3b.7 Development of a Daytime Polar Cloud Mask Using Theoretical Models of Near- Infrared Bidirectional Reflectance for Arm and Ceres

نویسندگان

  • Qing Trepte
  • Robert F. Arduini
  • Yan Chen
  • Sunny Sun-Mack
  • Patrick Minnis
  • Douglas A. Spangenberg
  • David R. Doelling
چکیده

Scene identification in satellite imagery over polar regions is difficult because clouds are often similar to the underlying surface in terms of temperature and visible (VIS) reflectance. During the day, however, the brightness temperature difference (BTD) between the 3.7 and 11.0-μm channels on many satellites is greater for clouds than for clear snow allowing for discrimination between clouds and snow. This difference is primarily in the reflected solar component of the solar infrared (SIR) 3.7-μm channel. Similarly, clouds are usually more reflective than snow at 1.6 μm, a near-infrared (NIR) channel on several new satellites. Typically, these channels are used for detecting snow based on either reflectance or temperature difference thresholds that are empirically established. While more accurate than using visible or infrared thresholds alone, these empirical NIR or SIR thresholds are less than satisfactory because of their wide variability, especially with viewing and illumination angles. To minimize the need for empirically adjusting the thresholds for a given set of conditions, to reduce the error accrued from using single-value thresholds, and to facilitate more accurate automated scene identification over snow-bound regions, better characterizations of the bidirectional reflectance patterns of snow at 1.6 and 3.7 μm are needed. This paper examines the use of theoretical models for improving the cloud masks used by the Clouds and Earth's Radiant Energy System (CERES) project and the Atmospheric Radiation Measurements (ARM) Program. Bidirectional reflectance models of snow at NIR and SIR wavelengths are constructed from the radiative transfer calculations. The NIR and SIR models are used together to classify polar scenes observed with the Terra Moderate-Resolution Imaging Spectroradiometer (MODIS). The resulting cloud mask images are compared with those from the empirical polar scene identification. By using reasonably accurate, theoretically based masks for these two wavelengths; it should be possible to consistently detect clouds over the Arctic, Antarctica, and other snow-covered areas using a variety of different satellites. ____________________________________________ *Corresponding author address: Q. Trepte, SAIC, Inc., Hampton, VA 23666; email: [email protected] 2. THEORETICAL MODELS

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تاریخ انتشار 2001