Anomalous propagation (AP) occurs when the refractive properties of the atmosphere bend radar energy from near the surface back towards the ground. This is caused by a very negative vertical gradient of the refractivity (dN/dh < -157 1/km), which can occur as the result of either rapid decrease in humidity or sudden rise in temperature with altitude. Meteorologically, AP occurs primarily with temperature inversions where air near the ground cools much faster than the air above it. The ability to predict these events is necessary to mitigate AP in the radar data. Atmospheric soundings can be used to forecast AP, but the data density, both temporal and spatial, is insufficient to provide real-time forecasts of the necessary three dimensional profiles. To compensate, PRA has developed a technique utilizing WRF to forecast the refractivity profile and a propagation model to predict the occurrence of AP.

This paper describes the results of the PRA AP forecasting product with respect to AP events observed with associated WSR-88D weather radar products.