Atmospheric ducts influencing radio frequency propagation develop when the vertical gradient of modified radio refractivity M is less than zero. The gradient of M can be related to the gradient of potential temperature θ and water vapor mixing ratio w

dM/dz=0.128 + c₁dw/dz - c₂dθ/dz

where c₁ and c₂ are functions of temperature, pressure and humidity.

The mean structure of stably stratified internal boundary layers (IBL) resulting when air heated over a land surface advects over a colder ocean has mostly been described in terms of the gradient of θ (Rotheram, 1983, Smedman et al, 1997, Skyllingstad, 2004). In IBLs where thermal stability suppresses turbulence and the vertical mixing of water vapor, the gradient of M has the potential to be less than zero.

This paper will present the results of a study that analyses the spatio-temporal structure of θ, w, and M in stably stratified IBLs produced on two days of offshore flow during the Wallops 2000 Microwave Propagation Measurement Experiment.

During the multi-agency Wallops 2000 Microwave Propagation Measurement Experiment, a wealth of sea surface and upper air meteorological observations were collected by helicopter, rocketsonde, and boat in order to document the spatial and temporal structure of the coastal atmospheric boundary layer near the Virginia Eastern Shore.

For the two days of this analysis, air-sea temperature differences of 2-5 degrees K where measured from 5 to 65 kilometers offshore. Onshore surface heating produced well-mixed layers later observed above the stable IBL. IBL heights generally increased with offshore range as stability increased in the overriding turbulent layer. Intermittent regeneration of mixing (Mahrt, et al, 2003) was observed well offshore. Decreasing values of w in the upstream synoptic flow increased the magnitude of the gradient of w in the IBL. Corresponding measurements of radio frequency propagation indicate severe ducting effects out to 65 kilometers offshore.