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Wind mixing down


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#1
hm8

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I hope I'm not asking this in a way that sounds totally stupid, but what are some things I should be looking at to determine how much winds may mix down to the surface? For example, is a model sounding shows 50 kts of wind at the 850 mb height, what else should I look at to determine if these winds will be able to make it to the surface?

#2
tmagan

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I hope I'm not asking this in a way that sounds totally stupid, but what are some things I should be looking at to determine how much winds may mix down to the surface? For example, is a model sounding shows 50 kts of wind at the 850 mb height, what else should I look at to determine if these winds will be able to make it to the surface?

Whether there is an inversion near the surface, if the surface winds and 850 mb winds are coming from the same direction...

#3
ohleary

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I hope I'm not asking this in a way that sounds totally stupid, but what are some things I should be looking at to determine how much winds may mix down to the surface? For example, is a model sounding shows 50 kts of wind at the 850 mb height, what else should I look at to determine if these winds will be able to make it to the surface?


You want a well-mixed boundary layer. Looking at a sounding, you want to see an unstable layer from the sfc to 850mb (temps approaching the dry adiabatic lapse rate)...and decent surface pressure gradient. Good daytime heating of the surface helps too as it adds to the instability and gets air parcels moving upward. Cold air advection at the 850 mb level too, as these parcels will "want" to descend.

#4
BillB

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Don't limit yourself to 850 mb. For many years this was the common level to use because there were few or no other useful levels (maybe 925 mb, if available). These days, many models produce data for intermediate levels that are closer to the actual top of mixing.

First step is to determine how high mixing will reach during the time period. Daytime heating (especially during the warm season) or colder differential temperature advection aloft (compared to the surface) are the two standard ways of deepening the mixed layer. The layer is usually deeper in the warm season than in the cold season, but deep layers can occur in the cold season under the right circumstances.

When the lapse rate in this layer approaches dry adiabatic (when the layer is dry) or moist adiabatic (when the layer is saturated) the potential increases for transfering that wind to the surface, normally in the form of gusts. Note the strongest winds in this layer.

A good (and free) tool to download/use is BUFKIT ( http://www.wdtb.noaa.gov/tools/BUFKIT/ ).

I hope I'm not asking this in a way that sounds totally stupid, but what are some things I should be looking at to determine how much winds may mix down to the surface? For example, is a model sounding shows 50 kts of wind at the 850 mb height, what else should I look at to determine if these winds will be able to make it to the surface?



#5
isohume

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A good (and free) tool to download/use is BUFKIT ( http://www.wdtb.noaa.gov/tools/BUFKIT/ ).

Agree, Bufkit is a great tool to assess momentum transfers to the sfc.

One thing I'd add is that during waa flow, much of the potential momentum that Bufkit indicates is utilized as upward vertical motion since the incoming air is more buoyant than the prevailing air mass. So...the wind gusts realized at the sfc will normally be significantly less than the same situation under a caa pattern.



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