Sensitivity of cloud radiative forcing to changes of microphysical parameters measured by the CLOUDS mission

CLOUDS (a Cloud and Radiation monitoring satellite) is a study for a satellite mission designed to provide the gross vertical profile, the internal structure, the radiative and the imaging features of clouds. This subject is addressed by several missions designed for process study intent. CLOUDS, instead, is designed for providing data of routine use in long-term Numerical Weather Prediction (NWP) and General Circulation Model (GCM). User requirements have been collected from various sources, and instruments concepts derived to meet those requirements. However, to establish the sensitivity of a GCM to the targeted parameters and confirm the soundness of the specified requirements (mainly accuracy and vertical resolution), special effort had to be placed. The present paper offers a rather complete assessment of the range of usefulness that CLOUDS measurements may have on the radiative calculation. To this purpose, the cloud forcing was computed as a function of cloud parameters by using a radiative model that has been applied in the GCM of the Laboratory for Atmospheres at the NASA Goddard Space Flight Center. The results show that, in most cases, the model response to the addressed cloud parameters is good if the error is within the specified limit. This is better demonstrated for relatively large particle sizes, for ice better than for liquid water, for low optical thickness and for low cloud cover. The model, however, suggests that more stringent requirements would be appropriate when small particles are considered.