The Coupled Air–Sea Processes and Electromagnetic Ducting Research (CASPER) project aims to better quantify atmospheric effects on the propagation of radar and communication signals in the marine environment. Such effects are associated with vertical gradients of temperature and water vapor in the marine atmospheric surface layer (MASL) and in the capping inversion of the marine atmospheric boundary layer (MABL), as well as the horizontal variations of these vertical gradients. CASPER field measurements emphasized simultaneous characterization of electromagnetic (EM) wave propagation, the propagation environment, and the physical processes that gave rise to the measured refractivity conditions. CASPER modeling efforts utilized state-of-the-art large-eddy simulations (LESs) with a dynamically coupled MASL and phase-resolved ocean surface waves. CASPER-East was the first of two planned field campaigns, conducted in October and November 2015 offshore of Duck, North Carolina. This article highlights the scientific motivations and objectives of CASPER and provides an overview of the CASPER-East field campaign. The CASPER-East sampling strategy enabled us to obtain EM wave propagation loss as well as concurrent environmental refractive conditions along the propagation path. This article highlights the initial results from this sampling strategy showing the range-dependent propagation loss, the atmospheric and upper-oceanic variability along the propagation range, and the MASL thermodynamic profiles measured during CASPER-East.

The sea surface temperature (SST) relevant to air‐sea interaction studies is the temperature immediately adjacent to the air, referred to as skin SST. Generally, SST measurements from ships and buoys are taken at depths varies from several centimeters to 5 m below the surface. These measurements, known as bulk SST, can differ from skin SST up to O(1°C). Shipboard bulk and skin SST measurements were made during the Coupled Air‐Sea Processes and Electromagnetic ducting Research east coast field campaign (CASPER‐East). An Infrared SST Autonomous Radiometer (ISAR) recorded skin SST, while R/V Sharp's Surface Mapping System (SMS) provided bulk SST from 1 m water depth. Since the ISAR is sensitive to sea spray and rain, missing skin SST data occurred in these conditions. However, SMS measurement is less affected by adverse weather and provided continuous bulk SST measurements. It is desirable to correct the bulk SST to obtain a good representation of the skin SST, which is the objective of this research. Bulk‐skin SST difference has been examined with respect to meteorological factors associated with cool skin and diurnal warm layers. Strong influences of wind speed, diurnal effects, and net longwave radiation flux on temperature difference are noticed. A three‐step scheme is established to correct for wind effect, diurnal variability, and then for dependency on net longwave radiation flux. Scheme is tested and compared to existing correction schemes. This method is able to effectively compensate for multiple factors acting to modify bulk SST measurements over the range of conditions experienced during CASPER‐East.

Some properties of sea surface albedo have been investigated. (1) With aircraft data of upward and downward hemispheric irradiances in three broad bands of the solar spectrum we have investigated albedo as a function of transmittance (variations in windspeed and solar altitude were small). Transmittance measures the depletion of the solar radiation on its path through the atmosphere. For short solar wavelengths, 0.28–;0.53 μm, surface albedo shows little dependence on transmittance, while for longer solar wavelengths, 0.53–2.8 μm, albedo increases with decreasing transmittance. This behavior results from the observation that the short wavelengths are mostly diffuse for all degrees of cloudiness, whereas longer wavelengths become more diffuse with increasing cloudiness. (2) With measurements from broadband pyranometers on a mast over Lake Washington (Seattle, Washington) and from ships in the North Atlantic and the Tropical Atlantic during summer we have investigated the variation of albedo with solar altitude and transmittance. Our results are in general agreement with those of Payne (1972), but some deviations are indicated at high solar altitudes with low transmittances and for solar altitudes below 20°. The effects of wind speed on normalized albedo measured in the Joint Air‐Sea Interaction (JASIN) experiment agree with previous findings of Payne (1972) and Simpson and Paulson (1979).

Accurate measurement of fluctuations in temperature and humidity are needed for determination of the surface evaporation rate and the air-sea sensible heat flux using either the eddy correlation or inertial dissipation method for flux calculations. These measurements are difficult to make over the ocean, and are subject to large errors when sensors are exposed to marine air containing spray droplets. All currently available commercial measurement devices for atmospheric humidity require frequent maintenance. Included in the objectives of the Humidity Exchange over the Sea program were testing and comparison of sensors used for measuring both the fluctuating and mean humidity in the marine atmosphere at high wind speeds and development of techniques for the protection of these sensors against contamination by oceanic aerosols. These sensors and droplet removal techniques are described and comparisons between measurements from several different systems are discussed in this paper. To accomplish these goals, participating groups devised and tested three methods of removing sea spray from the sample airstream. The best performance was given by a rotating screen device, the "spray flinger." Several high-frequency temperature and humidity instruments, based on different physical principles, were used in the collaborative field experiment. Temperature and humidity fluctuations were measured with sufficient accuracy inside the spray removal devices using Lyman-a hygrometers and a fast thermocouple psychrometer. Comparison of several types of psychrometers (using electric thermometers) and a Rotronic MP-100 humidity sensor for measuring the mean humidity illustrated the hysteresis of the Rotronic MP-I 00 device after periods of high relative humidity. Confidence in the readings of the electronic psychrometer was established by in situ calibration with repeated and careful readings of ordinary hand-held Assman psychrometers (based on mercury thermometers). Electronic psychrometers employing platinum resistance thermometers perform very well.

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