Analyzing the Connection among Aerosol Optical Depth, Precipitation and Temperature in Southern India

Abstract

Aerosols are fine suspended solid or liquid particles in the atmosphere. Anthropogenic aerosols from industrialization and urbanization are one of the prominent parameters in the climate change process. Aerosol Optical Depth (AOD) is a quantitative estimate of the amount of aerosol present in the atmosphere. The rise in concentration of aerosols in the atmosphere has affected the clear sky’s visibility in most of the metropolitan cities. The presence of aerosols has direct and indirect effects on the climate system. The direct effect of AOD was to influence temperature through the absorption or scattering of radiation. The radiative energy balance within the atmosphere of the Earth is highly affected by the presence of aerosols as they are capable of scattering and absorbing the incoming solar radiation. The indirect effect of AOD is on the formation of cloud condensation nuclei which in turn affects the precipitation of a region. The current research studied the impact of AOD on precipitation and temperature patterns in the study area along with the quantification of parameters that resulted in Extreme Precipitation Events (EPEs). The AOD data from satellite and ground observations are the main source in studying the effect of aerosol on spatial patterns of precipitation and temperature in Southern India. The suitability of satellite data was evaluated by validating various satellite observations with the ground data. The continuous AOD data was validated against ground observations at Pune and Kanpur statistically and it was observed that Moderate Resolution Spectrometer (MODIS) was closely correlated with ground observations. The coefficient of determination is 0.58 for Pune station and 0.7 for Kanpur station. Trend in AOD was assessed for two decades over Southern India using Mann Kendall test and Innovative Trend Analysis method. Both the tests showed a decrease in area of significant trend for second decade when compared to first decade. It was observed that 67% and 40% of area showed a significant positive trend in first- and second-decades using Mann-Kendall test. On the contrary, 42% and 30% of area showed a significant positive trend in first and second decades using Innovative Trend Analysis method. The impact of AOD on temperature in the study area was assessed by using Aerosol Direct Radiative Forcing (ADRF) method. It is a measurement utilized to comprehend the impact of cooling or warming up of the atmosphere. The present research examined the impact of aerosols during the COVID-19 pandemic by comparing them to the average from the preceding five-year period (2015-2019). The study was carried out on three distinct time frames: prior to lockdown, during lockdown, and post lockdown. It was observed that the ADRF increased in the pre-lockdown period of 2020 compared against the average of 2015-2019, and the other time scales experienced an increase in ADRF. However, a drop in temperature was noticed v prior to lockdown period when compared to the other two-time frames. During the pre lockdown period, the most notable rise in ADRF and decrease in temperature occurred in the tropical savanna and warm semi-arid climate regions. During lockdown, the increase in ADRF was seen throughout the study area, and a decrease in temperature was observed only in the tropical monsoon region. In the post-lockdown period, the decline in ADRF was accompanied by a fall in temperature in the tropical savanna region. The indirect effect of aerosols was assessed by correlation analysis among AOD, CF and precipitation for south west monsoon from 2005 to 2019 under different atmospheric stability states (K-index) over Southern India. The analysis was performed for light, moderate and heavy rainfall regimes. The low warm clouds were analysed based on cloud top pressure and cloud top temperature data. The impact of atmospheric stability on CF was greater than that on AOD. The results revealed a positive relationship between AOD and CF. This might be due to the presence of dispersive aerosols. The effect of atmospheric stability on development of clouds was evident for isolated thunderstorm state when K is between 20-25. The effect of AOD on CF was significant for 20<K<25 and 25<K<30. For a better understanding on the influence of AOD on precipitation an exclusive study on metropolitan cities was performed in the current research work. The influence of AOD in causing the EPEs was quantified by analyzing the remotely sensed data of aerosol, cloud, and K-index and precipitation for Peninsular India's major cities. The last 10 days remotely sensed data before the occurrence of EPE’s event was analyzed. The combination of factors influencing precipitation in each city were analyzed by regression analysis. All the observed EPEs had intensified precipitation due to a combination of middle level clouds (CTP in the range of 440 hPa to 680 hPa), low AOD (0.4), with many thunderstorm states (K>35oC), and CTT 0oC. Other elements that contributed to the occurrence of EPE included low level clouds (CTP>680 hPa), an AOD in the range of 0.4 0.6, an isolated thunderstorm state (20oC<K<25oC), and CTT>0oC. It was concluded that low AOD and moderate CF leads to precipitation under favorable cloud top properties.