Properties of anvil cirrus during CRYSTAL-FACE in the 8–12 µm window bands of MODIS Airborne Simulator: observation and simulation

The infrared radiances measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS) aboard the NASA ER-2 aircraft flying at about 20 km above the surface on 29 July 2002 during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE), in conjunction with the data acquired by Cloud Radar System (CRS) and the Cloud Physical Lidar (CPL), are used to study the radiative properties of anvil cirrus at the window bands of 8-12 µm. The brightness temperature difference between 8 and 11 µm (DT_b(8, 11)) and that between 11 and 12 µm (DT_b(11, 12)) present different characteristics for clouds associated with anvil cirrus. A trispectral brightness temperature method is suggested for detecting clear sky, thin cirrus, thick cirrus, and anvil cirrus. Clear sky can be discriminated by DT_b(11, 12) greater than 1 K and DT_b(8, 11) less than –2 K. Both DT_b(8, 11) and DT_b(11, 12) are less than 1 K for anvil cirrus. Thick cirrus clouds can be determined by DT_b(8, 11) greater than 1 K and positive difference between DT_b(8, 11) and DT_b(11, 12), while thin cirrus clouds for DT_b(11, 12) greater than 1 K, DT_b(8, 11) above –2 K, and negative difference between DT_b(8, 11) and DT_b(11, 12). Cloud ice optical thickness derived from the ice water contents estimated by the CRS data and scattering properties of ice particles from a database of the single-scattering properties of ice crystals, are input to the Discrete Ordinates Radiative Transfer model (DISORT) for simulating the influence of cloud ice optical thickness and effective size on the brightness temperatures at 8, 11, and 12 µm. A bispectral brightness temperature method between the window bands is suggested for the estimation of cirrus cloud effective size and optical thickness within 10.