An objective approach to breaks in the weather

Abstract

An objective approach to breaks in the weather is presented. A simple mathematical procedure is described that makes it possible to detect changes in a time series. The method is based on a comparison of a typical change, prescribed a priori, with an interval of a local record of daily temperature, for instance. A break quality is defined as the ratio of the amplitude of the change (computed by a least-squares method) to the corresponding rms difference between the typical and observed change. In this way for each day a break quality may he computed. The real breaks are found by requiring that the break quality have a maximum value with respect to time. Since the break quality is a dimensionless quantity, different weather elements may be compared immediately. Results are given of a break analysis of daily values of temperature, sunshine and precipitation as observed at De Bilt during the period 1949-74. Daily mean temperature shows the highest break qualities while precipitation hardly shows clear breaks. The correlation between break qualities of different weather elements appears to be small. The method was also applied to the geopotential height field over the North Atlantic Ccean. The existence of the preferred region of blocking over the northeastern part of the Atlantic is seen-very clearly by means of the break analysis. Finally, some remarks are made concerning the construction of a statistical significance test and some further possibilities of the break analysis are indicated.