A Study on Clouds, Precipitation and Radiation Response to Aerosols Over Different Parts of Indian Region

Atmospheric aerosols are one of the most complex and uncertain constituents due to their significance in governing weather and climate over a particular region besides health impacts. Cloud droplets form in the presence of aerosol particles acting as CCN (Cloud Condensation Nuclei). Aerosol concentration may influence the droplets' number in clouds, associated radiation fluxes, and consequent precipitation. The study presented in this thesis considers satellite-based observations of aerosols and clouds combined with other datasets and WRF-Chem outputs to examine how aerosols influence cloud properties, radiation, and precipitation over different parts of the Indian region. Several aerosols, clouds, and meteorological parameters considered for the study. The primary variables considered for the study include aerosol optical depth (AOD), angstrom exponent (AE), cloud effective radius (CER), cloud fraction (CF), cloud optical depth (COD), cloud top pressure (CTP), cloud water path (CWP), relative humidity (RH), lower-tropospheric stability (LTS), vertical velocity (VV), wind speed (WS), shortwave cloud radiative forcing (SWCRF), longwave cloud radiative forcing (LWCRF), net cloud radiative forcing (NetCRF) and rainfall. In the pre-monsoon months, AOD was found to be moderate to high over Indo-Gangetic Plains (IGP) and coastal regions and low over central, western, and southern parts of the Indian region. Low to significant values of AOD was seen over some parts of IGP and north-eastern Indian region during post-monsoon and winter, indicating the dominance of anthropogenic production and biomass and fossil fuel burning. The trend analysis implies a significant increase in anthropogenic aerosols over the Indian region during all the seasons..

Aerosol processes perturb cloud trends over Bay of Bengal: observational evidence

Abstract Although, the aerosol-cloud interactions and its impact on daily to seasonal radiation/temperature has been well observed over South Asia in last two decade, the role of aerosol-cloud interactions on cloud occurrence trends (and surface temperature) is yet not evident. Here, evidence of aerosol-induced control on cloud occurrence trends over the Northern Bay of Bengal (NBOB) during the monsoon onset period is presented. In last 15 years, increased aerosol emissions over North India have led to an increase in aerosol loading at an elevated altitude of 1–3 km over the NBOB outflow region in monsoon onset period. This elevated aerosol loading induces increases the air temperature at 1–2 km altitude and stabilizes the lower troposphere over the region in recent years. The enhanced atmospheric stability in the region caused low-level cloud occurrences (below 3 km) to increase in recent years by ~20%, potentially contributing to the observed non-intuitive cooling trends in sea surface temperatures. These aerosol-cloud-climate observations emphasize the crucial need for improved aerosol representations in coupled ocean-atmosphere models for accurate predictions of climate change over South Asia

Sensitivity analysis of convective and PBL parameterization schemes for Luban and Titli tropical cyclones

Tropical cyclones (TCs) are the most distractive natural weather phenomena and cause extensive damage and socioeconomic loss over the North Indian Ocean (NIO) region. Convection and planetary boundary layer (PBL) system play a vital role in the origin and strengthening of the TCs. The various convective and PBL parameterization schemes are available in the statistical model, which integrates these processes. The efficient incorporation of these schemes is vital to enhance the performance of the numerical weather prediction (NWP) model. In the present study, twelve experiments have been designed to carry out the numerical simulations using Advance Research Weather Research and Forecasting (ARW) model. The behavior and performance of the schemes have been evaluated to verify the instantaneous forecast of the TCs. The simulated cyclone track, which is assessed with the Indian Meteorological Department (IMD) best track data, indicates that the vector displacement error and RMSE for the experiment MWBM and YWBM are 0.4 for MLBM and up to 60 mm in Luban. However, it is > 0.6 for YLBM and up to 40 mm for Titli. Based on the results and keeping the cyclone track, intensity, and rainfall, the BMJ convective scheme with the YSU and MYJ PBL has better predicting skills over the NIO region. The KF scheme has better skills in the prediction of TC intensity

Assessing the aerosols, clouds and their relationship over the northern Bay of Bengal using a global climate model

Abstract Comprehensive evaluation of aerosol-cloud interactions (ACI) simulated by climate models using observations is crucial for advancing model development. Here, we use Moderate Resolution Imaging Spectroradiometer (MODIS) data to evaluate aerosol and cloud properties obtained from Community Atmosphere Model 5 (CAM5) over northern Bay of Bengal during winter season. We conduct simulations for default model setup as well as for prescribed, and nudged meteorology using Goddard Earth Observing System (GEOS5) reanalysis dataset in order to study the impact of meteorological parameters on simulated ACI. CAM5 captures the spatial variability of cloud optical depth (τc), cloud droplet number concentration (Nc), and liquid water path (LWP), although the values are overestimated in the model. Default model strongly simulates observed negative cloud effective radius (re)-Nc susceptibility but fails to reproduce the observed positive LWP-Nc susceptibility possibly due to evaporative cooling of the large number of smaller droplets. Compared to default and prescribed meteorology simulations, nudging specific humidity (Q) at 6-hr relaxation time scale leads to a positive LWP-Nc susceptibility, and an overall improved simulation of aerosol indirect effects. Increasing the relaxation time scale beyond 6-hr degrade the simulation of indirect effects suggesting high sensitivity of indirect effects to Q and serious deficiencies in Q simulated by the model. Improvement in simulation of aerosol and cloud characteristics are also noted when winds (UV) are nudged but it worsens some of the simulated ACI sensitivities due to increased transport of absorbing aerosols over the study region and a dominant semi-direct effect in the model