Use of AIRS and MODIS to retrieve ice cloud optical thickness and effective diameter

Cloud optical thickness and effective diameter are often used in climate models to quantify the radiative forcing of clouds. In this study, data from the Atmospheric Infrared Sounder (AIRS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) are used to retrieve ice cloud optical thickness and effective diameter. We use AIRS thermodynamic profiles and a radiative transfer model (DISORT) to simulate infrared window-channel brightness temperatures at the top of the atmosphere. The ice crystal models used in the radiative transfer calculations are the same as those used by the MODIS retrieval algorithms. We then construct look-up tables using bands at 8.5, 11.0, and 12.0 µm. Once the look-up tables are constructed, it is possible to retrieve ice cloud optical thickness and effective diameter by using the MODIS-derived brightness temperatures and brightness temperature differences. Using this method provides optical thickness and effective diameter at the same spatial resolution as the MODIS brightness temperature product, 5 km by 5 km. In addition, we investigate the effect of introducing a 3 K bias in the AIRS temperature profiles and a 20% bias in the water vapor profile. These biases affect brightness temperatures at the top of the atmosphere very little, justifying the use of AIRS profiles, which have a much coarser spatial resolution (13.5 km spatial resolution at nadir). Since infrared bands are used for the retrieval, this method can be used to retrieve ice cloud optical thickness and effective diameter at nighttime as well.