Theme Leaders: Viviek Arora and Changui Peng
Land, ocean and lake surfaces provide a lower boundary condition to the atmosphere. The exchange of turbulent fluxes of energy, water and momentum between these surfaces and the atmosphere affects the regional climate in a significant way. The partitioning of net solar radiation, that is primarily determined by surface albedo, into terrestrial radiation, latent- and sensible-heat fluxes strongly depends on surface characteristics. Near-surface temperatures, humidity and wind speeds are directly influenced by the surface fluxes of heat, water and momentum. The overall intensity and variability of synoptic weather systems simulated by an RCM is also highly dependent on the representation of surface momentum fluxes (Beljaars 1995, Jones et al. 2003). The ability of an RCM to simulate high-impact weather systems and associated extreme wind speeds is therefore directly dependent on the quality of the surface momentum fluxes. The simulation of clouds and convection are also directly linked to the quality of simulated surface heat and water fluxes. Over oceans and lakes, surface temperatures and the available energy play a primary role in surface flux partitioning, while over the land surface structural vegetation attributes are also important.
One aspect of the coupling between these various surfaces and the overlying atmosphere is that the interaction is bi-directional. Feedbacks between the two components determine their dynamic equilibrium state. However, in current regional climate models only a few land surface variables are prognostic (e.g. soil temperatures, soil moisture and snow cover) and therefore respond to changes in the overlying atmosphere. In contrast, lakes, regional oceans and the vegetation are generally treated as static (prescribed) components and, while they influence the overlying atmosphere within an RCM, they themselves are not influenced by the evolving RCM atmosphere.
This theme aims to improve the representation of surface fluxes (project 4.2.1) and feedback processes in regional climate models by modelling the land-surface vegetation (project 4.2.2), subsurface soils (project 4.2.3), lakes (project 4.2.4) and important regional oceans (project 4.2.5) as dynamic and interactive components of the regional climate system. These components are linked to the overlying atmospheric portion of an RCM through the surface-flux parameterization. This scheme will be evaluated, further developed and its links to various interactive surfaces analysed.