## Meteorology / Convert to ARL / ERA-40

This menu tab is intended to convert ECMWF reanalysis data archives (ERA-40) GRIB1 files to the ARL packed HYSPLIT compatible format. Data files must first be downloaded from the ECMWF web page . The user can select which fields and time periods to extract. A GRIB file may contain multiple time periods. No more than one month's data should be included in a GRIB file if it is to be converted to ARL format. All the time periods from one input file will be processed into one output file. The ECMWF download menu permits copying all the data on the global grid or extracting regional sub-grids. The ARL conversion program can handle either option.

The conversion menu is designed to select three different GRIB file types: upper air data, surface data, and time invariant data. The upper air and surface data files should contain data for the same time periods. To be able to run HYSPLIT, the upper air data file must contain at a minimum the following variables: geopotential, temperature, u-velocity, v-velocity, w-velocity, and relative humidity. The surface variable file should contain the 2m temperature, and the 10m u- and v- velocity components. The invariant file should contain the surface geopotential field (terrain height). Terrain height or surface pressure is required for HYSPLIT to be able to interpolate pressure level data to its internal terrain following coordinate system.

The ECMWF input data may be on pressure surfaces or its native hybrid coordinate system. The "number of levels", counted from the surface upwards, can be used to restrict the size of the output file. This may be particularly useful when the input file consists of 60 or more hybrid levels.

Four different grid conversions are available through the menu. The "Extract" option interpolates the data to a conformal map projection of 100x100 grid points at 100 km resolution at the center of the latitude and longitude selection of the slider bar. The Northern- or Southern Hemisphere options create a 95-km resolution polar stereo-graphic grid centered about the pole. The "Input" option results in no interpolation and the program just converts the original latitude-longitude data (global or regional) to the ARL packed format. The "Input" option must be used for latitude-longitude sub-grids because interpolation from input data sub-grids is not supported.

Table of Contents