Examining the Role of the Air-Sea Interaction for the Observed Variability Evaporation Duct Height during CASPER

Electromagnetic (EM) ducting is the phenomenon in which the EM energy propagated from a source is trapped in a layer of atmosphere in the lower troposphere, mostly associated with the atmospheric boundary layer. In the atmospheric boundary layer, the EM ducts are chiefly produced by the vertical moisture gradient. An important category of EM ducts, known as evaporations ducts, are formed in the marine atmospheric surface layer (MASL) due to the strong near-surface moisture gradient created by the evaporation from the ocean surface. The height of the evaporation ducts can vary from a few meters to a few tens of meters from the surface. The study of evaporation ducts is important for many applications including maritime communications and satellite remote sensing.

To improve our capability to predict properties of evaporation ducts, the Coupled Air-Sea Processes and EM ducting Research -East Coast field campaign (CASPER-East) was conducted offshore of the Duck, North Carolina during the period from 10 October to 06 November 2015. Simultaneous observations of MASL variables and radio frequency (RF) propagation properties were made during the field campaign period. Using the CASPER-East measurements and results from the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS), this study examines how the air-sea interaction processes affect the observed EDH variability and its effects on RF propagation.

Our analyses showed that in general, mean EDH was higher near the shore and decreased with distance from the shore. MASL thermal stability, represented by air-sea temperature difference (ASTD), has shown to be the major factor regulating the observed EDH variability. Consistently higher EDH (> 15-20 m) tend to occur in the stable surface layer conditions (ASTD > 0) near the shore. The advection of warm air over the cold water near the shore produced stable surface layer conditions which created the conducive environment for the occurrence of higher EDH. In addition, this study revealed that the sporadic intrusion of cooler surface water also facilitates the formation of very high EDH that often exceeded 35 m. Using the same measurements and COAMPS simulation results, we will also present results on the impact of the air-sea coupled processes in modulating the observed RF propagation.