study of the impact of aerosols on microphysics of clouds over tehran

. introduction not only have clouds and precipitation (snow, rain and hail) made up an important part of the subsystem atmospheric phenomena of terrestrial climate, but are also considered as the main components of the water cycle. aerosols play an important role in cloudiness and precipitation. in warm clouds with the same liquid water path, increase in aerosols concentration results in the reduction of effective radius of cloud droplets and increase of cloud albedo, which is referred to as “first indirect effect” or “twomey effects” (twomey, 1977;twomey et al., 1984). variation in concentration of aerosols causes variation in the thickness, life period and precipitation rate of the clouds, referred to as “second indirect effect” or “albrecht effect”, (albrecht, 1989; hansen et al. 1997; ackerman et al. 2000). the city of tehran, the capital of i. r. of iran, is a mega-polis with a population of more than 10 million. the city is located in the central part of the alborz mountain range and characterized by its complex topography and diverse climate in an area of about 700 km2. the city's long term annual average precipitation is 300mm. the growth of the city during the last 30 years has turned the city to one of the most polluted cities in the country, particularly during autumn and winter. the geographical conditions of the city are the other most important factors affecting the city's environmental condition and the nature of the particulate matters that may be found in its atmosphere. in this paper, the influence of aerosols on microphysics of clouds over tehran during the years 2003 to 2012 has been investigated. 2. material and methods in this paper, the influence of aerosols on microphysics of clouds over tehran throughout the years 2003 to 2012 was studied. correlations between microphysics of clouds and aerosols from the moderate resolution imaging spectroradiometer (modis) were examined over tehran. these are both from the collection 5.1 level 3 daily data, at 10×10 resolution. daily aod, cer, cot, cwp, ctt and ctp observed by modis on board the aqua satellite from 2003 to 2012 on 10×10 grids are used in this work. in this study, aerosol index (ai) and aerosol optical depth (aod) were used as a proxy for the cloud condensation nuclei (ccn). to reduce errors, water phase cloud is defined as having a cloud top pressure greater than 800hpa. 3. results and discussion pearson and spearman correlation coefficients were calculated and written for all cloud microphysical quantities with aod as well as ai by software spss-v13. for the study, scatter plot diagrams between cer, cwp, cf, ctt, ctp and cot with ai and aod were drawn. furthermore, scatter plot diagrams between cloud microphysics quantities were drawn. we used the ai and aod as our aerosol products, as ai includes size dependence and has been shown to correlate better with ccn. pearson and spearman correlation coefficients and scatter plot diagrams showed that there are positive correlation coefficients between the aerosol index and cloud top temperature and cloud top pressure. however, there are negative correlation coefficients between aerosol index and cloud fraction, cloud optical thickness and cloud water path. there are no significant negative correlation coefficients between cloud effective radius and ai over tehran. these correlations suggest the increase of aerosols in tehran in the last 10 years, because of over seeding. in these 10 years, there was reduced cloudiness over tehran. the correlations between quantities of cloud microphysics fully match with theoretical studies. 4. conclusion at first step the scatter plots of cer, cwp, cf (cloud fraction), ctt, ctp and cot versus ai and aod, which were considered as the representative of aerosols in our study, were created to provide an early picture of the possible interrelationship between those cloud microphysics and aerosols characteristics. the same diagrams were plotted to provide a picture of the way microphysical characteristics of clouds vary with each other. study on the relationship between the aod and ai with ccn showed that ai is a better representative of ccn .then, correlations between clouds microphysical properties and both aod and ai were examined by means of pearson and spearman correlation coefficients which were calculated usingspss-v13. the results showed that there are positive correlation between the ai and ctt and ctp. however, there are negative correlation between aerosol index and cf, cot and cwp. negative but insignificant correlation was observed between cer and ai over tehran. the correlations suggest the increase in aerosols concentration over tehran during the duration of study might be regarded as one of the main reasons for the reduced cloudiness and precipitation over tehran. the correlations between quantities of cloud microphysics fully match the theoretical studies.