05.08.2005 - 1 - COSMO General Meeting COSMO General Meeting Zürich, 20 - 23 Sept 2005 1 christoph.schraff@dwd.de Stefan Klink, Klaus Stephan and Christoph Schraff [email protected][email protected][email protected]Recent developments in Latent Heat Nudging at DWD • “blacklist” for radar data • latent heat nudging and “prognostic” precipitation • results of assimilation experiments • summary and aspects of future work
Recent developments in Latent Heat Nudging at DWD. Stefan Klink, Klaus Stephan and Christoph Schraff [email protected][email protected][email protected]. “blacklist” for radar data latent heat nudging and “prognostic” precipitation - PowerPoint PPT Presentation
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• Motivation: detection of ground clutter, which is not removed by the operational Doppler filter,clutter occurs at many of these grid points more or less permanently
• approach: consistency of radar derived precipitation rate and satellite derived cloud-type product(Met-8 Nowcasting-SAF) at each grid point over a longer time period (e.g. 15 days)
• blacklist criterion: at 20% of the cloud-free dates, a non-zero precipitation rate has to be found at this special grid point
• Usage during assimilation: blacklisted grid points are not treated by the LHN-algorithm
• Plan: update this mask once a month, taking at least the last 30 days into consideration
total sum of precipitation derived from the DX radar composite
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
• Integration within one column • Integration along a path
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• The precipitation rate reflects the integral of latent heat release along the path of precipitation particles through the atmosphere from the formation of cloud droplets until the contact with the ground.
-from: R. A. Houze, Jr.: Cloud DynamicsInternational 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 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
Experiment for 07 – 18 July 2004 (mainly convective precipitation events)
Model setup:
general settings• LM version 3.13• LMK configurations (2-TL-scheme, SL advection scheme)• continuous assimilation cycle, 00-, 12-, 18- UTC forecasts
LHN experiment• ‘undelayed’ reference precipitation (vertically averaged precipitation flux)• applying temperature increments ‘only in clouds’• upper limit of scaling factor = 2, lower limit = 0.5• analysed 3D precipitation fields not passed to the forecasts• laterally nested into GME
Experiment for 07 – 18 July 2004 (mainly convective precipitation events)
Model setup:
general settings• LM version 3.15• new LMK configurations (2-TL-scheme, Bott advection scheme)• continuous assimilation cycle, 00-, 12-, 18- UTC forecasts
LHN experiment• ‘undelayed’ reference precipitation (vertically averaged precipitation flux)• applying temperature increments ‘only in clouds’• upper limit of scaling factor = 1.7, lower limit = 0.3, + logarithmic scaling• analysed 3D precipitation fields passed to the forecasts• laterally nested into LM
CTRL experiment• laterally nested into LM
result of a 4-day intermediate experiment:FBI for 5.0 mm threshold during assimilation (LM 3.13, nested into GME):
revised version decreases overestimation of precipitation compared to previous LHN version
• ‘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 – strong gravity waves induced LHN forcing too strong (?)
• 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’)– frequency bias slightly improved, increased values from + 0 h to + 6 h,
slightly decreased values from + 6 h to +
12 h
• 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, temperature improved in PBL at + 6 h
• 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 )