Processes of Colliding Cold Pools Derived from a 356-m High Met-Tower during an Extremely Heavy Rainfall Event Associated with a Squall Line

Convectively generated cold pools are important mesoscale structures. These cold pools are key for convection initiation and development, but observations of their vertical structure are insufficient. In this study, quantitative evolutions of the vertical structure of cold pools during a high-impact heavy rainfall event near the south coast of China were examined using observations on a 356-m high Shenzhen Met-Tower, and their possible impacts on heavy rainfall were further discussed with the high-resolution surface station network and radar observations. On 11 April 2019, precipitation was significantly enhanced near metropolitan Shenzhen and lasted for 50 min at its southern downtown, resulting in 11 deaths.

During this event, a shallow cold pool was first observed with its “head” structure by the Met-Tower and yielded a long-lasting weak near-surface cooling of 2.6 K. Approximately one hour later, another deeper cold pool accompanied bya squall line was added from the west. This addition led to a more abrupt and intense surface temperature deficit (-5.1 K) and stronger gusty winds (23 m/s) at the Met-Tower. When the two cold pools collided near Shenzhen, the low-level wind converged at their intersection, dynamically enhancing the heavy-rain-producing squall line. Furthermore, the collision of the two cold pools decreased the temperature gradient at the northern leading edge of the merged cold pool, which could inhibit the development of the squall line. The area south of the squall line became a relatively preferred environment for triggering convection under the warm and moist oceanic environment. Consequently, the squall line turned northeast‒southwest and formed a training line mode in which the orientation was almost parallel to the eastward movement. The training line mode prolonged the duration of precipitation in the southern downtown area of Shenzhen.