Special issue |
Interactions between aerosols and the South West Asian monsoon
Editor(s): M. K. Dubey, D. Spracklen, A. Sorooshian, J. Srinivasan, and B. V. Krishna MurthyMore information
This special issue will include papers that guide the path forward in making reliable predictions of aerosol effects on the heat budget of the South East Asian subcontinent by improving our knowledge of aerosol processes and their influence on the Indian monsoon. Several recent major studies have aimed to provide a detailed determination of aerosol physical and chemical properties across India prior to and during the Indian monsoon, and results from these studies will be welcomed. Most recently the South West Asian Monsoon Interactions project used a combination of UK and Indian research aircraft and a network of ground-based measurement sites. The analysis of data will enable assessment of aerosol composition and mixing state, provide source characterization, and help quantify aerosol optical properties such as extinction, absorption, and single scattering albedo that drive regional climate. Results that use this detailed characterization to test representations of aerosol properties in regional and global climate models will be included. The special issue will report on evaluations of and improvements in model representations of aerosol properties with data over India.
Preprint under review for ACP(discussion: open, 1 comment)
Vertical distributions of atmospheric aerosols and the ability of aerosols to form clouds has been studied across the Indo-Gangetic Plain based on air-borne measurements carried out during the SWAAMI field campaign studying the Indian monsoon. The ability of the aerosols to act as cloud forming nuclei showed contrasting features, increasing with increase in altitude, prior to the onset of monsoon, reversing the trend to decrease with increase in altitude during the active phase of the monsoon.
We have estimated the aerosol radiative forcing (ARF) by employing the assimilated, gridded aerosol datasets over the Indian region. The present ARF estimates are more accurate and certain than those estimated using the currently available, latest satellite-retrieved aerosol products. Therefore, the present ARF estimates and corresponding assimilated aerosol products emerge as potential candidates for improving the aerosol climate impact assessment at regional, subregional and seasonal scales.
Extensive airborne measurements of aerosol number–size distribution and black carbon (BC) profiles are carried out for the first time across the IGP prior to the onset of the Indian summer monsoon. These measurements, combined with spaceborne sensors and model results, provided an east–west transect of the role of mineral dust (local and transported) in the aerosol loading across the IGP, with an increase in coarse mode concentration and coarse mode mass fraction with altitude.
Continuous aerosol and cloud condensation nuclei (CCN) measurements carried out at the ground observational facility of a CAIPEEX campaign, situated in the rain-shadow region of the Indian subcontinent, are illustrated. The variations in CCN characteristics within the monsoon period are investigated along with the aerosol physical and optical properties. The change in the dependency of CCN activity on aerosol size and composition due to the variations in air mass and meteorology is brought out.
This study explores the variability of the Asian summer monsoon anticyclone (ASMA) spatial variability and trends using long-term observational and reanalysis data sets. The decadal variability of the anticyclone is very large at the edges compared with the core region. We propose that the transport process over the Tibetan Plateau and the Indian region is significant in active monsoon, strong monsoon and strong La Niña years. Thus, different phases of the monsoon are important in UTLS analyses.
The study reports the observation of highly absorbing aerosol layers at high altitudes (1–2.5 km) prior to monsoon and during its development over the Indian region and quantifies its climate impacts. The absorption of solar radiation in these layers perturbs the onset of monsoon through the impact on the atmospheric stability. When height-resolved values of single scattering albedo (SSA) are used in a radiative transfer model, a maximum heating ~1 K d (~twice that using SSA) is obtained.
Extensive measurements of the aerosol and cloud condensation nuclei (CCN) properties in South Asian outflow to the northern Indian Ocean were carried out as a part of the ICARB-2018 experiment during winter. At high supersaturations, most of the aerosols in the South Asian outflow become activated as CCN, whereas the aerosol system over the equatorial Indian Ocean is less CCN efficient even at higher supersaturations.
Concurrent measurements of the altitude profiles of the concentration of cloud condensation nuclei (CCNs), as a function of supersaturation (ranging from 0.2 % to 1.0 %), and aerosol optical properties were carried out aboard an instrumented aircraft across the Indo-Gangetic Plain (IGP) just prior to the onset of the 2016 Indian summer monsoon (ISM). A high CCN concentration is observed up to 2.5 km across the IGP, indicating the significant possibility of aerosol indirect effects.
Our study presents an analysis of the vertical and horizontal black carbon properties across northern India using aircraft measurements. The Indo-Gangetic Plain saw the greatest black carbon mass concentrations during the pre-monsoon season. Two black carbon modes were recorded: a small black carbon mode (traffic emissions) in the north-west and a moderately coated mode (solid-fuel emissions) in the Indo-Gangetic Plain. In the vertical profile, absorption properties increase with height.
The present study investigates the plausible aspects which influence the probability of drought occurrences over three Indian regions during the southwest Asian mid-monsoon period. The investigation reveals that an increasing tendency of dry day frequency (DDF) over urbanized regions in the last few decades has significant association with the abundance of anthropogenic aerosols. Additionally, future projections of DDF indicate a five-fold rise which can be a crucial concern for policy makers.
Light-absorbing particles impact the Earth system in a variety of ways. They can warm the atmosphere by their very presence, or they can warm the atmosphere after they deposit on snow, warm it, and warm the overlying atmosphere. This paper focuses on these two processes as they pertain to black carbon and dust's impacts on the South Asian monsoon. It will be shown that these two aerosols have a significant effect on the monsoon.
We have developed quality-enhanced, gridded datasets for aerosol optical depth (AOD) and absorption AOD by assimilating highly accurate measurements from the dense network of ground-based stations, with respective satellite-retrieved datasets. The assimilated datasets demonstrate improved accuracy and reduced uncertainties as compared to respective satellite products. Thus, these assimilated products emerge as important tools to improve the accuracy of climate impact assessment of aerosols.
Recent studies have shown that simulation of monsoons can be improved with an exact representation of SST–precipitation relationship. The vertical structure of precipitation with SST is distinctly different over the Arabian Sea than over the Bay of Bengal. The reflectivity profiles show variation with SST over the Arabian Sea and do not show considerable variation with SST over the Bay of Bengal. The variations in reflectivity profiles seem to originate at the cloud formation stage itself.
We show substantial improvements in the near-surface BC mass concentrations simulated by a regional chemistry transport model, WRF-Chem, over the Indian region, upon scaling up the CMIP5 equivalent anthropogenic BC emissions by 3 and introducing a diurnal variation to those. The diurnality in emissions alone significantly controls the simulated near-surface BC mass concentration, with a mean delay of 3–4 h. The simulated AOD, however, is still underestimated.
James Brooks, James D. Allan, Paul I. Williams, Dantong Liu, Cathryn Fox, Jim Haywood, Justin M. Langridge, Ellie J. Highwood, Sobhan K. Kompalli, Debbie O'Sullivan, Suresh S. Babu, Sreedharan K. Satheesh, Andrew G. Turner, and Hugh Coe
Our study, for the first time, presents measurements of aerosol chemical composition and physical characteristics across northern India in the pre-monsoon and monsoon seasons of 2016 using the FAAM BAe-146 UK research aircraft. Across northern India, an elevated aerosol layer dominated by sulfate aerosol exists that diminishes with monsoon arrival. The Indo-Gangetic Plain (IGP) boundary layer is dominated by organics, whereas outside the IGP sulfate dominates with increased scattering aerosol.
Aircraft-based measurements of vertical profiles of aerosol properties, across the Indo-Gangetic Plain (IGP), prior to onset of the Indian summer monsoon reveal a highly absorbing aerosol system over the IGP. Aerosols over the west IGP are mostly natural, larger in size and scatter light efficiently. Those over the central and eastern IGP are mostly anthropogenic in origin, smaller in size and absorb more light. Elevated absorbing aerosol layers may modulate regional precipitation patterns.