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Characteristic Analysis of Generalized Potential Temperature and Potential Vorticity during Freezing

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    The spatial and temporal distribution characteristics of different types of thermodynamic variables and potential vorticity during precipitation were compared and analyzed in this study. On the basis of the heavy rainfall events in Jilin Province on 13–14 July 2017, the following five types of potential temperature were calculated with model outputs: conventional potential temperature (θ), equivalent potential temperature (θe), generalized potential temperature containing a condensation probability function (θGao), generalized potential temperature containing a freezing probability function (θWang), and potential temperature covering condensation and freezing (θGu). The relationships between five associated types of potential vorticity [PV(θ), PV(θe), PV(θGao), PV(θWang), and PV(θGu)]and precipitation were also analyzed. Results showed that the generalized potential temperature introducing a freezing probability function (θWang) and its potential vorticity [PV(θWang)] corresponded well with heavy rainfall. The differences between θWang and θGao were observed at 5–11 km in the mid-upper troposphere over the rainfall region. θWang was always greater than θGao, with the maximum difference reaching 2.5 K. Hence, the introduction of the freezing probability function extends the application scope of the generalized potential temperature and offers a reliable depiction of the thermodynamic state of nonuniform saturated moist air over rainfall regions. The differences among the five types of potential vorticity were mainly observed under 12 km over the rainfall region. The positive and negative anomaly centers for potential vorticity PV(θGao) and PV(θWang) respectively defined by θGao and θWang were increasingly visible. The anomaly value of PV(θWang) was greater than that of PV(θGao), and the differences could reach ±0.2 PVU. Such difference was due to the enhancement of the generalized potential temperature over the rainfall region resulting from the introduction of the freezing probability function. This condition led to the abnormal enhancement of the moist potential vorticity in the freezing region.

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  • Received:April 28,2019
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  • Online: July 28,2020
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