452 Precipitation
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452 Precipitation
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Rainshadow
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Rainshadow in westerly flow
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Annual Precipitation
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SW Olympic Slopes-Hoh Rain Forest: 150-170 inches yr-1
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Sequim: roughly 15 inches per year.
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Puget Sound Convergence Zone
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Progress in Precipitation Prediction in Terrain
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NGM, 80 km,1995
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NGM, 1995
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2001: Eta Model, 22 km
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36-km
12-km
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NWS WRF-NMM (12-km)
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2007-2008
12-kmUW MM5Real-time
12-km WRF-ARWand WRF-NMMare similar
December 3, 20070000 UTC Initial12-h forecast3-hr precip.
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2007-2008
4-km MM5Real-time
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10-km
Smaller Scale Terrain ModulatesPrecipitation
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12-km
4-km
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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
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Annual Climatologies of MM5 4-km domain
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Verification of Small-Scale Orographic Effects
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Flow over terrain can be highly 3D and complex
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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.)
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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
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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
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Hurricane Isabel Reflectivity at Landfall
Radar Composite
18 Sep 2003 1700 Z
41 h forecast from 4 km WRF
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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.
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Precipitable WaterFrom Mike Warner
Associated with extraordinarily narrow filaments of moisture
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A Recent Devastating Pineapple Express: November 6-7, 2006
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Dark Green: about 20 inches
November 6-9, 2006
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We know quite a bit about atmospheric rivers and heavy NW precipitation events, although there
are still gaps in our knowledge
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Synoptic Set-Up for Top Fifty Events at Forks
Courtesy of Michael Warner
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Precipitable Water
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500 mb height
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SLP
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850 mb Temp
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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.
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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.
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Human Versus Objective Skill for Precipitation Forecasts: NWS Offices Around the US
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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
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Brier Scores for Precipitation for all stations for the entire study period.
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Brier Score for all stations, 1 August 2003 – 1 August 2004. 3-day smoothing is performed on the data.
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Brier Score for all stations, 1 August 2003 – 1 August 2004, sorted by geographic region.
Precipitation
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Reliability diagrams for period 1 (a), period 2 (b), period 3 (c) and period 4 (d).
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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
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Humans Help with Heavier Precipitation
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The End
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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.
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