September 2008 NUI Galway's Raindrop Research to Inform Climate Change Predictions

An NUI Galway climate researcher has made a major breakthrough in understanding the effect of air pollution and climate change on precipitation. Dr Colin O'Dowd and a team of leading international experts propose a new theory explaining why atmospheric aerosol pollution plays a dichotomous role in increasing and suppressing rainfall under different levels of pollution. Their results are essential to providing an improved understanding of how air pollution and climate change will influence flood and drought patterns in the future. The high-profile article 'Flood or Drought: How do Aerosols Affect Precipitation?' was published in the latest edition of the international journal, Science. The article explains how, in order for clouds to form, there must be sufficient water vapour in the atmosphere to produce supersaturated vapour fields, and there must be nuclei for the vapour to condense on in order to form cloud droplets. Water vapour primarily comes from water evaporation at the Earth's surface, while the nuclei come from atmospheric aerosol particles. In clean clouds with low nuclei availability, the cloud droplets formed are large and coalesce into raindrops that readily rain out. However, this study points out that in polluted convective clouds, where cloud droplets are smaller, they do not precipitate out until they reach altitudes above the atmosphere's freezing level. At that stage the droplets freeze into ice precipitation that then falls and melts, releasing latent heat upward and reabsorbsing heat at lower levels. The result is a greater upward transport of heat for the same amount of surface precipitation. This causes invigoration of the convective clouds, leading to additional rainfall despite the slower conversion rate of cloud droplets into rain drops. According to Dr O'Dowd, Director of the Centre for Climate and Air Pollution Studies at NUI Galway's Environmental Change Institute and Senior Lecturer with the School of Physics, "What this research shows, in very simple terms, is that air pollution not only dramatically effects the type of rain that falls but ultimately how much and where. The smaller type of raindrop being formed when pollution is a factor effectively 'heats up' the cloud system leading to changing weather patterns. This research will lead to a better understanding of and predictions regarding climate change". The scientific article 'Flood or Drought: How do Aerosols Affect Precipitation?' looks at how, as the level of aerosol pollution increases above a critical threshold, aerosol components absorb sufficient incoming solar radiation to warm and stabilize the troposphere. This inhibits convection of cloud parcels, and reduces the amount of energy absorbed at the Earth's surface, thus reducing the release of water vapour. O'Dowd adds, "The net result is that in the most polluted areas, precipitation, and even convective clouds, may not form at all. Given that about 37% of the energy put into the atmosphere by the Earth is as water vapour, this must be balanced by the amount of precipitation returning to the surface. Therefore, disturbance of this balance in one region must be compensated for in another, leading to changes in precipitation patterns." In their study, O'Dowd and colleagues introduce the above phenomena as an aerosol thermodynamic forcing which does not necessarily change the net energy budget, but redistributes it internally. The study highlights the importance of taking account of the aerosol thermodynamic forcing in climate models in order to better predict changes to the hydrological cycle associated with air quality and climate change. O'Dowd and his co-authors comprise the Steering Group developing the international project on Aerosols-Clouds-Precipitation and Climate (ACPC). ACPC is a joint International Geosphere-Biosphere Programme - World Climate Research Programme (IGBP-WCRP) project. O'Dowd represents two IGBP sub programmes on ACPC, namely the International Global Atmospheric Chemistry (IGAC) programme, and Surface Ocean Lower Atmospheres Studies (SOLAS) programme. -ends-