452 Precipitation
-
Upload
lev-levine -
Category
Documents
-
view
29 -
download
1
description
Transcript of 452 Precipitation
452 Precipitation
Prob. Of Precip.– Cool Season(0000/1200 UTC Cycles Combined)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1966 1969 1972 1975 1978 1981 1984 1987 1990 1993 1996 1999 2002
Year
Brier Score Improvement over Climate
Guid POPS 24 hr Local POPS 24 hr
Guid POPS 48 hr Local POPS 48 hr
Orographic precipitation enhancement
Annual Precipitation
0
2
4
6
8
10
12
-600 -400 -200 0 200 400 600
Distance (km)
Pre
cip
itat
ion
Rat
e (
mm
/hr)
Upslope site at 1067m
Valley site
Orographic Enhancement of Precipitation
Convective precipitation
Upper-level frontal band
Surface frontal rainband
Post-frontal convection
Pre-frontal precipitation
Upslope site
(elev. 1067m)
Valley site
Pre
cipi
tatio
n R
ate
(mm
/hr)
Distance (km)
Rainshadow
Rainshadow in westerly flow
Annual Precipitation
SW Olympic Slopes-Hoh Rain Forest: 150-170 inches yr-1
Sequim: roughly 15 inches per year.
.
Rainshadows Shift with Approaching Flow Directions
and Depend on Stability• As the flow approaching a barrier changes
direction, so does the orientation of the rain shadow.
• Less rainshadow during warm frontal period, more post-frontal.
Progress in Precipitation Prediction in Terrain
NGM, 80 km,1995
NGM, 1995
2001: Eta Model, 22 km
36-km
12-km
NWS WRF-NMM (12-km)
2007-2008
12-kmUW MM5Real-time
12-km WRF-ARWand WRF-NMMare similar
December 3, 20070000 UTC Initial12-h forecast3-hr precip.
2007-2008
4-km MM5Real-time
10-km
Smaller Scale Terrain ModulatesPrecipitation
12-km
4-km
Small-Scale Spatial Gradients in Climatological Precipitation on the Olympic PeninsulaAlison M. Anders, Gerard H. Roe, Dale R. Durran, and Justin R. MinderJournal of Hydrometeorology Volume 8, Issue 5 (October 2007) pp. 1068–1081
Annual Climatologies of MM5 4-km domain
Verification of Small-Scale Orographic Effects
Convective Precipitation
• Need at least 2-4 km resolution to get most convection even half right.
• If grid spacing is more coarse must have cumulus parameterization--Kain-Fritsch is used for MM5/WRF.
• Convection is least skillful precipitation in virtually all models.
• High resolution models can potentially give a heads on type of precipitation a day ahead (squall line, supercell, etc.)
Real-time WRF 4 km BAMEX Forecast
Composite NEXRAD Radar
4 km BAMEX forecast 36 h Reflectivity
4 km BAMEX forecast 12 h Reflectivity
Valid 6/10/03 12Z
Real-time 12 h WRF Reflectivity Forecast
Composite NEXRAD Radar
4 km BAMEX forecast
Valid 6/10/03 12Z
10 km BAMEX forecast
22 km CONUS forecast
Hurricane Isabel Reflectivity at Landfall
Radar Composite
18 Sep 2003 1700 Z
41 h forecast from 4 km WRF
Atmospheric Rivers
• Meridional flow of moisture is often limited to relatively narrow currents of moisture and usually warm temperatures.
Most West Coast heavy precip events are associated with “atmospheric rivers”, a.k.a. the “Pineapple Express”
A relatively narrow current of warm, moist air from the subtropics…often starting near or just north of Hawaii.
Precipitable WaterFrom Mike Warner
Associated with extraordinarily narrow filaments of moisture
A Recent Devastating Pineapple Express: November 6-7, 2006
Dark Green: about 20 inches
November 6-9, 2006
We know quite a bit about atmospheric rivers and heavy NW precipitation events, although there
are still gaps in our knowledge
Synoptic Set-Up for Top Fifty Events at Forks
Courtesy of Michael Warner
Precipitable Water
500 mb height
SLP
850 mb Temp
Extreme Precipitation Events• The current of warm,
moist air associated with atmospheric rivers are found in the warm sector, parallel, near, and in front of the cold front.
• Thus, atmospheric rivers are closely associated with the jet core and the region of large baroclinicity.
Orographic Enhancement• Upslope flow
greatly increases precipitation rates on terrain.
• Thus, wind speed and angle of attach can greatly modify the extreme nature of the precipitation.
Predicting Clouds
• Zero-order approach: use relative humidity
• 70% 1000-500 mb RH often corresponds with thicker, middle-level clouds that have a serious impact on radiation.
• 700 and 850 mb RH is also used by some.
Direct Use of Model Clouds• All modern models predict clouds, specifically
mixing ratios of cloud liquid water and cloud ice.
• The quality varies…and keep in mind there are serious deficiencies of even the best microphysical schemes.
• And problems with other physical parameterizations: boundary layer turbulence or radiation can also mess up model clouds.
Direct Use of Model Clouds
• Most problematic: stratus, stratocumulus, and FOG.
• Remember, some models have spin-up issues: precipitation and clouds improve during the first 3-9 hours. Particularly true of UW WRF which is now cold-started (no clouds at initialization!)
Cloud/Precip Forecasting Strategy
• Very short-term (0-3 hr): temporal extrapolation (informed by human judgement) is HARD to beat. Nowcasting.– Use radar and satellite animation. Models
generally not that useful.– New data assimilation/modeling systems: e.g.,
EnKF, RUC/RR/HRRR may be viable soon and eventually will take this on successfully.
Strategy
• Short-term (2-6 hr): Satellite extrapolation becomes central. Model becomes more dominant at the end
• Daily (6-24 h): Satellite extrapolation and model, weighing model more at longer range.
• Remember model spin up issues.
Strategy• A wise forecaster ALWAYS evaluates
model’s initial moisture fields—often a failure mode and a good indicator of potential model failures.
• How? Compare initial model fields and RH to satellite imagery.
850 700
Strategy
• Know regional precip/cloud climatology– Diurnal and geographic features tend to be very
repeatable, particularly during the warm season– Example: front range convection
Human Forecaster Issues
• Precipitation is a parameter where in general forecasters add the least skill compared to objective guidance.
• There are examples: like Monday’s convection in eastern WA/OR
• Psychological issues for high and low precip probabilities
Human Versus Objective Skill for Precipitation Forecasts: NWS Offices Around the US
Brier Scores for Precipitation for all stations for the entire study period.
Brier Score for all stations, 1 August 2003 – 1 August 2004. 3-day smoothing is performed on the data.
Brier Score for all stations, 1 August 2003 – 1 August 2004, sorted by geographic region.
Precipitation
Reliability diagrams for period 1 (a), period 2 (b), period 3 (c) and period 4 (d).
More than a day out….Human Forecasters Tend to Overpredict 10-30%--exaggerating the threat of rain when it is not likely.“Its probably not going to rain…but I will throw in 10-30% anyway to cover myself”
And underpredict rain when it is fairly definite. “Well it looks like it will rain…but I am unsure…so I will knock down the probabilities to 60-80% to cover myself”
Rain Psychology
Humans Help with Heavier Precipitation
The End
Bias scores for the (a) 1.33- and (b) 4-km
model simulations for 1400 UTC 13 Dec 2001 through 0800 UTC 14
Dec 2001.
Puget Sound Convergence Zone
Flow over terrain can be highly 3D and complex