Infrared radiances at MODIS bands over a deep convective cloud system during TC4: observations and simulations

Retrieval of ice cloud properties using infrared measurements has a distinct advantage over the visible and near-infrared techniques by providing consistent data regardless of solar illumination. The infrared bands at 3.7, 6.7, 7.3, 8.5, 11.0, and 12.0 μm have been used to infer ice cloud parameters by various methods, but the reliable retrieval of optical thickness is limited to thin cirrus with a visible optical thickness less than 6. The objective of the present study is to investigate the infrared radiances at these bands over high thick ice clouds and thereby to investigate the potential of extending the retrieval of optical thickness to thicker ice clouds. The measurements over a deep convective cloud system from the MODIS Airborne Simulator (MAS), Cloud Physics Lidar (CPL), and Cloud Radar System (CRS) aboard the NASA ER-2 aircraft during TC4 experiment on 5 August 2007, are used to investigate the infrared radiances over a deep convective cloud system. It is found that the brightness temperature differences among 3.7, 6.7 μm and the window bands have sensitivity to higher optical thickness up to around 20. Radiative transfer simulations on the basis of the collected in-situ measurements of atmospheric profile, cloud particle size distributions, ice particle habits, and combined ice water contents from CRS and CPL measurements are performed to investigate the effect of both ice cloud particle size and optical thickness on these infrared bands. The simulations show features that are consistent with those of the measurements. With respect to other infrared bands, the 6.7 and 11.0 μm bands are less sensitive to ice particle sizes. So combining the brightness temperature differences among these infrared bands also has the potential to obtain particle size information associated with the ice clouds having larger optical thickness.