A fast linear semi-Lagrangian advection scheme coupled with spectral (bin) microphysics to simulate an idealized super cell storm in WRF

A new, computationally efficient semi-Lagrangian advection (SLA) scheme was used to simulate an idealized supercell storm using WRF coupled with spectral (bin) microphysics (SBM). SLA was developed to make complicated microphysical schemes more computationally accessible to cloud-resolving models. The SLA is a linear combination of semi-Lagrangian schemes of the first and the second order. It has relatively low numerical diffusion, a high level of mass conservation accuracy, and preserves the sum of multiple advected variables. In addition to idealized tests, comparisons were made with standard WRF higher-order, nonlinear advection schemes. Tests of the SLA were performed using different values of weighting coefficients gamma for the combination of the first- and second-order components. The results of SLA on grids of 1 km, 500 m, and 250 m agree well with those of the standard WRF advection schemes, with results most similar to simulations with 250-m grid spacing. At the same time, the advection CPU time required by the SLA was 2.2-3 times shorter than the WRF advection schemes. The speed-up occurred in part because of the utilization of the same advection matrix for the advection of all hydrometeor mass bins. The findings of this work support the hypothesis that cloud microphysical simulation is more sensitive to the choice of microphysics than to the choice of advection schemes, thereby justifying the use of computationally efficient lower-order linear schemes.