Effects of Atmospheric Low-frequency Variation on Typical Persistent Heavy Rains During Pre-flood Season in Fujian

The daily precipitation data of 66 stations in Fujian and NCEP reanalysis data during 1961-2010 are used to analyze the atmospheric low-frequency (LF) variability responsible for 27 typical persistent heavy rain processes (TPHRP) in Fujian during pre-flood season. Using Butterworth band-pass filtering method, low frequency band signals of 30-60 d intraseasonal oscillation (ISO) are extracted from filtered air circulation time series grid data. A physical conceptual model about the atmospheric LF disturbances during TPHRP is suggested as follows.In the upper troposphere, LF between the Korean Peninsula and the Bohai Gulf is low and weak high system dominates in the west of the Tibet Plateau, together with the subtropical westerly jet (SWJ) core located between the Yangtze River estuary and the East China Sea, leading to the LF divergence over Fujian. These low-frequency systems are relative to the Northeast cold vortex, South Asia high and the subtropical westerly jet core, respectively, which provide high-level divergence dynamic conditions and driving dynamic conditions of vertical circulation for persistent heavy rains.In the middle troposphere, when LF disturbance pattern as double block high enhanced type, the Ural block high enhanced type, the Lake Baikal block high enhanced type, or the Okhotsk Sea block high enhanced type, LF low may maintain over Fujian showing on the weather map as two troughs and one ridge type, two ridges and one trough type, one trough and one ridge type, one ridge and one ridge type, respectively. The cold air from the middle and east influences Fujian, providing continuous cold air for persistent heavy rain processes.In the lower troposphere, there is a LF cyclone over Fujian with its center located in South China Sea (SCS) or south of the Yangtze River. The current of air from subtropical high provide continuous moisture conditions, low level convergence dynamic conditions, instability stratification conditions and maintain low-level vertical circulation dynamic conditions for persistent heavy rain processes.In vapor flux fields, whenever one of the LF disturbance strengthens in the vapor passageways as Somali to the Bay of Bengal, the western flank of the Western Pacific Subtropical High (WPSH) or the westerly transport, the water vapor flux convergence over Fujian will be enhanced, providing continuous water vapor transport. TPHRPs are finally triggered by the abnormal LF circulation configuration from lower to upper troposphere as above with favorable synoptic systems for heavy persisted rains. Because of the LF system with continuous and periodicity, the physical conceptual model provides scientific basis for the extension of the TPHRP forecast in Fujian during pre-flood season.