briefly discuss the third

phase.

With use of a simulator, the goal is

to examine two aspects of forecast-

ers’ work, namely the meteorological

content and their working methods/routines.

Furthermore human factors such as communication

and decision-making can be monitored. The large

benefit of this way of training is the creation of a

zero-measurement: All forecasters operate under the

same conditions in the same environment.

Set up

In the schematic below the set up of the simulator

is simply shown. To maintain pressure during the

training, time runs twice as fast as in real life .

Higher speeds are also possible but have not

Image 1: Schematic overview simulator’s set-up

Image 2:Thetestingenvironment

of theforecaster


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proven to be comfortable. The forecaster works in a

separate room with the simulator and a system to

issue forecasts as would normally be done. Also a

telephone is present where incoming questions from

customers are brought into the training. In the

control room the student is monitored and time of

action and kind of action are registered. The control

room is also the place where all the incoming infor-

mation (injections) originates.

Technical details

The ‘Meteo-Simulator’ at KNMI is an update of

Cloudy Camel (Pelkwijk, Higgings, Mills)

Initially it was designed specifically for KNMI, but

now it is also usable for other institutes. It is based

on php/jquery&javascript and adaption to php

requires a webserver (local or company webserver),

some of which can be taken freely from the internet.

Experiences

At KNMI we have developed experience in simula-

tion training over the last few years. In 2014 the

training was evaluated with the use of the newlydesigned Meteo Simulator.

The subjects for training were shift leaders and

senior forecasters:

- Analysis & guidance

• Understanding of the situation

• Recognition of uncertainties

- Procedures

- Products

- Dealing with unexpected situations (chemical acci-

dents)- Handing over to the next shift (presentation)

The chosen meteorological situation was not a triv-

ial case, the main issues were:

- Model analysis of the position of a low was wrong

(about 30-40 nm, image 3)- Winds and precipitation prediction were very

uncertain. (image 3)

- A chemical accident had occurred in an industrial

area with questions about the dispersion with an

uncertain wind-forecast.

Evaluation and recommendations

We realise that this method of training is still in an

early stage at KNMI, and we expect to develop

further in the coming years. The first results arepromising and sat is fying in many ways.

Participants’ feedback was very good. Also the

above-mentioned fact that a zero-measurement can

be made gives a quite good impression of the varia-

tion of skills within the group tested. This helped

very much in finding out strengths and weaknesses

and where the focus for further education should be.

Also the use of a realistic setting within the simula-

tor was very much appreciated. However, objective

assessment is rather difficult and should better be

done by a third party.

Sources/references:

http://en.wikipedia.org/wiki/Simulation#Simulation_

in_education_and_training

Image 3: Screenshot of theMeteo

Simulator. In theleftupper cornerthe simulation time is running twice normalspeed.In the upper row,datasources can be selected.

Thesources arerefreshedautomaticallyas‘simulatortime’progresses.In this screenshot

the main models used atKNMI are shown.Theforecast isobars andwindspeed at

a certaintime (selected in orange in the left column) forHirlam andHarmonie areshown.


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A Global Forecast Quality Score

for Administrative PurposesD. Cattani, M. Matter, MeteoSwiss

Introduction

Since 1985, MeteoSwiss has used a global score for

systematically assessing the basic weather fore-

casts issued by the regional forecasting centres.

This assessment is done for two main reasons.

Firstly, it is used for administrative purposes as the

weather centres are expected to communicate in asimple way to the general public and to the govern-

ment the evolution of the quality of their forecasts.

On the other hand, the forecasters need to know

the performance of their predictions in order to

improve them. In 2013, we developed a new verifi-

cation scheme, called COMFORT (for COntinuous

MeteoSwiss FORecast qualiTy), which also accounts

for benefits from the evolution of the forecasting

system as well as of the present automated obser-

vation networks.

COMFORT is a global measure of accuracy which

verifies deterministic forecasts of quantities repre-

senting sensible weather in Switzerland, namely:

precipitation (without distinction of its type),

sunshine duration, minimum and maximum

temperatures, and wind speed. Specifically,

COMFORT assesses the generic fore-

casts which are edited numerically by

the forecasters. These forecasts serve

then as a basis for generating a variety

of products, ranging from web apps to

agriculture or support for TV broad-casts.

A requirement that COMFORT had

ideally to fulfil was to encode in a

single value the general forecast quali-

ty, together with the capability to

provide intelligible explanation for a

hig h/low g lobal score, typical ly

computed over a long period and over

a large territory, to people that are

neither experts in verification, nor fore-casters. A way of conciliating these

conflicting requirements was to make

it possible to focus on specific periods

and/or geographical areas in order to

detect and analyse forecasts whose accuracy devi-

ates from the average. Also, forecasts for all time-

ranges are verified using the same spatial and

temporal resolutions, which allows comparison

across different lead times. In parallel, COMFORT

can be applied to NWP forecasts, typically the

“First Guess” predictions which initialize the fore-

cast editing tool used by MeteoSwiss bench fore-casters, making it possible to measure forecasters’

added value with respect to NWP.

Data Used in the Verification

Bench forecasters working at MeteoSwiss edit their

predictions with a graphical interface named the

Matrix Editor . These are either numerical values or

categories (the latter only for relative sunshine

which is edited according to five classes) and repre-

sent deterministic forecasts for a number of regions. The spatial resolution of a forecast edited

in the Matrix Editor depends on the forecast’s time-

range. The Swiss territory is partitioned into 27

regions for short-range forecasts (time-ranges D1

and D2), into 11 regions for medium-range fore-

Figure1: Matrix tool

Tool used bythe forecaster bywhich they modify a firstguess, with a stationcorr

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