Entrainment layers capping well mixed layers are breeding grounds for radio frequency ducts that trap electromagnetic energy near the surface resulting in a mixture of beneficial and deleterious impacts on the performance of coastal communication systems and radar. Sea breeze circulations create well defined entrainment layers and are expected to increase in intensity and frequency during the 21st century.

The gradient of modified refractivity (M) must be less than zero in order for the entrainment layer to produce a radio frequency duct or trapping layer. Thermal stability in the entrainment layer can force the gradient of M towards zero, but it requires a strong negative vertical gradient of water vapor mixing ratio in the entrainment layer to force a negative gradient of M. This is not uncommon since the water vapor in the free atmosphere is typically less than that in the mixed layer.

The surface value of w is related to the sea surface temperature. During sea breeze events, the elevated value of w is controlled by the humidity of the air in the circulation originating on land that is often drier than the air in the free atmosphere. The resulting shallow dry tongue at the top of the entrainment layer enhances the gradient of M and strengthens the radio frequency duct especially at the height of the sea breeze.

NSWCDD has developed radio frequency propagation test beds over the Potomac River at Dahlgren and at Wallops Island, Virginia. Both locations are supported by an operational Coupled Ocean Atmosphere Mesoscale Prediction System numerical weather prediction model with a 1km horizontal resolution and 10 to 15m vertical resolution in the sea breeze circulation. Both locations are instrumented with surface meteorological validation observations that are not assimilated into the numerical weather prediction model.

This paper will describe the spatio-temporal development of the dry tongue at the top of the entrainment layer in sea breeze events on the Potomac River and over the ocean at Wallops Island where the over land boundary layer humidity varies relative to the humidity in the free atmosphere. The relative impacts on radio frequency systems will be described.