Rather than confronting the formidable task of assessing the partial derivative of cloud properties in a particular place and time, we have adopted an alternative approach where satellite retrievals are used to estimate the radiative response or sensitivity to some specified change in cloud droplet numbers. Global spatial and temporal distributions of the albedo sensitivity can be used to estimate the magnitude range of future radiative flux responses, as well as provide an additional constraint for the validation of indirect effect modeling studies. In essence, our method is therefore an attempt to quantify the impact of well-posed hypothetical cloud modification scenarios, and does not address cloud albedo modifications that may have already occurred.
In this regard, the previously introduced concept of "cloud albedo susceptibility", a sensitivity parameter referring to cloud albedo changes for a 1 cm-3 change in cloud droplet number concentration under constant liquid water content conditions, is appropriate. The new work to be presented here, broadens the study of cloud susceptibility which has so far been limited to narrowband cloud albedo changes in the visible part of the spectrum. In order to address broadband albedo susceptibility, one has to go beyond changes in cloud extinction stemming from modified effective droplet sizes and take into account perturbations in the cloud droplet asymmetry parameter and single scattering albedo as well. The sensitivities and dependencies of cloud susceptibility can be studied, of course, theoretically, and while some of these results will be highlighted, the main emphasis will be placed to susceptibility studies using MODIS operational liquid water cloud retrievals in conjunction with a NASA-Goddard broadband column radiation model. The MODIS susceptibility studies are being conducted on data granule scenes (Level-2) in order to understand radiative flux modification of particular cloud types, as well as on a global scale with gridded (Level-3) MODIS cloud retrievals as input. Our global investigation shows the geographic distribution and seasonal variability of water cloud sensitivities to increased droplet number concentrations.