Aerosol-cloud interaction inferred from MODIS satellite data and global aerosol models

摘要

We have used the MODIS satellite data and two global aerosol models toinvestigate the relationships between aerosol optical depth (AOD) and cloudparameters that may be affected by the aerosol concentration. Therelationships that are studied are mainly between AOD, on the one hand, andcloud cover, cloud liquid water path, and water vapour, on the other.Additionally, cloud droplet effective radius, cloud optical depth, cloud toppressure and aerosol Ångstr?m exponent, have been analysed in a fewcases. In the MODIS data we found, as in earlier studies, an enhancement inthe cloud cover with increasing AOD. We find it likely that most of thestrong increase in cloud cover with AOD, at least for AOD<0.2, is a resultof aerosol-cloud interactions and a prolonged cloud lifetime. Large andmesoscale weather systemsseem not to be a cause for the increase in cloud cover with AOD in thisrange. Sensitivity simulations show that when water uptake of theaerosols is not taken into account in the models the modelled cloudcover mostly decreases with AOD. Part of therelationship found in the MODIS data for AOD>0.2 can be explained bylarger water uptake close to the clouds since relative humidity is higher inregions with higher cloud cover. The efficiency of the hygroscopic growthdepends on aerosol type, the hygroscopic nature of the aerosol, the relativehumidity, and to some extent the cloud screening. By analysing theÅngstr?m exponent we find that the hygroscopic growth of the aerosolis not likely to be a main contributor to the cloud cover increase with AOD.Since the largest increase in cloud cover with AOD is for low AOD (~0.2) and thus also for low cloud cover, we argue that cloud contaminationis not likelyto play a large role. However, interpretation of the complex relationshipsbetween AOD and cloud parameters should be made with great care and furtherwork is clearly needed.