Remote sensing of cloud properties using MODIS airborne simulator imagery during SUCCESS: 1. Data and models

Abstract

We investigate methods to infer cloud properties such as cloud optical thickness, thermodynamic phase, cloud particle size, and cloud overlap by comparing cloud and clear‐sky radiative transfer computations to measurements provided by the Moderate Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS). The MAS scanning spectroradiometer was flown on the NASA ER‐2 during the Subsonic Aircraft Contrail and Cloud Effects Special Study (SUCCESS) field campaign during April and May 1996. The MAS bands chosen for this study correspond to wavelengths of 0.65, 1.63, 1.90, 2.15, 3.82, 8.52, 11, and 12 μm. Clear‐sky absorption due to water vapor, ozone, and other trace gases is calculated using a set of correlated k‐distribution routines developed specifically for these MAS bands. Scattering properties (phase function, single‐scattering albedo, and extinction cross section) are derived for water droplet clouds using Mie theory. Scattering properties for ice‐phase clouds are incorporated for seven cirrus models: cirrostratus, cirrus uncinus, cold cirrus, warm cirrus, and cirrus at temperatures of T = −20°C, −40°C, and −60°C. The cirrus are composed of four crystal types: hexagonal plates, two‐dimensional bullet rosettes, hollow columns, and aggregates. Results from comparison of MAS data from a liquid water cloud with theoretical calculations indicate that estimates of optical thickness and particle size are reasonably consistent with one another no matter which spectral bands are used in the analysis. However, comparison of MAS data from a cirrus cloud with theoretical calculations shows consistency in optical thickness but not with particle size among the various band combinations used in the analysis. The methods described in this paper are used in two companion papers to explore techniques to infer cloud thermodynamic phase and cloud overlap.