Remote sensing of surface and cloud properties in the Arctic from AVHRR measurements

Algorithms to retrieve cloud optical depth and effective radius in the Arct ic using Advanced Very High Resolution Radiometer (AVHRR) data are develope d, using a comprehensive radiative transfer model in which the atmosphere i s coupled to the snowpack. For dark surfaces AVHRR channel 1 is used to der ive visible cloud optical depth, while for bright surfaces AVHRR channel 2 is used. Independent inference of cloud effective radius from AVHRR channel 3 (3.75 mu m) allows for derivation cloud liquid water path (proportional to the product of optical depth and effective radius). which is a fundament al parameter of the climate system. The algorithms are based on the recogni tion that the reflection function of clouds at a nonabsorbing wavelength (s uch as AVHRR channel 1) in the solar spectrum is primarily a function of cl oud optical thickness, whereas the reflection function at a liquid water ab sorbing wavelength (such as AVHRR channel 3) is primarily a function of clo ud particle size. For water clouds over highly reflecting surfaces (snow an d ice), the reflectance in AVHRR channel 1 is insensitive to cloud optical depth due to the multiple reflections between cloud base and the underlying surface; channel 2 (0.85 mu m) must be used instead for optical depth retr ieval. Water clouds over tundra or ocean are more straightforward cases sim ilar to those found at lower latitudes, and in these cases a comprehensive atmospheric radiative transfer model with a Lambertian surface under cloud is used. Thus, for water cloud over tundra and ocean, channel 1 is used for cloud optical depth retrieval. In all cases, channel 3 is used for indepen dent retrieval of cloud droplet effective radius. The thermal component of channel 3 is estimated by making use of channel 4 (11 mu m) and is subtract ed from the total channel 3 radiance. Over clear-sky scenes, the bidirectio nal reflectance properties of snow are calculated directly by the coupled s nowpack-atmosphere model. This results in greater overall accuracy in retri eved surface properties as compared with the simplified approach that uses a Lambertian approximation for the surface albedo. To test the physical soundness of the algorithms the authors have applied t hem to AVHRR data over Barrow, Alaska, from April to August 1992. Downwelli ng irradiances at the surface calculated using the retrieved cloud optical depth and effective radius are compared with field irradiance measurements, and encouraging agreement is found. The algorithms are also applied to thr ee areas of about 100-km dimension around Barrow, each having a different u nderlying surface (ocean, tundra, snow).