I’m sitting in a presentation by William R. Cotton, of Colorado State University where he’s talking about the effect of Urban Heat Islands (UHI) on precipitation. He’s making a convincing pitch showing how the UHI factors into downwind delayed convection initiated by the city UHI along with a significant contribution of aerosols and ice nuclei that seed the precipitation. He’s been able to demonstrate that in St. Louis, downwind from the city (typically NE to SE based on prevailing winds) there are increased precipitation from thunderstorms by as much as 160% during the life cycle of the storm.
Yesterday, I saw a very similar study done by Indiana State Climatologist, Dev Nyogi, where he studied Indianapolis, IN and came to similar conclusions. The midwestern cities make good case studies because they are singular islands of urbanization (as opposed to sprawling cities like Los Angeles and Chicago) that essentially become point heat sources at the mesoscale level.
The summary is this: Urban and-use has the biggest control on locations and amounts of precipitation and that condensation nuclei added by the city also have a significant effect. Heat and particles contributed by the city can make bigger, more precipitating thunderstorms.
Of course studies by Parker tells us there is no significant UHI effect, so this presents yet another challenge to what is looking ore and more like a flawed study by Parker.