Using MODIS to study the sensitivity of global high cloud detection and property estimation to sensor resolution

The 1-km resolution infrared data from the MODIS (MODerate-Resolution Imaging Spectroradiometer) on the EOS Terra and Aqua platforms is used to study the effect of sensor resolution in determining cloud top properties (height and effective cloud amount) using the CO₂ slicing algorithm. This is important to provide a link to historical retrievals of similar cloud properties afforded by the High Resolution Infrared Radiometer Sounder (HIRS) instruments flown on the NOAA operational platforms since the 1978. The accuracy of the cloud retrieval depends on good calibration, knowledge of spectral response functions, and having an accurate and computationally fast radiative transfer model to simulate top-of-atmosphere radiances. For the Aqua MODIS imager, the knowledge of spectral response functions for the 13.9 and 14.2--μm bands is improved through comparison with AIRS spectra. To improve the use of the bands used in the CO₂ slicing algorithm, the difference between calculated and observed clear-sky radiances is mitigated with a radiance bias adjustment. This is necessary because there are always subtle differences between observed and calculated radiances due to instrument noise, knowledge of the atmospheric and surface state, and radiative model approximations. These aspects of the cloud property retrieval are discussed and comparisons of cloud top properties studied at 1, 5, 10, 15, and 20 kilometer resolution are presented.