____________________________________________*Corresponding author address :Lin H.Chambers,Atmospheric Sciences,Mail Stop 420,NASA Langley Research Center,Hampton,Virginia 23681-2199;e-mail
.NEW CERES DA TA EXAMINED FOR EVIDENCE OF TROPICAL IRIS FEEDBACKLin H. Chambers*, Bing Lin and David F . Y oung NASA Langley Research Center, Hampton, VA1. INTRODUCTIONNew data products from the Clouds and the Earth’s Radiant Energy System (CERES;Wielicki et al.,1996)instrument were recently released.The Single Scanner Footprint (SSF)data product combines radiation budget data from CERES with cloud property retrievals from an imager on the same platform to provide a vastly improved charac-terization of the state of the atmosphere.In addi-tion,the SSF incorporates new CERES angular distribution models (ADM;Loeb et al.,2002)based on improved scene identification to obtain more accurate top-of-atmosphere fluxes from the satel-lite-measured radiances.Together these advances allow the study of radiative fluxes for specific cloud types with unprecedented accu-racy.SSF data products are now available using the Visible and InfraRed Sensor (VIRS)for the Tropical Rainfall Measuring Mission (TRMM).These data are used here to explore the Iris hypothesis recently proposed by Lindzen et al.,2001 (hereafter LCH).2. THE CERES INSTRUMENTThe CERES instrument is a bi-axial scanning radiometer (Wielicki et al.,1996).On the TRMM satellite,CERES operated in a quasi-periodic cycle of 2days in Fixed Azimuth Plane Scan (FAPS)mode and 1day in Rotating Azimuth Plane Scan (RAPS)mode,with an occasional day of along-track scanning.FAPS mode is basically a cross-track scanning mode and obtains maximum geo-graphic coverage over the orbital swath.RAPS mode scans in azimuth in order to obtain informa-tion on the anisotropy of radiation in the full hemi-sphere.TRMM is a precessing spacecraft with an orbital inclination of about 35o .It samples all local times over a 46 day period.CERES measures radiative energy in 3broad channels:the shortwave (SW)channel measures reflected solar radiation,the window (WN)channelmeasures thermal radiation between 8.10and 11.79µm,and the total (TOT)channel measures the total energy leaving the Earth at all wave-lengths (0.3->100µm,Priestley et al.,2000).The longwave (LW)radiation is obtained by subtrac-tion: LW=TOT-SW.CERES is primarily a climate instrument,so great attention has been paid to its calibration.As a result,the radiative fluxes measured by CERES have been both stable and accurate over the life-time of the instrument.Uncertainties in measured radiances are generally below the 0.5%level (Priestley et al.,2000).The errors in the instanta-neous estimated fluxes of SW (13W/m 2)and LW (4.3W/m 2)radiation are mainly due to errors in the application of the angular distribution models (Loeb et al.,2002)which result from errors in the scene identification.3. DATA ANALYSISA CERES SSF contains about 130parameters describing each CERES footprint.These include time,position,and viewing angles,surface infor-mation,filtered and unfiltered radiances,radiative fluxes at the surface and top of atmosphere,and a variety of parameters describing the clear and cloudy portions of the footprint.The latter are obtained from imager (VIRS)information and from ancillary inputs such as numerical weather predic-tion models (http://asd-www.larc.nasa.gov/ATBD/ATBD.html).The nominal 2-km VIRS-pixel derived properties are then convolved using the CERES scanner point spread function to obtain cloud prop-erties for up to two cloud layers within the ~10km CERES footprint.Currently these layers are dis-tinct,with no overlap properties identified.For this analysis,since radiative fluxes are only available at the CERES footprint scale,the properties of the two layers are area-weighted to obtain footprint-mean cloud properties.A set of screening criteria are applied to the data to ensure that footprints with problems are not used.This leaves more than half a million region-ally distributed footprints per day to be analyzed.CERES TRMM SSF data are available for the period January 1to August 31,1998.This includes the peak and decay of the strong 1997-982.12
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