Layer‐wise formation mechanisms of an entire‐troposphere‐thick extratropical cyclone that induces a record‐breaking catastrophic rainstorm in Beijing

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.