Reply on RC3

Abstract

With the highest albedo of the land surface, snow plays a vital role in Earth’s surface energy and water cycles. Snow albedo is greatly affected by snow grain properties (e.g., size and shape) and light absorbing particles (LAPs) such as black carbon (BC) and dust. The mixing state of LAPs in snow also has large impacts on LAP-induced snow albedo reduction and surface radiative forcing (RF). However, most land surface models assume that snow grain shape is spherical and LAPs are externally mixed with the snow grains. This study improves the snow radiative transfer model in the land model (ELM v2.0) of the Energy Exascale Earth System Model version 2.0 (E3SM v2.0) by considering non-spherical snow grain shapes (i.e., spheroid, hexagonal plate and Koch snowflake) and internal mixing of dust-snow and systematically evaluates the impacts on surface energy and water balances over the Tibetan Plateau (TP). A series of ELM simulations with different treatments of snow grain shape, mixing state of BC-snow and dust-snow, and sub-grid topographic effects (TOP) on solar radiation are performed. Compared with two remote sensing snow products derived from the Moderate Resolution Imaging Spectroradiometer, the control ELM simulation with the default settings of spherical snow grain shape, internal mixing of BC-snow, external mixing of dust-snow and without TOP can capture the overall snow distribution reasonably. The estimated LAP-induced RF ranging from 0 to 21.9 W/m2 with the area-weighted average value of 1.3 W/m2 is comparable to reported values. Focusing on the snow-related processes and surface energy and water balances, Koch snowflake shape, among other non-spherical shapes, shows the largest difference from spherical shape in spring. The impacts of the mixing state of LAP-snow are smaller than the shape effects and depend on snow grain shape. Compared to external mixing, internal mixing of LAP-snow can lead to larger snow albedo reduction and snowmelt, which further affect surface energy and water cycles. Compared to the control simulation, the individual contributions of non-spherical snow shape, mixing state of LAP-snow, and local topography to the change of snow and surface fluxes have different signs and magnitudes, and their combined effects may be negative or positive due to complex and non-linear interactions among the factors. Overall, the changes of net solar radiation in spring due to individual and combined effects range from -28.6 to 16.9 W/m2 and -29.7 to 12.2 W/m2, respectively. This study advances understanding of the role of snow grain shape and mixing state of LAP-snow in land surface processes and offers guidance for improving snow simulations and RF estimates in Earth system models under climate change.