Dust Induced Radiative effect on Marine Clouds during Monsoon Breaks

Indian Summer Monsoon is an important component in global climate system. The active and break phases of the monsoon are the manifestation of northward propagating monsoon intra-seasonal oscillations. During monsoon breaks, dry air laden with dust aerosols intrude over central India through Arabian Sea (AS) from the desert regions of north and northwestern continents. Earlier studies over Indian sub-continent showed that the elevated dust layers can cause significant warming in the free troposphere. Such warming over the marine region may perturb the monsoon circulation. To understand the effect of dust aerosols on marine clouds over AS during monsoon breaks, Cloud-Aerosol Lidar with Orthogonal polarization (CALIPSO) and Cloud and the Earth's Radiant Energy System (CERES) data has been analyzed for 12 monsoon break days during the period 2006 to 2013. These cases are selected based on the criterion that the CALIPSO track is over AS, dust is present up to or more than 5 km and 80% of dust is present apart from other aerosol types in the selected study region (10-25 ºN and 55-75 ºE ). The present data analysis is segregated into two types representing different environments such as clouds with dust (CWD) and clouds without dust (CWOD). In the first type, clouds contaminated with dust or presence of dust above the clouds is considered. While in the second type, clouds far away from the dusty regions representing pristine are considered.

During monsoon breaks, the results depicted maximum aerosol optical depth (AOD) above 10 º N and varied from 0.3 to 1.5. The vertical profile of dust backscatter coefficient (DBC) over AS showed an elevated layer between 2 to 4.5 km having maximum probability of occurrence around 3.5 – 4 km. The heating rate profile computed from SBDART radiative transfer model by using the CALIPSO derived vertical profile of dust AOD, showed a maximum of 8 K/Day, where DBC maximum is observed. The vertical distribution of dust aerosols is consistent with the direction of air-mass back trajectories indicating that the elevated layer is a result of intrusion of dry desert air from north western countries.

Short wave flux (SWF) at top of the atmosphere (TOA) is found to be less with dust as compared to without dust events, also the difference is more in case of cold clouds as compared to warm clouds. At the same time long wave flux (LWF) at TOA is more for clouds present in dusty atmosphere. In case of cloud micro and macrophysical parameters also significant differences are observed with and without dust. The percentage difference is observed to be much higher for cold clouds as compared to warm clouds. The results also indicate that semi-direct effect is more pronounced in cold clouds as compared to warm clouds. Such changes could affect the dynamical as well as circulation patterns in the immediate meso-scale environment during monsoon breaks.