什么是 Equivalent potential temperature？

The equivalent potential temperature $\theta_e$ of a parcel is the temperature it would have if it were lifted until all its vapour had condensed and fallen out, then brought down dry adiabatically to $1000\,\mathrm{hPa}$. A good approximation is

with $\theta$ the potential temperature and $T_{\text{LCL}}$ the temperature at the parcel’s lifting condensation level.