Progress on Microwave Radiometer-Based Whitecap Fraction and Sea Spray Aerosol Products

Sea spray aerosols are the dominant natural aerosols over the ocean. Sea spray aerosols influence solar and terrestrial radiation, act as cloud condensation nuclei and act as a sink for some atmospheric gases. Oceanic whitecaps are the major source of sea spray. Whitecaps and sea spray aerosols also contribute surface fluxes of energy, heat and mass at the air-sea interface which can impact both climate and weather. The production of sea spray aerosols and surface fluxes are usually parameterized in terms of whitecap fraction. Therefore, timely, global observations of whitecap fraction are valuable for input into both climate and weather models.

Estimations of global sea spray production can be obtained with a parameterization of a sea spray source function using global wind speed data from satellites or numerical weather prediction models. These parameterizations utilize the relationship between wind speed, whitecap fraction and sea spray production. Whitecaps are strongly emissive at microwave frequencies. Consequently, the microwave sea surface emissivity increases with increasing whitecap fraction. This change in emissivity with whitecap fraction affects the brightness temperature measured by microwave radiometers over ocean scenes. Other satellite instruments that are used to measure wind speed over the ocean, such as scatterometers and synthetic aperture radars, are less sensitive to whitecap fraction because they primarily measure ocean roughness instead of emissivity.

We have used the relationship between the microwave emissivity and the whitecap fraction to develop a method for estimating the whitecap fraction from WindSat brightness temperatures. WindSat is a spaceborne microwave radiometer with channels at nominal frequencies of 6.8, 10.7, 18.7, 23.8 and 37 GHz. Our WindSat whitecap algorithm uses 10.7-37 GHz brightness temperatures collocated to a common resolution of approximately 25 km X 35 km at the ocean surface. We have also collocated WindSat brightness temperatures with in situ sea spray production measurements to verify the relationship between the brightness temperatures and sea spray production. We will discuss the conclusions from these studies and the advantages of using radiometer brightness temperatures to estimate whitecap fraction and sea spray production.

We have produced estimates of whitecap fraction using our WindSat-based algorithm and archived WindSat and ancillary data. We will discuss the potiential for providing global, near-real-time whitecap fraction estimate and our progress toward that goal. We will discuss the ancillary data needed for our algorithm and possible methods for obtaining it in near-real-time. Our current algorithm is only applicable to the open ocean because the accuracy in coastal regions is limited by the resolution of the WindSat brightness temperatures. We will discuss the results of a feasability study for using millimeter-wave (80-200 GHz) brightness temperatures to estimate whitecap fraction to provide higher resolution data for coastal regions and near sea ice. We will also discuss the potential for using brightness temperatures from other microwave radiometers, such as the Global Precipitation Measurement (GPM) Microwave Imager (GMI) and the Advanced Microwave Scanning Radiometers 2 (AMSR2), to improve the coverage and timeliness of whitecap fraction estimates.