equivalent potential temperature

[Jeffrey Duda]

Which model data are you using?  American models should have data for
surface pressure.  It's usually called pressfc.

Jeff Duda

On Tue, Jan 26, 2010 at 10:08 AM, Gisele Zepka <gzepka at dge.inpe.br> wrote:

> 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
>



-- 
Jeff Duda
Iowa State University
Meteorology Graduate Student
3134 Agronomy Hall
www.meteor.iastate.edu/~jdduda
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