| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 96 === Aerosol plays an important role in the radiation field of the earth-atmosphere system, but not much attention was paid to a similar effect on cirrus clouds. In fact, the role of aerosols in the formation of ice clouds is still an open issue (Seifert et al., 2004). Li et al. (2005) compared the observed data by Microwave Lamb Sounder (MLS) with ECMWF reanalysis data and found that the sampled ECMWF ice water content (IWC) values are significantly smaller than the MLS estimates over nearly all the tropical landmasses. We presume that is mainly due to the fact that ECMWF analysis does not consider the aerosol effect on the formation of cloud ice.
In this study, we examined two cases: 2007.4.25 and 2007.5.11 in Indochina, using the Fifth-Generation NCAR/PSU Mesoscale Model (MM5), with implementation of the CLR cloud microphysical scheme. Sensitivity tests were conducted using average background aerosol by multiplying the number concentration by ten times to discuss aerosol influence on cloud ice. Model results were compared with CloudSat data. The hypothesis to be examined is that increasing aerosol in the boundary layer affects deep convection by increasing cloud drop number concentration and reflectance, as well as the lifetime of the cloud; smaller cloud drop with lower collision efficiency suppresses the rainfall formation, allowing more cloud drops to reach the upper level and freeze into ice particles via homogeneous nucleation, which then influence the subsequent mixed-phase processes.
The simulated results indicate that using 10 times the aerosol number concentration doubles the number concentration and reduces the size of cloud drops. Aerosols may also affect the dynamic of the convection system, resulting in changes in the updraft velocity and cloud top temperature, which in turn affects cloud ice number concentration. Furthermore, the growth of ice particles via the Wagner-Bergeron- Findeisen process is also enhanced due to faster evaporation of the smaller cloud drops. However, smaller cloud drops restrict the riming of snow and graupel. In these two cases, more aerosol number concentration increase the cloud optical depth and the albedo.
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