Update on the Northwest Regional Modeling System 2017 Cliff Mass and David Ovens University of Washington

Goal and sponsors To produce state-of-the-art, high-resolution numerical weather forecasts over the Pacific Northwest. Supported by NW Modeling Consortium: a collection of local, state, and Federal Agencies AND the private sector (KING-5)

http://www.atmos.washington.edu/mm5rt/

NW High Resolution Regional Prediction Currently runs with 36, 12, 4, and 1.3 km grid spacing using the WRF (Weather Research and Forecasting) ARW model twice a day at 0000 and 1200 UTC (5 AM and 5 PM PDT) One of the highest resolution numerical weather prediction efforts in the U.S. Physics tested to work best in our region.

Major Changes This Year Doubling of the ultra-high resolution, 4/3 km domain Major increase in computer resources (bigger domain and available sooner) Major physics improvements, particularly near terrain

Old Domain 1.33 km

New Domain

Never forget A grid spacing of 1 Never forget A grid spacing of 1.3 km is barely defining features of a scale of 8 km

36 km

12 km

Verification System Shows No Problems With Transition

The Balancing Act Reliability versus cutting edge physics Generally, have tried to optimize reliability and general skill. Believe we should optimize more for stable boundary layer—important for stagnation, freezing rain, snow, gap winds

The Overmixing Problem A big problem for UW WRF (and other modeling systems) has been overmixing in the vertical, particularly when the lower atmosphere is stable

Classic Problem Warm Air Inversion Cool Air Overmixing Warm Air Inversion Cool Air Problem for snow forecasting, gap wind forecasting, air quality prediction

Dealing with the stable boundary layer over-mixing problem Higher horizontal resolution has not solved this. Greater vertical resolution near the surface has not fixed this. Some boundary layer schemes are a bit better, but the problem remained.

The Problem Rears it Head in the Columbia Gorge and Portland in January

Simulated Temperature and Winds Portland Troutdale Cascade Locks Min pass height is at far right of domain near Cascade Locks (same place as 4/3 domain) This plot is from 310 m. Note, vortex. See this on radar data too. Zoomed in plot will be from 150 m

Vertical Structure Also note increased wind speed near CZK. Possible where the flow becomes supercritical. Also possibly Venturi.

January 8, 2017, Portland (PDX)

Reality

UW WRF Too Warm

Diagnosis: Too Much Mixing in the Model Numerical models have a certain amount of mixing built into them. Attempts to mimic actual processes in the real atmosphere. But, the WRF model had much too much mixing between the cold air below and warm air above

The default is mixing along model surfaces But that can mix in the VERTICAL when model surfaces are tilted…as they are in and near terrain Worse at high resolution Worse in gaps and valleys

New WRF Option: Horizontal Mixing

Original

After Change

And there is some extra diffusion in the model (6th order diffusion) that could be dropped

Better Fog

Verification

Reduced Diffusion is Now Operational Next thing to fix: better moist processes (a.k.a. microphysics) Begin a 4-km ensemble of 10-15 independent runs to get a better idea of uncertainty in the forecasts

1.3 km Precipitation Bias over 2015-2106 Winter

The End