452 Precipitation

Prob. Of Precip.– Cool Season(0000/1200 UTC Cycles Combined)

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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

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-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.

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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