free grads

()atot

Thie atot function (in grads version 2.0.2+) duplicates the summing capability of asum but adds the latitude-weighting feature of aave. The syntax is:

atot(expr, xdim1, xdim2, ydim1, ydim2)

Usage Notes

Please see the reference page for asum for details on the command syntax and usage notes.

Examples

1. An instructive sanity check is to calculate the total area over globe on any grid of data that has a constant value of 1 and no missing values: 
   
   
   ga-> d atot(const(temp,1,-a),global) 
   Result value = 12.5664 
   
   
   The answer we get is 4*pi, and the unit of the result is steradians.

2. Suppose you have a data variable sea ice concentration (named 'sic') which is given as a % in each grid box, and you would like to calculate the total area in the Arctic covered by sea ice. To calculate the sea ice coverage in millions of km^2, we multiply the atot result by the square of the radius of the Earth in km:
   ga-> d atot(sic/100,lon=0,lon=360,lat=40,lat=90)*6371*6371*1e-6 
   Result value = 11.289