Second MSC/COMET Winter Weather Course, Boulder, Downslope Windstorms Yet another thing I do not understand but must try to forecast

Second MSC/COMET Winter Weather Course, Boulder, Overview Observations and Theory A few equations Forecasting downslope winds Online tutorial East and West forecast lab cases

Second MSC/COMET Winter Weather Course, Boulder, Downslope Windstorms Result from breaking mountain waves Are a special case of gravity waves where the phase speed of the wave is zero Are common in many locations –lee of Rocky Mountains ?? –lee of Richardson mountains –west coasts Cape Breton and NF, as well as locations in AK Are not the same as gap winds (like Taku winds)

Second MSC/COMET Winter Weather Course, Boulder, Important Equations Froude Number: where h = mountain height U = windspeed perpendicular to barrier For Fr > 1 vertical deflections are generated For Fr < 1 horizontal deflections dominate

Second MSC/COMET Winter Weather Course, Boulder, Wave Equation 2-D wave equation: where Scorer parameter In general, when decreases with height, wave energy can be trapped vertically and the wave can propagate horizontally

Second MSC/COMET Winter Weather Course, Boulder, Downslope Windstorm Factors strong winds near mountaintop stable layer near mountaintop weak stability above stable layer weak vertical shear or a wind reversal aloft U = 0 at some altitude above mountaintop can produce extremely strong highly vertical waves (called a critical level)

Second MSC/COMET Winter Weather Course, Boulder, Forecasting Misconceptions Pressure gradient fallacy –mountain waves can induce pressure gradients –using pressure gradient will cause timing errors –Table 1, Colle and Mass, MWR, 1998 Rate of change in windspeed –if mountain waves responsible for wind, increases/decreases will be sharp and sudden

Second MSC/COMET Winter Weather Course, Boulder, Downslope Westerlies True downslope windstorms rare in westerlies Much more common with easterly component. Why? Strong winds very common in westerlies thanks to gravity

Second MSC/COMET Winter Weather Course, Boulder, Critical Level Tend to be associated with the most intense windstorms (see Colle and Mass Table) Wave energy not spread over large area Downslope winds (like chinook winds) tend to spread downstream Reverse shear can result in a wave-induced critical level

Second MSC/COMET Winter Weather Course, Boulder, Other considerations Often sharp boundary between weak upslope flow and strongest downslope flow (hydraulic jump) Hydraulic jump can propagate downstream Cold air dome can prevent penetration of high winds to the surface or can keep highest winds further up the slope Some experts claim that eta coordinates are not very effective in high resolution models for downslope windstorms

Second MSC/COMET Winter Weather Course, Boulder, Summary Must think conceptually in three dimensions Waiting for pressure gradient to appear on progs will result in poor forecasts For chronic gap winds, breaking waves might contribute to most severe events Effective understanding can differentiate between strong and severe winds

Second MSC/COMET Winter Weather Course, Boulder, Bottom Line Even in NE Colorado, with the highest density of observations and atmospheric scientists on the planet, downslope windstorms and their contributing factors are still not fully understood