A parameterization of sub-grid topographical effects on solar radiation in the E3SM Land Model (version 1.0): implementation and evaluation over the Tibetan Plateau

Abstract

Abstract. Topography exerts significant influences on the incoming solar radiation at the land surface. A few stand-alone regional and global atmospheric models have included parameterizations for sub-grid topographic effects on solar radiation. However, nearly all Earth system models (ESMs) that participated in the Coupled Model Intercomparison Project (CMIP6) use a plane-parallel (PP) radiative transfer scheme that assumes that the terrain is flat. In this study, we incorporated a well-validated sub-grid topographic (TOP) parameterization in the Energy Exascale Earth System Model (E3SM) Land Model (ELM) version 1.0 to quantify the effects of sub-grid topography on solar radiation flux, including the shadow effects and multi-scattering between adjacent terrain. We studied the role of sub-grid topography by performing ELM simulations with the PP and TOP schemes over the Tibetan Plateau (TP). Additional ELM simulations were performed at multiple spatial resolutions to investigate the role of spatial scale on sub-grid topographic effects on solar radiation. The Moderate Resolution Imaging Spectroradiometer (MODIS) data was used to compare with the ELM simulations. The results show that topography has non-negligible effects on surface energy budget, snow cover, snow depth, and surface temperature over the TP. The absolute differences in surface energy fluxes for net solar radiation, latent heat flux, and sensible heat flux between TOP and PP exceed 20, 10, and 5 W m−2, respectively. The differences in land surface albedo, snow cover fraction, snow depth, and surface temperature between TOP and PP exceed 0.1, 0.1, 10 cm, and 1 K, respectively. 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. When compared to MODIS data, incorporating the sub-grid topographic effects overall reduces the biases of ELM in simulating surface energy balance, snow cover, and surface temperature, especially in the high-elevation and snow-covered regions over the TP. The inclusion of sub-grid topographic effects on solar radiation parameterization in ELM will contribute to advancing our understanding of the role of the surface topography on terrestrial processes over complex terrain.

Impacts of Sub‐Grid Topographic Representations on Surface Energy Balance and Boundary Conditions in the E3SM Land Model: A Case Study in Sierra Nevada

“…On one hand, uncertainties from the forcing data and model structure in ELM can contribute to the simulation bias. On the other hand, as discussed in Hao et al (2021), most MODIS algorithms neglect the topographic effects and thus there are some uncertainties in the generated MODIS data especially over rugged terrain. Besides, the cloud and cloud shadow, illumination-viewing geometrical effects, and instrumental errors can also affect the accuracy of MODIS data (Bair et al, 2021).…”

“…In addition, a parameterization of sub‐grid topographic effects on solar radiation has been implemented in ELM (Hao et al., 2021). This cost‐effective parameterization uses multiple linear regression methods to build the relationship between several topographic factors and sub‐grid topographic effects (Lee et al., 2011).…”

“…The topographic factors include the standard deviation of elevation, grid‐average cosine of the local solar incident angle, sky view factor, and terrain configuration factor, which are precomputed based on the 90 m SRTM data (Table 1). This parameterization has been successfully applied in the Weather Research and Forecasting (WRF), CLM4, and ELM at different spatial scales ranging from 800 m to 200 km (Hao et al., 2021; Lee, Liou, et al., 2019; Zhao et al., 2016).…”

“…How sub‐grid topographic representations in LSMs may affect the simulated surface energy balance and surface boundary conditions is unclear. Sub‐grid topography can affect the radiative transfer processes and hence influence the surface energy balance (Hao et al., 2021). The mean surface turbulent heat flux (e.g., latent heat and sensible heat fluxes) (Lyons & Halldin, 2004) and high‐order scalar (co‐)variances (i.e., the variances and covariances of scalar quantities, such as near‐surface temperature and humidity) are used in atmospheric turbulence parameterization of ESMs for land‐atmosphere coupling (Guo et al., 2015).…”

“…In addition, most existing LSMs adopt a two‐stream solar radiative transfer scheme with the plane‐parallel (PP) assumption that neglects the sub‐grid topographic effects on solar radiation (Sellers, 1985). Recently, a scheme (TOP) to account for the sub‐grid topographic effects on solar radiation has been implemented in ELM (Hao et al., 2021). The aforementioned improvements in ELM offer a great opportunity to study the sub‐grid topographic effects on land surface processes.…”