A Study on Local‐Scale Thermal and Dynamical Mechanisms in the Initiation of a Squall Line Under Weak Forcing

Abstract

A convective system was initiated under weak forcing on the plains near Beijing on the afternoon of 26 June 2009 and developed into a squall line that resulted in heavy precipitation. Prediction of the convective initiation (CI) of the system was challenging due to the lack of precursors detectable by conventional observation networks. In this study, we investigated the CI mechanism using high‐resolution analyses obtained from the Variational Doppler Radar Analysis System via the assimilation of radar and dense surface observations. It was found that the CI was resulted from the interactions of a local cold air mass and the outflows from a weakened storm propagating from the nearby mountains. The cold air mass built up an unstable condition and veered air flow to form a local convergence zone in which moisture was accumulated. New convection was triggered as a result of strengthened convergence when the outflows from the dissipating storm approached the existing convergence zone. To confirm the critical role played by the local cold air mass, sensitivity experiments were conducted using WRF‐FDDA data assimilation and forecast system. The results suggested that the local cold air mass along with the southerly environmental wind and storm outflows exerted a dominant influence on the CI.

Comparison of Cold Pool Characteristics of Two Distinct Gust Fronts over Bohai Sea Bay in China

“…The reliability of the 4DVar model's inversion results has been verified (Chen et al [54]). These studies found that VDRAS wind and temperature analyses yielded a reasonable agreement with observations made in severe weather situations for different regions of the world (Wu et al [56]; Zhang et al [57]; Xiao et al [58]; Cui et al [7]).…”

Comparison of Cold Pool Characteristics of Two Distinct Gust Fronts over Bohai Sea Bay in China

“…The reliability of the 4DVar model's inversion results has been verified (Chen et al [51]). These studies found that VDRAS wind and temperature analyses yielded reasonable agreement with observations in severe weather situations for different regions of the world (Wu et al [53]; Zhang et al [54]; Xiao et al [55]; Cui et al [7]).…”

A Typical Weakly Forced Mountain‐To‐Plain Extreme Precipitation Event Exacerbated by Urbanization in Beijing

“…Predicting their intensity change is a challenging task for weather forecasters. Some studies analyzed the thermodynamic characteristics of these convective systems and found that the convergence of the low-level outflows of the MTP rainstorms with ambient warm and moist winds on the adjacent plains is the main reason for the MTP precipitation intensification (Xiao et al, 2017(Xiao et al, , 2022. However, these studies did not consider the key role of the urban underlying surface in the evolution of the MTP storms after they migrate to the core urban areas.…”

“…This mechanism is particularly evident in the case of unstable stratification, high convective instability energy, and strong vertical wind shear. Recently, Xiao et al (2022) dissected a squall line system under weak synoptic forcing in Beijing and further pointed out that the reinitiating convection over plains is mainly due to the interaction between local cold air mass and the outflow of weakened storms coming from mountains. Despite prior studies underscoring the convergence between MTP storm's cold outflows and ambient warm and moist winds, the impacts of land-atmosphere interactions on the MTP storms moving to urban areas are not adequately addressed.…”