RapidScat Diurnal Cycles Over Land

RapidScat, which is a Ku-band scatterometer mounted on the International Space Station, observes the Earth's surface in a non-sun-synchronous orbit allowing for different local time-of-day (LTOD) observations as the orbit progresses. The unique orbit and different LTOD observations provide surface observations that are composited to describe the diurnal variability of Ku-band normalized backscatter (sigma(0)) measurements over land globally. Previous sun-synchronous scatterometers providing twice-daily surface observations have been used to demonstrate some diurnal changes in sigma(0) in several regions globally, but instrument cross-calibration concerns prevent identifying diurnal changes by combining sigma(0) observations from multiple sensors. As a result, the full extent of diurnal changes to sigma(0) has not been determined until now. In this paper, RapidScat is used to identify diurnal changes to sigma(0) globally. Vegetation type is discussed with respect to the diurnal changes in sigma(0) regionally. The global diurnal changes to sigma(0) are discussed with emphasis on the Amazon, Congo, and Upper Danube river regions. Diurnal cycles are described that could not previously be identified with sun-synchronous instruments. Global means and the magnitude of the diurnal cycle are discussed. With the diurnal changes identified and quantified, RapidScat can be used for future cross-platform calibrations using land targets.