Central Asian Precipitation Extremes Affected by an Intraseasonal Planetary Wave Pattern

Abstract

Abstract The characteristics and mechanisms of extreme precipitation events over central Asia are investigated based on daily data and percentile criteria. The composited extreme precipitation events have a life cycle of about 10 days. The precipitation signal is weak but significant from eight to two days before the peak. Then it amplifies rapidly and reaches its peak in two days. The earliest precipitation signal appears over the northwest of central Asia and moves southeastward. It maximizes over northern central Asia and the windward side of the mountainous region, suggesting the role of orography in the precipitation extremes. Diagnosis suggests that the precipitation extremes are closely tied to a quasi-stationary planetary wave train emanating from the North Atlantic and the resultant disturbed Asian subtropical jet. The sharpened and accelerated Atlantic jet 10 days before the precipitation peak feeds a cyclonic anomaly at its exit region, which disperses energy downstream and forms a well-defined barotropic Rossby wave train in the Asian subtropical jet, resulting in an anomalous cyclone to the west of central Asia. On the one hand, the cyclone-induced water vapor transport into central Asia enhances the moisture content and increases the convective instability in the lower troposphere, providing a favorable thermodynamical condition for deep convections. On the other hand, the vorticity and temperature advection induced by this cyclone and the mechanical lifting on the windward side of mountains provide a favorable dynamical condition for ascending motion, which triggers the release of convective instability and the occurrence of precipitation extremes in central Asia.