equivalent potential temperature

[Andrew Revering]

Well what you want to do is define what the surface LEVEL is... and have
that LEVEL return to you whatever the pressure is at that level. 

So if you can figure out what the surface level is, that "row" of data
should contain a pressure to use.

Not sure if that helps.

-----Original Message-----
From: GRADSUSR at LIST.CINECA.IT [mailto:GRADSUSR at LIST.CINECA.IT] On Behalf Of
Gisele Zepka
Sent: Tuesday, January 26, 2010 10:09 AM
To: GRADSUSR at LIST.CINECA.IT
Subject: Re: equivalent potential temperature

Well I have a problem then.
I run the wrf model and it has 42 levels. The surface pressure is 
defined as 1013. This is my lowest level when I set z=1.
I really don't know how to get this Mb.

Gisele


Em 26/1/2010 13:59, Andrew Revering escreveu:
> 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