Environmental Impacts of Urbanization in Tropical Coastal Zones

Urban sprawl in tropical locations is rapidly accelerating and it is more evident in coastal areas where a large percentage of the population resides. The interaction of a rapidly changing coastal climate with a dynamic land use in tropical regions and the combined net consequences on the environment are relatively unknown. Most tropical coastal regions for example have tropical montane cloud forests as primary sources of fresh water and the cloud physics of these forests is heavily dependent of low land conditions and sea surface temperatures. This paper will address fundamental questions related to relative effects of land use for urbanization and climate change in the context of tropical coastal zones. The focus of the research is on the Island of Puerto Rico, the city of San Juan, one of the largest cities in the Caribbean, and the El Yunque Rain Forest, located 40km south-east of San Juan.

Analysis of long-term historical data for air and sea surface temperatures (SST), and moisture reflected urban biases above global warming represented by increases in minimum temperatures consistent with the sea surfaces temperatures, and increases of maximum temperatures in the urban areas only. Historical trends are validated with high resolution infrared images obtained during an airborne mission using NASA's ATLAS sensor. The remote sensor images reflect daytime skin temperature gradients between the city and the low land forests in excess of 20ºC, and more than 10ºC during nighttime. The infrared images also reflect a relatively homogeneous built environment as indicated by the low variability of the surface albedo. Significant variations in albedo are observed when a mixture of built and forested surfaces is found within small geographical areas.

The long-term impacts of the land use biases are further investigated using mesoscale atmospheric modeling. The representation of the built environment makes use of the high resolution ATLAS data and reconstruction of the natural potential vegetation. The models are validated extensively and favorably with present observations and conditions for the entire coastal region that comprehends the tropical forest and the city. The impact analysis for the city showed that the presence of the urban landscape of San Juan has an impact reflected in higher air temperatures over the area occupied by the city, with positive monthly average values of up to 2.5ºC, for the simulations that have specified urban LCLU indexes. Changes in the hydrological cycle of this coastal region are also observed in the model results in the form of precipitation disturbances shown, as a difference in total accumulated rainfall between the present urban landscape and the natural potential vegetation. Results also indicate that the urban-enhanced cloud formation and precipitation development occur mainly downwind of the city, including the accumulated precipitation. No significant changes are observed as shown in the sea-breeze patterns may be due to the predominant easterly winds. Impact analysis for the forest area indicates significant increases in cloud base height due to development in the low lands. Preliminary results show that these increases in cloud base height may have a net reduction on the primary productivity of the forest reflected in decreases of fresh water production.