Observations of deep convective clouds as stable reflected light standard for climate research: AIRS evaluation

摘要

Changes of the order of 1 %/decade or less are expected in the tropical ocean cloud amount due to global warming. Since the ocean is very dark in the visible and near infrared region of the spectrum, a change in the Earth reflectance is equivalent to a measure of the change in the cloud cover. A reliable measurement of such a small change in the visible requires a reference source which is much more stable than 1 %/decade. A procedure is developed to use the sunlight reflected from Deep Convective Clouds (DCC) as a stable and readily available reference source for reflected light channels. The procedure uses the stability of the NIST traceable infrared radiometric calibration of the Atmospheric Infrared Sounder (AIRS) to create a stable DCC detection threshold, which assures a stable visible reflected reference signal. DCC are identified in the data as any IR footprint within ±30 degree latitude, where the brightness temperature in the 1231 cm~(-1) window channel is 210K or less. The 90%tile value of the observed visible signal is used as reference to minimize the effect of under-filling the footprint. Typically 2000 DCC are identified each day during the daylight part of the orbit. The stability uncertainty in the DCC reference signal measured from the first four years of AIRS data is 0.02%/year, I.e. 0.2%/decade. The stability of the procedure demonstrated with AIRS is thus already better than the 1 %/decade expected change in the cloud amount due to global warming. Extrapolated from four years to the expected 12 year lifetime of AIRS the trend uncertainty on the DCC measurements should decrease to 0.06%/decade. A 12 year record of the Earth reflectance stabilized with the DCC would allow for a very sensitive test of a change in the cloud amount.AIRS was launched on the EOS Aqua spacecraft in May 2002 into a 705 km polar sun-synchronous orbit with accurately maintained 1:30 PM ascending node. Essentially un-interrupted data are freely available since September 2002. The DCC are included in the AIRS Calibration Data Subset (ACDS).