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brunt_vaisala_atm

Compute the Brunt-Vaisala frequency which is a measure of bouyancy in a continuously stratified atmosphere.

Available in version 6.4.0 and later.

Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"  ; This library is automatically loaded
                                                             ; from NCL V6.2.0 onward.
                                                             ; No need for user to explicitly load.

	function brunt_vaisala_atm (
		th      : numeric,  ; float, double, integer only
		z       : numeric,  
		opt [1] : integer,  
		dim [1] : integer   
	)

	return_val [dimsizes(th)] :  float or double

Arguments

Potential temperature (K).

Geometric height (m). Same dimensionality as th.

opt

opt=0, Return only the Brunt-Vaisala frequency as a regular variable.
opt=1, Return the Brunt-Vaisala frequency and d(theta)/dz

dim

The dimension number of z which corresponds to th.

Return value

An array of the same size and shape as th. The output will be double if th or z is of type double.

Description

The Brunt-Vaisala (BV) frequency (1/s) is a measure of bouyancy. In a continuously stratified fluid, it is the natural frequency of the vertical oscillation of fluid parcels. Hence, it should always be positive. However, this function will return a negative value when the vertical gradient of thv is negative. This is done for informational purposes only. The user can force only positive values via

         BV = where(BV.lt.0, 0, BV)

For plotting, the returned values which have units (1/s) are often multiplied by 86400 (s/day) to get units of (1/day).

Examples

Example 1: Read data from a WRF file and calculate assorted quantities.

;----------------------------------------------------------
;                     WRF DATA
;----------------------------------------------------------

   a  = addfile("wrfout_d01_2013-05-17_12","r") ;[Time|1]x[bottom_top|40]x[south_north|324]x[west_east|414]
                                               ;     0            1              2              3

   th = wrf_user_getvar(a,"theta",-1)      ; potential temperature (degK)
   z  = wrf_user_getvar(a,"z",-1)          ; model height

   brunt = brunt_vaisala_atm(th, z, 0, 1)  ; opt=0, ndim=1
   printVarSummary(brunt)
   printMinMax(brunt, 0)

; variable dimensions

   dimth = printMinMaxdimsizes(th)
   ntim  = dimth(0)

; print all vertical values at an arbitrarily chosen grid point

   ix = 20        ; arbitrary
   jy = 21

   print( sprintf("%7.1f", z(ntim-1,:,jy,ix))     \
        + sprintf("%15.5e",brunt(ntim-1,:,jy,ix)) )

The (edited) output is: