Orographic influences during winter precipitation events on the Avalon Peninsula, Newfoundland

Abstract

AbstractPrecipitation enhancement over a low coastal hill in winter is demonstrated for particular associations of synoptically determined onshore airflow and local geography encountered over southeastern Newfoundland. Four such cases, involving a mixture of pre‐warm‐frontal precipitation types at surface temperatures just below freezing, are analysed using comparative surface gauge records from sites at the coast and hill summit and detailed volume scan data from a Doppler radar. Whilst precipitation at the hill summit and upwind coast was of similar overall duration in each case, the surface rates on the hilltop exceeded those at the coast by 1.0–4.0 mm h−1 during different stages of the events. Analysis of the Doppler reflectivity patterns reveals that intensities are especially enhanced near the windward hill crest, supporting the observed association of greatest enhancement with a strong local upslope wind component exceeding 20 m s−1. In the majority of these cases the enhancement is maintained primarily by the ‘seeder–feeder’ mechanism, which appears to be accelerated during precipitation transitions and with the surface warm front 120–150 km distant; however, a preliminary phase of enhancement due to topographically induced uplift of stable pre‐frontal air is also recognised in one case. Copyright © 2000 Royal Meteorological Society