Improving snow albedo modeling in the E3SM land model (version 2.0) and assessing its impacts on snow and surface fluxes over the Tibetan Plateau

[摘要] With the highest albedo of the land surface, snow plays a vitalrole in Earth's surface energy budget and water cycle. Snow albedo isprimarily controlled by snow grain properties (e.g., size and shape) andlight-absorbing particles (LAPs) such as black carbon (BC) and dust. Themixing state of LAPs in snow also has impacts on LAP-induced snow albedoreduction and surface radiative forcing (RF). However, most land surfacemodels assume that snow grain shape is spherical and LAPs are externallymixed with the snow grains. This study improves the snow radiative transfermodel in the Energy Exascale Earth System Model version 2.0 (E3SM v2.0) LandModel (ELM v2.0) by considering non-spherical snow grain shapes (i.e.,spheroid, hexagonal plate, and Koch snowflake) and internal mixing ofdust–snow, and it systematically evaluates the impacts on the surface energy budgetand water cycle over the Tibetan Plateau (TP). A series of ELM simulationswith different treatments of snow grain shape, mixing state of BC–snow anddust–snow, and sub-grid topographic effects (TOP) on solar radiation areperformed. Compared with two remote sensing snow products derived from theModerate Resolution Imaging Spectroradiometer, the control ELMsimulation (ELM_Control) with the default configurations ofspherical snow grain shape, internal mixing of BC–snow, external mixing ofdust–snow, and without TOP as well as the ELM simulation with new model features(ELM_New) can both capture the overall snow distributionreasonably. Additionally, ELM_New overall shows smallerbiases in snow cover fraction than ELM_Control in spring whensnowmelt is important for water management. The estimated LAP-induced RF inELM_New ranges from 0 to 19.3 W m −2 with thearea-weighted average value of 1.5 W m −2 that is comparable to thereported values in existing studies. The Koch snowflake shape, amongother non-spherical shapes, shows the largest difference from the sphericalshape in spring when snow processes related to the surface energy budget andwater cycle have high importance. The impacts of the mixing state of LAP insnow are smaller than the shape effects and depend on snow grain shape.Compared to external mixing, internal mixing of LAP–snow can lead to largersnow albedo reduction and snowmelt, which further affect the surface energybudget and water cycle. The individual contributions of non-spherical snowshape, mixing state of LAP–snow, and local topography impacts on the snowand surface fluxes have different signs and magnitudes, and their combinedeffects may be negative or positive due to complex and nonlinearinteractions among the factors. Overall, the changes in net solar radiationin spring due to individual and combined effects range from −28.6 to 16.9 W m −2 and −29.7 to 12.2 W m −2 , respectively. This study advancesunderstanding of the role of snow grain shape and mixing state of LAP–snowin land surface processes and offers guidance for improving snow simulationsand RF estimates in Earth system models under climate change.

[学科分类] 土木及结构工程学