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Klaus Stephan , Stefan Klink and Christoph Schraff and Daniel Leuenberger

Recent developments in Latent Heat Nudging at DWD. Klaus Stephan , Stefan Klink and Christoph Schraff and Daniel Leuenberger klaus.stephan@dwd.de stefan.klink@dwd.de christoph.schraff@dwd.de daniel.leuenberger@meteoswiss.ch. revision to grid point search and its impact

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Klaus Stephan , Stefan Klink and Christoph Schraff and Daniel Leuenberger

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  1. Recent developments in Latent Heat Nudging at DWD Klaus Stephan , Stefan Klink and Christoph Schraff and Daniel Leuenberger klaus.stephan@dwd.de stefan.klink@dwd.de christoph.schraff@dwd.de daniel.leuenberger@meteoswiss.ch • revision to grid point search and its impact • problem case with strong advection • latest test suite June – July 2006: • impact of LHN, comparison to LME • some conclusions 05.08.2005 - 1 -

  2. Adaptations for applying LHN with prognostic precipitation • use of a reference precipitation: vertically averaged precipitation flux • apply LHN-increments only where latent heat rates are positive • apply (upper and lower) limits to scaling factor  , logarithmic scaling (replace ( –1) by ln() effective limits: [0.3, 1.7]) • impose absolute limits to LHN-increments • search for nearby grid points, if model precipitation rate is too low but to use a moderate forcing of precipitation at these points Other options used • adjustment of specific humidity in order to maintain relative humidity • vertical filtering of profiles of LHN-increments • horizontal filtering of incoming variables (of small extent) 05.08.2005 - 2 -

  3. z z LHserach *  Tinclhn LHN: Grid-point search Example: RRobs= 3.1 mm/h RRref= 0.4 mm/h RRsearch = 2.1 mm/h but, how large should be the scaling factor  ? (want to add ‘0.7 * RRref = 0.28 mm/h) conditions: RRsearchclose to RRobs LHref small enough LHsearch large enough (revised version of RRmax; in the old version, RRmax and  could become very large) 05.08.2005 - 3 -

  4. impact of new version with revised (‘weaker forcing’) grid point search compared to old version (‘stronger forcing’) case study: assimilation at 21 May 2005 radarweaker forcingstronger forcing hourly precipitation (4, 6 UTC): old stronger forcing produces strong gravity waves 4 UTC 4 UTC spatial average of actual precipitation rate: old stronger forcing overestimates precipitation mm/h radar weaker stronger 6 UTC 6 UTC UTC 05.08.2005 - 4 -

  5. impact of revised (‘weaker forcing’) grid point search compared to old ‘stronger forcing’ ETS FBI ETS test period: 8 – 20 July 2004 , comparison to control without LHN(older LMK version) Assimilation 0.1 mm/h underestimation of model (control) largely but not fully corrected 2.0 mm/h FBI almost no bias any more for strong precip higher ETS despite lower FBI: better match of precip patterns 05.08.2005 - 5 -

  6. impact of revised (‘weaker forcing’) grid point search compared to old ‘stronger forcing’ 12 UTC FBI ETS undershooting delayed and strongly reduced positive forecast impact for 4 – 6 h test period: 8 – 20 July 2004 , comparison to control without LHN(older LMK version) free forecasts Threshold = 2.0 mm/h 05.08.2005 - 6 -

  7. scores for hourly precipitation : with latent heat nudging / without latent heat nudging 48 forecasts, different convective cases threshold = 0.2 mm/h threshold = 2.0 mm/h free forecasts FBI ETS +0h +6h +0h +6h in 80 stratiform cases, LHN has less impact (3 – 4 h) 05.08.2005 - 7 -

  8. hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds radar LMK ass without LHN LMK ass with LHN 05.08.2005 - 8 -

  9. radar sees precipitation → constant input of latent heat by LHN but takes time to produce rain advection of LH → influence of LHN too far downstream rain, high pressure build-up of LH, low pressure flow slowed down, positive feedback hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds strong low-level flow no precipitation simulated spurious small-scale pressure disturbance and heavy rain system eventually propagating upstream and producing strong gravity waves 05.08.2005 - 9 -

  10. radar weighting of LH incr. ‘weighting’: LH increments decreased linearly from 1 to zero when low-level wind speed increases from 20 to 30 m/s (low-level wind speed vll := ½ v950 + ¼ v850 + ¼ v700hPa ) hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds LMK ass with LHN duplicating LH incr. 05.08.2005 - 10 -

  11. small drop drop drop number of observed events rise rise scores for hourly precipitation : with latent heat nudging / without latent heat nudging new LMK version with revised droplet size distribution, reducing evaporation of precip weak precipitation enhanced 16 June – 30 July 2006 (45 days) assimilation threshold = 0.1 mm/h threshold = 2.0 mm/h overestimation in early morning well balanced FBI ETS 05.08.2005 - 11 -

  12. scores for hourly precipitation : with latent heat nudging / without latent heat nudging 16 June – 30 July 2006 (45 days) free forecasts 00 + 12 UTC runs threshold = 0.1 mm/h threshold = 2.0 mm/h FBI LMK: too little precip undershooting (w. resp. to no-LHN) higher ETS same ETS despite smaller FBI ETS +0h +4h +0h +4h 05.08.2005 - 12 -

  13. scores for hourly precipitation : with latent heat nudging / without latent heat nudging 16 June – 30 July 2006 (45 days) free forecasts 12 UTC runs 00 UTC runs threshold = 0.5 mm/h FBI ETS +0h +4h +0h +4h 05.08.2005 - 13 -

  14. scores for hourly precipitation : comparison LME  LMK with LHN, LMK without LHN 16 June – 30 July 2006 (45 days) free forecasts 00 + 12 UTC runs threshold = 0.1 mm/h threshold = 2.0 mm/h all models: strong precip underestimated FBI LMK: precip areas too small higher ETS despite smaller FBI ETS +0h +4h +0h +4h 05.08.2005 - 14 -

  15. Conclusions & outlook • due to revised ‘weaker forcing’ grid point search (and using all the other adaptations of LHN to prognostic precipitation): • FBI close to 1 during assimilation, (much) less undershooting in forecasts • LHN better balanced, less gravity waves (but still too much, too strong gusts, etc.) • duration of positive forecast impact enlarged ( ~ 4 hours) • however: Still rapid loss of benefit  need for better understanding of convection, in particular how the model develops convection, role of environment, what kind of information is required  further improve LHN, vertical distribution of LH (3D reflectivity ?), horizontal filtering, use of cloud info  need for use of radar radial velocity, GPS tomography, Ensemble DA ? • model bias: model produces too little precipitation by itself, wrong diurnal cycle LHN able to compensate this during the assimilation by activating the model to produce more rain, i.e. pushes model away from its climate, but at the price of: – cooling and drying of PBL – increasing mid-tropospheric stability – undershooting of precipitation in forecast – stronger limitation to duration of forecast benefit from LHN  need for improving model (particularly diurnal cycle and bias of precipitation) 05.08.2005 - 15 -

  16. -28h 0 +8h -10h 16UTC 00UTC 12UTC 06UTC ANA FC Nested high-resolution EPS: role of convective environment for LHN,investigate on nested EPS with best-member selection based on satellite + radar data Radar Rainfall Assimilation and Short-Range QPF in a High-Resolution EPS: A Case Study (Daniel Leuenberger, Marco Stoll, MeteoSwiss) ECMWF EPS COSMO LEPS (7km) HIRES LEPS (2.2km) 05.08.2005 - 16 -

  17. COSMO LEPS (7km) Meteosat 7 IR 16:00 UTC SAT ENS mean & spread 3 5 1 2 4 6 7 8 9 10 Ch. Keil, DLR 05.08.2005 - 17 -

  18. Precipitation at 18UTC: Forecast (+2h) det 1 2 RADAR 3 4 5 6 7 8 9 10 05.08.2005 - 18 -

  19. Mean Area Precipitation (Bavaria) RAD1 2 3 4 5 6 7 8 9 10 det  convective environment matters a lot 05.08.2005 - 19 -

  20. Best member selection possible ?  only to a limited degree in current case 05.08.2005 - 20 -

  21. Benefit of LHN ? in all cases very significantly Radar With LHN Without LHN (dashed: determininistic) 05.08.2005 - 21 -

  22. Findings • Substantial spread in QPF among fine-scale members during first 4 hours • Large benefit of radar assimilation with LHN • Ranking in QPF does not correspond well with that using satellite data of driving members (convective environment is not explained with the cloud structure alone!) • Large spread in humidity among coarse members, smaller in temperature and wind • Some spread in CAPE („good“ members with higher CAPE) • Some difference in upper-level flow (some of the „bad“ members exhibit upper level convergence->subsidence in lower levels) • Cloud-based best-member selection does not work well for this case 05.08.2005 - 22 -

  23. 05.08.2005 - 23 -

  24. radar reflectivity data for LHN at DWD • reflectivity from „precipitation scan“ (lowest elevation angle between 0.5° and 1.8) • spatial resolution: 1 km x 1°, max. range 120 km, time resolution: 5’ • data processing: • correction of ground clutter by doppler filter • correction of orographic attenuation • use of a variable Z-R-relation to get precipitation rate • quality product of „precipitation scan“, detection of non-rain echoes (by K. Helmert and B. Hassler): • corrupt image • ‘German Pancake’ • anomalous propagation • spokes (of positive or negative attenuation) • circular arcs (of positive or negative attenuation) • echos of small extension (< 9 pixels) caused by wind energy plants etc. to be done: detection of other errorsnon-rain echoes • precipitation and radome damping • bright band • compositing of the 16 German doppler radars: precipitation using quality information, then quality product, 1 x 1 km • gribbing: use quality product to mask precip, interpolate to 2.8 x 2.8 km • use of blacklist 05.08.2005 - 24 -

  25. ‘German Pancake‘ anaprop arcs original (Emden, 19 July 2005, 23 UTC) quality product after detection of spokes + clusters 05.08.2005 - 25 -

  26. bright band much more obvious in 24-hour precipitation than in reflectivity / precipitation rate obs 24-h precip radar not yet done precipitation damping 05.08.2005 - 26 -

  27. 05.08.2005 - 27 -

  28. strong low-level flow no precipitation simulated LH input later on LH input at beginning rain produced closer to radar border hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds idea: duplicate LH increments near inflow border of radar domain, depending on wind vectors (average at low levels) rapidly, (areas of) LH input are significantly reduced  hardly any positive feedback effects and pressure disturbances however: problems further downstream 05.08.2005 - 28 -

  29. hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds LMK ass with old LHN new LMK / LHN with weighting LME ass (without LHN) mm/h 05.08.2005 - 29 -

  30. scores for hourly precipitation : comparison LME  LMK with LHN, LMK without LHN 16 June – 30 July 2006 (45 days) free forecasts 12 UTC runs 00 UTC runs threshold = 0.5 mm/h LMK: too weak diurnal cycle, too little precip in afternoon, less bias at night FBI LMK better than LME for 12 UTC runs ETS +0h +4h +0h +4h 05.08.2005 - 30 -

  31. LHN-Assumption: vertically integrated latent heat release  precipitation rate + - • 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 - 31 -

  32. 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 - 32 -

  33. 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 - 33 -

  34. 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 - 34 -

  35. 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 - 35 -

  36. 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 - 36 -

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