Downslope Windstorms

1. Downslope Windstorms Yet another thing I do not understand but must try to forecast

2. Overview • Observations and Theory • A few equations • Forecasting downslope winds • Online tutorial • East and West forecast lab cases

3. 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)

4. 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

5. 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

6. 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)

7. 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

8. Downslope Westerlies • True downslope windstorms rare in westerlies • Much more common with easterly component. Why? • Strong winds very common in westerlies thanks to gravity

9. 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

10. 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

11. 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

12. 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