Topics about the HYSPLIT trajectory model.
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When running the HYSPLIT trajectory model on the website part of the input allows the user to select which model they would like to use. For each model choice the vertical pressure system used is explicitly stated (i.e. pressure or hybrid sigma pressure). I was wondering if anyone could clarify whether the pressure coordinate system used is just the standard pressure levels with no other method involved to adjust the vertical levels or is the method used actually an eta pressure scheme. The eta coordinate system has surfaces that remain relatively horizontal at all times. At the same time, it retains the mathematical advantages of the pressure based system that does not intersect the ground. It does this by allowing the bottom atmospheric layer to be represented within each grid box as a flat "step". The eta coordinate system defines the vertical position of a point in the atmosphere as a ratio of the pressure difference between that point and the top of the domain to that of the pressure difference between a fundamental base below the point and the top of the domain.
The HYSPLIT code remaps all input meteorological data to its own terrain following sigma-height coordinate system from whatever coordinate system was used to create the input data file. All of the current meteorological files provided by NOAA-NCEP for conversion to the HYSPLIT input format have either been interpolated to standard pressure levels by NCEP or remain in their native hybrid vertical coordinate system. The ETA coordinate system is no longer used in any of the NCEP models available for HYSPLIT calculations. Some of the historical model data was created on an ETA coordinate system model but it was also interpolated to standard pressure surfaces before being processed for HYSPLIT applications.
Thanks for the clarification. With the hybrid sigma vertical coordinate system having better resolution near the ground allowing for better modeling of boundary layer process to include turbulence, diurnal heating, low-level moisture/winds and static stability why not leave all the runs in their native hybrid vertical coordinate system?
Originally we just had access to pressure level data. Certainly meteorological model output on native hybrid-sigma levels has advantages for dispersion modeling. Meteorology data on pressure levels is used more by weather forecasters.