### Could you explain the equation (Zagl = ak2 + bk + c) used by HYSPLIT? What is the different between Ztop, Zgl, Zmsl and Zagl?

Meteorological model output fields usually cannot be directly used by the dispersion model without some pre-processing, primarily because the data may have been interpolated to a variety of different vertical coordinate systems prior to output. To maintain a larger degree of flexibility within the dispersion model structure, i.e., the ability to use different meteorological data sources for input, the meteorological profiles at each horizontal grid point are linearly interpolated to an internal dispersion model terrain-following (F) coordinate system,

F = (Ztop – Zmsl) / (Ztop – Zgl), (1)

where all the heights are expressed relative to mean-sea level and where Ztop is the top of the dispersion model’s coordinate system, Zgl is the terrain height, Zmsl is the level height above mean sea level. The model’s internal heights above ground level can be chosen at any interval, however a quadratic relationship between height (z) and model level was specified, such that each level’s height with respect to the model’s internal index, k, is defined by

z = ak2 + bk +c, (2)

where a=30, b=-25, and c=5. This relation results in decreasing resolution away from the surface, with the first level (k=1) at 10 m, the second level at 75 m, the third at 200 m, while by the 20th level, at 11500 m, the difference between levels is about 1200 m. Any vertical resolution can be specified by altering the constants in Eq. (2), however the model’s internal resolution should be at the same or better vertical resolution than the input data.

Roland Draxler