A Critical Examination of Satellite Cloud Retrieval from AVHRR in the Arctic Using SHEBA Data

Abstract

This study examines the validity and limitations associated with retrieval of cloud optical depth τ and effective droplet size re in the Arctic from Advanced Very High Resolution Radiometer (AVHRR) channels 2 (0.725?1.10 ?m), 3 (3.55?3.93 ?m), and 4 (10.3?11.3 ?m). The error in re is found to be normally less than 10%, but the uncertainty in τ can be more than 50% for a 10% uncertainty in the satellite-measured radiance. Model simulations show that the satellite-retrieved cloud optical depth τsat is overestimated by up to 20% if the vertical cloud inhomogeneity is ignored and is underestimated by more than 50% if overlap of cirrus and liquid water clouds is ignored. Under partially cloudy conditions, τsat is larger than that derived from surface-measured downward solar irradiance (τsurf) by 40%?130%, depending on cloud-cover fraction. Here, τsat derived from NOAA-14 AVHRR data agrees well with τsurf derived from surface measurements of solar irradiance at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp in summer, but τsat is about 2.3 times τsurf before the onset of snowmelt. This overestimate of τsat is mainly due to the high reflectivity in AVHRR channel 2 over snow/ice surfaces, the presence of partial cloud cover, and inaccurate representation of the scattering phase function for mixed-phase clouds.