### I have a question about section five “meteorological input data” in the HYSPLIT_4 USER’s GUIDE. In page 49, what are the meanings of “pole latitude & longitude”, “reference latitude & longitude”, “synch latitude & longitude”, “synch point X and Y” and the relation (or difference) between “reference grid size” and “number X & Y points”? Or would you tell me what are their values if I have gridded meteorological data within lat. 5 N to 60 N and long. 110 E to 140 E., one degree resolution and I want to plot the back-trajectory within the same area?

This configuration file is designed to serve two very different types of grid: (a) laid out on a Conformal Map Projection, such as Stereographic, Mercator or Lambert, or (b) a Lat-Lon grid where grid points are separated by uniform degree intervals of Latitude and Longitude.

**Conformal Maps**(see also the information on CMAPF available here and at http://www.ready.noaa.gov/ss/models/cmapf.html.)*Pole latitude & longitude*:Standard conformal projections are drawn around a central (polar) point. A*Normal Projection*is one where this point is either the North Pole (latitude = +90.°) or the South Pole (latitude = -90.°). Thus, Stereographic Projections are commonly drawn on a plane tangent to the North or South Pole, and Mercator projections on a cylinder parallel to the axis between the two poles. For Normal Lambert and Mercator projections, which require a cut to place the map on a plane, the cut will be 180° from the specified longitude.Most of our projections are normal, but there are some which are not. An*oblique*stereographic projection is drawn on a plane tangent to the globe at some arbitrary point where minimizing distortion is of great interest, and a*transverse*Mercator projection is drawn on a cylinder parallel to an axis that passes through the equator. In such cases, the latitude and longitude of the tangent point or axial point will be entered here.*Reference latitude & longitude, reference gridsize and orientation*: The latitude and longitude of the*reference point*, at which the scale (gridsize in km.) and orientation (degrees between local North and the y-axis) have the specified values. Mathematics dictates that at other locations, the size of a grid cell in km. will be greater or lesser than the reference gridsize, and the direction of “North”, parallel to the local meridian, will make different angles with the y-axis.*Synch latitude & longitude, synch point X and Y:*To align (synchronize) the grid coordinates with the map, we specify the latitude and longitude coordinates of a specific grid point, which could be the origin or could be the midpoint of the grid, or any other point in the grid. In the example in the Users Guide, the midpoint of the grid, with grid coordinates X=50.5, Y=50.5 will be located at latitude and longitude 45., -90. Since the size in km. and the orientation of the grid are already specified, mathematics dictates where all the other gridpoints are.*Number of X & Y points:*The numerical count of grid points being stored in each direction. The cartographic parameters provided above specify a map projection and grid layout which potentially extends to infinity in each direction; the actual grid values stored are for grid coordinates extending from 1 to NX in the x-direction and 1 to NY in the y-direction. They determine the size of computer storage dedicated to the grid values, whereas the “gridsize” value, in kilometers, of an individual cell at the reference point, determines the geographical density of grid points.

**Lat-Lon Grids**These are grids whose points are uniformly spaced in latitude and longitude, and have no particular relation to Conformal maps, above. Based on the last statement of your question, I believe this is the case with which you are concerned..Some of the specifications in (a) have no meaning here, and others have been redefined The*reference gridsize*is set to zero to allow hysplit to know this is a Lat-Lon grid. Other variables have been redefined as follows:*Pole latitude & longitude*: Redefined to contain the maximum (Northern) latitude and maximum (Eastern) longitude of the array. (In your case, (60., 140.))*Reference latitude & longitude*: Redefined to contain the spacing in degrees of the latitude and longitude. (In your case, (1., 1.))*Synch latitude & longitude*: Redefined to contain the minimum (Southern) latitude and minimum (Western) longitude of the grid. (In your case, (5., 110.)).

Albion Taylor