Stable boundary layers, cold extremes and the role of surface coupling

Understanding stably stratified boundary layers over land is of importance for the prediction of nocturnal frost extremes and fog events. With increasing stability, the role of turbulent fluxes diminishes in the surface heat budget, while the relative importance of the radiative contributions and the surface heat transport increase. Indeed, it is known from model intercomparison studies like GABLS that the ability of models to model cold extremes is very sensitive role of surface coupling: models with different formulations and/or surface-coupling parameters show a large variety of surface flux and temperature predictions.

Here, therefore we dig into the process of atmosphere-vegetation-soil heat exchange, with a focus on cold extremes over short grass. First, novel observational results are presented. Detailed temperature profiles over the atmosphere-vegetation-soil continuum were obtained at the Cabauw national Observatory. Coiled distributed temperature sensing with optical fibers allowed us to probe the vertical temperature profiles with mm to cm resolution, ranging from a few meters above the ground to a couple of decimeters into to the soil.

The observed temperature dynamics are mimicked with various conceptual models to describe the heat transfer through the atmosphere-grass system. It is shown that traditional skin-layer type of formulations are unable to describe the temperature dynamics in a realistic manner. In the future, we therefore need to find more realistic, and yet simple alternatives to mimic natural temperature dynamics near the ground under stratified conditions. Potential alternatives and their limitations will be discussed, as to explore the road to future model implementations to improve the prediction of cold extremes over short grass.