Urban Flood Risk Management in a Changing Climate:

Sustainable and Adaptation Challenges

Country Report -Hong Kong, China

ESCAP/WMO Typhoon Committee5th Integrated Workshop

Macao, China, 6-10 September 2010

Hilda LamHong Kong Observatory

Contents

• Quantitative precipitation estimate (QPE) & quantitative precipitation forecast (QPF)

• Current operational arrangement for warning of heavy rain & flooding in HK

• Climate Change and Typhoon Committee Activities

• Benefit of Typhoons

SWIRLS= Short-range Warning of Intense Rainstorms in Localized

Systems• a radar-based nowcasting system operated by HKO sinc

e 1999• 6-min update cycle synchronized with Doppler radars inc

orporating real-time calibration of:– radar reflectivity– rain gauge data

• Tracking algorithms: – TREC (tracking of radar echoes by cross-correlation);– MOVA (multi-scale optical flow by variational analysis)

• outputs– SWIRLS outputs (see slide severe wx map of SWIRLS) in suppo

rt of rainstorm-related warnings – QPF products : flood, landslip warning with DSD & GEO/CEDD– since 2008, GIS-based rainfall nowcast product for PRD (see

slide)

Severe weather map of SWIRLS

24 July 2006• analyzed (solid ellipses) • 30-min nowcast (dashed

ellipses) positions of 4 types of hazardous wx:– hail– severe wind gusts– heavy rain– cloud-to-ground lightning

• “+”, “=”, “o” : – actual lightnings detected

Rainfall nowcast product for Pearl River Delta

based on • SWIRLS QPF • open GIS standard of KML

nowcast rainfall maps (colour pixels in map window)

• can be animated, zoomed, navigated in 3D

• by web plugin software

optional overlaid on rainfall maps• additional geographical info.

– road networks – place names

2010 new - Atmospheric Integrated Rapid-cycle (AIR) forecast model

• In 2010, HKO introduced a new NWP system:• Atmospheric Integrated Rapid-cycle (AIR) f/c mo

del sys.• based on JMA Non-Hydrostatic Model (NHM) • AIR major advancements over RSM: model resolution 60km 10km for T+72 hr f/c

– introduce 2-km resolution NHM operated on hourly update cycle to provide timely guidance to forecasters on high impact weather (include QPF) up to T+15 hr (sample: slide - QPF map)

– operation of a 3DVAR data assimilation system & more sophisticated physical parametrization schemes simulate 3-D air flow; cloud, convective processes

QPF map from 2-km NHM vs. forecast time series at HKO

RAPIDS=Rainstorm Analysis and Prediction Integrated Data-proces

sing System• operation since 2005• 2-km resolution QPF 1-6 hr f/c• optimally blending the SWIRLS & NHM outputs • probabilities of precipitation for various thresholds• by time-lagged ensemble approach• dynamical weightings assigned to SWIRLS/NHM outputs

by real-time verification • phase & intensity correction schemes

– correct spatial errors and biases in rainfall intensity – which occasionally found in NHM f/c precipitation (see slides ble

nded …)

RAPIDS Blended QPF – 3 June 2009 importance of blending NWP QPF at diff. lead times (1 hr)

verifying radar reflectivity

SWIRLS radar-based nowcast

RAPIDS blended QFP

RAPIDS Blended QPF –3 June 2009 importance of blending NWP QPF at diff. lead times (6 hr)

verifying radar reflectivity

RAPIDS blended QFP

SWIRLS radar-based nowcast

Probabilistic rainfall f/c for diff. rain thresholds based on QMORPH global ppt. analysis &

ECMWF EPS perturbed TC tracks

Current research and development activities on QPE/QPF

• upgraded SWIRLS radar tracking algorithm from correlation-based to optical flow-based

• operation of a real-time QPF verification system for effective performance monitoring and algorithm tuning

• development of a new QPE scheme based on radar-raingauge co-Kriging for better rainfall analysis over gauge sparse areas, as well as potential application to raingauge data QC

• a feasibility study to investigate if 1-hour rainfall nowcast could be applied to flood forecasting over a very small catchment area (order of a few sq. km)

Regional rainfall variations could be large

Rainfall distribution map (2-3 am on 15 March 2002)

GOWISE - District Rainfall for HADwith zoom function shown on small panel

Prototype of “Raingauge Black”

Home Affairs Department (HAD) – HAD GOWISEtailor-made wind info.

Tai O storm surge alert - Background

Flooding at Tai O caused by storm surge during the passage of Hagupit in September 2008 (source: Oriental Daily News / on.cc)

Traffic disruption at Lantau cauzed by landslides during the 7 June 2008 rainstorm (source: ISD)

Early Storm Surge Alerts for 5 new locations(x) in New Territories starting 2010

HK Automatic Raingauge Network

Annual TC activities in western North Pacific and South China Sea

0

5

10

15

20

25

30

35

40

45

50

19

61

196

4

196

7

197

0

19

73

19

76

197

9

198

2

198

5

198

8

199

1

19

94

19

97

200

0

200

3

200

6

20

09

No

. of T

C in

wN

P a

nd

SC

S

Annual Total

1960s:35 TCs

after 2000: 27 TCs

Annual TC no. making landfall south China coast ~300 km HK 1961-2008

1960s:3 TCs

1990-2008: 2.5 TCs

1960s:3 TCs

1990-2008: 2.5 TCs

Annual Typhoon no. making landfall south China coast ~300 km HK 1961-2008

~1 typhoon/year

Climate Change and Typhoon Committee Activities

• HKO supported the Typhoon Committee’s initiative in assessing the change in frequency and intensity of TCs in the ESCAP/WMO Typhoon Committee region

• in context of climate change

• by providing staff– Dr. TC Lee– to serve on its expert team

Climatologically, TCs contribute ~30% rainfall in Hong Kong

0

500

1000

1500

2000

2500

3000

3500

400019

61

1964

1967

1970

1973

1976

1979

1982

1985

1988

1991

1994

1997

2000

2003

2006

2009

rain

fall

(m

m)

TC rainother rain

Thank you

• Lionrock – a mountain• TC named by Hong Kong, China• (Source: CEDD)

Sea level rise

Hong Kong 14 cm since 1954

IPCC global prediction (2007) 18 – 59 cm by 2100

Estimates by other independent approaches

Simple correlation between sea level and temperature : + 0.5 to 1.4 m

Non-linear relation with multiple positive feedback : + 0.8 to 2 m

Sources : Rahmstorf, S., 2007. A Semi-Empirical Approach to Projecting Future Sea-Level Rise. Science 315, 368–70.Pfeffer, W.T., J.T. Harper and S.O. Neel, 2008. Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise. Science 321, 1340 – 1343.

Flooding of the coastal areas becomes easier

under tropical cyclone situations

Sea level rise, plus storm surge

waves caused by typhoon rise in sea level

coast coast

Table 1 : Magnitude of extreme sea levels at Victoria Harbour based on past data and a projected rise of 0.41 m (due to thermal expansion of seawater alone)

Extreme sea-level at Victoria Harbour

Return period (year) Extreme sea-level (mCD) based on past data

Extreme sea-level (mCD) after a mean sea-level rise of 0.41 m

2 2.9 3.3

5 3.1 3.5

10 3.3 3.7

20 3.4 3.8

50 3.5 4.0

Note: mCD = metres above Chart Datum.Chart Datum is 0.146 metre below Principal Datum.

Magnitude of extreme sea levels at Victoria Harbour based on past data, a projected rise of 0.59 m and of 1.4 m.

Extreme sea-levels at Victoria Harbour

Return period (year)

Extreme sea-level (mCD) based on

past data

Extreme sea-level (mCD) after a mean of sea-level rise of 0.59 m

Extreme sea-level (mCD) after a mean of sea-level rise of 1.4 m

2 2.9 3.5 4.3

5 3.1 3.7 4.5

10 3.3 3.8 4.7

20 3.4 4.0 4.8

50 3.5 4.1 4.9

Note: mCD = metres above Chart Datum.Chart Datum is 0.146 metre below Principal

Datum.

          Recorded Tides at Tai Po Kau on 12 January 2005

Spring Tide in Hong Kong

Spring Tide : Near New Moon or Full Moon each month, the Earth, Moon and Sun are aligned along a straight line and the sea-level rise and fall will have a larger range.

(courtesy of TVB)

Flooding in Tai O after Typhoon Hagupit (September 2008)

(Image Source: Civil Engineering and Development Department. )

Anti-flood structures