A tailored observational campaign for orographic precipitation—STOPEX

During 7 weeks in Fall 2005, phase 1 of STord Orographic Precipitation EXperiment (STOPEX) was conducted. Detailed observations, both temporally and spatially, were recorded at an island (25km north-south extension and 10km west-east) on the west coast of Norway. Data sampled from a dense network of tipping-buckets and weather stations in addition to a weather radar, revealed extreme precipitation gradients over short distances. The distribution of precipitation indicated that terrain disturbances with horizontal scales of 5km or more, gave raise to significant variations in precipitation. The strongest orographic intensification of precipitation was found on the mountain top at 600 m above sea level(1236mm) and on the lee-side (850mm), and the weakest was found on the windward side (342mm).

In the last day of the campaign (14th Nov), a record high precipitation event took place, giving a maximum of 243mm over 20 hours. A 'state of the art' numerical model (MM5) with 1km grid spacing was capable of producing half the observed amount, and tests with a linear model showed that conversion times in the micro-physics had to be very short (Tau=200s) in order to reproduce the amount.

The location of this island enable us to perform 'clean' studies of orographic precipitation as it is exposed to open ocean and undisturbed upstream air. The phase 1 of the campaign was designed in order to test strong gradients in precipitation across the island, and with some adjustments such as adding a vertically integrated water vapour GPS-system, phase 2 will take place this Fall.