Confusion about the Wind drag coefficient

EFDC_Explorer
1

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

2

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

3

Hi, Janesh

Thank you for your reply.
I still have a little confusion. What if i don’t need to calculate the evaporation since i have observed evaporation? For the “Atmospheric Parameters” settings, if i choose “False” for the “Internally compute Evaporation”, will the “1000Evap Transfer Coeff, <0 use WSP Drag" still work? Or could i set any value and the value will not affect the results?
Also, if "1000Evap Transfer Coeff, <0 use WSP Drag” is used for evaporation calculation, then I’m still confused about where to set the “wind drag coefficient”? Since i see some literatures referring to the Calibration with adjusting the wind drag coefficient in EFDC.

Thanks again! Any suggestion will be appreciated.

Bin