Regional Climatology and Statistics of Surface Radiation Budget

The Earth’s surface radiation budget (SRB) is strongly coupled to the climatology of a region. The SRB quantities of primary importance to regional climate are net shortwave (SW) radiative flux and net longwave (LW) radiative flux. Net SW is the amount of solar energy which is absorbed at the surface and is available. Net LW describes the radiative exchange between the atmosphere and the surface. The difference between net SW and net LW is the net amount of heat which is available for warming/cooling the surface and for driving the weather/climate system. A SRB Data Base has been developed at Langley Research Center by use of satellite measurements. This data base uses a 2.5 degree quasi-equal area grid to define regions, and 6596 regions are required to cover the Earth. The data set covers 8 years. Climatological mean monthly values were formed of the net SW and net LW for the regions.

The plot of monthly-mean net LW as a function of monthly-mean net SW through the yearly cycle of a region forms a closed 12-sided figure. These figures have various shapes: some are nearly linear, some are nearly circular, some are figure 8’s. Their characteristics are related to the climatology of the region and have been used as a basis for the classification of regional climate by use of simple criteria. Although representative regions can be examined to study their behavior, in order to describe the totality of 6596 regions it is necessary to define quantitative descriptors, which can then be summarized statistically. The purpose of these statistical descriptions is to lead to insight into the relations between SRB and regional climate.

The first measure used to describe the 12-sided figure is its centroid, i.e. its annual mean net SW and LW. A 2-D histogram in terms of these quantities shows that within each class land regions are quite closely clustered. This domain is a basis for climate classification of land regions in terms of SRB. The figure can be further described in terms of correlation, slope and area. The correlation tells how nearly the points lie on a line. Correlation is high for most extratropical non-polar ocean regions and for most extratropical land regions. Some figures are quite circular and are distinguished by small correlations. The area enclosed by the figure tells whether the figure is a broad loop instead of a narrow shape in the net SW-net LW domain. Area has the advantage of being a signed quantity, so that a clockwise loop is negative and a counterclockwise loop is positive. This directionality of the figure is related to the processes governing the yearly cycle. For example, the India-Burma regions have large negative areas, which are due to the South Asian monsoon.

In this paper, these various statistical measures are defined and related to physical processes effecting radiation at the earth's surface. Global maps are used to display the ranges of these parameters and identify regions of similar climatological characteristics.