The influence of pollution on deep convective dynamics continues to be a controversial topic. Convective invigoration, an increase of the updraft speed in polluted environments, has been argued to originate from enhanced latent heating when large amounts of liquid water freeze after being transported through the freezing level by convective updrafts. This has been agued to occur when collision–coalescence is suppressed in the lower portions of polluted deep convective clouds as a result of high droplet concentrations and reduced droplet sizes. More recently, the proponents of the invigoration hypothesis began to separate the invigoration as described above, the "cold invigoration", from the invigoration that can happen below the freezing level, the "warm invigoration". In section 2 of our recent paper, Grabowski and Morrison (J. Atmos. Sci. 2020) we discuss the two invigoration mechanisms. We show that the cold invigoration is simply not possible because the freezing merely recovers the reduction of cloud buoyancy associated with carrying the liquid water across the freezing level. We also show that a common interpretation of the warm invigoration, the increase of latent heating due to larger surface area of cloud droplets in polluted clouds, is incorrect. The key is the increased cloud buoyancy due to lower supersaturations in polluted clouds.
This lecture will review selected high-impact studies arguing about the invigoration, both observations and modeling, pointing out their key flaws. We will discuss our above arguments in more detail. We will also present an appropriate framework in which the impact of pollution on convective dynamics needs to be considered, focusing on the vertical distribution of the latent heating and its impact on cloud-environment interactions.