Mesoscale Variability of Surface Ducts During Santa Ana Wind Episodes

Abstract

Mesoscale variability of the marine boundary layer characteristics and the corresponding radio wave ducting conditions over the Southern California Bight (SCB) during three Santa Ana wind events have been examined based on the analysis of field observations and diagnosis of mesoscale model simulations. Under the influence of a dry and the stably stratified airflow from inland, a shallow moist internal boundary layer (MIBL) develops over the coastal waters. The MIBL consists of a thin well‐mixed moist layer capped by an inversion layer where the specific humidity decreases with height sharply. Accordingly, a surface duct (SD) is virtually ubiquitous with its depth compared with the MIBL depth. The wind direction and the SD characteristics exhibit diurnal variations over the SCB area associated with land and sea breezes. In addition, the MIBL interacts with the coastal topography, Channel Islands, and sea surface temperature gradients, resulting in spatial mesoscale variabilities in SD characteristics. Specifically, the SD height and strength grow with the offshore distance owing to the surface latent heat flux over sea, and in the meantime, exhibit substantial along‐shore variations. The shallow MIBL tends to split and flow around the higher elevation of the islands, leading to the formation of wakes with reduced modified refractivity. These island wakes may extend dozens of kilometers downstream with the SD height and strength growing accordingly.

Utö Observatory for Analysing Atmospheric Ducting Events over Baltic Coastal and Marine Waters

“…The abrupt, horizontal changes in the surface, in terms of roughness, moisture and thermal forcing, affect the marine boundary layer structure [43], resulting in regions with variable propagation conditions. If the boundary layer is shallow, the topography can prevent horizontal flow and affect the boundary layer structure [43], and island wakes can also affect signal propagation [44]. This highlights the difficulty in producing a reliable ducting forecast for the Archipelago Sea.…”