[gradsusr] Vertical Integrated Moisture Transport (VIMT)

[Eric Altshuler]

Hi Muhammad, 


I think there are two issues here, but the problem might not be caused by either of them. First, you close file 1 after defining a variable. I don't think this is a good idea, since it leaves grads without a default file and the subsequent behavior of grads might be strange or incorrect (I ran into something like this in the past). Similarly, if there are several files open, the current default file should not be closed, for the same reason (e.g. if you have 'set dfile 2', don't close file 2 until you've set the default file to something other than 2). In your script, you should leave file 1 open and use 2,3,4 to refer to the files you open later. 


There is also an error in the calculation. The vertically integrated moisture transport is NOT equal to the product of the vertical integrals of wind and moisture. You need to calculate it this way: 


'define zon=vint(surfpres,shum.2*uwnd.3,500)' 
'define mer=vint(surfpres,shum.2*vwnd.4,500)' 


Best regards, 



Eric L. Altshuler 
Research Scientist 
Center for Ocean-Land-Atmosphere Studies 
253 Research Hall, Mail Stop 6C5 
George Mason University 
4400 University Drive 
Fairfax, VA 22030 USA 

E-mail: ela at cola.iges.org 
Phone: (703) 993-5725 
Fax: (703) 993-5770 

----- Original Message -----
From: "Muhammad Latif" <latif_space at yahoo.com> 
To: gradsusr at gradsusr.org 
Sent: Friday, May 9, 2014 4:06:19 PM 
Subject: [gradsusr] Vertical Integrated Moisture Transport (VIMT) 



Hi all, 


I am trying to calculate Vertical Integrated Moisture Transport (VIMT) from surface to 500 hPa in units of kg/m/sec using grads function "vint(psexpr,expr,top)" only on the one time step. 
I used the following script to get VIMT: 


'sdfopen spr-daily.nc' ;* spr is the surface pressure in Pa. 

'set time 20jun1970' 
'define surfpres=pres/100' ;* /100 to convert in hPa 
'close 1' 


'sdfopen shum-daily.nc' 
'sdfopen uwnd-daily.nc' 
'sdfopen vwnd-daily.nc' 


'set lat 0 45' 
'set lon 50 100' 


'set time 20jun1970' 
'define vintq=vint(surfpres,shum.1,500)' ;*shum is the specific humidity in kg/kg 
'define vintu=vint(surfpres,uwnd.2,500)' ;*uwnd is the u component of wind in m/sec 
'define vintv=vint(surfpres,vwnd.3,500)' ;*vwnd is the v component of wind in m/sec 
'define zon=(vintq*vintu)' 
'define mer=(vintq*vintv)' 


'set gxout vector' 
'd zon;mer' 


Script runs successfully showing VIMT, but the magnitude of vector appears (4e+06) in the vector scale bar. 
I guess, this magnitude is not right. The value should lie around 20 or 40 (kg/m/sec). 


I'll be very thankful if anyone could help me to sort out the issue mentioned above. 


Regards 


Muhammad Latif 
PhD Scholar 
Department of Meteorology 
CIIT, Islamabad 


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