Abstract.The The product of all the group confidences is used to determine the confidence of finding clear-sky conditions. This paper outlines the MEDIS cloud masking algorithm. While no present sensor has all of the spectral bands necessary for testing the complete MEDIS cloud mask, initial validation of some of the individual cloud tests is presented using existing remote sensing data sets. IntroductionThe Moderate-Resolution Imaging Spectroradiometer (MODIS) is a keystone instrument of the Earth Observing System (EeS) for conducting global change research. The MEDIS provides global observations of Earth's land, oceans, and atmosphere in the visible and infrared regions of the spectrum. Measurements at 36 wavelengths, from 0.4 to 14.5 /xm, will allow investigators to study the Earth in unprecedented detail. Biological and geophysical processes will be recorded in the MEDIS measurements on a global scale every 1 to 2 days. Many of the atmospheric and surface parameters require cloud free measurements. The MEDIS cloud mask provides an estimate that a given MEDIS field of view (FeV) is cloud free. It is a global level 2 product generated daily at 1 km and 250 m spatial resolutions. Copyright 1998 by the American Geophysical Union.Paper number 1998JD200032.0148-0227/98/1998JD200032509.00 resolution. Radiances from 14 spectral bands (Table 1) 2. Storage requirements are a concern. The cloud mask is more than a yes/no decision. The cloud mask consists of 48 bits of output which include information on individual cloud test results, the processing path, and ancillary information (e.g., land/sea tag). The first eight bits of the cloud mask provide a summary adequate for many processing applications; however, some applications will require the full mask at 4.8 Gb of storage per day.3. The cloud mask must be easily understood but provide enough information for wide use; it must be simple in concept but effective in its application. This paper describes the approach for detecting clouds using MODIS observations and details the algorithms. Section 2 presents a very brief summary of some current global cloud detection algorithms and discusses the wavelengths used in the MODIS cloud mask algorithm. Section 3 discusses the approach employed by the algorithm. Section 4 details the input 32,141

The Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Earth Observing System (EOS) Terra and Aqua platforms provides unique measurements for deriving global and regional cloud properties. MODIS has spectral coverage combined with spatial resolution in key atmospheric bands, which is not available on previous imagers and sounders. This increased spectral coverage/spatial resolution, along with improved onboard calibration, enhances the capability for global cloud property retrievals. MODIS operational cloud products are derived globally at spatial resolutions of 5 km (referred to as level-2 products) and are aggregated to a 1°equal-angle grid (referred to as level-3 product), available for daily, 8-day, and monthly time periods. The MODIS cloud algorithm produces cloud-top pressures that are found to be within 50 hPa of lidar determinations in single-layer cloud situations. In multilayer clouds, where the upper-layer cloud is semitransparent, the MODIS cloud pressure is representative of the radiative mean between the two cloud layers. In atmospheres prone to temperature inversions, the MODIS cloud algorithm places the cloud above the inversion and hence is as much as 200 hPa off its true location. The wealth of new information available from the MODIS operational cloud products offers the promise of improved cloud climatologies. This paper 1) describes the cloud-top pressure and amount algorithm that has evolved through collection 5 as experience has been gained with in-flight data from NASA Terra and Aqua platforms; 2) compares the MODIS cloud-top pressures, converted to cloud-top heights, with similar measurements from airborne and space-based lidars; and 3) introduces global maps of MODIS and High Resolution Infrared Sounder (HIRS) cloud-top products.

[1] AIRS and MODIS on the EOS Aqua spacecraft collect global observations of the Earth's upwelling infrared radiance for numerous remote sensing and climate related applications. This paper presents comparisons of the AIRS and MODIS radiance observations and illustrates the utility of using high-spectral resolution observations to create a highly accurate assessment of broadband sensor calibration. In the analysis, the high-spectral resolution AIRS spectra are reduced to MODIS spectral resolution, and the high-spatial resolution MODIS data are reduced to AIRS spatial resolution for global data collected on 6 September 2002 and 18 February 2004. Spatially uniform scenes are selected, and the observed differences are characterized as a function of several parameters including scene temperature, sensor scan (view) angle, and solar zenith angle. The comparisons are in general very good with respect to the expected radiometric accuracies of the sensors, with mean brightness temperature differences of 0.1 K or less for many of the MODIS bands. Uncertainties of these determinations range from near 0 K for window region bands to as large as 0.2 K for other bands. For MODIS water vapor bands 27 (6.8 mm) and 28 (7.3 mm) and temperature sounding bands 34 (13.7 mm), 35 (13.9 mm), and 36 (14.2 mm), the differences exhibit a dependence on scene temperature, with peak differences exceeding 1 K for bands 27 and 36. Differences as a function of scan angle are 0.4 K or less for all bands, and scan angles but clear trends are defined. Results for the 2 days demonstrate good reproducibility with changes in mean differences of 0.1 K or less for most bands.