Vertical salinity gradients in the top few meters of the ocean surface can exist due to the freshwater input from rain. If present, surface gradients complicate comparing salinity measured at depths of a few meters to salinities retrieved using L‐band microwave radiometers such as SMOS and Aquarius. Therefore, understanding the spatial scales and the frequency of occurrence of these vertical gradients and the conditions under which they form will be important in understanding sea surface salinity maps provided by microwave radiometers. Salinity gradients in the near‐surface ocean were measured using a towed profiler that profiled salinity in the top 2 m of the ocean with a minimum measurement depth of 0.1 m. In addition, an Underway Salinity Profiling System was installed on the R/V Thomas G. Thompson. This measured near‐surface salinity at depths of 1 and 2 m. Both the towed profiler and the underway system found the occurrence of negative salinity anomalies (i.e., salinity decreasing toward the surface) was correlated with the presence of rain. The magnitude of the anomaly (i.e., the difference between salinity at 0.1 m and the salinity at 0.26 m) was proportional to the cube of the rain rate for rain rate, R, greater than 6 mm h⁻¹. From this, for R > 15–22 mm h⁻¹, depending on the areal extent of the salinity anomalies, rain can cause scene‐averaged salinity offsets that are as large as the accuracy goal for Aquarius of 0.1‰.

During the second Salinity Processes in the Upper Ocean Regional Study (SPURS‐2) 2017 tropical Pacific cruise, two drifters were deployed on 9 November. The drifters measured temperature and salinity in the top 36 cm, wave spectra, and the noise of rain drops. During a short nearly circular survey with a 1.8‐km radius around the drifters, the R/V Revelle measured air‐sea fluxes, as well as temperature and salinity stratification in the top 1 m from a towed surface salinity profiler (SSP). A C‐band weather radar measuring rain rate within 1‐ to 100‐km range of the ship observed discrete rain cells organized in a system moving from the southeast to the northwest. Some of the intense rain cells were small scale (1 km in diameter or less) with short lifetimes yet dropped more than 5 cm of water in half an hour near the drifters, whereas the ship measured short rain episodes totaling 1.3 cm of rainfall mostly accompanied by very low wind. The data indicate a large spatial heterogeneity in temperature and salinity, with near‐surface freshening of up to 9 psu measured at different times by the two drifters (separated by less than 500 m) and by the SSP. The drifters indicate deepening of the fresh and cool surface layer during the rain which then thinned during the following 40 min with very low wind speed (<2 m/s). Patchy surface‐trapped cold and fresh layers were also observed by the SSP east of the drifters. The high spatial and temporal variability of rainfall and surface‐trapped fresh pools is discussed.

Air‐sea gas fluxes are generally defined in terms of the air/water concentration difference of the gas and the gas transfer velocity,k_L. Because it is difficult to measure k_Lin the ocean, it is often parameterized using more easily measured physical properties. Surface divergence theory suggests that infrared (IR) images of the water surface, which contain information concerning the movement of water very near the air‐water interface, might be used to estimatek_L. Therefore, a series of experiments testing whether IR imagery could provide a convenient means for estimating the surface divergence applicable to air‐sea exchange were conducted in a synthetic jet array tank embedded in a wind tunnel. Gas transfer velocities were measured as a function of wind stress and mechanically generated turbulence; laser‐induced fluorescence was used to measure the concentration of carbon dioxide in the top 300 μm of the water surface; IR imagery was used to measure the spatial and temporal distribution of the aqueous skin temperature; and particle image velocimetry was used to measure turbulence at a depth of 1 cm below the air‐water interface. It is shown that an estimate of the surface divergence for both wind‐shear driven turbulence and mechanically generated turbulence can be derived from the surface skin temperature. The estimates derived from the IR images are compared to velocity field divergences measured by the PIV and to independent estimates of the divergence made using the laser‐induced fluorescence data. Divergence is shown to scale withk_Lvalues measured using gaseous tracers as predicted by conceptual models for both wind‐driven and mechanically generated turbulence.

Vibrational sum-frequency spectroscopy (VSFS) has been used for some time as a laboratory-based surface chemical analytical tool. Here, theoretical considerations in applying the method as a remote-sensing probe for detecting trace levels of chemicals adsorbed on surfaces are presented. Additionally, a VSFS instrument is configured to operate at a stand-off distance of 2.2 m using near-nadir incidence angles. This system was used to measure VSFS spectra for films of pure 1-amino-4-nitrobenzene ( p-nitroaniline, PNA) and pure 2-hydroxy-1,3,5-trinitrobenzene (picric acid, PA) adsorbed on polished T-6061 aluminum alloy. These spectra are used to investigate the effect of optical polarization on the sum-frequency response of these compounds at nadir optical geometries. Detection limits for each compound are also estimated and found to be 0. 51 ig cm² for PNA and 0.89 μg cm² for PA. The implications of these results regarding remote sensing applications of VSFS are discussed.