Improving Near-Surface Retrievals of Surface Humidity Over the Global Open Oceans From Passive Microwave Observations

Ocean evaporative fluxes are a critical component of the Earth's energy and water cycle, but their estimation remains uncertain. Near-surface humidity is a required input to bulk flux algorithms that relate mean surface values to the turbulent fluxes. Several satellite-derived turbulent flux products have been developed over the last decade that utilize passive microwave imager observations to estimate the surface humidity. It is known, however, that these estimates tend to diverge from one another and from in situ observations. Analysis of current state-of-the-art satellite estimates provided herein reveals that regional-scale biases in these products remain significant. Investigations reveal a link between the spatial coherency of the observed biases to atmospheric dynamical controls of water vapor vertical stratification, cloud liquid water, and sea surface temperature. This information is used to develop a simple state-dependent bias correction that results in more consistent ocean surface humidity estimates. A principal conclusion is that further improvements to ocean near-surface humidity estimation using microwave radiometers requires incorporation of prior information on water vapor stratification and sea surface temperature.