oacres (gexpr, sexpr <,radii>)
A Cressman objective analysis is performed on the station data to arrive at a gridded result that represents the station data.
gexpra valid grid expression
sexpra valid station data expression
radiioptional radii of influence. Defaults are: 10,7,4,2,1
The Cressman Analysis scheme is described in a 1959 paper in Monthly Weather Review. Multiple passes are made through the grid with increasingly smaller radii of influence. At each pass, a new value is calculated for each grid point based on a correction factor that is determined by looking at each station within the radius of influence.
For each such station, an error is defined as the difference between the station value and a value arrived by interpolation from the grid to that station. The correction factor is based on a distance weighted formula applied to all such errors within the radius of influence. The correction factors are applied to each grid point before the next pass is made.
Any grid boxes that do not have stations within the third specified radius are set to the missing data value.
oacresfunction can be quite slow to execute, depending on grid and station data density.
oacresand compare them to the station data to insure they meet your needs.
oacresfunction is more of a quick-look function rather than a rigorous analysis scheme. If you have specific analysis requirements, consider doing your objective analysis outside of GrADS with a user-defined function.