Quasi-biweekly mode and its modulation on the diurnal rainfall in taiwan forecasted by the CFS

Abstract

The occurrence of diurnal afternoon convection in Taiwan undergoes substantial modulation from tropical intraseasonal oscillations in the westernNorth Pacific, including the quasi-biweekly (QBW)mode. By analyzing surface station observations and the Climate Forecast System (CFS) Reanalyses (CFSR), as well as the NCEP CFS version 2 (CFSv2) reforecast data over 18 summers from 1993 to 2010, it was found that the QBW mode plays a significant role in the formation of episodic diurnal convection. When the cyclonic circulation of the QBW mode is located west of Taiwan, followed by an anticyclonic circulation to the east, Taiwanrs diurnal convection activity tends to intensify and persists for about 4-7 days. Synoptically, this situation reflects the enhanced subtropical anticyclone leading to fairweather conditions and increasedmonsoon southwesterlywinds moistening the lower troposphere, all ofwhich are conducive to thermally induced diurnal convection inTaiwan. The opposite situation tends to suppress the diurnal convection activity for a sustained period of time. Based upon this synoptic linkage, an empirical relationship between the precipitation diurnal amplitude and low-level circulation fields of the CFSv2 is derived. It was found that the CFSv2 forecast exhibits an effective lead time ranging from 16 to 24 days for the QBW mode and, subsequently, diurnal convection episodes in Taiwans. © 2013 American Meteorological Society.