Diurnal and seasonal characteristics of precipitation at an upland site in Scotland

Abstract The diurnal and seasonal characteristics of precipitation at Eskdalemuir, in southern Scotland, were studied by representing the hour of rainfall as a circular variable and applying statistical methods for directional data. Hourly precipitation data for 1970–86 were analysed for four different precipitation intensities. Concurrent observations of wind direction and speed were used to help in the interpretation of the results. The occurrence of precipitation of light intensity, 0.1–0.4 mm h−1, has a bimodal distribution with early‐morning and afternoon maxima in both summer and winter. Heavier precipitation in summer is concentrated in the afternoon, presumably reflecting convective activity in response to the diurnal heating cycle. Very heavy precipitation (≥5.0 mm h−1) in winter occurs significantly more often in the early‐morning hours (02:00–08:00 GMT) than during the rest of the day. This is attributed to orographic enhancement of precipitation because of the associated wind direction. For both seasons, very heavy precipitation in the early‐morning hours occurs predominantly in airflow from the south or south–south‐east, i.e. roughly perpendicular to the topographical barrier formed by the Southern Uplands. In contrast, very heavy precipitation during the afternoon, especially in summer, is more associated with south–south‐westerly winds coming from the sea. Copyright © 2002 Royal Meteorological Society.