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Stefan Klink, Klaus Stephan and Christoph Schraff and Daniel Leuenberger

Recent developments in Latent Heat Nudging. Stefan Klink, Klaus Stephan and Christoph Schraff and Daniel Leuenberger stefan.klink@dwd.de klaus.stephan@dwd.de christoph.schraff@dwd.de daniel.leuenberger@meteoswiss.ch. latent heat nudging and “prognostic” precipitation

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Stefan Klink, Klaus Stephan and Christoph Schraff and Daniel Leuenberger

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  1. Recent developments in Latent Heat Nudging Stefan Klink, Klaus Stephan and Christoph Schraff and Daniel Leuenberger stefan.klink@dwd.de klaus.stephan@dwd.de christoph.schraff@dwd.de daniel.leuenberger@meteoswiss.ch • latent heat nudging and “prognostic” precipitation • ( strong overestimation of precipitation during LHN) • results of assimilation experiments 05.08.2005 - 1 -

  2. 55km y x Latent heat nudging and “prognostic” precipitation LHN-Assumption: vertically integrated latent heat release  precipitation rate The correlation between the vertically integrated rate of latent heat release and the surface precipitation rate is significantly smaller in the simulation with prognostic precipitation 05.08.2005 - 2 -

  3. + - • main part of positive latent heat release occurs in updrafts, • strong precipitation rates are often related to downdrafts • at x < 3 km , with prognostic treatment of precipitation (model resolves large clouds): • model is able to distinguish between updrafts and downdrafts inside convective systems • horizontal displacement of areas with strong latent heating resp. to surface precipitation, modified spatial structure of latent heat release in the model • scheme will notice only with temporal delay if precipitation already activated by LHN from: R. A. Houze, Jr.: Cloud Dynamics International Geophysics Series Vol. 53 05.08.2005 - 3 -

  4. possible adaptations I • horizontal displacement of verticallyintegrated latent heating and surface precip.: • strong horizontal smoothingapplied to 2D fields of observed and modelled surface precipitation rate and to 3D field of latent heat release,in order to get higher correlation of precipitation and latent heating • (is found to have limited impact, tends to decrease precipitation amounts) 05.08.2005 - 4 -

  5. possible adaptations II • change of the spatial structure of latent heat release in the model: • updraft regions (at the leading edge of a convective cell): very high values of latent heat release TLHmo, little precipitation RRmo • higher values of the scaling factor  and of LHN increments often occur • reduce upper limit of the scaling factor • adapt grid point search routine • downdraft regions (further upstream): high precipitation rate, weak latent heat release (often negative in most vertical layers) • LHN increments are inserted only in the vertical layers where the model latent heating rates are positive (approx. in cloudy layers) (to avoid e.g. negative LHN increments and cooling where the precipitation rate should be increased) 05.08.2005 - 5 -

  6. vertically averaged precipitation flux (more consistent, however it does not eliminate the temporal delay completely) • for LHN: temporal delay effect found to be much more important than spatial displacement • possible adaptations III: • temporal delay effect (generated precipitation reaches the ground with some delay): • an immediate reference information, on how much precipitation the temperature increment has initialised already, is required within each time step • use of a ’reference precipitation’RRref: • diagnostically calculated precipitation rate (by additional call of diagnostic precipitation scheme without any feedback • on other model variables) 05.08.2005 - 6 -

  7. continuous assimilation cycle for 07 – 18 July 2004, 3 daily forecasts LMK configurations (x = 2.8 km, SL advection), adapted LHN (-limits : 2.0 / 0.5) nested into GME nested into LM Hourly accumulated precipitation height 05.08.2005 - 7 -

  8. continuous assimilation cycle for 07 – 18 July 2004, 3 daily forecasts LMK configurations (x = 2.8 km, SL advection), adapted LHN (-limits : 2.0 / 0.5) nested into GME nested into LM Hourly accumulated precipitation height 05.08.2005 - 8 -

  9. 08 – 18 July 2004 -limits : 1.7 / 0.3 nested into LM Bott advection scores for hourly precipitation : with latent heat nudging / without latent heat nudging assimilation 0.1mm threshold values 2.0mm ETS FBI 05.08.2005 - 9 -

  10. scores for hourly precipitation : with latent heat nudging / without latent heat nudging 12-UTC forecasts 0.1mm threshold values 2.0mm ETS FBI … and similar results for 18-UTC forecasts 05.08.2005 - 10 -

  11. scores for hourly precipitation : with latent heat nudging / without latent heat nudging 0-UTC forecasts 0.1mm threshold values 2.0mm ETS FBI 05.08.2005 - 11 -

  12. moister drier more stable colder verification against German radiosondes, 11-day period (8 – 18 July 2004): dashed: with latent heat nudging / solid: without latent heat nudging bias +0h +6h +12h +18h relative humidity temperature 05.08.2005 - 12 -

  13. verification against German radiosondes, 11-day period (8 – 18 July 2004): dashed: with latent heat nudging / solid: without latent heat nudging r m s e +0h +6h +12h +18h r e l a t I v e h u m I d I t y t e m p e r a t u r e worse better 05.08.2005 - 13 -

  14. Role of low-level Environment OBS CTRL from aLMo ANA 12UTC LHN from aLMo ANA 12UTC LHN from aLMo ANA 15UTC 05.08.2005 - 14 -

  15. LHN 12 - 20 UTC (starting from aLMo ANA) 3h sum of precipitation 21 - 24 UTC (+1 to +4 h free forecast) Additional three hours of conventional aLMo assimilation improve environment and thus precip forecast started from LHN! Nudging 12 - 15 UTC, LHN 15 - 20 UTC 05.08.2005 - 15 -

  16. Response of model dynamics to forcing OBS CTRL 05.08.2005 - 16 -

  17. Summary of Results • ‘blacklist’ for radar data: avoids introduction of spurious rain at radar locations • several adaptations to LHN to cope with prognostic precipitation; most important: use of an ‘undelayed’ reference precipitation (vertically averaged precipitation flux) • revised LHN, assimilation mode: • simulated rain patterns in good agreement with radar observations, • overestimation of precipitation strongly reduced • subsequent forecasts, impact on precipitation (10-day summer period): • large positive impact for 4 hours (longer than insimulations with diagnostic precip) • mixed ETS impact beyond + 6 h (interpretation yet unclear, need verification without ‘double penalty’) • upper-air verification (11-day summer period): • LHN cools and dries PBL, increases mid-tropospheric stability and upper-tropospheric moisture • overall neutral impact on rmse of forecasts • strong gravity waves induced during assimilation LHN forcing too strong 05.08.2005 - 17 -

  18. Aspects of Further Work • more case studies for both summer and winter periods (up to 10 days), (further problems ?) • further diagnosis of LHN results, in order to better understand some problems (e.g. performance of 0-UTC runs, decrease of forecast impact, too strong LHN forcing) and improve / tune LHN scheme • role of gravity waves • vertical structure of precipitating cells (e.g. wind field) • vertical distribution of LHN increments • environment of precipitation cells (moisture convergence) • horizontal filtering • introduction of PI-data (international composite) outside the German DX-area • further use of cloud type product of Nowcasting-SAF within LHN (humidity adjustment) • LMK test suites (periods up to three months), with comprehensive verification • ( possibly: use of 3D reflectivity data in order to determine more precisely (both in space and time) the areas of positive latent heat release • height-dependent scaling of the latent heat rate ) 05.08.2005 - 18 -

  19. 05.08.2005 - 19 -

  20. test case with “prognostic” precipitation Hourly accumulated precipitation height 05.08.2005 - 20 -

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