20030611: Can we calculate Cape from a gridded model data?

Lewis,

I created a CAPE routine in GEMPAK 5.6.J that allows the computation of
CAPE Grids using pressure levels for data sets that don't have CAPE computed 
already.

The usage is:
GFUNC    = cape(P_bottom,Z_bottom,T_bottom,Td_bottom)

The 4 input variables are grids used to define the bottom of integration.
Generally, they are at the surface, but could be elevated if desired.
The integration uses data on pressure levels found in the data set and
computes THTA_parcel - THTA_environment areas. 

For example, the NGM data set we receive doesn't have CAPE. It does have
surface pressure and surface elevation grids (PRES@0%NONE and HGHT@0%NONE),
and Temperature and Relative Humidity at 2 meters (Dewpoint can
be calculated from temperature and relative humidity internally within GEMPAK),
so I can calculate CAPE with:

GLEVEL = 2
GVCORD = hght
GFUNC    = cape(pres@0%none,hght@0%none,tmpc,dwpc)

The quality of result depends on how many pressure levels are in your data set
(eg, if you only have 1000, 700, 500, 300 levels, the result is likely to
differ from a value that is computed within the model). I get very good
comparison with data at 50 mb intervals when compared against the value
output from the ETA model we have access to.

In general, we don't have all 4 parcel bottom variables on the same level and
coordinate, so I typically have to use teamperature and dewpoint on levels
near the surface (like 2 meter, or .9950 sigma).

Steve Chiswell



>From: "Poulin,Lewis [CMC]" <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200306111853.h5BIrfLd012747

>Hi Unidata Support..
>
>I have successfully converted our Canadian Models into Gempak format and can
>view them in Garp and Nmap.
>
>The models I am viewing are not sounding data but rather actually model data
>on various grids. Some grids are local to north america, other grids are
>global in coverage. 
>
>I have successfully done addtional calculations on those grids to add
>addtional fields to the gempak file like Thta level calcs and precipitation
>interval calcs.
>
>Our model output dataset does not include CAPE as the American models do.
>We do include a wide range of Height, Temperature and Dewpoint depression
>data for a large number of levels from which I believe it would be possible
>to calculate CAPE.
>
>I was researching a bit on how to use Gempak to calculate CAPE but my
>interpretation of the info suggests that we need sounding data on which to
>do the calculation?
>
>I was curious if there was a way I could generate CAPE values from our 3-D
>gridded model data sets. Ideally it would be a Cape value for each grid
>point which I could then 
>contour in Gempak. 
>
>Any advice you would have that could point me in the right direction would
>be greatly appreciated. 
>
>Many thanks..
>
>Lewis Poulin
>CMC-CMOI.
>