A mesoscale model intercomparison of coastal refractivity

Advancements in mesoscale models, data-assimilation systems, and use of multiprocessor, parallel computing architecture have allowed more detailed representation of the coastal marine atmospheric boundary layer (MABL). The vertical structure of the MABL is known to have a pronounced effect upon radar and communications systems particularly that of water vapor for which spatial variations can either disrupt or extend microwave signals well beyond the normal radar horizon. High-resolution numerical weather prediction (NWP) forecast fields of temperature, pressure, and vapor pressure are used to diagnose the refractive index of the atmosphere as it is influenced by evolving synoptic conditions and due to local pressure gradients that develop with changes in sea surface temperature, diurnal heating, and orographic effects.

In an effort to evaluate the current ‘state-of-the-art' prognostic capability in a challenging littoral setting, an international collaboration developed between several countries to assess their operational forecasting systems in the context of representing mesoscale variability in the MABL refractivity field. These models include the U.S. Naval Research Laboratory's COAMPS®1, Great Britain's Unified Model (UM), New Zealand's MM5, and Canada's GEM, each run for a seven day period over Wallops Island, Virginia, during which extensive field measurements were collected along radials extending up to ~100 km from shore. The field data include helicopter vertical profiles, hourly surface met tower observations, and surface buoy time-series, along with a boat-mounted transmitter and shore-based receiver measuring propagation pathloss.

Model intercomparisons of the domain-wide ducting characteristics such as duct strength, thickness and base height provides insight into the spatial evolution of the refractive environment over the 7-day period. An evaluation of standard statistical metrics of the predicted refractivity and ducting quantities suggests potential for model forecast system improvements that will have the greatest impact on refractivity. Ample mesoscale features characterized the coastal and offshore ducting resulting from the high-resolution SST analysis and complex topographic sea/land breeze flows in an around the coastal bays and rivers, within rapidly changing large-scale forcing. A detailed investigation of these refractive patterns links them to variations in the vertical distribution of water vapor associated with the myriad of mesoscale forcing mechanisms that contribute to structure in the surface, boundary and entrainment layers. The relative importance of several key forcing mechanisms will be considered at the conference.

1 COAMPS is a registered trademark of the Naval Research Laboratory