Which model data are you using? American models should have data for surface pressure. It's usually called pressfc.<br><br>Jeff Duda<br><br><div class="gmail_quote">On Tue, Jan 26, 2010 at 10:08 AM, Gisele Zepka <span dir="ltr"><<a href="mailto:gzepka@dge.inpe.br">gzepka@dge.inpe.br</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Well I have a problem then.<br>
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.<br>
I really don't know how to get this Mb.<br>
<br>
Gisele<br>
<br>
<br>
Em 26/1/2010 13:59, Andrew Revering escreveu:<div><div></div><div class="h5"><br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
If you're using GRIB data, you should find a SFC level. You don't want to<br>
set it to 1013 because that isn't necessarily the suface. In many cases that<br>
will be an imaginary, extrapolation below ground (in the mountains for<br>
example).<br>
<br>
Look for 'SFC' in the inventory of your GRIB data for the proper level. You<br>
should be able to find Tc, Td, RH for the 'SFC' level in there to do the<br>
calculations.<br>
<br>
So to clarify... MB would be the MB at the Surface. How that’s defined<br>
exactly in your data set you'll have to figure out, but you don’t want it to<br>
calculate it for 1013, unless you wanted a geopotential height like 850mb<br>
done.<br>
<br>
Andrew Revering<br>
Convective Development, Inc.<br>
<a href="http://www.convectivedevelopment.com/" target="_blank">http://www.convectivedevelopment.com/</a><br>
<br>
<br>
-----Original Message-----<br>
From: <a href="mailto:GRADSUSR@LIST.CINECA.IT" target="_blank">GRADSUSR@LIST.CINECA.IT</a> [mailto:<a href="mailto:GRADSUSR@LIST.CINECA.IT" target="_blank">GRADSUSR@LIST.CINECA.IT</a>] On Behalf Of<br>
Gisele Zepka<br>
Sent: Tuesday, January 26, 2010 9:50 AM<br>
To: <a href="mailto:GRADSUSR@LIST.CINECA.IT" target="_blank">GRADSUSR@LIST.CINECA.IT</a><br>
Subject: Re: equivalent potential temperature<br>
<br>
Thanks a lot Andrew.<br>
<br>
As I said before, I will use model data to calculate theta-e. If I set<br>
the surface in the model, it is 1013 hPa. Is this the level that I need<br>
to consider in Mb (station pressure). If not, what is Mb? All Tc, Td and<br>
RH are catch in Mb level, aren't they?<br>
<br>
Gisele<br>
<br>
<br>
Em 26/1/2010 12:59, Andrew Revering escreveu:<br>
<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
You can calculate the Theta-E from ANY level. The typical levels of<br>
<br>
</blockquote>
interest<br>
<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
for severe weather are 850mb and the surface. Values of 330K or greater<br>
<br>
</blockquote>
are<br>
<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
usually indicative of instability great enough for severe weather. If<br>
<br>
</blockquote>
you're<br>
<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
looking at severe weather, I probably would use the surface. Likewise, you<br>
can use the gradient you get from Theta-E, overlay wind barbs at the level<br>
you're calculating (surface or 850) and you get a real nice idea of where<br>
the boundaries are at that level.<br>
<br>
The disclaimer is there are a LOT of other parameters needed to really<br>
identify a situation as severe-worthy though.<br>
<br>
I don't know if GrADS has a function for it, but the 'saturated' potential<br>
temperature (Theta-E) can be calculated with either of the following:<br>
<br>
Tc= temperature in degrees C<br>
Td = dew point temperature in degrees C (if needed)<br>
Mb = station pressure (be careful to note this is not sea level pressure<br>
<br>
</blockquote>
or<br>
<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
altimeter, they are all different)<br>
M = Mixing Ratio (calculations below if needed)<br>
RH = relative humidity<br>
Ms = Saturation Mixing Ratio (if needed, calculations below)<br>
Es = Dry Saturation Vapor Pressure (calculations below if needed)<br>
<br>
ThetaE = (Tc + 273.15) * ( 1000 / Mb ) ^ 0.286 + (3 * M)<br>
<br>
OR<br>
<br>
ThetaE = (273.15 + Tc) * ( 1000 / Mb ) ^ 0.286 + (3 * (RH * (3.884266 * 10<br>
<br>
</blockquote>
^<br>
<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
(( 7.5 * Tc ) / ( 237.7 + Tc )) ) /100 ))<br>
<br>
<br>
M= RH*Ms/100<br>
OR<br>
M= ((0.622*E)/(Mb-E))*1000<br>
<br>
Ms = ((Val(RH) / 100) / Val(M)) * 100<br>
OR MORE ACCURATELY<br>
Ms = 0.622 * Es/(P - Es)<br>
<br>
Es = 6.1078 * exp([(9.5939 * Td) - 307.004]/[(0.556 * Td) + 219.522])<br>
<br>
Andrew Revering<br>
Convective Development, Inc.<br>
<a href="http://www.convectivedevelopment.com/" target="_blank">http://www.convectivedevelopment.com/</a><br>
<br>
-----Original Message-----<br>
From: <a href="mailto:GRADSUSR@LIST.CINECA.IT" target="_blank">GRADSUSR@LIST.CINECA.IT</a> [mailto:<a href="mailto:GRADSUSR@LIST.CINECA.IT" target="_blank">GRADSUSR@LIST.CINECA.IT</a>] On Behalf<br>
<br>
</blockquote>
Of<br>
<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Gisele Zepka<br>
Sent: Tuesday, January 26, 2010 8:40 AM<br>
To: <a href="mailto:GRADSUSR@LIST.CINECA.IT" target="_blank">GRADSUSR@LIST.CINECA.IT</a><br>
Subject: equivalent potential temperature<br>
<br>
Dear all,<br>
<br>
I am interested to analyse the atmospheric instability when a storm<br>
occurrs using equivalent potential temperature from mesoscale model.<br>
What atmospheric level do I need to check to get an idea of instability?<br>
I don't have a theta_e output, so I will calculate it from temperature<br>
and humidity data from model.<br>
<br>
Please, any help will be nice.<br>
<br>
Thanks.<br>
Gisele<br>
<br>
<br>
<br>
</blockquote>
<br>
<br>
</blockquote>
<br>
<br>
-- <br>
Msc. Gisele dos Santos Zepka<br>
Atmospheric Electricity Group (ELAT)<br>
National Institute for Space Research (INPE)<br>
Av. dos Astronautas, 1758<br>
São José dos Campos/ SP/ Brazil CEP 12227-010<br>
Phone ++55(12)39456841<br>
</div></div></blockquote></div><br><br clear="all"><br>-- <br>Jeff Duda<br>Iowa State University<br>Meteorology Graduate Student<br>3134 Agronomy Hall<br><a href="http://www.meteor.iastate.edu/~jdduda">www.meteor.iastate.edu/~jdduda</a><br>