The anthropogenic aerosol effect on rainfall has been studied since the late 1960s when it was first suggested that pollution might inhibit rainfall in some cloud types. Since then, a number of direct and indirect aerosol effects on cloud microphysics have been proposed. The concept of the second aerosol indirect effect is that smaller, more numerous pollution particles lead to the formation of cloud droplets not large enough to precipitate from warm phase stratiform cloud.
Further research on aerosol, and its part in the climate system, is needed because this key influence on climate is not yet fully understood according to the Intergovernmental Panel on Climate Change. Most aerosol effect studies have used only model output or coarse spatial scale (typically 1° latitude/longitude) multi-seasonal or decadal data sets. Spatial analysis of aerosol effects at less aggregated spatial and temporal scales using atmospheric observations are rare. Despite this knowledge gap, hypotheses to geo-engineer climate change reversal using anthropogenic aerosol are appearing in the literature.
The aim of this research was to explore and assess the relationship between aerosol, cloud development and rainfall at city size, and cloud resolving scales for selected stratiform and cumuliform precipitation events. Research was conducted in the subtropical city of Brisbane, Australia where previous research of 35 year rainfall trends indicated a possible anthropogenic aerosol effect on rainfall. As Brisbane grows beyond a predicted 4 million inhabitants in the next decade, an understanding of influences on its weather at this time of uncertain future climate and global warming impacts is critical, particularly in regard to security of freshwater supplies, air quality, climate extremes and human health.
The four main objectives of the study were: (1) to identify a statistic that quantifies spatial co-variation between aerosol, cloud, rain and atmospheric parameters; (2) to use the Weather Research and Forecasting (WRF) model to map aerosol characteristics over and downwind of the Brisbane region; (3) to apply WRF and geospatial statistical methods to three rain events to determine possible aerosol effects; and (4) to critically assess the relationships between aerosol, clouds and rainfall identified under objectives 1-3.
Because optically based remote sensing of aerosol in the atmospheric column is not possible during rain events, the chemistry transport version of WRF was used to estimate aerosol parameters during rain events. The model also provided wind fields and vertical air movement data for the study. The cloud and rain parameters were provided from meteorological observations. Spatial statistics data exploratory methods of Universal co-kriging were selected to research the covariance between key meteorological parameters investigated by this study because data transformation was necessary to account for issues of the form and trend in the data distribution functions.
In agreement with an emerging consensus in the literature, results of the cross-variogram analysis suggest that the presence of anthropogenic aerosol was associated with areas of less intense rain out to 22 km from aerosol ingestion for the stratiform system studied. For the two cumuliform systems studied, anthropogenic aerosol emissions were associated with enhanced rain out to an average of 18 km downwind of aerosol ingestion. Rain events with a single layer of cloud were difficult to find for the period 2009 to 2011, and so correlation between pollution and cloud droplet size could not be established.
The aerosol size distribution and rain rate associations were further tested in the context of other meteorological and cloud dynamics parameters that are known to influence rainfall. Generally, the findings indicate that the geostatistical association of anthropogenic aerosol and rain rate are unlikely to be counter explained by confounding parameters and processes – cloud water content, atmospheric stability, orographic processes, urban environment enhanced heating and general wind convergence.
The research is limited to three rain events around the city of Brisbane. Since the results of the research show that geostatistics provides useful investigative methods to quantify patterns in atmospheric properties, these techniques should be applied to a wider range of rain events to establish any definitive pollution impact on rainfall trend for Brisbane and surrounding regions. Further, the application of these methods to other urban areas would confirm the second aerosol indirect effect at city size, cloud resolving, rain event scales and point to any general potential deleterious impacts from aerosol on weather and climate which could impact regions in proximity to large urban centres.