Diurnal characteristics of heavy precipitation according to weather type at an upland site in Scotland

Abstract

AbstractThe diurnal characteristics of heavy 3 h and 24 h precipitation events exceeding a certain threshold (o‐called peak‐over‐threshold or POT events) were studied according to synoptic weather type. Hourly precipitation data for 1970–86 from Eskdalemuir, an upland site in southern Scotland, and the (daily) Mayes weather type classification for Scotland were used. Through cluster analysis, the weather types were divided into five groups according to the overall diurnal distribution of precipitation. Subsequently, 272 POTs for each duration were extracted and analysed using circular statistics. The diurnal distributions of the 3 h POTs were found to be significantly non‐uniform for four of the five weather type groups.The 3 h POTs in the group that includes the Cyclonic weather type show a bimodal distribution with early‐morning and afternoon maxima. A bias towards increased temperatures and a shift in the wind direction suggest that early‐morning POTs may be caused mainly by warm fronts. This is consistent with warm fronts providing favourable conditions for orographic enhancement, which occurs predominantly during the early morning. The afternoon maximum may represent cold frontal precipitation enhanced by thermal convection. Because a cyclone passage generally involves a warm front followed by a cold front, 24 h POTs were found to start significantly more often in the first 4 h after midnight than during the rest of the day. Copyright © 2002 Royal Meteorological Society.