# Confusion about the Wind drag coefficient

Hi, all: For the setting of the wind drag coefficient (see attached), i’m confused about the meaning of “1000*Evap transfer coeff, <0 use wind drag”. For example, if i set this value to be 1.5, what does that mean? Does it means the wind drag coefficient to be 1.5? Any help will be appreciated. Thanks! Bin Attached files In EFDC_DSI, "1000Evap transfer coeff, <0 use wind drag" is used to calculate the wind function CLEVAP for the evaporation calculations.

In the subroutine CALTSXY.f90, CLEVAP is first initialized as
CLEVAP (L) = 0.001 * ABS(REVC)

Then a condition is checked if REVC is less than zero or not.

`! *** EVAPORATION WIND FUNCTION (FW)	      IF( ISTOPT(2) == 1 .OR. IEVAP == 2 )THEN	        IF(REVC < 0.)THEN 	          CLEVAPTMP=0.001ABS(REVC)	          DO L=LF,LL	            CLEVAP(L)=1.E-3*(0.8+0.065*WINDST(L)) 	            CLEVAP(L)=MAX(CLEVAP(L),CLEVAPTMP) 	          ENDDO 	        ENDIF	      ENDIF`

So, if REVC is smaller than zero, then the evaporation function will be calculated using the formula CLEVAP(L) = 1.E-3 (0.8 + 0.065WINDST(L)) and the maximum of them will be used.

I hope it helps. Let me know if you have any confusion.

Best,
Janesh

Hi, Janesh