Impacts of Aerosols and Urban Heat Island on Clouds and Precipitation

This study addresses the impact urbanization on clouds and precipitation, and in particular the effects of land cover and Urban Heat Island (UHI), and of aerosol pollution. The UHI effect takes place when, due to surface properties, the air in a city is hotter than the surrounding countryside, altering local weather patterns. Aerosols affect radiative forcing both directly, by scattering and absorbing incoming solar radiation, and indirectly, by affecting the cloud's optical and microphysical properties. By altering cloud microphysics, aerosols can lead to a decrease in the precipitation efficiency, an increase in cloud lifetime, and an increase in the amount of clouds. Previous works show contradictory impacts especially on precipitation over urbanized areas. Because of the large spatial and temporal variability of aerosols, remote sensing observation from satellites delivers the most reliable information about global aerosol distributions. UHI analyses reveal seasonal patterns, and elevated UHI at night, and in the winter season. In a case study, a set of very cold days with off shore winds was chosen from January 2004. Offshore winds reliably formed clouds as air moved from the coastline to the ocean. Three differenced images show streaks of clouds with larger/ more ice-particles than those downwind of outlying regions. Ongoing work includes seasonal climatologies with cloud particle size for the study area.