Aerosol-cloud-precipitation interaction during a winter time hot weather episode over Indian region using satellite observations and WRF-Chem

Abstract

Aerosol-cloud-precipitation (ACP) interaction during a wintertime hot weather event in February 2016 over India is analyzed and presented. Moderate Resolution Imaging Spectroradiometer (MODIS) datasets from the Aqua satellite are used to discuss the relationship between aerosol and cloud parameters viz. aerosol optical depth (AOD), Angstrom Exponent (AE), cloud fraction, ice and liquid cloud effective radius, cloud optical depth, cloud top temperature, and ice and liquid cloud water path. Further, the Tropical Rainfall Measuring Mission (TRMM) dataset is used for analyzing the rainfall during the dissipating phase of the heatwave episode. Prevailing meteorological conditions during pre-mature, mature and dissipating phases of the episode are analyzed using ERA-Interim datasets by considering near-surface temperature, relative humidity, and vertical integrated moisture flux convergence. Back–trajectory analysis is carried out for determining the origin of air mass and possible aerosol transport. Hot and dry westerly winds are found to be dominating during pre-mature and mature phases, and consequently, significant aerosol transport is observed. The variation of AOD is up to 1.6 during the three phases with AE being in the range 0.8-1.7, indicating the presence of both fine and coarse-mode aerosols. Clouds respond to aerosols reasonably well in the presence of favorable dynamic and thermodynamic atmospheric conditions through relevant microphysical processes. Suppression of precipitation during the pre-mature and mature phase is primarily attributed to weak aerosol-cloud interaction. A major rainfall event over a region comprising northern Odisha, eastern Jharkhand and most parts of West Bengal, and situated in the north of minimum vertical integrated moisture flux zone, is realized during the dissipating phase. This rainfall event resulted from unseasonal moisture transport from neighboring Bay of Bengal, the presence of appreciable aerosol loading and the consequent ACP interaction. The modeling studies carried out using WRF-Chem. Near-surface air temperature is found to be relatively higher during the mature and dissipating phases from the simulations without considering chemistry (named as 'NOCH' simulation), with a combination of direct and implicit indirect effect (named as 'DEF' simulation) and with indirect effect (called as 'INDEF' simulation). All three sets of simulations overestimated rainfall over some of the locations, whereas underestimation by the model is realized over most of the regions. DEF and INDEF simulated results indicate that AOD is underestimated compared to MODIS retrieved values in premature and mature phases, while overestimated during the dissipating phase. Vertically integrated cloud fraction value is found to be moderate to high during the dissipating phase over the location where precipitation occurred. Positive AOD and rainfall relationship indicates invigoration effect, which possibly enhanced the precipitation during the dissipating phase. However, the variability of cloud condensation nuclei within 600hPa from the surface does not necessarily correspond to the hydrometeor variations to impact the rainfall occurrence, amount and spatial distribution.