PRECIPITATION AND AIR-FLOW INDEXES OVER THE BRITISH-ISLES

The relationships between regional daily precipitation time series in the British isles and 3 indices of air flow are examined with a view t o assessing their potential for use in GCM downscaling. These indices, calculated from daily grid-point sea-level pressure data, are as foll ows: total shear vorticity, a measure of the degree of cyclonicity; st rength of the resultant flow; and angular direction of flow. The 3 ind ices, particularly vorticity, exert a strong control over daily precip itation characteristics such as the probability and amount of precipit ation. 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 relat ionships between air flow indices and regional and 2 single site preci pitation series in England shows they are similar, although at the sit e-scale local factors may play an important role in affecting the rela tionships with the indices. Two models for generating daily precipitat ion series from vorticity are presented and evaluated by their ability to reproduce the following characteristics of precipitation over an i ndependent validation period: annual totals and interannual variabilit y, wet day probability and spell duration, and size of daily precipita tion amounts. Model 1 is based on empirical relationships between vort icity and precipitation. Model 2 is based on user-defined categories o f vorticity. The results for 2 sites (Durham and Kempsford) show that both models reproduce key characteristics of the observed daily precip itation series. Differences in model structure and number of parameter s 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 a re continuous variables. Previous downscaling techniques have relied u pon classification techniques that impose artificial boundaries to def ine classes that may contain a wide range of conditions and no informa tion about the intensity of development of the weather system concerne d. 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 po tential to apply this technique to other such regions. An example is s hown of the relationships between daily precipitation in Switzerland a nd the air flow indices. The models may also be applied to the develop ment of future daily precipitation scenarios using the coarse-scale ou tput of GCM pressure fields.