452 Precipitation

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

Orographic precipitation enhancement The basic idea is simple. The details of the dynamics and microphysics are more complicated. Flow over topography is a special case of dynamics that has to be considered separately or in addition to the other processes producing lifting and condensation. The pattern of fallout of the orographic precipitation is sensitive to the microphysical processes. How fast can the precip fall out?

Annual Precipitation

SW Olympic Slopes-Hoh Rain Forest: 150-170 inches yr-1

Sequim: roughly 15 inches per year.

Orographic Enhancement of Precipitation Upslope site (elev. 1067m) Valley site Precipitation Rate (mm/hr) Distance (km)

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.

Rainshadow

Rainshadow in westerly flow

Rainshadow Amplitude and Structure Depends on Synoptic Situation and Stability Less rainshadow during warm frontal and pre-occlusion periods, when deep and generally stable precipitation. Also winds tend to more southerly (terrain parallel) during this period. More rainshadow and less lee precipitation post-frontally, when flow is less stable and more westerly The former produces more shallow windward convection and greater westerly flow increases low-level upslope on windward side and drying downslope on leeward side.

Pre-occlusion, less profound rainshadow (15 UTC 23rd)

Moist warm southerly flow at 850 mb

Visible sat photo…clouds over region, with some suggestion of rain/cloudshadowing

Progress in Precipitation Prediction in Terrain NWP can handle terrain impacts on precipitation fairly well if sufficient resolution and state-of-the-art microphysics But there are still some issues (such as too much snow aloft over barriers)

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-ARW and WRF-NMM are similar December 3, 2007 0000 UTC Initial 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 Olympic Peninsula 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 4-km domain

Verification of Small-Scale Orographic Effects

Convective Precipitation Until recently, subject approaches have dominated. Both subjective and modeling approaches more more skillful with stratiform. Convection is least skillful precipitation in virtually all models Need at least 2-4 km resolution to get most convection even half right. If grid spacing is more coarse must have cumulus parameterization—SAS is used for MM5/WRF. High resolution models can potentially give a heads up 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

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

Diurnal Precipitation Variation During the Warm Season

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.

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.

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!)

Imperfect Predictions

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. Radar apps now available on smartphones to do this (e.g. Dark Sky) 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. Rapid refresh modeling systems. 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.

700 850

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. 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 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) 1.33- 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