Urban induced modification of precipitation - Findings from a systematic literature review with focus on studies from the last decade

Changing land surfaces can have a profound impact on local and regional climates. Cities in particular are associated with a comprehensive transformation of the local surface structure, which, in combination with anthropogenic emissions of radiative active gases and particles, leads to a significant modification of the energy balance of the urban boundary layer. One of the most well-known urban climate phenomena is the urban heat island effect. In addition to the thermodynamic effect, a clear urban influence on the wind field and the modification of precipitation above and around cities is documented by many studies. Observational and modeling studies provide convincing evidence that precipitation patterns over and/or around urban areas are altered, and convective precipitation and flash flood events may be enhanced or even triggered. The urban influence often is result of a combination of thermal effects in connection with the heat island, obstacle effects an aerosol influences on microphysical processes and on the heating profile over the city. Precipitation can not only be increased by cities, it can also be weakened or even averted, especially in connection with particle emissions. Inprinciple urban effects on precipitation are fairly well known, but the many published studies on them show a wide range in terms of the magnitude of precipitation changes and their location relative to the urban area. Simple statements concerning urban precipitation modification cannot currently be made, if this is at all possible given the complexity resulting from the overlaying influencing factors. In many cases, the lack of standardized reporting of study results makes it considerably more difficult to compile generalized statements. As part of the Cluster of Excellence "Climate, Climatic Change and Society" (CliCCS), a project is focussing on sustainable adaptation scenarios of cities with regard to hydrological pressures in connection with climate change. Within this project, we conducted a systematic literature review on precipitation modification by cities with a focus on the last 10 years. The reason for this was that the last comprehensive review on the subject was several years ago and the number of publications is constantly increasing. Here we report the first results of our systematic review broken down by spatial scale, dominant processes and possible relevance to climate change. In doing so, we highlight the influence of cities on heavy precipitation and point out the development of the research landscape and alleged research gaps. Selected results from recent publications are shown for illustrative purposes.