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 (σ 0 ) measurements over land globally. Previous sun-synchronous scatterometers providing twice-daily surface observations have been used to demonstrate some diurnal changes in σ 0 in several regions globally, but instrument cross-calibration concerns prevent identifying diurnal changes by combining σ 0 observations from multiple sensors. As a result, the full extent of diurnal changes to σ 0 has not been determined until now. In this paper, RapidScat is used to identify diurnal changes to σ 0 globally. Vegetation type is discussed with respect to the diurnal changes in σ 0 regionally. The global diurnal changes to σ 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.