Multidecadal Consistent Ocean Vector Winds: from QuikSCAT to RapidScat and Beyond

There are multiple indications that wind circulation characteristics may have been changing over the last decades. However, obtaining a globally consistent high-accuracy assessment of these changes from spaceborne remote sensing data has proved elusive in many cases, even though ocean vector wind data has been collected from space starting in the 1990's. The reason that a consistent assessment of these changes is still a work in progress is due to multiple factors. Firstly, many of the scatterometer instruments were designed to meet accuracies satisfactory for weather prediction, not for the smaller magnitude changes associated with climate change. Nevertheless, these instruments have proven at least self-consistent over long periods and suggest that climate signals may be retrievable from their data. Secondly, the instruments have used different microwave frequency bands, and uncertainties still remain regarding the consistency of the measurement physics across frequency bands. Thirdly, the data have been processed differently with different, and potentially inconsistent, model functions. Finally, and perhaps most importantly, all of these satellites have been in sun-synchronous orbits, with observations being made at different local times. These differences in observation time leads to aliasing of diurnal and semi-diurnal observations into the climate data record. In the first part of this talk, I review our current state of knowledge of the intercalibration of the three major scatterometers of the past decades: NASA's QuikSCAT, EUMETSAT's ASCAT (A & B), and ISRO's OSCAT. In the second part, I describe the design of the NASA ISS-RapidScat cross-calibration mission. This mission uses the non-sun-synchronous characteristics of the International Space Station (ISS) orbit to provide coincident data with scatterometers in the constellation every revolution. It is expected that it will be able to provide cross-calibration cross section and wind data for QuikSCAT, ASCAT, and the forthcoming ISRO ScatSat mission. RapidScat is expected to launch in September, 2014, and the final part of this talk will present the first results of the mission on orbit.