COSMO General Meeting Bucharest, Sept Klaus Stephan, Stefan Klink and Christoph Schraff and Daniel Leuenberger Recent developments in Latent Heat Nudging at DWD 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

COSMO General Meeting Bucharest, Sept 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)

COSMO General Meeting Bucharest, Sept LHN: Grid-point search Example: RR obs = 3.1 mm/h RR ref = 0.4 mm/h RR search = 2.1 mm/h LH serach z *  z Tinc lhn but, how large should be the scaling factor  ? (want to add ‘0.7 * RR ref = 0.28 mm/h) (revised version of RR max ; in the old version, RR max and  could become very large) conditions: RR search close to RR obs LH ref small enough LH search large enough

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

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

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

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

COSMO General Meeting Bucharest, Sept radar LMK ass without LHN LMK ass with LHN hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds

COSMO General Meeting Bucharest, Sept hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds strong low-level flow no precipitation simulated build-up of LH, low pressure rain, high pressure 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 flow slowed down, positive feedback spurious small-scale pressure disturbance and heavy rain system eventually propagating upstream and producing strong gravity waves

COSMO General Meeting Bucharest, Sept LMK ass with LHN hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds duplicating LH incr. 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 v ll := ½ v ¼ v ¼ v 700hPa ) radar

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

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

COSMO General Meeting Bucharest, Sept ETS free forecasts FBI threshold = 0.5 mm/h 12 UTC runs scores for hourly precipitation : with latent heat nudging / without latent heat nudging +0h +4h 00 UTC runs 16 June – 30 July 2006 (45 days)

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

COSMO General Meeting Bucharest, Sept 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)

COSMO General Meeting Bucharest, Sept ECMWF EPS COSMO LEPS (7km) HIRES LEPS (2.2km) ANA FC Radar Rainfall Assimilation and Short-Range QPF in a High-Resolution EPS: A Case Study (Daniel Leuenberger, Marco Stoll, MeteoSwiss)-28h+8h -10h 0 12UTC06UTC 16UTC00UTC Nested high-resolution EPS: role of convective environment for LHN, investigate on nested EPS with best-member selection based on satellite + radar data

COSMO General Meeting Bucharest, Sept Meteosat 7 IR 16:00 UTC ENS mean & spread SAT Ch. Keil, DLR COSMO LEPS (7km)

COSMO General Meeting Bucharest, Sept det21 RADAR Precipitation at 18UTC: Forecast (+2h)

COSMO General Meeting Bucharest, Sept RAD det Mean Area Precipitation (Bavaria)  convective environment matters a lot

COSMO General Meeting Bucharest, Sept Best member selection possible ?  only to a limited degree in current case

COSMO General Meeting Bucharest, Sept Radar With LHN Without LHN (dashed: determininistic) Benefit of LHN ?  in all cases very significantly

COSMO General Meeting Bucharest, Sept 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

COSMO General Meeting Bucharest, Sept

COSMO General Meeting Bucharest, Sept 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

COSMO General Meeting Bucharest, Sept original (Emden, 19 July 2005, 23 UTC) after detection of spokes + clusters quality product anaprop‘German Pancake‘arcs

COSMO General Meeting Bucharest, Sept precipitation dampingbright band much more obvious in 24-hour precipitation than in reflectivity / precipitation rate obs 24-h precip radar not yet done

COSMO General Meeting Bucharest, Sept

COSMO General Meeting Bucharest, Sept hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds strong low-level flow no precipitation simulated LH input at beginning 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 rain produced closer to radar border LH input later on

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

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

COSMO General Meeting Bucharest, Sept from: R. A. Houze, Jr.: Cloud Dynamics International Geophysics Series Vol. 53 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 displacementof 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 LHN-Assumption: vertically integrated latent heat release  precipitation rate

COSMO General Meeting Bucharest, Sept 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  T LHmo, little precipitation RR mo  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)

COSMO General Meeting Bucharest, Sept 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’ RR ref : diagnostically calculated precipitation rate (by additional call of diagnostic precipitation scheme without any feedback on other model variables) 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

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

COSMO General Meeting Bucharest, Sept 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 t e m p e r a t u r e r e l a t I v e h u m I d I t y + 0 h+ 0 h+ 6 h+ 6 h+ 12 h+ 18 h worse better

COSMO General Meeting Bucharest, Sept ‘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 in simulations 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 Summary of Results