Assessment of the extent of the contamination of measurements taken on Porto Santo during ASTEX

February 1994.Also issued as Cara-lyn Lappen's thesis (M.S.) -- Colorado State University, 1994.Includes bibliographical references.The object of the Atlantic Stratocumulus Experiments(ASTEX) was to measure and examine properties of the marine atmosphere. Since instruments were placed on the island of Porto Santo, however, some degree of contamination of pure marine conditions was experienced due to the local effects of island topography. In order to assess the expected differences between a pure marine environment and measurements taken on the island of Porto Santo, a numerical model- the Regional Atmospheric Modelling System (RAMS) was used in direct comparison with observational data for the case of June 10, 1992. Specifically, this study focuses on the mean wind fields simulated by RAMS and compares them to the winds measured by the United Kingdom's C130 Meteorological Research Flight, a 400Mhz wind profiler, and rawinsondes. The model's inability to resolve a 100m cliff on the windward side of the island was found to cause a phase shift between the model-produced and the actual wind fields. This was determined to be a l-2km upward phase shift and a 300m to 500m windward phase shift for the RAMS data. After applying this correction, and comparing these four sources of data, the extent of the island's effects in the horizontal as well as the vertical was determined. In the horizontal, the effects decrease with distance from the island until approximately 2km upwind or downwind where the effect was minimal. In the vertical, the effect of the island was detectable up to 3.5km, but not felt continuously. The maximum effect was found at the ground and at approximately 1.2km. Wind data taken at Porto Santo must be filtered at the ground, and near the 1.2km and 3.5km levels. In between these levels, wind measurements taken on the island would appear to provide an accurate representation of the pure marine environment.Sponsored by the National Oceanic and Atmospheric Administration NAG-1-1146; the Office of Naval Research N00014-91-J-1422, P00004; and the Department of Energy DE-FG02-90ER60970