Re: NCL Skew-T CAPE calculation

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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Received on Tue Apr 13 14:57:07 2010