Multi-scale characteristics of moisture transport during a rainstorm process in North China

A rainstorm process that occurred in North China from July 24-25, 2011 was accurately simulated using the Weather Research and Forecasting model, and the multi-scale characteristics of moisture transport were studied based on the simulated results. The results indicated that water vapor was carried to North China mainly by the southwest low-level jet and easterly flow, with the former playing a principal role. The enhancement and northward extension of the southwesterly wind were consistent with the increase of magnitude and northward propulsion of the moisture flux. The variation of the winds mirrored fluctuations in the amount of precipitation. In addition, the water vapor from low latitudes to North China was transported first near the boundary layer over 15°N-21°N and then primarily at 850 hPa over 21°N-30°N, 900 hPa over 30°N-39°N, and 800 hPa over the region north of 39°N. The net budget of water vapor in North China was always positive during the rainstorm process because the zonal deficit was much smaller than the meridional surplus. The contribution of the water vapor advection was larger than that of the water vapor convergence in the prior period of rainfall, and the subsequent moisture aggregation relied on the water vapor convergence. The rainband in North China presented frontal mesoscale characteristics, and the short-term aggregation of moisture was closely related to the genesis and development of the mesoscale convective system that was triggered mainly by the cold air intrusion near the boundary layer. The underlying cold air not only lifted the warm air to trigger the convection, but it also influenced the development of the low pressure system in the lower levels, which further intensified the convergence and benefited the rapid accumulation of moisture to the convective zone near the boundary layer. The moisture transport reached its maximum an hour before the rainstorm occurred.