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© Crown copyright 2007 Page 1
Rainfall predictionDr. Brian Golding
Head of Meteorology Research & DevelopmentMet Office
June 2007
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Outline
Extreme rainfall eventsRadar observationExtrapolation nowcastingNWP nowcasting
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Extreme rainfall totals of the UK, Japan & the world
Takara, 1996, 97, 2004
Maximum Precipitation records of the world
Preston 1893
Eskdalemuir1953
Maidenhead 1901
Halifax 1989
Martinstown1955
From Takara (2004)
UK record (mm)
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Extreme rainfall events, 1900-2005, by weather type
Convection
Organised Convection
Front with embedded convection
Front
OrographicBoscastle, Cornwall, 2004
Kent, 1973
Lynmouth, Devon, 1952
Ffestiniog, Wales, 1973
Hampstead, London,1975
Halifax, Yorkshire, 1989
Horncastle, Lincolnshire, 1960
Martinstown, Dorset, 1955
+ = convective
X = convective with frontal forcing
ж = orographic
∆= frontal with embedded convection
□ = frontal
Carlisle, 2005
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Scales of Motion & Predictability
Hail shaft
Thunderstorm
Front
Extratropical Cyclone
Space Scale
Lifetime
Predictability
Nowcast
10mins 1 hr 12hrs 3 days
30mins 3 hrs 36hrs 9 days
2mins 15min 3hrs 18hrs
1000km
100km
10km
1km
MCS*
* Mesoscale Convective System
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Hampstead, 14th August 1975
Extreme rainfall, 170.8mm in 3 hrs - largest record in LondonStorm consisted of successive clouds, forming to south then drifting north along a stationary convergence lineHigh ground may have been triggerRapid runoff due to hard urban surfaceMajor disruption and several deaths
0
20
40
60
80
100
120
140
160
180
0 60 120 180minutes
Cumulative rainfall (mm)Rainfall rate (mm/hr)
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West London flood, 3rd August 2004
Silverlink rail services affected between Willesden & Wembley Signals out of action at Charing Cross & London Bridge due to lightning strikeShop ceilings collapsed in StainesDisruption at Heathrow
Parts of Metropolitan, District, Piccadilly and Central Lines closed by floods.
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Rain rate profile on 3rd August 2004 at High Wycombe
0
20
40
60
80
100
120
140
160
180
200
14:52
14:54
14:56
14:58
15:00
15:02
15:04
15:06
15:08
15:10
15:12
15:14
15:16
15:18
15:20
15:22
15:24
15:26
15:28
Time (UTC)
Rain
rat
e in
mm
/hr
(1 minute average rates)
41mm total
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InformationInformationContentContent
Forecast LengthForecast Length
LogLogScaleScale
Log Scale (days)Log Scale (days)0.010.01 0.10.1 11 1010
NowcastNowcast(Extrapolation Forecast)(Extrapolation Forecast)
LimitLimit((““Perfect ForecastPerfect Forecast””))
NWPNWP(Model Forecast)(Model Forecast)
Blending extrapolation & NWP
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The role of Nimrod
A rapid update cyclefor
very short range forecasting of
hazardous weather variables using
high resolution observationsmerged with
NWP forecasts
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Outline structure of Nimrod processing
Each cycle (15 mins)Image processing (radar & satellite)Data fusion (images, lightning fixes, in situ obs)Extrapolation (to T+6 hours)Merging with downscaled NWP (4km grid)Diagnosis
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Apply gauge adjustment factor
Correct for variations in the vertical reflectivity
profile
Remove spurious echoes
Check for corrupt imageInput Image
Quality controlled and
corrected image
Radar data processing
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Raw radar data Radar data (qualitycontrolled and corrected)
25 50 75 100 150 200 250 300 500 mm
Rain gauges(3494 gauges used)
Monthly rainfall for February 1997
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Raw Radar data Radar data (qualitycontrolled and corrected)
25 50 75 100 150 200 250 300 500 mm
Rain gauges (3463 gauges used)
Monthly rainfall for June 1997
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Generate analysis for full domain by blending:
Corrected radar compositeSatellite visible and infrared imagery calibrated against radarLightning fixes (minimum rain rate)A very short range forecast Surface in situ weather reports
Radar is used where it has good visibilityA variational analysis, combining the
remaining data sources, is used elsewhere
Producing a rain analysis
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“Optic Flow” extrapolation method
Given the “Optic flow” equationdp/dt = ∂p/∂t + u.∂p/∂x + v.∂p/∂y = 0with p the precipitation rate
Calculate estimates of ∂p/∂t, ∂p/∂x & ∂p/∂y in each nxn pixel block containing precipitationFind values of u,v that minimise the residualSmooth the results and extend to non-precipitating areas
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3-hr Gandolfforecast
Observed 06:45
Object forecast
Observed 09:45
Optic flow forecast
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Merge with NWP model forecast
NWP provides forecast information in areas where the extrapolation forecast has areas of “missing data”Introduces an element of growth and/or decay into the forecastsEnsures continuity between forecast products
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Accuracy of 15km rain accumulation vs radarover UK, Oct 03 - Sep 04
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3
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7
0 60 120 180 240 300 360 420 480Lead tim e in m inutes
RMSF
mesoscale model
advection
Nimrod
Persistence
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Performance Improvements
RMS surface pressure error over the NE Atlantic
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Influence of gridlength on Boscastle forecasts
60km forecast from 00UTC
Forecast rainfall accumulations for
1200-1800UTC 16/8/2004
12km forecast from 00UTC 4km forecast from 00UTC 1km forecast from 00UTC
5km radar actual
Clark, 2006
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Lower scale limit of skill 3rd Aug 2004
Uncertainty scale above which forecast of exceedanceof 8mm has useful skill: 12km (blue) and 1km (red) models initialised at 0900UTC 3rd August 2004
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Lower scale limit of skill for 13/8/2006
Uncertainty scale above which forecast of exceedance of 4mm has useful skill: 12km (blue)
and 1.5km (red) models initialised at 0900UTC 13th August 2006
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Summary
Nowcasting, based on extrapolation, can provide useful very short range forecasts of high impact weather variables
Convective scale Numerical Weather Prediction should be able to provide more accurate nowcasts at longer lead times
Success will require fundamental scientific research, as well as bigger computers. In particular:
New approaches to data assimilation using new data sources such as radar
Improved models, which represent both boundary layer and convective cloud processes more accurately
Better products which convey the information required by the user