Radiative effect of dust aerosols on cloud microphysics and meso-scale dynamics during monsoon breaks over Arabian sea
Introduction
Dust aerosols play an important role in global and regional climate. They have direct forcing on climate through absorption and scattering the incoming solar radiation (Kaufman et al., 2002). High concentrations of dust act as cloud condensation nuclei in water clouds (DeMott et al., 2003, Kaufman and Fraser, 1997, Levin et al., 1996) and ice nuclei in cold clouds (Isono et al., 1959; DeMott et al., 2003, Mohler et al., 2003). Subsequently, aerosols have indirect forcing on climate by modifying cloud properties through increasing cloud albedo and suppressing precipitation (Albrecht, 1989, Twomey, 1977). Indirect effects due to dust over both warm and cold clouds were shown in different studies (Li et al., 2010, Min et al., 2009). In addition to the above two effects, dust is also responsible for semi direct effects, which is associated with the changes in cloud properties due to the presence of absorbing aerosols. Highly absorbing aerosols can generate local heating that in turn changes the relative humidity and the stability of the troposphere and thereby influence cloud formation and lifetime. Therefore, they induce an increase or a reduction of cloud cover and cloud albedo, depending upon the vertical distribution of aerosols below, within or above the clouds (Koch and Del Genio, 2010). Studies on semi-direct effect of dust aerosols showed suppression in the growth of cloud thickness (Huang et al., 2006a, Huang et al., 2006b, Su et al., 2008, Yorks et al., 2009).
During Asian summer monsoon (June to September), desert dust is the most frequently occurring aerosol type over AS and Indian sub-continent, and most of it is mainly due to the long range transport from the desert regions of Africa and West Asia (Rahul et al., 2008, Prijith et al., 2013, Kaskaoutis et al., 2014a). The source and advection of dust aerosols depends up on the weather conditions and the strength of the monsoon circulation (Rahul et al., 2008, Manoj et al., 2011, Kaskaoutis et al., 2014b). The spatial and vertical variability of aerosol optical depths (AOD) over AS was studied during monsoon and found largest in the month of July (Prijith et al., 2013). Dust aerosols can change the radiative balance both at the surface and top of the atmosphere (TOA) significantly. According to global climate model simulations, dust-induced heating of the atmosphere over North Africa and West Asia rapidly modulate monsoon rainfall over central India (Vinoj et al., 2014 and references therein).
Over the marine region, the warming due to dust aerosols may perturb the monsoon circulation. There are periods during monsoon season, when there is striking reduction of rainfall over most parts of India and increase near Himalayas and parts of north east and south east peninsular India. Such periods are termed as ‘Monsoon breaks’ (Rao, 1976). Breaks are permanent features of monsoon intra seasonal oscillations. Dust induced heating during monsoon breaks may cause significant changes in local/regional scale processes in the absence of large scale processes. In the present study, satellite data from different sources has been combined to investigate the interaction of dust aerosols with clouds and radiation and their possible effect on meso-scale dynamics during monsoon breaks over AS.
Section snippets
Synoptic conditions during monsoon breaks
Indian summer monsoon is an important component in the global climate system. Strong westerlies generated by North–South temperature gradient in the surface and lower tropospheric levels bring abundant amount of moisture and cloudiness to the land, which eventually produce large amount of rainfall during monsoon season over the Indian sub-continent (Rao, 1976, Webster et al., 1998). The active and break phases of the monsoon are the manifestation of northward propagating monsoon intra-seasonal
Data & methodology
The CALIPSO mission uses crucial lidar and passive sensors to obtain unique data on aerosol, cloud vertical structure and optical properties (Winker et al., 2009, Winker et al., 2007). CALIPSO science team developed different aerosol models for different aerosol types by combining AERONET and ground based lidar observations using Lidar Ratio as main constraint. By combining these models and Cloud-Aerosol Discrimination Algorithm, the satellite retrieved backscatter data is divided into six
Analysis
The geographical area selected for the present study is 0–25° N and 60–75° E over the AS. A total of 12 monsoon break days are considered for the analysis which satisfied the criterion that the CALIPSO track is over AS, and dust is the major aerosol component present (based on the images of CALIPSO aerosol types) as compared to other aerosols in the selected region. These 12 days are not equally distributed along the years. To detect dust induced cloud modifications, the present data analysis
Vertical distribution and radiative effect of dust aerosol over AS
The information on vertical distribution of dust aerosol and the height up to which it can reach is sparse over AS and now became evident with the advent of CALIPSO. AOD along the CALIPSO tracks for the 12 cases are shown in Fig. 1a. AOD gradient is seen from north to south. Maximum AOD is observed north of 10° N and varied from 0.3 to 3. During summer monsoon, monthly mean AOD showed maximum value towards northern and western parts of AS and decreased towards south (Prijith et al., 2013).
Conclusions
Dust aerosol plays a significant role in altering the climate system through their direct, indirect and semi-direct effects. This study addresses the changes in cloud micro-macrophysics and meso-scale dynamics due to the radiative effects of dust aerosol during monsoon breaks for the period 2007 to 2013, covering the spatial domain of 0–25° N and 60–75° E over the AS. The data sets used are CALIPSO, CERES and NCEP/NCAR Reanalysis data. The main findings of the study are summarized as follows.
Acknowledgements
The authors wish to thank Director IITM and Ministry of Earth Sciences (MoES). CALIPSO, CERES and NCEP science teams for making available the data products. Authors also acknowledge D.A. Ramu and Raju Attada, IITM for useful discussions in some of the calculations.
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