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Use of radar data in modelling at the Met
Office (UK)
Bruce Macpherson
Mesoscale Assimilation, NWP
Met Office
EWGLAM / COST-717 Joint Session
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Outline Radar data for assimilation into:
– atmospheric models
– land surface models
Radar data for NWP model verification Radar data for model validation
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Rainfall Assimilation at the Met Office
1996 - Operational assimilation of 3-hourly rain rates from UK weather radar network into Mesoscale Model
1997 - NWP forecast impact studies on radar data 1997 - test assimilations of lightning data as proxy rainfall 1998 - Assimilation of hourly radar data 2000 - Radar Quality Estimate used in assimilation 2001/2 - French & German radars added to UK composite
2003 - Retuning for ‘New Dynamics’ model version
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Radar Data Impacts - objective scores
(v radar analyses)
Period 1 - significant benefit up to t+6/12
Period 2 - neutral signal
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Radar Data Impacts - subjective
OPER
t+15
NO radar
assimilation
t+15
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Radar Data Impacts: hourly v 3-hourly data
3-hourly
radar data
Hourlyradar data
T+3 T+3
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Radar Data Impacts: hourly v 3-hourly data
3-hourlyhourly
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Relative importance of radar assimilation for short-period rain
forecasts
Radar &
satellite
aircraft
surface
No of Cases
showing
benefit
sonde
Light rain
Heavy rain
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Radar Coverage and Quality
Estimate(Nimrod system)
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Mesoscale Model Rainfall Assimilation
Data: hourly surface rain rates, pure radar -no gauge/satellite
QC & processing (with help from NWP):– clutter & anaprop removal -- bright band correction
– range correction -- orographic enhancement
– weekly radar calibration v gauge Monthly
Totals
Raw radar
Corrected
radar
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Mesoscale Model Rainfall Assimilation (II)
Averaging: 5km raw data ==> 15km (model ~12km)
Assimilation: latent heat nudging (LHN)
– target rainfall analysis Ran = (Rmod + W Robs) / (1+W)
– Robs interpolated from hourly values to model timestep
– assimilation increments ( T) filtered on scale 2x
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Land Surface Assimilation For agricultural and hydrological products, soil
moisture input to mesoscale model Current system: “MORECS”
– rainfall data ~150 daily gauges over UK
– weekly analysis on 40km grid
– interpolated variable is % of average annual rainfall
New system: “Soil State Diagnosis Model (SSDM)”– rainfall data: Nimrod radar analysis
– hourly analysis on 5km grid (same as SSDM)
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UK Mes soil moisture
Relax to climatology
Radar
artefacts
MORECSNimrod SSDM
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Radar wind assimilation
VAD profile availability
(CWINDE project)
• operational since 2001
• monthly monitoring
• quarterly quality reports
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Radar data to verify NWP rainfall forecasts
on monthly timescale, gauges and ‘corrected’ radar tell roughly same overall story
Model skill
Month ==>
v gauge
v radar
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Global Model precipitation verification over U.K
Models 6 models, mostly 09/2000-10/2002 12Z runs up to 72h 24h accumulation up-/down-scaled to 96*96 km2 by box
averaging Observations corrected radar data upscaled to 96*96 km2 by box averaging
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Global Model precipitation verification over U.K., 0-24h
forecasts
Frequency bias
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LAM verification v UK radar
UK Mesoscale DWD LM Hirlam reference …any more?
Work in progress for:
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For short NWP trials and batches of case studies, radar provides – better spatial sampling than gauges
– clearer link with forecaster’s subjective assessment
v radar
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…..radar may allow study of model behaviour with higher resolution in time
Mesoscale Model trial verification
Frequency bias for
hourly totals > 0.17mm
(v Nimrod radar)
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and maps of verification v radar can begin to show land/sea differences
v radar
Rain/no-rain (>0.4 mm/6hrs)Frequency bias t+12-18 hr forecasts
autumn 2000
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Observations of Evaporation
02 Apr 2000 11 Dec 1999
94GHz Radar-Derived Ice Water Content (below 0ºC)
Radar data provided by Robin Hogan (Reading Univ.) and RCRU (RAL)
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Model/Obs Comparison
Average ice evaporative depth scale from the Chilbolton 94 GHz cloud radar and the operational UM for 20 separate days in Oct, Nov, Dec 1999.
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FASTEX IOP 16: ValidationComparison of the reference and modified model ice evaporative depth scales with
94GHz radar observation statistics
Average depth scale
Reference:1260 m
Modified: 780 m
Obs: 640 m (160m)
Modified model = higher vertical resolution, double ice evaporation rate and two thirds of ice fall speed
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Impact of including rain advection on rainfall distribution.
Rainfall rate (mm/hr) Orography (m)Rainfall rate difference (advection-no advection)
10hr model forecast.
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VerificationDartmoor River Catchment Rainfall
3 Hour Accumulations
Avon & Erme
Dart
Teign
Tamar
Exe
With Rain AdvectionNo Rain Advection
Radar 2km Model Forecasts
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VerificationCorrelation between model surface rainfall
and NIMROD radar-derived surface rainfall for Dartmoor
With Rain AdvectionNo Rain Advection
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Future Plans
Limited area 4D-Var operational 2005– with radar derived surface rainfall data in 2006
Experimental 4D-Var assimilation of radar radial winds
– project with Salford University Partial Dopplerisation of UK radar network
– 7 radars by 2006 Convective scale model by 2008
– develop radar verification techniques– establish viable radar assimilation method
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Questions?