Abstract ID: 3.13533 | Accepted as Talk | Talk | TBA | TBA

Mountains create and enhance their own clouds, which both scatter and absorb shortwave radiation from the sun and absorb and re-emit longwave radiation from the ground and atmosphere. However, the impacts of clouds on the surface radiation balance in high elevation, snow-covered mountain terrain are poorly quantified. Capturing these effects are among the primary challenges faced by physically based snow energy balance modeling. In this study, we use ground observations of clouds and surface radiation collected by the Surface Atmosphere Integrated Field Lab (SAIL) campaign and partner organizations in the upper elevations (2880 m.a.s.l) of the Upper Colorado River Basin (UCRB) over a 21-month period from September 2021 to June 2023 to estimate Cloud Radiative Forcing (CRF) in the shortwave, longwave, and the net effect for a single, intensively monitored site. Longwave warming dominates over snowpacks in the winter when snow albedos are high (0.8-0.9) and the background atmospheric precipitable water vapor is low (<0.5 cm), yielding a maximum monthly average net CRF of +34.7 W·m−2 in winter, meaning that clouds increase the surface net radiation relative to clear skies during this time period. Perhaps paradoxically, clouds generally increase net radiation over bright snowpacks even at solar noon. However, for a brief two-to-three week period over melting, low-albedo snowpacks (0.5-0.6) impacted by dust impurities, CRF switches sign, and clouds reduce the net radiation available for melt production relative to clear skies. In the summer over non-snow covered ground, CRF reaches a minimum monthly average -47.6 W·m−2 with hourly minima of -600 W·m−2. Sensitivity tests elucidate the role of the surface albedo on the net CRF. The results suggest that net CRF will both increase in magnitude and lead to a more persistent cooling effect on the net radiation budget as snow cover declines across mountains.