DEVELOPMENT OF 1- 4KM RESOLUTION DATA ASSIMILATION FOR NOWCASTING AT THE MET OFFICE

© Crown copyright 2005

DEVELOPMENT OF 1- 4KM RESOLUTION DATA ASSIMILATION

FOR NOWCASTING AT THE MET OFFICE

Sue Ballard, September 2005

Z. Li, M. Dixon, S.Swarbrick, O.Stiller and H. Lean

Met Office, JCMM, Reading University


Contents

Aim of high resolution convective scale system4km UK 2005, 1km ~ 2008-2010

Trial system – small domains

Data assimilation options

Trial results Impact of data assimilation Impact of relative humidity and latent heat nudgingScale selective assimilation

Conclusions


High Resolution Trial Model

1 km76 levelsResolved convection

4 km38 levelsMass-limited convection


Current HRTM Assimilation Options

12 km 3D-Var Data Assimilation With or without moisture and Latent Heat Nudging (LHN)

using AC scheme (referred to as MOPS data – moisture observation processing system)

i.e. spin up 4km, 1km from 12km T+1 each cycle.

4km 3D-VAR with continuous cycles with or without MOPS 1km with nudged reconfigured 4km increments using IAUWith or without LHN and moisture nudging using AC scheme

IAU – increments output from 3D-Var and fixed over time window

AC scheme – increments depend on latest model fields so vary with timestep through weighting factor and model evolution/impact of data


T-3 T-1 T+0 T+1 T+3

AC scheme/UM3D cloud fractionSurface rainrate

IAU3 hour f/c: background Hourly ModelOb

Next analysisPrevious analysisNudging RH & Latent heat

T+2T-2

Conventionalobservations

3D-Var (FGAT)

Obs window

3D-Var system including MOPS RH and LH nudging via AC scheme

6hr accumulations from13Z to 19Z 16/8/04 from 12UTC analysis

Rain rates at 14.30 UTC from 12UTC analysis

With 4km 3D-Var +MOPS radarSpun-up from 12km T+1

Accumulated precipitation19-20UTC 27th April 2004From 18UTC analysis

radar

With MOPS

No MOPS

Impact of MOPS data


Spin - up Assimilation12 and 4km withMOPS

Assimilation12 and 4kmNo MOPS

1km domain averageRainrate15UTC13th May 1003


Domain average rain rates

4 Cases x 8 cycles summer 20031km no MOPS

Black – radar Blue 12 km Green – 4km Orange -1km Dashed – spinup

7 Cases x 4 cycles summer 2004Prognostic rain (1 and 4km)Increased wt MOPS (12,4,1km)


Domain average rain rates



Black – radar Blue 12 km Green – 4km Orange -1km Dashed – spinup


T+2 Forecast precipitation rate for 20UTC 27April 2004

WaterLoading+ReducedMOPS wts

12km 4km 1km radar


T+4 Forecast precipitation rate for 22UTC 27April 2004

12km 4km 1km radar

WaterLoading+ReducedMOPS wts


Statistics for 4 cycles of assimilation and forecast data time9,12,15,18UTC 27 April 2004

Area averagePrecipitation rateFor radar area

Skill score for radar area for accumulation greater than 2mm against sampling radius

Orange dashed4km

Orange 4kmWater loadingReduced MOPS wt

Blue 12km

Black radar

Orange 4kmblue12km

Orange 4kmWater loadingReduced wt MOPSblue 12km


theta inc

12km analysis

“new” 4km analysis

/12km back

“new” 4km analysis

/4km back & half length

standard 4km analysis doesn’t see obs outside domain

Zero at boundary

4km short wave analysis 12km long waves

Problem of small domain Scale selective analysis


Impact of S-band radar radial wind datav-component of velocity at 410m

12km analysis 4kmstandard

Scale selectiveRadar dataIn 12km and 4km

ScaleSelectiveRadar in 4km only


Conclusions

Need 1-4km to capture small scale severe events Need DA to overcome spin-up of explicit convection

However long range forecasts can be better than short range MOPS RH & LHN improves locations but amounts too high

At 1-4km and worse with prognostic rain Needs further development but ideally want to use assimilation of

cloud and precipitation in 3D-Var and 4D-Var Model explicit convection overdoes rates (?) Need to combine synoptic scale and convective analysis Need to move towards full analysis for 1km model

Ideally want 4D-Var or EnKF - Start with 3D-Var need to resolve problems seen with 4km first Use reduced vertical resolution (and horizontal?) for analysis? High resolution data Balance and background errors


Questions & Answers

DEVELOPMENT OF 1- 4KM RESOLUTION DATA ASSIMILATION FOR NOWCASTING AT THE MET OFFICE

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