A 10-yr Climatology of Tibetan Plateau Vortices with NCEP Climate Forecast System Reanalysis

A plateau vortex refers to a shallow meso--scale cyclonic vortex that is usually confined to near-surface levels (500 hPa) over the Tibetan Plateau during warm seasons. It is the major precipitation-producing weather system over the plateau, but the knowledge of its climatology and understanding of generation mechanisms are limited because of the lack of adequate observations in this harsh mountainous region. In this study, the high-resolution NCEP Climate Forecast System Reanalysis data have been used to perform a statistical survey of these vortices over 10 warm seasons (April-October of 2000-09). The purpose is to document their climatological features, including genesis, size, life cycle, propagation, and diurnal variation.Results show that similar to 103 plateau vortices occur on average every year. Most are detected from May through August, with the maximum monthly count in July. The primary area of origin exhibits a west-east orientation in correspondence with a large-scale confluence zone, and the most concentrated source lies in the area of 33 degrees-36 degrees N, 84 degrees-90 degrees E in the high elevated central and western plateau. Significant diurnal variations are observed, characteristic of a preferential genesis during late afternoon to evening hours and a late night dissipation peak. The vortex events have an average life span of similar to 15 h and an average horizontal dimension (effective diameter) of similar to 280 km. In accordance with the steering environmental flow, an overwhelming majority travel eastward with a mean translation speed of similar to 10 m s(-1). A small fraction of systems (approximately nine cases annually) move off the plateau, predominantly from the eastern edge.