Effect of a Sea Spray Layer on Ocean Surface Signal at GPS Frequencies

Surface fluxes of momentum, heat, gases, and particles parameterize well air-sea interaction processes taking place at the ocean surface. The surface fluxes realize the coupling between the ocean and the atmosphere and are thus useful in weather forecast and climate prediction models to represent boundary conditions. A suite of geophysical variables is necessary for studying and parameterizing air-sea fluxes including ocean surface wind speed, sea surface temperature, precipitation, and sea surface salinity. Satellite-based passive and active remote sensing observations provide global, long-term data of these geophysical variables. These are observations made with radiometers and radar scatterometers (for passive and active data, respectively) operating at microwave frequencies from 6 to 90 GHz. However, there are two limitations of the satellite observations at these frequencies: (i) The sensors do not perform well in hurricane conditions due to strong attenuation by clouds and within rain bands; and (ii) Collected twice a day, the data have low temporal resolution.

The Global Navigation Satellite Systems reflectometry (GNSS-R) rectifies these limitations and complements the observations from radiometers and scatterometers by providing data through hurricane rain bands at high temporal resolution. GNSS-R uses Global Positioning System (GPS) sensors, which operate at low frequencies (also known as L band frequencies) L1 = 1.57542 GHz and L2 = 1.22760 GHz. The long wavelengths of L1 and L2 penetrate clouds and rain and are thus able to probe the ocean surface during hurricane conditions, while the ubiquity of GPS sensors provides abundance of data at any time. Therefore, GNSS-R data enable the observation and study of hurricane genesis and evolution. The injection of sea spray into the air under gale force winds alters the atmospheric profiles of momentum, moisture, and enthalpy (sensible plus latent heat). These energetic and thermodynamic influences of the sea spray affect the intensity and structure of hurricanes. Therefore, the ability to recognize and evaluate the effect of sea spray on remote sensing signals has the potential for better observing and forecasting hurricane intensification.

The wind speed determines the sea spray concentration, while turbulent transport, gravitational settling, and spray evaporation control the droplet heights. Depending on the sea spray production, the signal detected at microwave frequencies from the rough ocean surface covered with foam could be either augmented or completely masked. To assess the sea spray effect, we have developed a radiative transfer model (RTM) computing the microwave reflectivity and emissivity of a vertically stratified sea spray layer. Effective medium theory is used to obtain the dielectric properties of the sea spray in terms of layer thickness and seawater content. We describe the processes taking place within a sea spray layer and establish requirements for modeling spray dielectric properties. The RTM is formulated and first results are reported.