University of Washington Mesoscale Ensemble (UWME) Information

General Information: A group in the Department of Atmospheric Sciences at the University of Washington is running a mesoscale, short-range (0-3 day forecasts) ensemble with the Weather Research and Forecasting (WRF) mesoscale model twice per day (at the 0000 UTC and 1200 UTC cycles) in order to produce skillful probabilistic mesoscale meteorological forecasts for the Pacific Northwest. This effort, supported by a consortium of local, state, and federal agencies, has several goals:

• To test the applicability and effectiveness of multianalysis short-range ensemble forecasting (SREF; 0-72 hours lead time) at mesoscale ( O(10km) ) grid resolutions.
• To determine whether mesoscale SREFs have more or less value in regions of large orography and weak convection (northwestern U.S.) than in regions with mostly flat topography and more frequent, deep convection.
• To determine whether the mesoscale SREF mean possesses greater skill than any of its individual members.
• To determine whether the mesoscale SREF spread can be used as a predictor of both deterministic and probabilistic forecast error.
• To determine whether the mesoscale SREF can be used to produce skillful probabilistic forecast guidance for surface weather parameters, especially for temperature, winds, and precipitation.
• To determine the relative skill of the mesoscale SREF members in real-time and during the recent past to potentially derive appropriate weightings, that when applied to each ensemble member, would result in more accurate deterministic and probabilistic weather forecasts.
• To determine whether model error plays a significant role in the prediction of surface weather parameters for a region subject to large topographic influences.
• To determine whether incorporating model uncertainty into a mesoscale SREF is worthwhile, even for a region where the mesoscale forecast uncertainty is often dominated by the synoptic-scale uncertainty.
• To determine whether the multianalysis ensemble size can be expanded using linear combinations of the synoptic-scale ICs in order to improve the chance of encompassing truth and to provide more skillful probabilistic forecast guidance.

Model Configuration: The WRF mesoscale ensemble forecasts currently feature a 36-km (151x127) grid that covers much of western North America and the northeastern Pacific, a nested 12 km (184x142) grid that covers the entire Pacific Northwest U.S. and Southwest Canada, and a 4 km (406x283) grid covering Washington, Oregon, Idaho and parts of the neighboring states and provinces. The model utilizes 37 vertical eta levels, and is non-hydrostatic in order to limit pressure gradient force errors in the complex terrain. An upper-radiative boundary condition is used to allow gravity waves to radiate through the model top without being reflected. The sub-grid scale parameterizations include:

• Yonsei University planetary boundary layer scheme
  (see Hong and Pan, 1996)
• Thompson explicit moisture scheme including mixed phase processes (see Thompson et al., 2004)
• Grell-Freitas ensemble scheme for cumulus convection (see Grell and Freitas, 2014)

Detailed terrain and land use information for each domain was derived from the 1-km U.S.G.S. digital database.

Initial Condition Selection Strategy: MULTIANALYSIS
Initial conditions and lateral boundary conditions for the WRF mesoscale ensemble are currently generated by interpolation of separate global/synoptic-scale model analysis and forecast fields obtained from several operational weather prediction centers worldwide. A current list of these models includes:

• The National Center for Environmental Prediction (NCEP) Global Ensemble Forecast System (GEFS)
• The control (c01) and 9 randomly chosen members of the Ensemble Kalman Filter perturbation forecasts are chosen. More info on the GEFS is here.
• The Canadian Meteorological Centre (CMC) Global Environmental Multiscale (GEM) model
• The Bureau of Meteorology (Australia) Global AnalysiS and Prediction (GASP) model
• The Japan Meteorological Agency (JMA) Global Spectral Model (GSM)
• The Fleet Numerical Meteorology and Oceanography Center (FNMOC) Navy Global Environmental Model (NAVGEM)
• The Central Weather Bureau (CWB) (Taiwan) Global Forecast System
• The United Kingdom Met Office (UKMO) Unified Model (UM)

See the summary of members.

Hardware: Forecasts are computed on a cluster of computers:

1. A Linux PC Beowulf cluster of 28 possible machines with Intel Xeon ES-2650 and Intel Xeon Silver 4116 CPUs.

TIMING
Each 36/12/4 km 72-hour forecast finishes in approximately 4.25-4.75 hours. Each global/synoptic-scale IC becomes available at a different time after 0000/1200 UTC. Thus, some UWME ensemble members finish before others.

All UWME members plus the centroid run (CENT) are usually completed by 1430/0230 UTC (0630/1830 PST) provided all initializations are in on time. Ensemble mean and spread calculations are made at that time and they usually finish by 0030/1230 UTC (0830/2030 PST).

Future Improvements: The ensemble modeling system is constantly being improved, so expect changes and delays. Among the changes we hope to make in the future are:

• Adding graphics of near real-time performance and historical verification over extended periods to the UWME Verification & Performance page.
• Adding an array of post-processed forecast products to the website, including calibrated (via Bayesian Model Averaging) probability plots and data range products.