Modelling low-frequency rainfall events using airflow indices, weather patterns and frontal frequencies

Abstract

Low-frequency, high-magnitude daily rainfall amounts recorded at several sites in central and southern England were related to the prevailing Lamb Weather Types (LWTs), daily weather front and airflow data. Three statistically distinct weather-type clusters were indentified and used to construct a simplified frontal model of daily precipitation occurrence/amount. The model was calibrated against station data for the period 1970–1990 and used to reconstruct observed daily precipitation between 1875 and 1969 given the historic sequence of LWTs. Although the model reproduced the incidence of low-frequency, high-magnitude events, it failed to capture variations in mean wet day probabilities and wet/dry spell persistence. This inability was attributed to the general limitations of the weather classification methodology, which did not capture all aspects of the precipitation regime with equal levels of proficiency. Therefore, the prospects for downscaling high-resolution precipitation series directly from indices of mean sea-level pressure rather than via weather patterns was discussed. Preliminary results indicate that relationships can be established between mean daily precipitation occurrence and airflow indices such as vorticity and strength of air flow. However, further research is required to establish the value of such indices for modelling low-frequency, high-magnitude precipitation events.