Impacts of a revised surface roughness parameterization in the Community Land Model 5.1

The roughness of the land surface (z(0)) is a key property, exerting significant influence on the amount of near-surface turbulent activity and consequently the turbulent exchange of energy, water, momentum, and chemical species between the land and the atmosphere. Variations in z(0) are substantial across different types of land cover, ranging from typically less than 1 mm over fresh snow or sand deserts up to more than 1 m over urban areas or forests. In this study, we revise the parameterizations and parameter choices related to z(0) in the Community Land Model 5.1 (CLM), the land component of the Community Earth System Model (CESM). We propose a number modifications for z(0) in CLM, guided by observational data. Most importantly, we find that the observations support an increase in z(0) for all types of forests and a decrease in the momentum z(0) for bare soil, snow, glaciers, and crops. We then assess the effect of those modifications in land-only and land-atmosphere coupled simulations. With the revised parameterizations, diurnal variations of the land surface temperature (LST) are dampened in forested regions and are amplified over warm deserts. These changes mitigate model biases compared to MODerate resolution Imaging Spectroradiometer (MODIS) remote sensing observations. The changes in LST are generally stronger during the day than at night. For example, the LST increases by 5.1 K at 13:30 local solar time but only by 0.6 K at 01:30 during boreal summer across the entire Sahara. The induced changes in the diurnal variability of nearsurface air temperatures are generally of the opposite sign and of smaller magnitude. Near-surface winds accelerate in areas where the momentum z(0) was lowered, such as the Sahara, the Middle East, and Antarctica, and decelerate in regions with forests. Overall, this study finds that the current representation of z(0) in CLM is not in agreement with observational constraints for several types of land cover. The proposed model modifications are shown to considerably alter the simulated climate in terms of temperatures and wind speed at the land surface.

Automated summary

The roughness of the land surface is an important factor in the exchange of energy and other variables between land and atmosphere. This study proposes modifications to the parameterizations of land roughness in a land component of a climate model. The modifications improve the model's representation of land surface temperature and wind speed, bringing them into better agreement with observations.