Some issues in studies on the atmospheric instability of convective storms

Abstract

Atmospheric instability is one of the necessary conditions for the occurrence of severe convective weather, which is characterized by its intricacies. This paper first briefly reviews the air parcel theory and points out its limitations in application, for example, changes in pressure and vorticity of the environmental atmosphere inevitably caused by the strong upward motion of parcels in convective storms. Then, the concepts of static instability, symmetric instability, and other types of instability are reviewed. A special focus is given to summarizing the conditions for conditional instability, moist absolute instability, and conditional symmetric instability, as well as their relationship with the occurrence and development of convective storms, with some misunderstandings being clarified. The most effective method for determining conditional instability is to make a finite virtual displacement of the parcel and then use convective available potential energy (CAPE) for identification. However, the calculation of CAPE and convective inhibition is sensitive to the temperature and moisture of the parcel, and they should be computed and corrected using virtual temperature. The optimal CAPE value has better representativeness than that of the surface-based CAPE. In strong vertical wind shear and low CAPE environments, the acceleration effect of dynamic disturbance pressure gradient caused by rotation is crucial for the development of severe convective storms. Convective instability does not necessarily correspond to conditional instability. A straightforward method for distinguishing conditional symmetric instability is to use saturated equivalent geostrophic potential vorticity. We further summarize the mesoscale rainband characteristics caused by this type of instability.

Keywords

• convective storms
• instability
• parcel
• shear
• conditional instability
• convective available potential energy
• symmetric instability