Examining the atmospheric radiative and snow-darkening effects of black carbon and dust across the Rocky Mountains of the United States using WRF-Chem

Abstract

Abstract. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) is run to quantify the in-snow and atmospheric radiative effects of black carbon (BC) and dust on a convective-allowing (4 km) grid for water year 2009 across a large area of the Rocky Mountains. The snow-darkening effect (SDE) due to the deposition of these light-absorbing particles (LAPs) on surface snow enhances snowmelt by 3 to 12 mm during late spring and early summer, effectuating surface runoff increases (decreases) prior to (after) June. Meanwhile, aerosol–radiation interactions (ARIs) associated with LAPs generally dim the surface from incoming solar energy, introducing an energy deficit at the surface and leading to snowpack preservation by 1 to 5 mm. Surface runoff alterations brought forth by LAP ARI are of opposite phase to those associated with LAP SDEs, and the BC SDE drives a majority of the surface energy and hydrological perturbations. More generally, changes in snow water equivalent (SWE) brought forth by LAP effects are more a result of perturbations to the surface energy budget rather than changes in precipitation amount or type. It is also found that perturbations to the surface energy budget by dust ARI can differ in sign from those of BC ARI, with the former being positive, enhancing snow melting, and changing runoff.

Black carbon dominated dust in recent radiative forcing on Rocky Mountain snowpacks

“…These were based on 2009 simulations across a large area of the Rocky Mountains using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) on a 4 km grid. [37] found that BC dominated perturbations to the snow surface energy balance compared to dust for 2009 when integrated over mountainous terrain by more than 2 W m −2 . Consistent with our results, [37] found dust concentrations in snow decreased northward, unlike BC concentrations in snow, due to the southern Rockies proximity to dust emission sources.…”

“…[37] found that BC dominated perturbations to the snow surface energy balance compared to dust for 2009 when integrated over mountainous terrain by more than 2 W m −2 . Consistent with our results, [37] found dust concentrations in snow decreased northward, unlike BC concentrations in snow, due to the southern Rockies proximity to dust emission sources. The WRF-Chem modeling concluded that BC radiative forcing dominated dust radiative forcing, leading to BC-related snow darkening which lead to earlier snowmelt, earlier snow disappearance, and earlier associated streamflow across the Rocky Mountains [37].…”

“…LAP concentrations at isolated locations may not accurately represent larger regions, however, or the associated regional radiative forcing [21,34,35]. To place the point snowpack measurements in larger spatial perspective, we also compared the snowpack measurements with modelled BC and dust values in snow previously reported by [37]. These were based on 2009 simulations across a large area of the Rocky Mountains using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) on a 4 km grid.…”

“…Recent understanding of BC and dust concentrations in western snowpacks are based on few and spatially limited snow measurements often from areas of relatively high atmospheric dustiness [27,34,35]. Moreover, BC and dust rarely have been measured concurrently in western seasonal snow so climate model simulations of the effects of LAP in snow on radiative forcing, snowmelt, and hydroclimate [36,37] generally have been evaluated against sparse measurements of either dust or BC snow concentrations, but seldom both [11]. To reduce snow radiative forcing and preserve valuable snow-water resources in the semi-arid western United States, findings from these studies have been used to promote reductions in activities that lead to increased dust emissions including over-grazing and off-road vehicle use in sensitive areas such as the Colorado Plateau [21].…”

A Quantitative Model‐Based Assessment of Stony Desert Landscape Evolution in the Hami Basin, China: Implications for Plio‐Pleistocene Dust Production in Eastern Asia

“…The increase 10.1029/2020GL090064 in total downwelling radiation over the Hami Basin but decrease in the surrounding regions implies that the elevated downwelling SW radiation produced via changes to albedo overwhelms the reduction of downwelling SW radiation by enhanced dust production. This feature likely arises due to the effect of the albedo and increased aerosol production working to iteratively scatter SW radiation between the surface and the dust layer (Rahimi et al, 2020). As such, the feedback between evolving surface albedo and dust production may be just as vital to altering SW radiation as aerosol emission variability through changes in roughness and erodibility alone.…”