equivalent potential temperature

[Gisele Zepka]

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