The UK weather radar network current and future ... UK weather radar network current and future...

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The UK weather radar network current and future capabilities including the upgrade to dual polarisation.

Dr Jacqueline Sugier, Radar R&D, Observations, Met OfficeRMetS National Meeting, 20th March 2013


Very important:• Underpin many Met Office services• Used for weather and flood forecasting• Provide vital information for emergency

response teams• It is our only observing system with the

spatial and temporal resolution to resolve convective cells.

Historically, the UK weather radar network has provided detail information regarding the intensity, the location, distribution and evolution of precipitation.

The Importance of Weather Radar

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Early days of the UK Radar Network

The network declared operational in 1985. It was comprised of 5 radars covering England and Wales (2 S-band and 3 C-band single polarisation non-Doppler radars).

The network was delivering in real-time:

• 2km resolution single site rainrate data every 5 mins to remote users such as the regional water boards.

• And 5km resolution composite rainfall map of England and Wales every 15 mins.


UK Radar NetworkStatus February 2013

UK radar network now comprised of 15 radars:• All the network but 1 are Doppler

radars• 2 dual polarisation Doppler radars

(yellow dots).• All C-band

With increased telecomms and computing capabilities, raw data is now sent directly to Exeter for central processing (limited processing at site).


Outline

• Radar Network upgrade to dual polarisation

• UK Weather Radar Network products and capabilities

• Continuous development


Outline• Radar Network upgrade to dual

polarisation



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Network Renewal: Top-level Aims

• Continuity - avoid running into age–related failures. (Hameldon Hill radar is 33 years old and has completed ~14M scans!)

• Use latest technology to increase radar capability ( e.g. dual-polarisation to improve quality control and rainfall rate accuracy).


Weather Radar Network Renewal Design

Radar systems and software have been developed in partnership with academia and industry to create a bespoke and fully customisable radar system and network


OSA radar design overview

PC for

Radar control,

GUI &

Product

Generation

Aerial mounted

Receiver &

Digitiser

C-band

Transmitter

Waveguide

switch:

Split power H & V

or Full power

H polarisation

Central processor

Radarnet

Met Office HQ

Exeter

Radar cabin


Analogue receiverDual Channel Analogue receiver design - Based on current UK network single channel standard (Met Office /Pascall. UK)

Performs down-conversion from C-band to intermediate frequency (IF) –30MHz


DigitiserVHDL digital down converters are used to convert I & Q to baseband prior transferring them to the Cyclops PC for further signal processing and product generation.

Met Office design based on Pentek (US) card:

• 4 channels A/D – 200 MSPS 16 bits analogue to digital converters, controlled by Virtex-6 FPGA

• 30 channels of digital IO


Cyclops-D4 PC system

Cyclops-D4 provides:• Radar control • Signal processing/Product

generation: PC based C++ software. Produces Z, V, SQI, CI, CPA, N, ZDR, ρHV, ΦDP, and LDR, at a resolution of 1 deg per 300 / 75 m for long / short pulse respectively.

• System monitoring• FTP/ remote desktop

connection

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Renewal on the ground

• Replacement of the receiver, antenna, radome and waveguide system, and refurbishment of the pedestal.


Re-engineering of the pedestal

• New motors, gearboxes, servos, supplied by Danaher motion (Germany) Brushless AC Motor with epicylic gearbox

• Control via CanOpen interface

• Hengstler 14 bit Synchronous Serial Interface encoder for absolute position information


Re-engineering of the pedestal

Old motor (~ 25 kg)and gear head (~ 35 kg)

New motor (11 kg)and gear head (13 kg)


Dual polarisation antenna

• Reuse the Precision (UK) 3.7m dish• New feedhorn design for dual

polarisation (Q-Par, UK) • Cross polar isolation design goal of -

40dB,• Gain 43dB minimum,• Sidelobe design goal of -30dB within

+/- 10 deg and -35deg outside +/- 10 deg.

• 1 degree beam width


Weather Radar Network Renewal Implementation

• Upgrade to a 3rd and 4th operational sites are underway (Castor Bay-NI; and Predannack -Cornwall). 5 installations planned for FY 13/14.

• Completion late FY 15/16

• Doppler capability rolled-out to all but 1 UK radar.

• Dual polarisation design is running at 2 Ops sites (Chenies – NW London; and Thurnham –Kent) and 1 R&D site (Wardon Hill – Dorset).



polarisation




Operational radar products

Rainfall productsderived from radar reflectivity

Doppler productsderived from the phase shift caused

by moving reflecting targets

During the 5 mins cycles, the radar perform 10 scans at different elevation. Data is averaged to 1° x 600m polar resolution.

Rainfall products generated every 5 minsat 5km (Op), 1km (Op), and 500m (R&D)

resolution.

Radial winds are been assimilated into 1.5km NWP model (UKV).


Flagging of Raw Data Single Site Rain Rate Compositing

Rainfall products – Radarnet processing


R&D product:Radar refractivity

• Fabry (1997) demonstrated that refractivity changes can be derived by monitoring change over a time period of the phase of the electromagnetic return signal from stationary targets.

Radar refractivity can provide useful information to NWP models regarding the evolution of the humidity field 30-60km radius around each radar sites, which in turn can provide observation of convergence before a storm forms.

Station obs agree with radar, Nicol et al 2012b).


• The bigger the drop the more oblate it becomes.

• The shape of the raindrop affects how it reflects the vertically and horizontally polarised radar beam.

• ZDR is the ratio between the received power measured in the horizontal (H) and vertical (V) plan:

ZDR = 10* log10( ZH / ZV)

Differential reflectivity, ZDRprovide mean particle shape

~ 3 mm ZDR ~ 1.5dB

Small drops (1mm) ZDR ~ 0dB

Large raindrop > 6mm ZDR ~ 6 dB;

Raindrop size against ZDR


Reflectivity, dBZ Differential Phase Shift, degree

Example of Differential Phase, φDPChenies – 17 Mar 2013

φDP Indicates the relative delay between the Horizontal and Vertical wave → Increase in differential phase related to attenuation of the radar signal.

As rain becomes heavier raindrop become oblate. The horizontally polarised wave will be more affected by more water than the vertically polarized wave.

φDP = φH - φV


Example of ρHV data Wardon Hill - 5 Feb 2013Reflectivity (Z), dBZ Co-polar correlation coefficient (ρHV)

• Correlation between the H and V backscattered field.• Decorrelation occurs if both orthogonal backscattered field do not vary simultaneously. • Good indicates of spurious echoes, bright-band.• Good indicator of the quality of the radar system.


Dual polarisation quality: ρHVassessment

• During light uniform rainevents, the correlationbetween the H and Vchannels should beclose to 1.

• Small imperfection in thecross channel isolationresult in the degradationof the quality dualpolarisation parameters,with their precisiondecreasing as peak ρHVfalls away from 1.

Radar Systems Peak ρHV

EEC Thurnham (C-band, 4.3m antenna)

0.97

Vaisala (C-band, 4.3m dish) 0.997

Colorado State University, CSU-CHILL (S-band, 8.5m antenna)

0.997

Met Office Wardon Hill (C-band, 3.7m antenna)

0.997


Example of LDR dataWardon Hill 5 Feb 2013

Reflectivity (Z), dBZ Linear depolarisation ratio (LDR), dB

LDR = 10* log10( ZHV / ZHH)• Measure of the depolarisation of the H wave• LDR is sensitive to the particle shape, dielectric

constant and orientation of the particle major axis with respect to the plane of the radar polarisation.


Dual polarisation quality: LDR assessment

• LDR used to assess the quality of the dual polarisation measurements

• Lower LDR in rain means better isolation between the H and V channels i.e. better dual polarisation performance

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-42 -41 -40 -39 -38 -37 -36 -35 -34 -33 -32 -31 -30 -29 -28 -27 -26 -25Linear depolarisation ratio, dB

Met

Off

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Freq

uenc

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occ

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ce, %

0.00

0.03

0.06

0.09

0.12

0.15

0.18

Vais

ala

Freq

uenc

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occ

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polarisation




Beyond network renewal

Great opportunity:• Dual polarisation

Signal Processing (Cyclops D4) and receiver was developed in-house – we have complete control over the signal processing.


Our goal: Deliver optimum radar products to improve short-range forecast particularly of severe weather.

Radar reflectivity

Radar refractivityRadial wind

Rainfall

Working with our academic partners, our research focus on:

• Extracting maximum information from the radar network.

• Developing new signal processing and central processing techniques for improving the accuracy of the radar products.

NWP (DA, STEPS, UKPP, VER)

FSD

EA, FFC


Spurious echoes identification

Problem: A major limitation to assimilating radar products into NWP and hydrological models is the presence of non-precipitation echoes in the data.

POD Jan 2012 POD Jan 2013 Average dBZ Jan 2013Improved scheme was introduced last year based on Nicol

et al (IAHS Publ 2012). This technique show clear improvement to the data; however some limitations with sea clutter and RLAN interference.

Going forward: use dual polarisation parameters to improve this scheme further.


Characteristics of weather radars microwave frequencies

From Delrieu et al, 2000: Quantification of Path-Integrated Attenuation for X-band and C-band Weather Radar Systems Operating in the Mediterranean Heavy Rainfall

Extr

eme

rain

Higher frequency: more severe Attenuation during critical events

Severe attenuation of the radar return is a major problem in intense rainfall, particularly during event likely to cause floods.


Dept. of MeteorologyPassive emissions

• Attenuators are emitters (Prevost 1818).

• Radar signal Attenuating storms emit at the radar frequency.

• Emission can be expressed as the total attenuation along the path of the radar beam.

• Radar receiver is sensitive enough to act as a radiometer and measure emissions (transmitter switched off)

Our goal: to bring new emission techniques combined with dual polarisation parameter into operational use for improving the identification and correction of attenuation of the radar return caused by rain and wet radome.


Improved QPE accuracy during heavy rainfall

Problem: Severe attenuation of the radar reflectivity in heavy rainfall cause severe under estimation of rainfall.

New opportunity: use R=f(KDP) instead of R=f(Z). KDP can be used to identify area of heavy rain and derive rainfall estimate without using the reflectivity.

KPD is a phase parameter immune to the error related radar calibration or reduction of the reflectivity factor caused by partial beam blockage, attenuation by precipitation or wet radome. KDP is also less sensitive than reflectivity to changes in the DSD.


Objective: to improve fine-scale measurement and prediction of rainfall and to enhance urban pluvial flood prediction. This will enable urban water managers to adequately cope with intense storms, so that the vulnerability of populations and critical infrastructure can be reduced.

RAINGAIN

Sites: The activity includes the implementation and use of advanced radar technologies (dual polarisation C band) over central London, and (X Band) in Leuven, Paris, and Rotterdam.

Partners: The project gathers 13 partners in Belgium, France, Netherlands and the UK (local authorities, universities and enterprises).


Super-resolution processing

• Can signal processing techniques refine resolution of the rainfall estimates for urban catchments?

• Our goal: 100m or better resolution over central London by 2014

500m data on Invent


Gauge-Radar merging techniques

• The project is assessing the influence of the following parameters on the quality of the merged product:

• Choice of merging method.• Gauge density.• Degree of rainfall correlation.• Polling time

Radar only Gauge only

Merged

Radar onlyRadar only Gauge onlyGauge only

MergedMerged

• Objective: The development and implementation of a high resolution radar-raingauge merged product for use in real-time of quality controlled raingauge data. The scheme is required to run using a 15 minute accumulation time for use in flood forecasting.


Summary

The new UK radar system is delivering top class parameters including dual polarization radar refractivity and passive emission to support research activities focusing to improving short-range forecast particularly of severe weather.


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