Eidgenössisches Departement des Innern EDI Bundesamt für Meteorologie und Klimatologie MeteoSchweiz Institut für Physik der Atmosphäre On the Value of Radar-Derived Rainfall Assimilation on High-Resolution QPF Daniel Leuenberger 1, Christian Keil 2 and George Craig 2 1 MeteoSwiss, Zurich, Switzerland 2 DLR, Oberpfaffenhofen, Germany COSMO GM 2008, Cracow

2 Value of Radar Assimilation for COSMO GM 2008, Cracow Introduction Convective-scale assimilation of radar rainfall data Latent Heat Nudging (LHN) Results of a 7 month test suite at MeteoSwiss What determines the impact of LHN on QPF?

3 Value of Radar Assimilation for COSMO GM 2008, Cracow ECMWF IFS MeteoSwiss Model Setup COSMO-7 6.6km, 60 levels Param. deep convection Assimilation of conv. obs. COSMO-2 2.2km, 60 levels Explicit deep convection Assimilation of conv. obs. and radar rainfall COSMO-2 Radar ~600 km

4 Value of Radar Assimilation for COSMO GM 2008, Cracow Setup of Experiments 2.2km assimilation cycle with/without LHN Forecasts out to +12h, initialized at 00 and 12 UTC 11. June 2007 – 15. January 2008 (346 forecasts)

5 Value of Radar Assimilation for COSMO GM 2008, Cracow Examples of Improvement 6-12h Precipitation Forecast ( ) Verifying Radar LHNNOLHNRadar LHNNOLHNRadar 0-6h Precipitation forecast ( ) Verifying Radar

6 Value of Radar Assimilation for COSMO GM 2008, Cracow Verification against Radar 346 Forecasts, 11. June January 2008, hourly sums

7 Value of Radar Assimilation for COSMO GM 2008, Cracow Verification against Radar (Summer) 9 Forecasts, 11. June July 2007, hourly sums

8 Value of Radar Assimilation for COSMO GM 2008, Cracow Verification of other Variables Pa Surface Pressure Time UTC m/s m Wind speed Time UTC deg Wind direction Time UTC RMS of 74 12UTC Forecasts (Reference: ca. 60 Swiss Sfc. Stations) NOLHN LHN

9 Value of Radar Assimilation for COSMO GM 2008, Cracow Verification of other Variables % Cloud cover Time UTC K 2m Temperature Time UTC K m Dewp. Temperature Time UTC RMS of 74 12UTC Forecasts (Reference: ca. 60 Swiss Sfc. Stations) NOLHN LHN

10 Value of Radar Assimilation for COSMO GM 2008, Cracow What determines the impact of LHN? Use high-resolution NWP ensemble (2.8km mesh size) Driven by regional COSMO-LEPS ensemble 10 members with LHN, 10 members without Different mesoscale environment in each member 3 differently forced convection cases forced frontal non-forced frontalairmass 31.July June July 2006

11 Value of Radar Assimilation for COSMO GM 2008, Cracow Example: Airmass convection NOLHN Radar NWPEnsembleNWPEnsemble Time UTC Timelines of observed and simulated area-averaged surface rainfall LHN Time UTC Assimilation mm Forecast

12 Value of Radar Assimilation for COSMO GM 2008, Cracow Definition of Time Scales LHN impact factor LHN time scale  LHN Convective time scale Done et al. (QJ 2006) F LHN time  LHN

13 Value of Radar Assimilation for COSMO GM 2008, Cracow Stratification of Simulations Results suggest 2 different regimes: equilibrium situation: short  c precipitation only redistributed short-lived impact of LHN  LHN [h]  c [h] forced frontal, non-forced frontal airmass non-equilibrium situation: long  c LHN triggers convection long lasting impact of LHN

14 Value of Radar Assimilation for COSMO GM 2008, Cracow Findings LHN improves high-resolution NWP forecasts QPF improvement in the first 3-12h (dependent on score and rainfall intensity) Other variables slightly improved, particularly in summer More realistic rainfall input for soil moisture Impact on QPF dependent on Precipitation forcing (equilibrium vs. non-equilibrium) Life time of precipitation system (predictability!) Mesoscale environment of convection (e.g. stability) Extent of NWP model domain and radar data coverage

15 Value of Radar Assimilation for COSMO GM 2008, Cracow Thank you for your attention

16 Value of Radar Assimilation for COSMO GM 2008, Cracow LHN Impact Factor forced frontal forecast hour F LHN non-forced frontal forecast hour airmass forecast hour

17 Value of Radar Assimilation for COSMO GM 2008, Cracow Radar Rainfall Assimilation Latent Heat Nudging (LHN) Jones and Macpherson, 1997, Leuenberger and Rossa, 2007 Adjusts model temperature and moisture to match radar rainfall Modifications for advection of precipitation (DWD) Stephan et al., 2008 Jones, C. D. and B. Macpherson, 1997: A Latent Heat Nudging Scheme for the Assimilation of Precipitation Data into an Operational Mesoscale Model, Meteorol. Appl., 4, Leuenberger, D. and A. Rossa, 2007: Revisiting the latent heat nudging scheme for the rainfall assimilation of a simulated convective storm, Meteorol. Atmos. Phys., 98, Stephan, K., S. Klink and C. Schraff, 2008: Assimilation of radar derived rain rates into the convective scale model COSMO-DE at DWD, submitted to QJRS