A Conceptual Model for a Generalized Canopy Parameterization for Mesoscale Models

Abstract:

The canopy layer is the lowest part of the planetary boundary layer (PBL). It ranges from the surface to the top of obstacles (trees, buildings, etc.) and acts as the interface between processes in the boundary layer influenced by weather and surfaces as well as plants, human-made structures, soil, and humans. Forest canopy and urban canopy are two vertically most extended canopies and therefore can have a large influence on PBL processes. Although these two canopies differ in some aspects, e.g. form, structure, and function, they are not as different as current parameterizations suggest. These days, the greening of cities is one of the objectives for mitigating climate change effects. For urban climate scenario simulations, it would be desirable to treat both canopies in a unified way. Based on that, we explored the possibility of developing a generalized scheme that unifies urban and forest canopy parameterizations and developed a conceptual model for the generalized canopy parameterization.

To achieve our goal we firstly reviewed existing urban and forest parameterizations. Similarities between the parameterization of these two canopy types have been identified. For example, aerodynamic effects of the wind speed reduction and the turbulence intensity enhancement are considered in both urban and forest canopy parameterizations. Thermodynamically, both parameterizations represent the absorption, trapping, and shadowing effects of radiation. They evolved from slab models via single layer models to multilayer models. However, while urban canopy parameterizations consider the anthropogenic emission and increased surface runoff, forest canopy parameterizations consider the process of photosynthesis and uptake of water by plants and roots. Based on the literature review a conceptual model for a generalized canopy parameterization was developed that systematizes the relevant variables, processes, canopy morphology and canopy effects, and includes the external environment components interacting with the canopy system, e.g. atmosphere and pedosphere. The conceptual model of this study will serve as a design outline for a parameterization to be used in numerical models of the forested urban canopy and enables a more general and abstract consideration of modeling future cities.