>From: "Jennie L. Moody" <address@hidden> >Organization: UVa >Keywords: 200205232127.g4NLRTa11165 McIDAS GRDCOPY DERIVE= Jennie, >I am just trying to re-derive (follow) what Owen and I have >done before in calculating PV from grids. I am trying to >find out exactly how math keywords work, like DDX and DDY, >when one uses the DERIVE keyword in a GRDCOPY command. OK, a lofty goal ;-) >In particular I wanted to see if these are centered differences. >I found a comment that said these are defined in the McIDAS >User's Guide, but I cannot locate them. Do you know where this >might be? The McIDAS-X 7.80 User's Guide section on GRDCOPY has a table for DERIVE= parameters: http://www.unidata.ucar.edu/packages/mcidas/780/users_guide/McHTML-136.HTML This page does not, however, specify exactly how DDX and DDY are defined other than saying: DDX - derivative with respect to x DDY - derivative with respect to y >C ? A variety of common meteorological parameters can be derived using >C ? the DERIVE keyword. These include VOR (vorticity), ABV (absolute >C ? vorticity), DSH (shear deformation), DST (stretching deformation), >C ? DVG (divergence), SPD (speed), DIR (direction), TD (dewpoint), >C ? COR (Coriolis parameter), and BETA (Beta parameter). >C ? >C ? The MATH keyword defines the operation to perform on the specified >C ? Gn grids. For example, if G1='PARAM Z;LEV 500;TIME 12;SRC MRF' >C ? G2='LEV 1000' MATH='G1-G2' is entered, a 1000 mb height grid will be >C ? subtracted from a 500 mb height grid, creating a 1000-500 mb thickness >C ? grid for the 12 UTC MRF model run. The math operators valid in the MATH >C ? keyword are +,-,/,*,**,SQRT,EXP,LOG,LOG10,SIN,COS,TAN,ASIN,ACOS,ATAN, >C ? ABS,MIN,MAX,DDX,DDY,DELSQ,COR,BETA,LAT, and LON. >C ? >C ? See the McIDAS User's Guide for definitions of the operators. >Thanks. I could try to find out exactly how the derivatives are done by reading the code if you need the info. Please let me know. >be well, You too! Tom
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