equivalent potential temperature

[Andrew Revering]

If you're using GRIB data, you should find a SFC level. You don't want to
set it to 1013 because that isn't necessarily the suface. In many cases that
will be an imaginary, extrapolation below ground (in the mountains for
example).

Look for 'SFC' in the inventory of your GRIB data for the proper level. You
should be able to find Tc, Td, RH for the 'SFC' level in there to do the
calculations.

So to clarify... MB would be the MB at the Surface. How that’s defined
exactly in your data set you'll have to figure out, but you don’t want it to
calculate it for 1013, unless you wanted a geopotential height like 850mb
done.

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 9:50 AM
To: GRADSUSR at LIST.CINECA.IT
Subject: Re: equivalent potential temperature

Thanks a lot Andrew.

As I said before, I will use model data to calculate theta-e. If I set 
the surface in the model, it is 1013 hPa. Is this the level that I need 
to consider in Mb (station pressure). If not, what is Mb? All Tc, Td and 
RH are catch in Mb level, aren't they?

Gisele


Em 26/1/2010 12:59, Andrew Revering escreveu:
> 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