Modelling Of Atmospheric Refractivity In The Marine Atmospheric Boundary Layer Using GEM-LAM

Radio frequency (RF) propagation is highly sensitive to the vertical structure of the coastal marine atmospheric boundary layer (MABL). Abrupt gradients changes in the temperature and humidity profiles can result in bending electromagnetic energy away from or toward the earth's surface, leading to enhanced or attenuated naval surface sensor and communication system performance.

Understanding the changes of RF propagation conditions in complex coastal environments is vital to support littoral naval operations of national defence agencies around the world and thus requires an ability to predict the evolution of atmospheric refractivity in time and space at high resolution. To develop such modelling capabilities, a working group was formed under the ABCANZ (America/Britain/Canada/Australia/New Zealand) military information exchange agreement.

This presentation will highlight current work within the ABCANZ group using a limited-area version of the Global Environmental Multiscale (GEM-LAM) model. The model simulations were compared with measurements taken during two field measurement campaigns. These studies have illuminated ducting layer sensitivity to sea surface temperature and surface layer representation. An analysis of the characteristics of observed vertical profiles showed a high occurrence rate of surface ducts. The vertical scale of these phenomena is typically on the order of a few meters, which is much higher than the usual vertical resolution of mesoscale numerical models. To overcome this difficulty, levels are added in the surface layer in order to obtain an accurate representation of the important refractivity features above the water surface.