The Convective Boundary Layer and the Terra Incognita

Numerical simulations of the convective boundary layer (CBL) are performed to investigate model behavior in the gray zone, also known as the 'terra incognita' (Wyngaard 2004). The gray zone of the CBL refers to a range of model grid spacing that is comparable to the size of the most energetic convective eddies, which are on the order of the boundary layer depth. Using Rayleigh-Benard thermal instability theory as a reference, it is shown that the gray zone modeling is not just a numerical challenge, but also poses dynamical difficulties. When the grid spacing falls within the CBL gray zone, grid dependent convection can occur. The size of the initial instability structures is set by the grid spacing rather than the natural state of the flow. The resulting artificial convective structures change the higher order flow statistics and pose fundamental difficulties for gray zone modeling applications. If convection in the gray zone is different from its natural state, we are then faced with the question: should we, or should we not allow convection to happen in the gray zone? This work attempts to shed some light on this issue.