A Modified Double-Moment Bulk Microphysics Scheme Geared toward the East Asian Monsoon Region

Representation of cloud microphysical processes is one of the key aspects of numerical models.An improved double-moment bulk cloud microphysics scheme(named IMY)was created based on the standard Milbrandt-Yau(MY)scheme in the Weather Research and Forecasting(WRF)model for the East Asian monsoon region(EAMR).In the IMY scheme,the shape parameters of raindrops,snow particles,and cloud droplet size distributions are variables instead of fixed constants.Specifically,the shape parameters of raindrop and snow size distributions are diagnosed from their respective shape-slope relationships.The shape parameter for the cloud droplet size distribution depends on the total cloud droplet number concentration.In addition,a series of minor improvements involving detailed cloud processes have also been incorporated.The improved scheme was coupled into the WRF model and tested on two heavy rainfall cases over the EAMR.The IMY scheme is shown to reproduce the overall spatial distribution of rainfall and its temporal evolution,evidenced by comparing the modeled results with surface gauge observations.The simulations also successfully capture the cloud features by using satellite and ground-based radar observations as a reference.The IMY has yielded simulation results on the case studies that were comparable,and in ways superior to MY,indicating that the improved scheme shows promise.Although the simulations demonstrated a positive performance evaluation for the IMY scheme,continued experiments are required to further validate the scheme with different weather events.

Changes in Tropical Cyclone Number in the Western North Pacific in a Warming Environment as Implied by Classical Thermodynamics

Observational analyses show that the equatorial trough in the western North Pacific (WNP) is a well-known origin for tropical cyclones (TC) which have tended to weaken in intensity and decrease in number during the last several decades under global warming. A scientific problem then arises as to why higher sea surface temperatures (SSTs), one of the necessary conditions for typhoon genesis, can cause a weakened equatorial trough and a decreased TC number. In this paper, the WNP is taken as an example to illustrate a possible mechanism for the above-mentioned seemingly counterintuitive phenomena and explain the causality between the unusually heterogeneous pattern of SSTs in a warming environment and TC number in the WNP. This mechanism is based substantially on the second law of thermodynamics.