Vertical Diffusion Scheme (VDS)

VDS(Vertical Diffusion Scheme) was developed to investigate the vertical CO2 transport in the PBL and to evaluate CO2 vertical rectification. The VDS was driven by the net ecosystem carbon flux and the surface sensible heat flux, simulated using the Boreal Ecosystem Productivity Simulator (BEPS) and a land surface scheme--Ecosystem-Atmosphere Simulation Scheme (EASS). These three components are linked through two prognostic variables: land surface sensible heat fluxes affecting the mixed layer development, and net ecosystem productivity (NEP) driving vertical CO2 transfer, which are calculated using EASS and BEPS, respectively, at each computing time step. The VDS is designed to simulate scalar diffusion processes in the planetary boundary layer (PBL). These processes modify the lowest 100 to 3000 m of the atmosphere, though the troposphere extends from the ground up to an average of 11 km [Stull, 1993]. The maximum top boundary height in VDS is 2520 m. Generally, over the land surface under a high-pressure weather system the PBL has a well-defined structure that evolves in a diurnal cycle [Stull, 1993]. The four major components of this structure are the surface layer, the stable boundary layer, the convective boundary layer, and the residual layer. Many researchers use second-order closure or higher-order closure methods to study/ simulate the complex diurnal evolutions of the PBL at the expense of high computation power. First-order closure is often called the gradient transport theory or well-known K-theory. Although it is one of the simplest parameterization schemes, it is only applicable in situations dominated by small-eddy. Unfortunately, it frequently fails when large eddies are present. Furthermore, in the real atmosphere, there are occasions where transport occurs against the gradient (i.e., counter gradient) [Stull, 1993]. Thus, K-theory is not applicable for use in convective mixed layers. Hence to minimize the problem, we selected different schemes to treat different situations of the PBL structure. One is a stable/nocturnal module in which K-theory is used; another is a free-convection module which is based on Estoque's principles [Esoque, 1968; Blackadar, 1976, 1978]. The criteria that determine which module is applicable, as shown in Figure 1, are the sign and magnitude of the bulk Richardson number Rb in the surface layer and the magnitude of the height of the mixed layer and the Monin-Obukhov length[Zhang and Anthes, 1982].

The VDS model is integrated with the surface fluxes calculated using coupled BEPS-EASS at 1-min computing time steps. This model includes four major components (Figure 1):

  1. the surface scaling parameterizations,
  2. convective boundary layer (CBL) sub-model,
  3. stable/nocturnal module, and
  4. free convection module.

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© Revised: Feb., 2006