Characterization of Diurnal Cycle and Land-Sea Flow Effects on Refractivity at Microwave Frequencies during CASPER-West

Sharp vertical gradients in moisture and temperature within the atmospheric boundary layer (BL) impact electromagnetic (EM) propagation at microwave frequencies via changes to refractivity gradients. Modified refractivity can be defined by M = A/T(P + B*e/T) +C*z/R, where T, e, z, and P are temperature, vapor pressure, height, and pressure, respectively, and A, B, and C are constants. Atmospheric Layers where the vertical refractivity gradient, dM/dz, is negative can trap, or duct, EM energy directed at a low elevation angles, potentially increasing radar detection ranges near the Earth. By contrast, large positive dM/dz can lead to subrefraction, which pulls microwave energy away from the Earth, potentially reducing radar detection ranges. More generally, conditions that deviate from reference atmospheric conditions often assumed for radar and communications systems are termed nonstandard or anomalous propagation.

Propagation conditions within littoral BLs are challenging to characterize and predict. Complex coastlines and topography combined with strong horizontal gradients of near-surface heat and moisture content can lead to complex BL vertical structure (e.g. land-sea breezes, katabatic flows, internal BLs, and low clouds/fog) associated with nonstandard refraction. The Coupled Air-Sea Processes and Electromagnetic (EM) ducting Research (CASPER)-West intensive operation period (IOP) was a multi-institutional campaign conducted at Pt. Mugu, California, and adjacent waters of the Southern California Bight during September/October 2017. The focus of CASPER-West was to investigate ducting layers associated with intrusions and capping of the littoral marine BL by adjacent dry air. The Naval Surface Warfare Center, Dahlgren Division, supported CASPER-West by launching multiple daily rawinsondes onshore both day and night over the 28 day test period. The soundings were coordinated with two shipboard launch platforms and concurrent surface observations. In this study, the evolution of the propagation environment associated with offshore and onshore flow regimes and the diurnal cycle is characterized. The impacts of associated nonstandard refraction features will be placed in the context of existing climatologies of the Southern California Bight.