SPECIFICATION OF TEMPERATURE AND PRECIPITATION IN TERMS OF CIRCULATION PATTERNS

Abstract

High-speed calculation techniques using Whirlwind I, the digital computer at the Massachusetts Institute of Technology, make practical the use of an objective approach to the classification of climatic data based on the atmospheric circulation. Circulation patterns over the region bounded by 30°N, 60°N, 65°W and 125°W are approximated by a linear combination of Tschebyscheff orthogonal polynomials. The coefficients of the polynomials are used as circulation indices and are related to the weather through the use of a linear operator. The usefulness of this new method is illustrated by the comparison of a classification of temperature and precipitation data for January 1948 to 1952 at a number of localities with a classification of the same data based on previous methods. Nearly twice as much information is available for the specification of the weather data with use of the new method. In addition, a measure is obtained of the relative amounts of information available in the sea-level and 700-mb circulation patterns over the United States and Canada for the specification of temperature and precipitation at a number of stations in the central and eastern United States. The sea-level pattern explains a greater amount of the variance of the 1500 GCT temperature than does the 700-mb pattern (55 per cent versus 46 per cent), but the 700-mb patterns explain slightly more of the variance of 24-hour precipitation (50 per cent versus 47 per cent). The results indicate the applicability of the large-scale circulation patterns, either near the surface or aloft, for the classification of climatic data.