How does mid-tropospheric dry air affect the evolution of supercellular convection?

•The effect of midlevel dry air on supercellular convection is examined.•Dry air aloft suppresses the development of supercells via two processes.•One is entrainment and the other is the dry-air penetration process.

To investigate the influence of mid-tropospheric dry air on the evolution of supercell storms, idealized numerical experiments with several moisture profiles were performed. In an environment with (without) a mid-level dry layer, supercellular convection decays immediately (persists for a long period). A set of trajectory analyses revealed that two suppression processes contribute to the convection decay in the environment with the mid-tropospheric dry layer. One is the entrainment process within the mid-tropospheric dry layer, and the other is the dry-air penetration process. In the latter process, dry air penetrates into the low-level updraft region, so that the supply of warm, moist air for convection is reduced. Neither of the processes contributes effectively in an environment with a dry layer located at a higher altitude. The dependence of the results on the environmental shear profile, evaporation rate, and the amount of convective available potential energy (CAPE) was also examined by additional experiments.