Refractive Conditions under different mesoscale flow regimes during CASPER-East Field Campaign

The Coupled Air Sea Processes and Electromagnetic ducting Research (CASPER) experiment took place from October 10 to November 06 2016. During this field campaign observations were taken at a long pier at Duck NC and in the offshore region east of Duck, NC. Among the observation platforms used during the field campaign were two research vessels, two research aircraft, pier-based tower and remote sensing measurements, surface buoys, and under water gliders. There were also receivers and antennas in order to measure electromagnetic (EM) properties through the atmosphere during coordinated efforts between stationary and moving (ship) observation platforms. During CASPER, simulations by the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) were made by the Naval Research Lab, Monterey. These simulations, along with sounding data are used in this study. In this work, first we present an evaluation of COAMPS's 2 km resolution forecasts against observations. Our evaluation of the model shows that COAMPS performs well in forecasting the meteorological conditions during the entire campaign. The simulations are able to capture a variety of boundary layer vertical structure such as a well-mixed boundary layer or a stable layer, including the presence of sharp inversions. This model vs. observations intercomparison gives us the confidence to use COAMPS results to investigate the spatial and temporal distribution of refractive properties under different mesoscale flow conditions. The main focus of this research is on the characteristics of different type of ducts and their variations revealed by COAMPS simulations. These ducting conditions vary depending on the mesoscale flow regime and spatial distribution of surface temperature (ocean-to-shore and ocean-to-ocean under sea surface temperature (SST) gradients). We have obtained the evaporation duct height by applying the modified COARE surface flux algorithm in the surface layer using the lowest level of COAMPS forecasts as input to the COARE algorithm. Our results show that the evaporative duct height increases with the sea surface temperature (away from the coast) with mean variations of around ~3.8 m between locations close to the coast (7.7 m) and around the Gulf Stream location (to the east of the coast, 11.5 m). Elevated duct heights are usually higher where SST temperatures are higher. The later leads to a sloped trapping layer profile in the coastal region, which is present most of the time. This sloped profile is linked to a surface based duct close to the interphase between the ocean and the coast in the shore region. We will also present results from analyses of the evolution of ducting properties for three special cases involving meteorological conditions of interests to mesoscale model simulations. These cases include 1) transitions from relatively calm winds, warm and moist conditions to strong winds associated with cool and dry air (October 17 and 18, 2016), 2). atmospheric stable conditions under moist, warm temperatures and strong winds (October 28 and 29, 2016), and 3). Across the transition region between the Gulf Stream and the coastal flow (October 30 and 31, 2016).