Precipitation and air flow indices over the British Isles

Abstract

The relationships between regional daily precipitation time series in the British lsles and 3 indices of air flow are examined with a view to assessing their potential for use in GCM downscaling.These indices, calculated from daily grid-point sea-level pressure data, are as follows: total shear vorticity, a measure of the degree of cyclonicity; strength of the resultant flow; and angular direction of flow. The 3 indices, particularly vorticity, exert a strong control over daily precipitation characteristics such as the probability and amount of precipitation. There are significant regional differences in the relationships with precipitation, particularly between the England and Wales series and the Scotland and Northern Ireland series. Comparison of the relationships between air flow indices and regional and 2 single site precipitation series in England shows they are similar, although at the site-scale local factors may play an important role in affecting the relationships with the indices. Two models for generating daily precipitation series from vorticity are presented and evaluated by their ability to reproduce the following characteristics of precipitation over an independent validation period: annual totals and interannual variability, wet day probability and spell duration, and size of daily precipitation amounts. Model 1 is based on empirical relationships between vorticity and precipitation. Model 2 is based on user-defined categories of vorticity. The results for 2 sites (Durham and Kempsford) show that both models reproduce key characteristics of the observed daily precipitation series. Differences in model structure and number of parameters affect their accuracy in simulating the interannual variability and daily characteristics of precipitation. The air flow indices represent a significant advantage over traditional weather types because they are continuous variables. Previous downscaling techniques have relied upon classification techniques that impose artificial boundaries to define classes that may contain a wide range of conditions and no information about the intensity of development of the weather system concerned. As the 3 air flow indices are the basic determinants for describing the day's weather in many parts of the world, there is significant potential to apply this technique to other such regions. An example is shown of the relationships between daily precipitation in Switzerland and the air flow indices. The models may also be applied to the development of future daily precipitation scenarios using the coarse-scale output of GCM pressure fields.

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