klaus stephan , stefan klink and christoph schraff and daniel leuenberger
DESCRIPTION
Recent developments in Latent Heat Nudging at DWD. Klaus Stephan , Stefan Klink and Christoph Schraff and Daniel Leuenberger [email protected] [email protected] [email protected] [email protected]. revision to grid point search and its impact - PowerPoint PPT PresentationTRANSCRIPT
05.08.2005 - 1 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
Klaus Stephan , Stefan Klink and Christoph Schraff
and Daniel [email protected]
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
05.08.2005 - 2 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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
mo
obsLHmoLHN RR
RRTT , 1
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 - 3 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
LHN: Grid-point search
0 0 0 0 0
1 0 0 0 0
1,7 0 0,4 0,3 0
2,1 0,2 0,1 0,2 0
1,5 0,8 0 0 0
Example:
RRobs = 3.1 mm/h
RRref = 0.4 mm/h
RRsearch = 2.1 mm/h
LHserach
z
*
z
Tinclhn
but, how large should be the scaling factor ?(want to add ‘0.7 * RRref = 0.28 mm/h)
17.0,min,7.1max
1
obsrefmax
search
refmax
RRRRRR
RR
RRRR
(revised version of RRmax ; in the old version,
RRmax and could become very large)
conditions: RRsearch close to RRobs
LHref small enough
LHsearch large enough
05.08.2005 - 4 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
radar weaker forcing stronger forcing
spatial average of actual precipitation rate:old stronger forcing overestimates precipitation
radarweaker
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 2005
4 U
TC
4 U
TC
6 U
TC
6 U
TC
mm/h
hourly precipitation (4, 6 UTC):old stronger forcing produces strong gravity waves
05.08.2005 - 5 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
ETS
ETSAssimilation
FBI
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
05.08.2005 - 6 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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
05.08.2005 - 7 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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 +0h +6h+6h
in 80 stratiform cases, LHN has less impact (3 – 4 h)
05.08.2005 - 8 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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
05.08.2005 - 9 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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 LHNbut takes time to
produce rainadvection of LH → influence
of LHN too fardownstream
flow slowed down,positive feedback
spurious small-scale pressure disturbance
and heavy rain
system eventually propagating upstream
and producing strong gravity waves
05.08.2005 - 10 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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 zerowhen low-level wind speed increases from 20 to 30 m/s(low-level wind speed vll := ½ v950 + ¼ v850 + ¼ v700hPa )
radar
05.08.2005 - 11 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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
overestimationin early morning
well balanced
new LMK version with revised droplet size distribution,
reducing evaporation of precip weak precipitation enhanced
drop
rise
drop
risenumber ofobserved
events
smalldrop
05.08.2005 - 12 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
ETS
free forecasts
FBI
threshold = 2.0 mm/h
16 June – 30 July 2006 (45 days)
00 + 12 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 +0h +4h+4h
higher ETS
undershooting(w. resp. to no-
LHN)
LMK: too little precip
05.08.2005 - 13 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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 +0h +4h+4h
00 UTC runs
16 June – 30 July 2006 (45 days)
05.08.2005 - 14 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
ETS
free forecasts
FBI
threshold = 2.0 mm/h
16 June – 30 July 2006 (45 days)
00 + 12 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 +0h +4h+4h
LMK: precip areas too small
all models: strong precip underestimated
05.08.2005 - 15 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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 modeldevelops 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, butat 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 - 16 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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-28h +8h+8h-10h-10h 00
12UTC12UTC 06UTC06UTC 16UTC16UTC 00UTC00UTC
Nested high-resolution EPS: role of convective environment for LHN,investigate on nested EPS with best-member selection based on satellite + radar data
05.08.2005 - 17 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
Meteosat 7 IR 16:00 UTC
ENS mean & spread
1 2 3 4 5
6 7 8 9 10
SAT
Ch. Keil, DLR
COSMO LEPS (7km)
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det 21RADAR
3 54 6
7 98 10
Precipitation at 18UTC: Forecast (+2h)
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RAD 1 2 3 4 5 6 7 8 9 10 det
Mean Area Precipitation (Bavaria)
convective environment matters a lot
05.08.2005 - 20 -COSMO General MeetingCOSMO General Meeting
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Best member selection possible ? only to a limited degree in current case
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10
Ranking Satellite (non-linear pattern recognition)
Ra
nk
ing
QP
F
05.08.2005 - 21 -COSMO General MeetingCOSMO General Meeting
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RadarWith LHN Without LHN (dashed: determininistic)
Benefit of LHN ? in all cases very significantly
05.08.2005 - 22 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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 - 23 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
05.08.2005 - 24 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
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 - 25 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
original (Emden, 19 July 2005, 23 UTC) after detection of spokes + clustersquality product
anaprop‘German Pancake‘ arcs
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precipitation damping bright band
much more obvious in 24-hour precipitationthan in reflectivity / precipitation rate
obs 24-h precip radar
not yet done
05.08.2005 - 27 -COSMO General MeetingCOSMO General Meeting
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05.08.2005 - 28 -COSMO General MeetingCOSMO General Meeting
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hourly precipitation on 21 July 2005, 11 UTC: problem case with very strong low-level winds
strong low-level flow
no precipitation simulated
LH inputat beginning
idea:duplicate LH increments
near inflow border of radar domain, depending on wind vectors
(average at low levels)
rapidly,(areas of) LH inputare significantly
reduced
hardly anypositive feedback
effects and pressure disturbances
however:problems further
downstream
rain produced closer to
radar border
LH inputlater on
05.08.2005 - 29 -COSMO General MeetingCOSMO General Meeting
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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
05.08.2005 - 30 -COSMO General MeetingCOSMO General Meeting
Bucharest, 18 - 21 Sept 2006 [email protected]
ETS
free forecasts
FBI
threshold = 0.5 mm/h 12 UTC runs
+0h +0h +4h+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
05.08.2005 - 31 -COSMO General MeetingCOSMO General Meeting
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+
-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
• LHN-Assumption: vertically integrated latent heat release precipitation rate
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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
mo
obsLHmoLHN RR
RRTT , 1
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)
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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: refRR
RR
RR
RR obs
mo
obs
• 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
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verification against German radiosondes, 11-day period (8 – 18 July 2004): dashed: with latent heat nudging / solid: without latent heat nudging
bias
temperature
relativehumidity
+ 0 h + 6 h + 12 h + 18 h
more stable
colder
drier
moister
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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 + 6 h + 12 h + 18 h
worse
better
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• ‘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