Re: NCL Skew-T CAPE calculation

From: Dennis Shea <shea_at_nyahnyahspammersnyahnyah>
Date: Tue Apr 13 2010 - 15:12:20 MDT

Hi Rachel,

The 'sounding' you sent has an LCL of about 935 hPa.

The (fortran) code that computes the CAPE has a comment:

c lclmb approximate pressure at lifting condensation
c level (mb). the pressure ACTUALLY USED will
c be the first value on the sounding grid greater
c than or equal to lclmb.

Given that the 1st 2 levels of your data are 1009.5 and 925, the
better level to start would have been 925. Even better, it would
have been better to interpolate and start at the exact LCL.
Unfortunately, the code uses the 1009.2 level as a start.
This would lead to larger CAPE values.

---
FYI: a number of years ago, I ran a few tests on several codes
that compute CAPE. The results had a much wider spread than
would be expected.
---
Do you have the specific humidity, geopotential height,
surface geopotential [station elevation] and surface pressure?
or ... could these quantities be calculated from the information
you have at hand? Really, having P, T(P), TD(0) is rather minimal
in terms of data.
If so, you could use ...
http://www.ncl.ucar.edu/Document/Functions/Built-in/wrf_cape_2d.shtml
D
On 04/13/2010 02:56 PM, Correia, James wrote:
>
> Hi Rachel-
> It looks like the call to cape_thermo from within the skewt_func.ncl script looks for the nearest model level closest to the ground and reassigns the lcl to that value then computes CAPE leaving the reported lcl alone.  I am not sure where the code resides to actually see if that is what is happening. But this highlights that CAPE is approximate in this calculation, and lcl is more precise.
> Jimmyc
>
>
> On 4/13/10 11:33 AM, "Rachel G. Mauk"<mauk.20@buckeyemail.osu.edu>  wrote:
>
> Hello,
>
> I am working with the NCL Skew-T routine to plot NCEP/NCAR reanalysis temperatures over the North Atlantic Ocean. The program is producing CAPE values much too high for the environment (~3000-4000 J when my calculations estimate 600-1200 J).
>
> I noticed something on the Skew-T plots that might be part of the problem. The CAPE curve is plotted along the pseudo-adiabat starting at the first pressure level, not at the LFC. I am really puzzled, especially since the NCL-calculated LCLs match the LCLs on my hand-plotted Skew-Ts (usually between 925-960 hPa). Do I need to set another attribute or three in my program, or in the original skewt_func.ncl code?
>
> I have attached a sample text file (I don't have dewpoints for pressures above the first level, so the remainder are filler values) and the resulting plot in jpg and ps format. My version of routine skewt_2.ncl is included below in this email.
>
> I really appreciate any assistance. Please let me know if more information is needed.
>
> Rachel G Mauk
> Masters Candidate
> Atmospheric Sciences Program
> The Ohio State University
> 1036 Derby Hall
> 154 N. Oval Mall
> Columbus, OH 43210
>
> ;**************************************************
> ; skewt_new.ncl, based on skewt_2.ncl from NCL lib,  ncl.ucar.edu<http://ncl.ucar.edu>
> ;**************************************************
> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/skewt_func.ncl"
> ;**************************************************
>
>   begin
>
> ; --- Read Data; no winds; set to _FillValue------------;
> diri = "$NCARG_ROOT/textfiles/"
> fili = "jose_2918_trial.txt"
> nlvl = 12
> ncol = 3
> TestData = asciiread (diri+fili, (/nlvl,ncol/), "float")
>
> p  = TestData (:,0)
> tc = TestData (:,1)
> tcd = TestData (:,2)
>
> z = (/-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999./)
> wspd = (/-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999./)
> wdir = (/-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999.,-999./)
>
> tcd@_FillValue = -999.
> z@_FillValue = -999.
> wspd@_FillValue = -999.
> wdir@_FillValue = -999.
>
> ;*************************
> ; create plot
> ;*************************
>    wks  = gsn_open_wks ("ps", "skewt_jose_2918")
>
> ; --- Create background skew-T; plot sounding ---------------------
>    skewtOpts                 = True
>    skewtOpts@DrawColAreaFill = True    ; default is False
>    skewtOpts@tiMainString    = "Jose 10/29/81 18Z; Filler Dewpoints"
>
>    dataOpts   = False        ; set options
>    dataOpts@colTemperature = "black"
>
>    skewt_bkgd = skewT_BackGround (wks, skewtOpts)
>    skewt_data = skewT_PlotData   (wks, skewt_bkgd, p,tc,tcd,z,wspd,wdir,dataOpts)
>    draw (skewt_bkgd)
>    draw (skewt_data)
>    frame(wks)
>
>   end
>
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-- 
======================================================
Dennis J. Shea                  tel: 303-497-1361    |
P.O. Box 3000                   fax: 303-497-1333    |
Climate Analysis Section                             |
Climate & Global Dynamics Div.                       |
National Center for Atmospheric Research             |
Boulder, CO  80307                                   |
USA                        email: shea 'at' ucar.edu |
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Received on Tue Apr 13 15:12:29 2010

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