2022-06-23 All-Hands Presentation Meeting Notes

Shortwave radiation scheme quantitatively describes the interaction between light and land surface. Nearly all Earth system models (ESMs) that participated in the Coupled Model Intercomparison Project (CMIP6) adopt simple plane-parallel (PP) two-stream schemes to represent the shortwave radiative transfer processes over vegetation and snow. However, the PP schemes assume that the terrain is flat and neglects the sub-grid topographic effects on solar radiation. The snow radiative transfer models assume that snow grain shape is spherical and light-absorbing particles (LAPs, e.g., black carbon and dust) are externally mixed with the snow grains.

In this study, we incorporated a well-validated sub-grid topographic (TOP) parameterization in the Energy Exascale Earth System Model (E3SM) Land Model (ELM). Using the Tibetan Plateau as a testbed, we found that topography has non-negligible effects on surface energy budget and snow cover. The magnitude of the sub-grid topographic effects is dependent on seasons and elevations and is also sensitive to the spatial scales. Although the sub-grid topographic effects on solar radiation are larger with more spatial details at finer spatial scales, they cannot be simply neglected at coarse spatial scales.

We also improved the snow radiative transfer model in ELM by considering non-spherical snow grain shapes (i.e., spheroid, hexagonal plate and Koch snowflake) and internal mixing of dust-snow. We found that Koch snowflake shape, among other non-spherical shapes, shows the largest difference from spherical shape in spring. 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. All the non-spherical snow shape, mixing state of LAP-snow, and local topography contribute to the change of snow and surface fluxes, and they have non-linear interactions. Our study highlights the importance of realistically representing shortwave radiative transfer processes in land surface models.