Forecasting the Nighttime Evolution of Radio Wave Ducting in Complex Terrain Using the MM5 Numerical Weather Model

This study tests the effectiveness of using mesoscale models to forecast operationally those atmospheric conditions that cause anomalous propagation of radio waves. Providing early warning about where and when such conditions will develop would be particularly useful for military communications. To test mesoscale models' ability to provide such early warning, the Pennsylvania State University / National Center for Atmospheric Research Fifth Generation Mesoscale Model (MM5) simulates atmospheric conditions around the mountainous, desert terrain near Nellis Air Force Base in Nevada for five forecast periods during the summer and fall of 2002. The extent of ducting regions predicted by the model and the mechanisms that result in the development of those regions are examined. The sensitivity of model predictions to changes in horizontal and vertical resolution is also tested. Results of these analyses reveal that rapid radiational cooling around sunset and subsequent cold air pooling establish vertical temperature gradients that, along with pre-existing moisture gradients, are compacted by gravity waves along the lee sides of mountains within the model domain. This gradient compaction drives changes in the electromagnetic refractivity of the atmosphere that can trap radio waves near the Earth's surface. The sensitivity tests show that increasing the model's horizontal resolution increases the area of predicted ducting significantly. Increasing the vertical resolution does not increase the area of predicted ducting by much, but does sharpen the edges of larger predicted ducting regions. The original document contains color images. All DTIC reproductions will be in black and white.