Abstract
A numerical transport scheme that avoids the problem of spurious generation of negative mixing ratios has been developed. The scheme is computationally simple in any number of spatial dimensions can be used with nonperiodic boundary condition and preserves shapes reasonably well. It is based on the idea of formulating a quadratically conservative finite-difference advection equation and then advecting the square root of the concentration instead of the concentration itself. The requirement of step-by-step quadratic conservation in time places restrictions on the time-differencing method. A modified Lax-Wendroff procedure is shown to be suitable under this constraint and is used here. The transport scheme has properties comparable to fourth- order centered spatial differencing combined with leapfrog time-stepping and filliing.
