452 Precipitation
Rainshadow
Rainshadow in westerly flow
Annual
Precipitation
SW Olympic Slopes-Hoh Rain Forest: 150-170 inches yr-1
Sequim: roughly 15 inches per year.
.
Puget Sound Convergence Zone
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-km
UW MM5
Real-time
12-km WRF-AR
and WRF-NMM
are similar
December 3, 200
0000 UTC Initia
12-h forecast
3-hr precip.
2007-2008
4-km MM5
Real-time
Smaller Scale Terrain Modulates
Precipitation
10-km
12-km
4-km
Small-Scale Spatial Gradients in Climatological Precipitation on the Olym
Alison M. Anders, Gerard H. Roe, Dale R. Durran, and Justin R. Minder
Journal of Hydrometeorology
Volume 8, Issue 5 (October 2007) pp. 1068–1081
Annual Climatologies of MM5 4km domain
Verification of Small-Scale
Orographic Effects
Flow over terrain can be highly 3D and complex
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
Valid 6/10/03 12Z
4 km BAMEX forecast
36 h Reflectivity
4 km BAMEX forecast
12 h Reflectivity
Composite
NEXRAD Radar
Real-time 12 h WRF Reflectivity Forecast
Valid 6/10/03 12Z
4 km BAMEX
forecast
10 km BAMEX
forecast
22 km CONUS
forecast
Composite
NEXRAD Radar
Hurricane Isabel Reflectivity at Landfall
18 Sep 2003 1700 Z
Radar Composite
41 h forecast from 4 km WRF
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.
Associated with
extraordinarily narrow
filaments of moisture
Precipitable Water
From Mike Warner
A Recent Devastating Pineapple Express:
November 6-7, 2006
November 6-9,
2006
Dark Green: about 20 inches
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.
Human Versus
Objective Skill for
Precipitation
Forecasts: NWS
Offices Around the
US
Prob. Of Precip.– Cool Season
(0000/1200 UTC Cycles Combined)
0.7
Brier Score Improvement over Climate
0.6
Guid POPS 24 hr
Local POPS 24 hr
Guid POPS 48 hr
Local POPS 48 hr
0.5
0.4
0.3
0.2
0.1
0
1966
1969
1972
1975
1978
1981
1984
Year
1987
1990
1993
1996
1999
2002
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.
Precipitation
Brier Score for all stations, 1 August 2003 – 1 August
2004, sorted by geographic region.
Reliability diagrams for period 1 (a), period 2 (b),
period 3 (c) and period 4 (d).
Rain Psychology
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”
Humans Help with
Heavier Precipitation
The End
Bias scores for the (a) 1.33and (b) 4-km model
simulations for 1400 UTC 13
Dec 2001 through 0800
UTC 14 Dec 2001.