Difference Map and Difference Time-Series
The difference operation in Giovanni-3 (which is referred to by the nickname “g3diff”) calculates the difference between parameter values in area maps. This operation can also calculate parameter time-series for area-averaged values in time-sequenced difference maps.
To create a difference area map, the operation takes the parameter value at each defined location in a map, and calculates the difference between that value and the value of a parameter in a different map at the same map location. The identity* of the parameters in the two maps can be the same, or they can be two different parameters. The difference operation simply calculates the difference between the values at each location in the maps.
To create a difference time-series, the operation first calculates a set of difference maps for a designated time period, i.e, a difference map is created for each time-step in the period. Then the area-averaged value for any area in each difference map (this can be the entire area of the difference map) is calculated. The result is a time-ordered sequence of area-averaged values derived from the difference maps.
If there is missing data in either of the maps, then the calculation cannot be performed and the output will show missing data points.
The schematic diagram below illustrates this mathematically simple but very important operation.
There are actually two ways that a time-series of area-averaged difference values could be created. The differences could be calculated before or after the area-averaging computation. While the result should theoretically be the same, the complication of missing data would actually change the output. In Giovanni-3, the difference values are calculated before averaging, as this should better represent the statistics of the differences. However, if the two parameters are plotted separately and then the differences are calculated outside of Giovanni, those results may not be the same as the results provided by Giovanni.
* Identity: In Giovanni-3, the parameters are usually geophysical quantities, like air pressure, temperature, aerosol optical depth, rain rate, etc. The use of “identity” above means the geophysical quantity that is represented by the parameter.
This Technical Summary page is a draft version -- for information on this operation while the documentation is being completed, please send a message to GES DISC Help.