!copyright (C) 2001  MSC-RPN COMM  %%%RPNPHY%%%
*** FUNCTION DTFRZ

      FUNCTION DTFRZ (T,P,Q,SD,D273) 3
#include "impnone.cdk"
      REAL DTFRZ,T,P,Q,SD,D273
*
*Author
*          Fritsch and Chappell  (Sept 25, 1986)
*
*Revision
*
*Object
*     to calculate the difference of T due to freezing
*
*Arguments
*
*          - Input -
* T        temperature
* P        pressure
* Q        specific humidity
* SD       difference of moisture
* D273     = 1/273.16
*
*Notes
*
*
**
*
      REAL RLC,RLS,TOTRU,BT,DT,QS1,QS2,QS,X
*
C 
C...COMPUTE FREEZING EFFECT.
C 
      RLC = 2.5E6-2369.276*(T-273.16) 
      RLS = 2833922.-259.532*(T-273.16) 
      TOTRU = SD+Q
      BT = 1./T 
      DT = D273-BT
      QS1 = 611.*EXP(RLC*DT/461.) 
      QS2 = 611.*EXP(RLS*DT/461.) 
      QS = .622*RLS*QS2/(P-QS2) 
      X = 2.1672E-3*RLS*QS*BT*BT
      DTFRZ = (SD*(RLS-RLC)+.622*RLS*QS1/(P-QS1)-QS)/(1004.+1952.*TOTRU+
     *   X) 
      RETURN
      END