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Quasi-Biweekly Mode and Its Modulation on the Diurnal Rainfall in Taiwan Forecasted by the CFS

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  • 1 Utah Climate Center, and Department of Plants, Soils, and Climate, Utah State University, Logan, Utah
  • | 2 Central Weather Bureau, Taipei, Taiwan
  • | 3 Utah Climate Center, and Department of Plants, Soils, and Climate, Utah State University, Logan, Utah
  • | 4 Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, Los Angeles, California
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Abstract

The occurrence of diurnal afternoon convection in Taiwan undergoes substantial modulation from tropical intraseasonal oscillations in the western North 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, Taiwan's diurnal convection activity tends to intensify and persists for about 4–7 days. Synoptically, this situation reflects the enhanced subtropical anticyclone leading to fair weather conditions and increased monsoon southwesterly winds moistening the lower troposphere, all of which are conducive to thermally induced diurnal convection in Taiwan. 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 Taiwan.

Current affiliation: WeatherRisk Explore Inc., Taipei, Taiwan.

Corresponding author address: Shih-Yu Wang, Utah Climate Center, Utah State University, 4825 Old Main Hill, Logan, UT 84322. E-mail: simon.wang@usu.edu

Abstract

The occurrence of diurnal afternoon convection in Taiwan undergoes substantial modulation from tropical intraseasonal oscillations in the western North 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, Taiwan's diurnal convection activity tends to intensify and persists for about 4–7 days. Synoptically, this situation reflects the enhanced subtropical anticyclone leading to fair weather conditions and increased monsoon southwesterly winds moistening the lower troposphere, all of which are conducive to thermally induced diurnal convection in Taiwan. 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 Taiwan.

Current affiliation: WeatherRisk Explore Inc., Taipei, Taiwan.

Corresponding author address: Shih-Yu Wang, Utah Climate Center, Utah State University, 4825 Old Main Hill, Logan, UT 84322. E-mail: simon.wang@usu.edu
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