Hybrid Mass Flux Cumulus Scheme (HYMACS) as a Step to Unified Cumulus Parameterization and Its Application to Tropical Cyclone Intensity Prediction

In this study, a hybrid mass flux cumulus scheme (HYMACS) adapted from the Kain-Fritsch scheme (KFS) is investigated with the weather research and forecasting model (WRF) in terms of the dependence on horizontal grid resolution and the effect on simulated tropical cyclone intensity. The aim is to reduce the resolution dependence of cumulus parameterization (CP) schemes between 3 km and 20 km, the so called gray zone, and to lessen the tendency of the KFS to overestimate the intensity of tropical cyclones in the gray zone.

The HYMACS solve convective drafts implicitly but compensating motions explicitly by introducing a subgrid-scale mass source/sink term to the grid-scale continuity equation in a fully compressible model. In contrast, the KFS assume local compensation of convective mass fluxes. The HYMACS is compared with the KFS in mass lifting experiments with prescribed mass and heat source/sink of a subgrid-scale convective draft in a horizontal resolution of 3, 9 and 27 km. Of the simulations with different resolution, the column air mass decrease is 6 times on average more significant with the KFS than the HYMACS. On the other hand, the column air mass decrease difference between 3 km and 27 km resolution is 7 times larger with the KFS than the HYMACS.

To look into the effect of the HYMACS on tropical cyclone intensity, simulations of an idealized tropical cyclone on an f-plane with constant sea surface temperature are conducted in those horizontal resolution settings with the HYMACS, the KFS and an explicit scheme without any CP. Using the explicit scheme as reference, the preliminary results suggest that the HYMACS reduces the overestimation of intensity by 10 hPa in terms of central sea level pressure in the gray zone (9 km horizontal resolution) as compared to that with the KFS.

Keywords: hybrid mass flux cumulus scheme, tropical cyclone intensity prediction