Error in WRF winds rotated to Earth coordinates

From: Andrea N. Hahmann <ahah_at_nyahnyahspammersnyahnyah>
Date: Mon Jan 09 2012 - 05:51:10 MST

I think I have found a bug in the calculation of the cone used for the calculation of the winds rotated to the Earth coordinates in a Lambert Conformal projection. The error is only for the Lambert Conformal projection. As far as I know, alog10 (i.e. log10) is not the same as 10^…

In WRFUserARW.ncl:

         if( map_projection .eq. 1) then ; Lambert Conformal mapping
           if( (fabs(true_lat1 - true_lat2) .gt. 0.1) .and. \
               (fabs(true_lat2 - 90. ) .gt. 0.1) ) then
               cone = 10^(cos(true_lat1*radians_per_degree)) \
                     -10^(cos(true_lat2*radians_per_degree))
               cone = cone/(10^(tan(45. -fabs(true_lat1/2.)*radians_per_degree)) - \
                            10^(tan(45. -fabs(true_lat2/2.)*radians_per_degree)) )
           else
               cone = sin(fabs(true_lat1)*radians_per_degree)
           end if

In WRF (fortran; subroutine lc_cone in share/module_llxy.f90):

      IF (ABS(truelat1-truelat2) .GT. 0.1) THEN
         cone = ALOG10(COS(truelat1*rad_per_deg)) - &
                ALOG10(COS(truelat2*rad_per_deg))
         cone = cone /(ALOG10(TAN((45.0 - ABS(truelat1)/2.0) * rad_per_deg)) - &
                ALOG10(TAN((45.0 - ABS(truelat2)/2.0) * rad_per_deg)))
      ELSE
         cone = SIN(ABS(truelat1)*rad_per_deg )
      ENDIF

Furthermore, when doing the calculation in NCL it does not match the values of COSALPHA and SINALPHA in the wrfout file.

Regards, Andrea

--
Andrea N. Hahmann
Senior Scientist
DTU Wind Energy
Technical University of Denmark
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Department of Wind Energy
Risø Campus Frederiksborgvej 399
Building 118
4000 Roskilde
Direct +45 46775471
Mobile +45 2133 0550
ahah@risoe.dtu.dk<mailto:ahah@risoe.dtu.dk>
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Learn more about DTU Wind Energy on www.vindenergi.dtu.dk<http://www.vindenergi.dtu.dk/>.

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Received on Mon Jan 9 05:51:21 2012

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