Interpolates from high resolution rectilinear grids to low resolution recitlinear grids using local area averaging.
function area_hi2lores ( xi [*] : numeric, yi [*] : numeric, fi : numeric, fiCyclicX  : logical, wy [*] : numeric, xo [*] : numeric, yo [*] : numeric, foOption  : logical ) return_val : float or double
A one-dimensional array that specifies the X coordinates of the fi array. It must be monotonically increasing. It may be unequally spaced. For geo-referenced data, xi is generally the longitude array.yi
A one dimensional array that specifies the Y coordinates of the fi array. This may be a monotonically increasing or decreasing array. It may be unequally spaced. For geo-referenced data, yi is generally the latitude array.fi
An array of two or more dimensions. The two rightmost dimensions must of size nyi x nxi, and are the dimensions used in the interpolation. If missing values are present, the attribute fi@_FillValue must be set appropriately.fiCyclicX
An option to indicate whether the rightmost dimension of fi is cyclic. This should be set to True only if you have global data, but your longitude values don't quite wrap all the way around the globe. For example, if your longitude values go from, say, -179.75 to 179.75, or 0.5 to 359.5, then you would set this to True.wy
A scalar or a one dimensional array containing the weights for the yi coordinate. If no weighting is to be used, set to 1. Otherwise, wy could contain (say) gaussian weights or cosines of the latitude.xo
A one-dimensional array that specifies the X coordinates of the return array. It must be monotonically increasing, but may be unequally spaced. For geo-referenced data, xo is generally the longitude array.yo
A one-dimensional array that specifies the Y coordinates of the return array. It may be monotonically increasing or decreasing. It may be unequally spaced. For geo-referenced data, yo is generally the latitude array.foOption
If the variable is True then optional arguments may be present. Currently, only one option is available. There is an internal parameter, critpc, that specifies the percentage of input grid points within the scope of the low resolution grid point that must be present. The default is critpc=100 (percent). This means that if any missing values (_FillValue) are encountered the output grid point will be set to _FillValue. The user may change the default value by setting the foOption variable attribute, critpc, to any number [0 to 100]. A value of (say) critpc=70 (percent) means that a value will be returned if 70% of the high resolution grid points within the scope of the low resolution grid point are non-missing.
The returned value will have the same dimensions as fi, except for the rightmost two dimensions which will have the same dimension sizes as the lengths of yo and xo. The return type will be double if fi is double, and float otherwise.
area_hi2lores uses local area averaging to interpolate from a high resolution rectilinear grid to a low resolution rectilinear grid. The method accounts for fractional contributions of the input high resolution grid points to the scope of each low resolution grid point. This is not strictly conservative remapping. However, it is reasonably quick and yields grids that are adequate for most purposes. Use the SCRIP (Spherical Coordinate Remapping and Interpolation Package) for conservative remapping.
In default mode, if missing values (_FillValue) are present in the input high resolution grid points within the scope of a low resolution grid point, a missing value will be returned. Use the opt@critpc to alter this behavior.
If the output coordinates (xo,yo) are outside those of the input coordinates (xi,yi), then the fo values at those coordinates will be set to missing (i.e. no extrapolation is performed).
Use area_hi2lores_Wrap if retention of metadata is desired.
Assume fi is dimensioned niy x nix (100 x 400), and that the rightmost dimension is not to be treated as cyclic. The returned array will be of size noy x nox, where noy and nox are the sizes of the one-dimensional arrays xo and yo:
fo = area_hi2lores (xi,yi,fi, False, 1, xo,yo, False)
Consider high resolution grid, fi(ntim,nlat,mlon), where nlat=400 and mlon=1440. The input latitudes span -49.875 to +49.875 (degrees_north). The input longitudes span -179.875 to +179.875 (degrees_east). The grid is cyclic and missing values are present. Create a 2x3 grid over the same spatial range using the default critpc [=100%] and then a second grid that requires only say) 50% of the high resolution grid points be non-missing.
f = addfile ("foo.nc", "r") p = f->PRC printVarSummary( p ) LON = fspan(-178.5,178.5, 120) LAT = fspan(-48,48, 49) po = area_hi2lores (p&lon,p&lat, p , True, 1, LON, LAT, False) ; (ntim,49,120) opt = True opt@critpc = 50 ; require only 50% of the values to be present PO = area_hi2lores (p&lon,p&lat, p , True, 1, LON, LAT, opt ) ; (ntim,49,120)