Abstract
Abstract. Monsoonal rainfall is the primary source of surface water in India. Using 12 years of in situ and satellite observations, we examined the association of aerosol loading with cloud fraction, cloud top pressure, cloud top temperature, and daily surface rainfall over the Indian summer monsoon region (ISMR). Our results showed positive correlations between aerosol loading and cloud properties as well as rainfall. A decrease in outgoing longwave radiation and an increase in reflected shortwave radiation at the top of the atmosphere with an increase in aerosol loading further indicates a possible seminal role of aerosols in the deepening of cloud systems. Significant perturbation in liquid- and ice-phase microphysics was also evident over the ISMR. For the polluted cases, delay in the onset of collision–coalescence processes and an enhancement in the condensation efficiency allows for more condensate mass to be lifted up to the mixed colder phases. This results in the higher mass concentration of larger-sized ice-phase hydrometeors and, therefore, implies that the delayed rain processes eventually lead to more surface rainfall. A numerical simulation of a typical rainfall event case over the ISMR using a spectral bin microphysical scheme coupled with the Weather Research Forecasting (WRF-SBM) model was also performed. Simulated microphysics also illustrated that the initial suppression of warm rain coupled with an increase in updraft velocity under high aerosol loading leads to enhanced super-cooled liquid droplets above freezing level and ice-phase hydrometeors, resulting in increased accumulated surface rainfall. Thus, both observational and numerical analysis suggest that high aerosol loading may induce cloud invigoration, thereby increasing surface rainfall over the ISMR. While the meteorological variability influences the strength of the observed positive association, our results suggest that the persistent aerosol-associated deepening of cloud systems and an intensification of surface rain amounts was applicable to all the meteorological sub-regimes over the ISMR. Hence, we believe that these results provide a step forward in our ability to address aerosol–cloud–rainfall associations based on satellite observations over the ISMR.
Highlights
Aerosol–cloud–rainfall interactions and their feedbacks pose one of the largest uncertainties in understanding and estimating anthropogenic contribution of aerosols to climate forcing (Forster et al, 2007; Lohmann and Feichter, 2005)
We found that the mean relative humidity in the lower troposphere remains high over the Indian summer monsoon region (ISMR) during moderateand heavy-rainfall events (RF > 6 mm) using the Global Data Assimilation System (GDAS) data
An important finding is that the Moderate Resolution Imaging Spectroradiometer (MODIS)-retrieved cloud properties (CF, cloud top pressure (CTP), cloud top temperature (CTT)), India Meteorological Department (IMD) in situ surface-accumulated rainfall, and Tropical Rainfall Measuring Mission (TRMM)-retrieved precipitation rate illustrated a positive association with increasing aerosol loading
Summary
Aerosol–cloud–rainfall interactions and their feedbacks pose one of the largest uncertainties in understanding and estimating anthropogenic contribution of aerosols to climate forcing (Forster et al, 2007; Lohmann and Feichter, 2005). The present of higher concentrations of absorbing aerosols over northern India is shown to induce a stronger north– south temperature difference, which fosters an enhancement in moisture convergence from the ocean and a transition of a break spell of the Indian summer monsoon (ISM) into an active spell of ISM (Manoj et al, 2011) This aerosol radiative effect causes an increase in the moist static energy, the invigoration of convection, and eventually more rainfall over India during the following active phase (Hazra et al, 2013; Manoj et al, 2011). This comprehensive effort to understand aerosol– cloud–rainfall interactions over India will likely illustrate the significance of aerosols’ impact on monsoonal rainfall via the microphysical pathway under continental conditions
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