Mesoscale Modeling for Radar Propagation Prediction during the Wallops-2000 Experiment

In coastal regions, advection of an air mass from the land over the water may result in a large change in temperature and humidity which has a significant impact on the occurrence of radar ducting. Radar ducting occurs when modified refractive index decreases with height where sharp changes in humidity and temperature appear. Understanding the changes of radar propagation conditions in such complicated coastal environments is vital to national defence agencies around the world and thus requires an ability to predict the evolution of atmospheric refractivity in time and space. To this end, this study aims to assess the feasibility of using a Numerical Weather Prediction (NWP) high resolution model to predict duct existence, height and intensity in relation to synoptic and local weather features. The NWP model used here is the Met Office Unified model (MetUM) running with grid lengths of 12km, 4km and 1km. Hourly model results were then evaluated and assessed against the field measurements (Wallops-2000 experiment) collected over 7-day intensive observations between 28 April and 4 May 2000 near Wallops Island off the shores of Virginia, USA. The Wallops-2000 experiment was made available through the collaboration between ABCANZ countries (America, Britain, Canada, Australia and New Zealand).

This study focuses on the verification of near-shore and over-water surface and boundary layer temperature and moisture and ducting characteristics. The model predictions are compared and evaluated with buoy, helicopter measurements in terms of time-series and vertical profiles. The characteristics of ducts occurring at the locations of helicopter profiles were also examined statistically, indicating high occurrence rate of surface ducts. The model time-height profile at a buoy site shows that surface ducts occur during the whole experiment period with different degree of strength. Strong and thick surface ducts were also found in both observations and model results at the shallow marine boundary layer (MBL) while the most elevated ducts were only predicted at the deep MBL where observations were not available. The high resolution NWP model exhibits its ability to capture these trends and characteristics in the ducting trapping layer as compared with the measurements. These statistics are discussed in association with local meteorological conditions and weather systems affecting the region.