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wrf_cape_2d

Computes maximum convective available potential energy (CAPE), maximum convective inhibition (CIN), lifted condensation level (LCL), and level of free convection (LFC).

Prototype

	function wrf_cape_2d (
		p       : numeric,  
		t       : numeric,  
		q       : numeric,  
		z       : numeric,  
		zsfc    : numeric,  
		psfc    : numeric,  
		opt [1] : logical   
	)

	return_val [4,...] :  float or double

Arguments

An array containing full model pressure values [must be Pa], Could be ordered bottom_top or top_bottom. This array must be the same dimensionality as t, q, and z. See the description section for more information on dimension requirements.

An array containing temperature values [K]. Must be ordered similar to p, q, and z. This array must be the same dimensionality as p, q, and z. See the description section for more information on dimension requirements.

An array containing water vapor mixing ratio [kg/kg]. Must be ordered similar to p, t, and z. This array must be the same dimensionality as p, t, and z. See the description section for more information on dimension requirements.

An array containing full modelheight [m]. Must be ordered similar to p, t, and q. This array must be the same dimensionality as p, t, and q. See the description section for more information on dimension requirements.

zsfc

A scalar or array containing surface height (terrain) [m]. Must be the same dimensionality as psfc. See the description section for more information on dimension requirements.

psfc

A scalar or array containing surface pressures (hPa). Must be the same dimensionality as zsfc. See the description section for more information on dimension requirements.

opt

Set to False for pressure level data. Set to True for terrain-following data.

Return value

A multi-dimensional array whose leftmost dimension is 4 (0=CAPE, 1=CIN, 2=LCL, 3=LFC). The rightmost dimension sizes depend on the input dimension sizes. The type will be double if any of the input is double, and float otherwise.

In version 6.1.0, this function was upgraded to return a "_FillValue" attribute. See the description section below for more information.

Description

This function replaces the now obsolete rip_cape_2d function.

This function uses the RIP [Read/Interpolate/plot] code to calculate potential energy (CAPE) and convective inhibition (CIN) (in m**2/s**2 or J/kg) only for the parcel with max theta-e in the column (i.e. something akin to Colman's MCAPE). CAPE is defined as the accumulated buoyant energy from the level of free convection (LFC) to the equilibrium level (EL). CIN is defined as the accumulated negative buoyant energy from the parcel starting point to the LFC. The word "parcel" here refers to a 500 meter deep parcel, with actual temperature and moisture averaged over that depth.

There are two possible cases for the input and output dimension sizes:

p, t, q, z (time,lev,lat,lon) and psfc, zsfc (time,lat,lon)
--> return_val(4,time,lat,lon)
p, t, q, z (lev,lat,lon) and psfc, zsfc (lat,lon)
--> return_val(4,lat,lon)

The leftmost dimension of the return array represents four different quantities:

return_val(0,...) will contain CAPE
return_val(1,...) will contain CIN
return_val(2,...) will contain LCL
return_val(3,...) will contain LFC

The return variable will contain the following attribute:

return_val@description = "mcape_mcin_lcl_lfc"

LCL is the lifted condensation level in (m), and is the LCL for the parcel in each column with maximum qe below 3000 m AGL.

LFC is the level of free convection (LFC), m (AGL). This is the LFC for the parcel in each column with maximum qe below 3000 m AGL.

Previous to version 6.1.0, this function returned a value of -0.1 for out-of-range values. This has been replaced with a missing value, and a "_FillValue" attribute is also set. The value of the missing value depends on the type of the return array (float or double).

This routine does not do anything special with missing values. If you pass in missing values, they will get used as if they were valid values in the calculations. So, make sure your data contains no missing values before you call this routine.

Examples

Note that if your netCDF file doesn't have a ".nc" suffix, you must include it in the call to addfile so it knows what kind of file to open. The addfile call below will cause NCL to look for both a file called "wrfout_d01_2005-12-14_13:00:00.nc" and "wrfout_d01_2005-12-14_13:00:00".

Example 1

begin
  a = addfile("wrfout_d01_2005-12-14_13:00:00.nc","r")

  T  = a->T
  P  = a->P
  PB = a->PB
  QV = a->QVAPOR
  PH  = a->PH
  PHB = a->PHB
  HGT = a->HGT
  PSFC = a->PSFC

  T = T + 300.
  P  = P + PB 
  tk = wrf_tk( P , T )
  PH =  PH + PHB
  z = wrf_user_unstagger(PH,PH@stagger)
  z = z/9.81

  cinfo = wrf_cape_2d( P, tk, QV, z, HGT, PSFC, True )

  ;mcape = cinfo(0,.....)
  ;mcin  = cinfo(1,.....)
  ;lcl  = cinfo(2,.....)
  ;lfc  = cinfo(3,.....)

end

Example 2

The function wrf_user_getvar, (available in the $NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl script) can also be used to calculate many diagnostics in one step.

  load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
  load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"

  a = addfile("wrfout_d01_2000-01-24_12:00:00.nc","r")

  cape2d = wrf_user_getvar(a,"cape_2d",-1)

  ;mcape = cape2d(0,.....)
  ;mcin  = cape2d(1,.....)
  ;lcl  = cape2d(2,.....)
  ;lfc  = cape2d(3,.....)

You can see some other example scripts and their resultant images at: