Layer‐Wise Formation Mechanisms of an Entire‐Troposphere‐Thick Extratropical Cyclone That Induces a Record‐Breaking Catastrophic Rainstorm in Beijing

Abstract

A rarely seen entire‐troposphere‐thick (ETT) extratropical cyclone that induced a record‐breaking catastrophic rainstorm in Beijing (the largest since 1963) on 20 July 2016 was reproduced reasonably by a convection‐permitting Weather Research and Forecasting Model. This cyclone was the result of a vertical coupling of a lower, a middle, and an upper tropospheric cyclone, which formed at different times and locations. Dry air descending from the stratosphere through a tropopause folding process capped moist air to the middle and lower troposphere, making convection associated with the cyclone relatively shallow. Overall, cyclonic vorticity associated with the ETT cyclone enhanced much rapider than its wind kinetic energy (KE), and formation mechanisms of the ETT cyclone were characterized by layer‐wise features: (i) The formation of the middle and upper tropospheric cyclones was dominated by cyclonic vorticity transport, instead of cyclonic vorticity production, whereas the lower tropospheric cyclone was generated through both cyclonic vorticity transport and convergence‐related cyclonic vorticity production. (ii) Consistent with the cyclonic vorticity intensification, the formation of the lower and middle tropospheric cyclones featured an enhancement of rotational wind KE (in western section of the cyclone, this was mainly due to the work done by the pressure gradient force, but in eastern section, it was mainly due to the inward transport of wind KE by rotational wind), whereas the formation of the upper tropospheric cyclone featured a decrease in the rotational wind KE (particularly in its eastern section), which was in contrast to its increasing cyclonic vorticity. The export of wind KE by a southwesterly wind within the eastern section of the cyclone governed the rotational wind attenuation.

On the Relationship of a Low-Level Jet and the Formation of a Heavy-Rainfall-Producing Mesoscale Vortex over the Yangtze River Basin

“…Their results showed that the interaction of this DBV with a middle-level cyclone enhanced its intensity and precipitation. It should be noted that there is a common feature for the studies mentioned above, i.e., an LLJ was remarkable during the formation period of the DBVs [7,[17][18][19]. This feature has also been confirmed as significant by a 14-year statistic [16] and a semi-idealized simulation of DBV [20].…”

“…For example, Dong and Zhao [17] investigated five DBVs embedded in the Meiyu front and found that a couple of upper-level westerly jet and low-level jet (LLJ) was a favorable condition for the development of these vortices. Sun et al [18] and Fu et al [19] investigated the formation mechanisms of two heavy-rainfall-producing DBVs. They found that the vorticity production (due to convergence) and the vorticity transport (due to convection) were dominant factors for the vortices' generation.…”

On the vertical extending of the explosive extratropical cyclone: A case study

“…During the EEC's rapid development, its top level extended upward swiftly (from 850 hPa to 200 hPa in 21 hr). The mean VSS was about −0.84 PaÁs −1 , which was much higher than that of conventional extratropical cyclones (Fu et al, 2015;Li et al, 2019). This EEC was used to answer the two scientific questions raised above.…”

“…For example, Fu et al (2015) analyzed an extratropical cyclone in the Yangtze River Basin and found that the top level (i.e., the uppermost level of a cyclone's vertical range) of the cyclone extended from 750 hPa to 550 hPa within 12 hr. Li et al (2019) investigated a cyclone that caused a catastrophic rainstorm in Beijing and found that it took $15 hr for the top level of this cyclone extended vertically from 500 hPa to 200 hPa. On average, the vertical stretching speed (VSS; which is calculated by the variation of a cyclone's top level divided by corresponding time) was about −0.46 PaÁs −1 for the former cyclone and around −0.56 PaÁs −1 for the latter cyclone.…”

Surface wind and vertical extent features of the explosive cyclones in the Northern Hemisphere based on the ERA‐I reanalysis data

“…A cyclone is a three-dimensional system (Kouroutzoglou et al, 2012;Jiang et al, 2020), and thus it has a vertical extent, within which each level has a remarkable closed cyclone centre belonged to the same cyclone (Li et al, 2019). The vertical extent of a cyclone is closely related to its evolution.…”