A CLASS OF THE VANLEER-TYPE TRANSPORT SCHEMES AND ITS APPLICATION TO THE MOISTURE TRANSPORT IN A GENERAL-CIRCULATION MODEL

A generalized form of the second-order van Leer transport scheme is de rived. Several constraints to the implied subgrid linear distribution are discussed. A very simple positive-definite scheme can be derived d irectly from the generalized form. A monotonic version of the scheme i s applied to the Goddard Laboratory for Atmospheres (GLA) general circ ulation model (GCM) for the moisture transport calculations, replacing the original fourth-order center-differencing scheme. Comparisons wit h the original scheme are made in idealized tests as well as in a summ er climate simulation using the full GLA GCM. A distinct advantage of the monotonic transport scheme is its ability to transport sharp gradi ents without producing spurious oscillations and unphysical negative m ixing ratio. Within the context of low-resolution climate simulations, the aforementioned characteristics are demonstrated to be very benefi cial in regions where cumulus convection is active. The model-produced precipitation pattern using the new transport scheme is more coherent ly organized both in time and in space, and correlates better with obs ervations. The side effect of the filling algorithm used in conjunctio n with the original scheme is also discussed, in the context of ideali zed tests. The major weakness of the proposed transport scheme with a local monotonic constraint is its substantial implicit diffusion at lo w resolution. Alternative constraints are discussed to counter this pr oblem.