The interaction between low-level jet evolution and severe convective rainstorms under topographic effect

Observations of radars, satellite and wind profile radars are combined with data collected at regional automatic weather stations to analyze the initiation, development, structure evolution and propagation characteristics of three mesoscale convective systems (MCSs) occurred in 2016. These systems induced an extremely severe precipitation in eastern Hubei during the Meiyu period. This paper focuses on the trigger, development and maintenance mechanism of the three MCSs and the mesoscale dynamic relationship between them. In particular, the interaction between the low-level jet and the severe convective rainstorms under the topographic effect is discussed. The results are as follows: (1) Different to most of heavy rain bands caused by the Meiyu front, which are distributed along the lower-shear line, the extreme rain bands presented in this study show a slanted n-shape, and the two main rain bands are nearly perpendicular to the lower-shear line. The extremely severe rain is caused by the northwest—southeast oriented MCS on the windward slope of Dabie Mountain, the northwest—southeast oriented MCS in the central and eastern plain area of Hubei province, and the northeast—southwest oriented MCS on the east side of Tongbai—Dahong Mountains. These three MCSs move slowly and have the characteristic of back-propagating. (2) The convective rainstorm associated with the MCS on the windward slope of Dabie Mountain is triggered by topographic forcing during the continuous downward expansion of the lower boundary of the low-level jet. And the cold pool produced by the MCS in Dabie Mountain accelerates down the slope, forming the outflow boundary. The strong convergence produced by the outflow boundary and the environmental airflow leads to the initiation, development and strengthening of the MCSs in the central and eastern plain of Hubei province and the eastern side of Tongbai—Dahong Mountains. (3) The low-level southwesterly jet perpendicular to Dabie Mountain has a strong supporting effect on the convective cold pool, which not only balances the downward force generated by the density airflow of the cold pool, but also further strengthens the intensity of convergence in the mountain area. As a consequence, the heavy rainstorm system on the windward slope of Dabie Mountain maintains and develops over a long time. However, once the convective cold pool on the hillside accumulates to sufficient thickness, or the lower boundary of the low-level jet rises rapidly, the balance is broken. The cold pool dives downhill and advances rapidly in the plain area, resulting in a large-scale thunderstorm gale in central and eastern plain of Hubei province. Meanwhile, the MCS strengthens and organizes gradually along the cold pool frontier, generating the southeast—northwest oriented severe rain bands.