Analysis of an Extreme Short-Duration Rainstorm under Background of Weak Synoptic Forcing in Hangzhou
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Abstract:
On the morning of August 26, 2017, an extreme convective heavy rainfall event occurred in the main urban area of Hangzhou, with a maximum hourly precipitation rate reaching 127.0 mm, breaking the historical record for hourly rainfall in the region. Both mesoscale regional models and global large-scale numerical models failed to forecast this extreme rainfall event. This study utilises conventional observational data, high-resolution surface observations, weather radar data, and NCEP/GFS 0.25°×0.25° 3-hourly analysis fields to investigate the synoptic background, physical parameter fields, and the development mechanisms of meso- and micro-scale systems during this event. The main findings are as follows. (1) Synoptic Background and Large-Scale Conditions: The rainfall event occurred under the influence of the western Pacific subtropical high (WPSH). At the 500 hPa level, Hangzhou was located near the ridge line of the WPSH, while in the lower troposphere and at the surface, it was situated within a saddle-shaped pressure field. This configuration resulted in weak steering flows, and the saddle-shaped structure restricted the movement of weather systems, allowing slow-moving or quasi-stationary mesoscale convective systems (MCSs) to persist, which was highly favourable for heavy rainfall. (2) Favourable Thermodynamic and Moisture Conditions: The environmental conditions exhibited strong convective instability, with a low lifting condensation level (LCL) and a deep warm cloud layer, enhancing precipitation efficiency. The boundary layer was nearly saturated, and extreme values were observed in parameters such as precipitable water (PWAT) and surface dew point temperature. Additionally, weak vertical wind shear in the environment contributed to high precipitation efficiency by allowing convective updrafts to remain vertically aligned without significant disruption. (3) Boundary Layer Convergence and Dynamic Lifting Mechanisms: The northeasterly flow at the base of a surface high-pressure system intensified and was obstructed by local topography, leading to cyclonic convergence and wind speed convergence in the boundary layer over Hangzhou and northern Shaoxing. A convergence line formed 3 hours before precipitation initiation and persisted for an extended period, providing sustained dynamic lifting for convective triggering and development. (4) Low-Level Jet and Moisture Transport: A boundary-layer easterly ultra-low-level jet (ULLJ) combined with southeasterly warm and moist flows at 850 hPa provided continuous moisture transport and energy supply, sustaining the convection and contributing to the extreme rainfall intensity. (5) Quasi-Stationary MCS: The generation, merging, and intersection of convective cells, along with the parallel but opposite directions of advection and propagation, caused the MCS to remain nearly stationary. This led to multiple heavy rainfall cores overlapping in the same region, which was a crucial factor in the extreme precipitation accumulation.
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Supported by:Beijing E-Tiller Technology Development Co., Ltd.
Supported by:Beijing E-Tiller Technology Development Co., Ltd.