Using the DRS Library

Data in the MCD are written in DRS format. If you are not using a package (such as GrADS) which can read DRS format you can write a program in FORTRAN (or some other language, such as C or IDL) to access the data using the DRS library. We supply several FORTRAN subroutines which are also used in the MarsGRAM-style interface to the MCD (see Section 4.2) to help in this process. The advantage of this method of access over GrADS, for example, is that the large- and small-scale variability models may be used if the appropriate subroutines are called.

Opening and Closing Files

To access the data you must first open the file. An example of opening an MCD file within a FORTRAN program is shown here.

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#include "drsdef.h"
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integer udrs ! DRS file unit number
integer ierr
character*256 datfile ! DRS data file
character*256 dicfile ! DRS dictionary file
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udrs=60
datfile='/FULL/PATH/NAME/mcd/data/viks04me.dat'
dicfile='/FULL/PATH/NAME/mcd/data/viks04me.dic'
ierr=aslun(udrs,dicfile,udrs+1,datfile,IDRS_READ)
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! read some DRS data
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ierr=cllun(udrs) ! close the file again
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Reading and Manipulating Data

Once the file has been opened you can read data from the MCD either by using the DRS routines directly or by using subroutines from the mcdgm directory. The following routines may prove particularly useful. Each is commented within the code to indicate the type and size of arguments which it expects; note that in some cases the number of arguments has changed since earlier versions of the MCD. The MCDGM program provides one example of how to call these routines (examine the dstep.f routine in particular).

• var2d.f Reads one value of a 2-D field from the MCD. Uses bilinear interpolation to translate the database fields to the user specified longitude and latitude.
• var3d.f Reads one value of a 3-D field from the MCD. Uses trilinear interpolation to translate the database fields to the user specified longitude, latitude and $\sigma$ level. The subroutine getsi.f can be called first to translate geometric height into the $\sigma$ levels used in the MCD.
• profi.f Reads a vertical profile from a 3-D field on model $\sigma$ levels. Uses bilinear interpolation to translate the database fields to the user specified longitude and latitude.
• getsi.f Solves the hydrostatic equation to find the value of $\sigma$ (the MCD vertical coordinate) corresponding to the user specified height above the surface.
• mount.f Returns the model orographic height in metres at a particular longitude and latitude.
• atime.f This routine takes the local time at a particular longitude and turns it into database prime meridian time. Database times always correspond to True Solar Times, as opposed to a Mean Solar Time.
• eofpb.f This routine computes a large-scale EOF perturbation to a variable (density surface pressure, temperature, zonal wind and meridional wind). Note that this routine has changed substantially since version 1.0.
• grwpb.f This routine computes a small-scale gravity wave perturbation to a variable (density, temperature, zonal wind and meridional wind). Note that this routine has changed since version 1.0.