Sensitivity of Thermal Infrared Radiation at the Top of the Atmosphere and the Surface to Ice Cloud Microphysics

Abstract

This paper reports on the sensitivity of the brightness temperatures associated with radiances at the surface and the top of the atmosphere, simulated for the Imaging Infrared Radiometer (IIR) 8.7-, 10.6-, and 12-?m channels under ice cloudy conditions, to the optical and microphysical properties of ice clouds. The 10.6- and 12-?m channels allow simultaneous retrieval of ice cloud optical thickness and effective particle size (Deff) less than 100 ?m. It is illustrated that the particle shape and size distributions of ice crystals have noticeable effects on the brightness temperatures. Using the split window technique based on the 10.6- and 12-?m channels in conjunction with cloud properties assumed a priori, the authors show that the influence of the cloud microphysical properties can lead to differences on the order of ±10% and ±25% in retrieved effective particle sizes for small (Deff < 20 ?m) and large particles (Deff > 40 ?m), respectively. The impact of cloud model on retrieved optical thickness is on the order of ±10%. Different particle habits may lead to ±25% differences in ice water path (IWP). Theoretically, the use of an additional channel (i.e., 8.7 ?m) can give a stronger constraint on cloud model and improve the retrieval of Deff and IWP. The present simulations have confirmed that cloud microphysics has a significant impact on the 8.7-?m brightness temperatures mainly because of particle shape. This impact is larger than the errors of the IIR measurements for cloud optical thicknesses (at 12 ?m) ranging from 0.3 to 8. Furthermore, it is shown that the characterization of optical and microphysical properties of ice clouds from ground-based measurements is quite challenging. Especially, water vapor in the atmosphere has an important impact on ground-based cloud retrievals. Observation stations at higher altitudes or airborne measurements would minimize the atmospheric effect.