On the role of humidity in estimating marine surface layer stratification and scatterometer cross section

Estimates of any given flux (momentum, heat, or moisture) that use the full set of diabatic profile relations applicable to the marine atmospheric surface layer require an accurate representation of flux coefficients, diabatic parameters, roughness lengths, and mean values for all meteorological quantities. Calculation of the momentum flux, for example, requires that the stratification function include both temperature and humidity effects and that the solution to the equation set be based on full set iteration. We find that over warm water, particularly during cases of low humidity, the momentum flux and stratification estimates are very sensitive to the reported relative humidity. Similarly, we find that the ERS 1 radar cross section, when treated as a function of the wind stress (or momentum flux), varies significantly with relative humidity, particularly when surface temperatures are warm. This study suggests that any calibration/validation campaign using remote-sensing observations, in particular the ERS 1 scatterometer, should utilize high-quality ground truth relative humidity measurements in addition to the traditional suite of high-quality wind and temperature gradient measurements.