Examining the relationship between cloud and radiation quantities derived from satellite observations and model calculations

Abstract

This study examines the consistency and inconsistency in shortwave (SW) top-of-atmosphere (TOA) reflectances and albedos obtained from satellite measurements of the Earth Radiation Budget Experiment (ERBE) and radiation modeling based on cloud properties retrieved from the Advanced Very High Resolution Radiometer (AVHRR). The examination focuses on completely overcast scences covered by low-level, single-layered, maritime stratus with uniform cloud-top heights as determined from AVHRR measurements at near nadir. A radiation model was then applied to the retrieved cloud optical depths, droplet effecctive radii, and top temperatures to compute the SW TOA reflectances and albedos that are compared with coincident ERBE observations. ERBE-observed and AVHRR-based modeled reflectances show excellent agreement in terms of bh trend and magnitude, but the two albedos exhibit significant differences that have a strong dependence on cloud optical properties and solar zenith angle (SZA). To unravel the differences, two major factors, that is, scence identification and angular dependence model (ADM), involved in converting reflectance to albedo are examined. It is found that the dependence is mainly caused by the use of a single ERBE-ADM for all overcast scenes, regardless of cloud optical properties. The mean difference in SW TOA flux is about 4-12 W m-2, depending on SZA, but individual differences may reach up to 40-50 W m-2 for persistent large or small cloud optical depths. Nearly all of the uniform low-level overcast scenes as determined by AVHRR are identified as mostly cloudy by ERBE, but the misidentification does not have any adverse effect on the albedo differences. In fact, replacing the ERBE mostly cloudy ADM with the overcast ADM exacerbates the albedo comparisons. The mean fluxes obtained with the two ADMs differ by ∼8 W m-2 at SZA ≈ 33° and by 30 W m-2 at SZA ≈ 60°.