I will be converting ECMWF grib files into ARL-packed format for use in calculating backward trajectories in Europe using your Hysplit model. What fields do I need to have for HYSPLIT to properly calculate my trajectories?

Preparing input data for Hysplit can be very challenging task. You need to understand the structure of the data that you will be using to convert for input to the model. At a minimum Hysplit requires U,V,W,T , (RH or Q optional), at various model sigma levels, or if the input date are on pressure levels then the geopotential height at each level. At the surface either surface pressure or terrain height are required, all other variables are optional (e.g. 10 meter winds, 2 m temperature).

You should examine the sample ECMWF data converter program (ecm2arl.f) supplied with the PC distribution in the \data2arl\ecmwf directory. It will require some modification to match the data you obtain, but it does show various vertical level options and lists the grib variable identification numbers for the various variables used by the model. Further when you run the program you need to define the limits of a conformal map. ECMWF data are usually provided at latitude-longitude points, the program interpolates these to a polar stereographic, lambert conformal, or mercator projection. You need to specify the location, resolution, and domain limits for this grid.

Additonal subroutines that you will need can be found in the \data2arl\source and \metdata\source directories. You will also need the GRIBEX program and associated libraries from ECMWF to read their grib files.

Roland Draxler