Broadband cloud susceptibility inferred from MODIS

Studies investigating the modification of cloud microphysical and optical properties by anthropogenic aerosol have become more numerous in recent years, in no small part because of the availability of new ground- and space-based observational systems, as well as the development of models that attempt to account for aerosol sources and transport. The radiative impact of altered cloud fields, one of the focal points of indirect effect climate change research (i.e., the Twomey effect), is addressed in this presentation. While modeling investigations that elucidate the physical mechanisms and processes that ultimately determine the nature of the cloud radiative modification are potentially useful for advanced cloud model parameterizations, satellite observations have a unique role to play. Unfortunately, the observational challenges in determining an actual change are many, and originate mainly from the difficulties in trying to quantify a “partial derivative”. These challenges include the need to identify and then quantify the background (pre-modified) state of the cloud, the relationship between column aerosol optical depth and CCN concentrations at cloud level, and cloud-dependent dynamic/thermodynamic changes that coexist and can correlate with changes in aerosol amounts.

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.