Improving the behavior of the cumulus parameterization for tropical cyclone prediction: Convection trigger

Abstract

Numerical studies have consistently shown the importance of moist convection in the evolution of tropical cyclones. However, a systematic evaluation of cumulus parameterization in the simulation of tropical cyclone is rare. The objective of this study is to 1) assess the performance of various subgrid-scale cumulus parameterization schemes in the simulation of tropical cyclones. Emphasis is placed on the intensity, distribution, and character of precipitation and on the forecast location and intensity of tropical cyclones; 2) improve the behavior of one of the cumulus parameterization schemes by modifying the parameterization, the weakness of which is identified in 1). The distribution and intensity of precipitation, its partitioning into grid-resolvable and subgrid-scale portions, the location and intensity of tropical cyclone were extremely sensitive to the choice of cumulus parameterization scheme. The scheme developed by Betts and Miller [Betts, A.K., Miller, M.J., 1993. The Betts–Miller scheme. In:Emanuel, K.A., Raymond, D.J. (Eds.), The Representation of Cumulus Convection in Numerical Models. Amer. Meteor. Soc., 246 pp] reproduced most of the features of rainfall distribution over the land, it tends to overestimate the rainfall coverage and make false alarm of intense rainfall. The mass flux scheme developed by Kain and Fritsch [Kain, J.S., Fritsch, J.M., 1993. Convective parameterization for mesoscale models: the Kain–Fritsch scheme. The Representation of Cumulus Convection in Numerical Models, Meteor. Monogr., No. 46, Amer. Meteor. Soc., 165–170] gave the best simulation of tropical cyclone on the 15 km grids, while the quasi-equilibrium scheme proposed by Grell [Grell, G., 1993. Prognostic evaluation of assumptions used by cumulus parameterization. Mon. Weather Rev., 121, 764–787] tended to underestimate sub-grid scale rainfall due to its deficiency in removing instability. Finally, it is also suggested that the Kain–Fritsch scheme can be improved for the case of weak synoptic forcing. In particular, the parameterization of the convective parcel's temperature perturbation by the environmental vertical velocity in the convective trigger function seems not quite robust. To improve this deficiency for our application, the effect of moisture advection is taken into account in determining the convective parcel's temperature perturbation. Preliminary results show that, the modified scheme can eliminate reasonably well the convective instability under weak synoptic forcing, change the response of the disturbance in the lower troposphere disturbance to environmental humidity and in favor of simulation of tropical cyclone. The results of this study should be of potential use for improving the performance of operational tropical cyclone prediction.