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HOME > Monitoring_and_Data > Oceanic and Atmospheric Data > Reanalysis: Atmospheric Data > wgrib2 -cress_lola

wgrib2: -cress_lola


The -cress_lola option is similar to the -lola option in that it creates a regular LOngitude-LAtitude grid. The former uses a Cressman analysis and the latter option uses a nearest-neighbor interpolation.

You need to specify the lower left corner of the grid, the number of points in the zonal and meridional directions and the latitude/longitude increments. Finally you need to specify the output file and the format. WARNING: winds and other vector fields will not be rotated. If the vector fields use a grid relative orientation, then your interpolated winds will be using the original grid.

Interpolation scheme

The interpolation to the lola grid is by a Cressman analysis. The Cressman analysis is a multipass system which uses a user-specified "radius" for each pass. A Cressman analysis can be computationally expensive so you may want to explore multiprocessing techniques.


-cress_lola LonSW:#lon:dlon LatSW:#lat:dlat file radius1:radius2:..:radiusN

LonSW        Longitude of the South-West point, values from 0 .. 360
#lon         number of longitude points
dlon         spacing of the points in the zonal direction in degrees

LatSW        Latitude of the South-West point, values from -90 .. 90
#lat         number of latitude points
dlat         spacing of the points in the meridional direction in degrees

file         name of the output grib file

radiusM      The radius in km for M-th pass.

Prelim - Cressman Analysis

defintion: input grid = observations = grid from the input grib file
           output grid = grid as defined by  LonSW:#lon:dlon LatSW:#lat:dlat

(1) compute mean of observations,save mean on background grid (output)
    background(i,j) = average(observation)

(2) Repeat N times:  PASS-M

   (a) find background value for observations by bilinear interpolation of background grid
   (b) remove the background from the observations: obs' = obs - interpolated_background
   (c) inc'(i,j)  = weighted average of obs'
           inc'(i,j) is increment on output grid
           weighted average of obs' depends on the distance between grid point and obsservation
   (d) new background = background + inc'

Warning, this scheme doesn't handle handle rotated winds in a useful manner. There will also be problem with analyzing winds near the poles.

See alse: -lon -lola

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