Convection initiation (CI) associated with a cold pool gust front (GF) of a mesoscale convective system (MCS) occurred in the Bohai Bay region, North China, is investigated using a real‐data, high‐resolution Weather Research and Forecasting model (WRF) simulation. Observations revealed that many CIs occurred successively near the GF, most of which developed significantly in both size and intensity and eventually merged with the MCS. The WRF simulation captured the general features of the CIs, GF, and MCS. Lagrangian vertical momentum budgets were conducted along the backward trajectories of air parcels within a convective cell initiated near the GF. The total vertical acceleration was decomposed into dynamic and buoyant components. The results showed that the dynamic acceleration played a decisive role for the CI, which can be further decomposed into five terms. The horizontal curvature term was rather small, with the height variation of air density almost zero. Although both vertical and horizontal extension terms played a positive role during the CI process, the vertical twisting term had the greatest contribution to the CI. This reflects the vertical tilting (to the forward of the GF) of an elevated updraft which was generated by the convergence of a forward‐tilting updraft at the GF leading edge and another airflow coming from ahead of the GF.

(a) Full map of China including the location of Tianjin; (b) the terrain height (m) of Tianjin with the urban boundary (red line); (c) photograph of Tianjin city.

An isolated convection was unexpectedly initiated in the evening of 1 August 2019 around the Tianjin urban region (TUR), which happened at some distance from the shear line at lower level and the preexisting convection to the South, analyzed by using ERA5 reanalysis data and observations from surface weather stations, and a S-band radar. The results show that, 42 min before the initiation of the convection, the atmospheric thermodynamic conditions around TUR were favorable for the initiation of the isolated convection, although the southerly and vertical shear of the horizontal wind at the lower level was weak. A sea-breeze front approached the TUR and continued to move West, leading to the triggering of the isolated convection in the context of the urban humid heat island (UHHI) effect. Subsequently, the gust front, which was formed between the cold pool away from the TUR and the warm and humid air of the UHHI, moved northward, approached the convection, and collided with sea breeze front, resulting in five reflectivity centers of isolated convection being merged and the convection’s development. Finally, the isolated convection split into two convections that moved away from the TUR and disappeared at 20:36 Beijing Time. The isolated convection was initiated and developed by the interaction of the sea breeze front and gust front in the context of the UHHI effect. The sea breeze front triggered the isolated convection around TUR in the context of the UHHI effect, and the gust front produced by the early convective storms to the south played a vital role in the development of the isolated convection.

To investigate the characteristics of Bohai High, the daily precipitation data of Beijng-Tianjin region during June to September from 2006 to 2015, national basic automatic weather stations data of 5 minute, NCEP/NCAR 1°×1° reanalysis data as well as CINRAD/SA data of Tianjin, from the definition of and the distribution characteristics of precipitation in Beijng-Tianjin region, the concept of regional precipitation reduction in summer in the context of Bohai High was given. This was done by classifying large-scale synoptic circulation and synthetic diagnosis method. The results show that: (1) Bohai High has the most significant effect of abnormal weaken precipitation in the eastern of Tianjin, while has a relatively mild effect in the northern of Tianjin. (2) According to the characteristics of large-scale circulation situation, the synoptic processes over heavy rain are classified into three patterns: upper-level trough pattern (I), cold vortex pattern (II) and the interaction between subtropical high and western trough (III). Except that water vapor come from the Bohai Sea and the Yellow Sea for Type II, for the other two patterns the water vapor were related to remote supplies from the South China Sea and the East China Sea. The Bohai High of Type Iand III were thick warm negative vorticity system whereas the Type II was a shallow negative vorticity system. (3) The abnormally weakened of Bohai High were related to their depths and duration, the deeper and the longer duration of Bohai High, the stronger weakening effect on the precipitation in Tianjin.