Numerical Simulation Study of Moist Baroclinic Instability Mechanism during a Yellow River Cyclogenesis Event

Abstract

As the main mechanism of extratropical cyclogenesis， moist baroclinic instability plays a central role in the study of cyclone thermodynamics， which can be further divided into four categories： dry baroclinic instability， moist instability， diabatic Rossby wave and Type C cyclogenesis （tropopause intrusion）.The '7·20' heavy rainstorm was caused by the eastward movement of a Yellow River cyclone into North China after its rapid formation on July 18， 2016.Compared with the mature stage of the cyclone， the mechanism of the initial stage is still unclear.This article uses ERA5 reanalysis data and WRF model to study the moist baroclinic instability of the cyclogenesis event numerically.The results show that mid-lower troposphere presented diabatic Rossby wave pattern， that is， the eastward movement of the system was mainly driven by the cycle of vertical motion and diabatic effect.The vertical motion on which the wave relied was more provided by vorticity advection.The PV sink and the ageostrophic wind in the upper layer delayed the eastward movement of the tropopause intrusion PV， maintaining the phase difference between upper and lower layers.Finally， a PV column formed throughout the troposphere in front of dry intrusion.Using piecewise PV inversion， several sensitivity runs are designed to remove unbalanced circulation， tropopause dry intrusion PV and lower-level diabatic-produced PV from the initial field， respectively.Combined with the analysis of generalized omega equation， it shows that the baroclinic wave in this process must be coupled with the diabatic process with the help of sufficient water vapor to develop strongly.The cyclogenesis was suppressed when the latent heat was turned off.Dry baroclinic instability cannot explain this process.The removal of initial unbalanced field did not affect the baroclinic instability but will delay development of the system.Limited by humidity and mesoscale circulation structure， the active area of lower-level unbalanced flow was controlled by dry baroclinic dynamics.In this case， the gradient of θbottom was too small to organize eastward diabatic Rossby wave by relying only on the initial lower-level PV.Nor can strong lower-level diabatic heating generate as in Type C cyclogenesis by tropopause intrusion.For this Yellow River cyclogenesis case， it is required the initial lower-level PV anomaly to be strong enough to counteract the suppression of the cooling subsidence in front of dry intrusion； on the other hand， it is also required that the dry intrusion， in an appropriate initial phase difference with the lower system， strengthened the ascending motion east of the low-level PV in form of vorticity advection through vertical penetration， so as to promote the eastward momentum of diabatic Rossby wave to enter north China with more saturated environment.None of dry baroclinic instability， diabatic Rossby wave and Type C cyclogenesis could independently explain this cyclogenesis event， which was an initial optimal perturbation growth under the combined effect of diabatic Rossby wave and tropopause dry intrusion.

Keywords

• moist baroclinic instability
• cyclogenesis
• heavy rain
• potential vorticity inversion
• generalized omega equation