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Atmospheric aerosols partly compensate for the warming due to greenhouse gases by perturbing the radiation balance of the Earth–Atmosphere system. In this study, the impacts of aerosols on surface temperature are examined over the Indo-Gangetic Plain (IGP) and Himalayan Tibetan Plateau (HTP), where diverse aerosol and climatic conditions prevail. Both regions have significant impacts on the regional climate and hydrological cycles in South Asia. The IGP experiences high aerosol loading throughout the year and is expected to affect surface temperature significantly. In contrast, the HTP exhibits relatively pristine or lower aerosol loading, whose effects on surface temperature are highly uncertain due to snow albedo feedback. Climate model simulations are used to decompose the surface temperature changes due to aerosol forcing to its radiative and non-radiative components over the IGP and HTP. The shortwave cooling due to aerosols is mostly compensated by the decrease in sensible heat over the IGP. On the other hand, HTP experiences surface cooling due to the direct effects and surface warming due to aerosol-induced snow-darkening effects (deposition of absorbing aerosols on snow). The net effect of aerosols on shortwave radiation is further redistributed into non-radiative heat fluxes. This study provides a better understanding of aerosol-induced surface temperature change and its partitioning into radiative and non-radiative components.

Abstract: This study assessed the drought characteristics in various districts of Assam using a standardized precipitation index (SPI). Assam has been traditionally affected by floods; however, in the last few decades, droughts have occurred due to erratic rainfall patterns. In this study, gridded rainfall data provided by India Meteorological Department for 70 years (1951–2020) was used. The SPI was calculated at a 3-month time scale (SPI03) using accumulated rainfall of a 3-month sliding window to reflect seasonal or, to some extent, agricultural drought. In Assam, the percentage of drought in August and September that represented monsoonal drought was about 15% during the course of 70 years. Additionally, Assam’s overall percentage of drought was at 14.5%, indicating that dryness will occur throughout the year regardless of the season. The most hit district in terms of the number of times was Bishwanath Chariali, and the least affected was Karbi Anglong. Furthermore, the Sivasagar district was found to be experiencing the highest number of droughts in the most intense category. The average drought in Assam lasted for roughly 5.2 months. Dhubri district, which had the lowest inter-arrival time (IAT) of drought, experienced drought spells more frequently. It is concluded that drought mitigation planning should be implemented in areas with a higher frequency of droughts, longer drought spells, and shorter IATs. The findings of this study will aid the efforts being made by various departments in proper planning, action, and management of multiple entities with regard to the state’s drought. Research highlights:

3-month SPI can be very effective in detecting agricultural drought.

The 3-month SPI for August and September month can be used to monitor drought conditions for Kharif crops.

The average inter-arrival time of drought can be used to indicate how frequently a drought spell resurfaced.

Aircraft measurements of the vertical profiles of aerosol total number concentrations and size distributions (in the size range of 0.5–20 μm) were made over seven geographically diverse locations of the Indian mainland during two contrasting seasons, winter (December 2012) and spring (April–May 2013), as a part of the regional aerosol warming experiment (RAWEX). Our observations revealed an increase in the vertical extent of aerosol loading during spring having a significant enhancement in coarse mode aerosols in the lower free-troposphere (FT) over western and central parts of India and the Indo-Gangetic plains (IGP). The particulate depolarisation ratio (PDR) derived from the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) over the same region showed the presence of dust (including polluted dust) at higher altitudes in spring. Concurrent and collocated measurements of aerosol scattering and absorption properties aboard the aircraft revealed that the FT enhancement in coarse mode aerosol loading during spring is associated with a decrease in single scattering albedo and an increase in columnar absorption aerosol optical depth. This confirms that the elevated layers of coarse mode aerosols seen during spring are absorbing in nature, especially over the IGP. The presence of such coarse-mode absorbing aerosols plays a crucial role in governing the radiation balance over the IGP in spring through the diabatic heating of the upper atmosphere.