Comparison of MODIS cloud microphysical properties with in-situ measurements over the Southeast Pacific

Utilizing the unique characteristics of the cloud over the Southeast Pacific (SEP) off the coast of Chile dur- ing the VOCALS field campaign, we compared satellite re- mote sensing of cloud microphysical properties against in- situ data from multi-aircraft observations, and studied the ex- tent to which these retrieved properties are sufficiently con- strained and consistent to reliably quantify the influence of aerosol loading on cloud droplet sizes. After constraining the spatial-temporal coincidence between satellite retrievals and in-situ measurements, we selected 17 non-drizzle compari- son pairs. For these cases the mean aircraft profiling times were within one hour of Terra overpasses at both projected and un-projected (actual) aircraft positions for two differ- ent averaging domains of 5 km and 25 km. Retrieved quan- tities that were averaged over a larger domain of 25 km com- pared better statistically with in-situ observations than aver- ages over a smaller domain of 5 km. Comparison at projected aircraft positions was slightly better than un-projected air- craft positions for some parameters. Overall, both MODIS- retrieved effective radius and LWP were larger but highly correlated with the in-situ measured effective radius and LWP, e.g., for averaging domains of 5 km, the biases are up to 1.75 µm and 0.02 mm whilst the correlation coefficients are about 0.87 and 0.85, respectively. The observed effec- tive radius difference between the two decreased with in- creasing cloud drop number concentration (CDNC), and in- creased with increasing cloud geometrical thickness. Com- pared to the absolute effective radius difference, the corre- lations between the relative effective radius difference and CDNC or cloud geometric thickness are weaker. For averag- ing domains of 5 km and 25 km, the correlation coefficients between MODIS-retrieved and in-situ measured CDNC are 0.91 and 0.93 with fitting slopes of 1.23 and 1.27, respec- tively. If the cloud adiabaticity is taken into account, better agreements are achieved for both averaging domains (the fit- ting slopes are 1.04 and 1.07, respectively). Our comparison and sensitivity analysis of simulated retrievals demonstrate that both cloud geometrical thickness and cloud adiabaticity are important factors in satellite retrievals of effective radius and cloud drop number concentration. The large variabili- ties in cloud geometrical thickness and adiabaticity, the de- pendencies of cloud microphysical properties on both quan- tities (as demonstrated in our sensitivity study of simulated retrievals), and the inability to accurately account for either of them in retrievals lead to some uncertainties and biases in satellite retrieved cloud effective radius, cloud liquid water path, and cloud drop number concentration. However, strong correlations between satellite retrievals and in-situ measure- ments suggest that satellite retrievals of cloud effective ra- dius, cloud liquid water path, and cloud drop number con- centration can be used to investigate aerosol indirect effects qualitatively.