Dust aerosol responsible for snowmelt at higher elevation in western Himalayan Region

While black carbon has a large effect on snow darkening and resultant melting of snow, dust particles transported from as far as Saudi Arabia that gets deposited in the Western Himalayan Region has a large role to play in melting of snow, particularly at higher elevations.

While black carbon has a large effect on snow darkening and resultant melting of snow, dust particles transported from as far as Saudi Arabia that gets deposited in the Western Himalayan Region — Hindu Kush, Karakoram and western Himalaya ranges — has a large role to play in melting of snow, particularly at higher elevations, a study published in the journal Nature Climate Change has found.

This is the first time the role of long-distance transported dust in elevational heterogeneity of snow melting in the Himalayas has been made, says Dr. Chandan Sarangi from the Department of Civil Engineering at IIT Madras, who is one of the corresponding authors of the paper. Dr. Yun Qian from the the Pacific Northwest National Laboratory (PNNL), Richland, Washington, is the other corresponding author of the paper. The work was done when in collaboration with PNNL, University of Colorado, Boulder and University of California, Los Angeles.  

While dust transported as elevated aerosol layers get deposited at 1-5 km elevation, black carbon emission is mostly a surface phenomenon and influences melting of snow from surface to about 3 km elevation. Based on remote sensing data of spatial distribution of dust aerosol concentration over the Indian subcontinent and dust-induced snow albedo reduction over Himalayas during the period 2011-2016 (March-August) and simulations, the authors show that the relative impacts of dust and black carbon vary with surface elevation of snowpack. This is in addition to snowmelt caused by warming due to climate change.

Earlier studies have shown that the magnitude of snow mass decrease is about 1 mm per year at 1 km elevation, about 5 mm per year at 4.5 km elevation and about 3 mm per year at 6 km elevation.

Though black carbon has a larger snow albedo darkening effect than dust due to a larger mass absorption efficiency, the study found that the radiative effects of dust deposited on snow are comparable to black carbon in the Western Himalayan Region at higher elevations. This is mainly because the deposition of dust by mass is 100-1,000 times more than black carbon.

“As the elevation increases, the influence of dust becomes greater than black carbon, and this coincides with maximum intensity of snowmelt reduction seen at 3-5 km elevation. Between these two, black carbon mainly contributes to snowmelt at lower elevation while dust is the major contributor for snowmelt at higher elevation,” he says.

Westerlies transport dust particles as elevated aerosol layers at maximum intensities mostly during the pre-monsoon period (March to May) and this gets deposited at higher elevations in the western Himalayan region.  

“Due to global warming, snow cover at lower elevations in the Himalayas will occur less frequently or totally disappear compared with snow cover at higher elevations. The annual contribution of dust to snowmelt will therefore likely increase in the future as the black carbon effect at lower elevation weakens with dwindling snowpack,” they write. “Snowmelt is the main source of water for many major Himalayan rivers. Dust deposition during the pre-monsoon period causes early snowmelt water in the Himalayan rivers and a reduction of snowmelt water during peak summer months,” Dr. Sarangi says.

Published in The Hindu on October 6, 2020