equivalent potential temperature

[Andrew Revering]

You can calculate the Theta-E from ANY level. The typical levels of interest
for severe weather are 850mb and the surface. Values of 330K or greater are
usually indicative of instability great enough for severe weather. If you're
looking at severe weather, I probably would use the surface. Likewise, you
can use the gradient you get from Theta-E, overlay wind barbs at the level
you're calculating (surface or 850) and you get a real nice idea of where
the boundaries are at that level.

The disclaimer is there are a LOT of other parameters needed to really
identify a situation as severe-worthy though. 

I don't know if GrADS has a function for it, but the 'saturated' potential
temperature (Theta-E) can be calculated with either of the following:

Tc= temperature in degrees C
Td = dew point temperature in degrees C (if needed)
Mb = station pressure (be careful to note this is not sea level pressure or
altimeter, they are all different)
M = Mixing Ratio (calculations below if needed)
RH = relative humidity
Ms = Saturation Mixing Ratio (if needed, calculations below)
Es = Dry Saturation Vapor Pressure (calculations below if needed)

ThetaE = (Tc + 273.15) * ( 1000 / Mb ) ^ 0.286 + (3 * M)

OR

ThetaE = (273.15 + Tc) * ( 1000 / Mb ) ^ 0.286 + (3 * (RH * (3.884266 * 10 ^
(( 7.5 * Tc ) / ( 237.7 + Tc )) ) /100 ))


M= RH*Ms/100
OR
M= ((0.622*E)/(Mb-E))*1000

Ms = ((Val(RH) / 100) / Val(M)) * 100
OR MORE ACCURATELY
Ms = 0.622 * Es/(P - Es)

Es = 6.1078 * exp([(9.5939 * Td) - 307.004]/[(0.556 * Td) + 219.522])

Andrew Revering
Convective Development, Inc.
http://www.convectivedevelopment.com/

-----Original Message-----
From: GRADSUSR at LIST.CINECA.IT [mailto:GRADSUSR at LIST.CINECA.IT] On Behalf Of
Gisele Zepka
Sent: Tuesday, January 26, 2010 8:40 AM
To: GRADSUSR at LIST.CINECA.IT
Subject: equivalent potential temperature

Dear all,

I am interested to analyse the atmospheric instability when a storm 
occurrs using equivalent potential temperature from mesoscale model. 
What atmospheric level do I need to check to get an idea of instability? 
I don't have a theta_e output, so I will calculate it from temperature 
and humidity data from model.

Please, any help will be nice.

Thanks.
Gisele

-- 
Msc. Gisele dos Santos Zepka
Atmospheric Electricity Group (ELAT)
National Institute for Space Research (INPE)
Av. dos Astronautas, 1758
São José dos Campos/ SP/ Brazil CEP 12227-010
Phone ++55(12)39456841