Warming trends in Asia amplified by brown cloud solar absorption.

Atmospheric brown clouds are mostly the result of biomass burning and fossil fuel consumption. They consist of a mixture of light-absorbing and light-scattering aerosols and therefore contribute to atmospheric solar heating and surface cooling. The sum of the two climate forcing terms-the net aerosol forcing effect-is thought to be negative and may have masked as much as half of the global warming attributed to the recent rapid rise in greenhouse gases. There is, however, at least a fourfold uncertainty in the aerosol forcing effect. Atmospheric solar heating is a significant source of the uncertainty, because current estimates are largely derived from model studies. Here we use three lightweight unmanned aerial vehicles that were vertically stacked between 0.5 and 3 km over the polluted Indian Ocean. These unmanned aerial vehicles deployed miniaturized instruments measuring aerosol concentrations, soot amount and solar fluxes. During 18 flight missions the three unmanned aerial vehicles were flown with a horizontal separation of tens of metres or less and a temporal separation of less than ten seconds, which made it possible to measure the atmospheric solar heating rates directly. We found that atmospheric brown clouds enhanced lower atmospheric solar heating by about 50 per cent. Our general circulation model simulations, which take into account the recently observed widespread occurrence of vertically extended atmospheric brown clouds over the Indian Ocean and Asia, suggest that atmospheric brown clouds contribute as much as the recent increase in anthropogenic greenhouse gases to regional lower atmospheric warming trends. We propose that the combined warming trend of 0.25 K per decade may be sufficient to account for the observed retreat of the Himalayan glaciers.

Atmospheric rivers fueling the intensification of fog and haze over Indo-Gangetic Plains.

Indo-Gangetic Plains (IGP) experiences persistent and widespread rise of fog and haze during the winter season. This has been attributed to the rise in pollution levels and water vapor, but the reason for enhancement in latter is not clear yet. We detect moisture incursion from Arabian Sea, a phenomenon called atmospheric rivers (AR), land-falling intermittently along 12-25° N corridor of the west-coast of India during winter; using satellite and reanalysis data. The total vertically integrated horizontal water vapor transport in AR-landfalls ranging from 0.7 × 108 to 2.2 × 108 kg/s; nearly five-orders of magnitude larger than the average discharge of liquid water from Indus River into Arabian Sea. These AR events are playing prominent role in enhancing water vapor over IGP region by 19 ± 5%; in turn fueling the intensification of fog and haze through aerosol-water vapor interaction. We found that AR events enhanced aerosol optical depths over IGP by about 29 ± 13%. The progression of moist-laden winds in ARs onto Himalayan Mountains contributes to the precipitation that explains the observed rise in the extreme flow of western Himalayan Rivers in winter. We conclude that these ARs likely contribute to the decline of snow albedo as pollution-mixed-ARs encounter Hindukush-Karakoram-Himalayan mountain region.