Robert Conrick, Qi Zhong, and Cliff Mass An evaluation of precipitation forecasts during November 12-15, 2015 and November 16-19, 2015 Robert Conrick, Qi Zhong, and Cliff Mass University of Washington Pacific NW Weather Workshop 2017

Cases: November 12-15, 2015 November 16-19, 2015 Atmospheric river events, producing over 400 mm of precipitation in some areas. Divided into two periods: - Prefrontal: Warm period prior to frontal passage - Postfrontal: Cooler period following frontal passage Modeled at 36-12-4-1.33 km resolutions using the WRF-ARW v.3.7.1 - Microphysics: WSM5 & 6, Thompson, Goddard, SBU-YLin

Why evaluate model microphysics? Accurate microphysics representation is important for: Hydrologic impacts Water resource management Forecasting of high-impact events

Why evaluate model microphysics? WRF has more than 20 microphysics (MP) options. What problems exist in schemes?

Which microphysics deficiencies are known? Variability in location and amount microphysical species and precipitation Ice and snow representations - Many schemes still use spherical snow. Riming and graupel - Most schemes have ice as discrete catagories Photo credit: NOAA NSSL

36 km 1.3 km Does resolution improve forecast accuracy? Increasing resolution improves precipitation forecasts in PNW. - We’ll revisit this in a few slides. Nov. 01, 2015 – Feb. 01, 2016 36 km 1.3 km

Does elevation influence forecast accuracy? Nov. 12 - 15: Lower elevations tend to be better forecast with less spread. Higher elevations are underforecast by 20-30%. Percent of Observed ASOS & Mesowest stations

Does elevation influence forecast accuracy? Nov. 16 - 19: Lower elevations: Still less spread. All elevations have large underprediction. Percent of Observed ASOS & Mesowest stations

Olympic Mountains: Windward vs. Leeward Underforecasting precipitation is common on both windward and leeward slopes. Nov 12-15 has worse windward than leeward forecasts; Nov 16-19 is opposite. Percent of Observed Percent of Observed credit: Qi Zhong Nov 12-15 Nov 16-19

Olympic Mountains: Windward vs. Leeward Microphysics (MP) shows less resolution dependence than PBL. Nov 12-15 Percent of Observed Percent of Observed credit: Qi Zhong Microphysics PBL (Thompson MP)

Quinault Area Forecasts SW-NE transect up the Quinault Goals: Do microphysics and precipitation forecasts differ as the flow is modified by terrain? Is pre- and post-frontal forecast accuracy different?

Quinault Area Forecasts Nov. 12-15 Nov. 16-19 Percent of Observed Percent of Observed

Are synoptic differences responsible? Melting levels do not substantially differ by MP scheme. Beach Nov. 16-19 Height (m; agl) Bishop

Are synoptic differences responsible? 850 mb wind speed & direction (not shown) do not substantially differ by MP scheme. Beach Nov. 16-19 Speed (m/s) Bishop

Are differing microphysical species responsible? Hypothesis: Different schemes produce different amounts of species and hydrometeors, which influences precipitation forecasts. Cloud ice Snow Nov. 12-15 Case: Pre-frontal Average

Conclusions Increasing resolution improves model accuracy substantially. Low elevations tend to have less spread in forecasts, but elevation does not determine how accurate a forecast is. Underforecasting precipitation is a problem, especially on windward slopes where observed precipitation is high. - No resolution dependence when varying MP schemes. - Varying PBL schemes shows strong resolution dependence. In the Quinault region, sensitivity to partitioning of MP species yields very poor forecasts.