Simulations of annual degree day impacts of urban vegetative augmentation

One approach for reducing summertime energy consumption in cities is-through implementation of urban vegetation planting programs. While the direct effect of such programs is to cool individual buildings and air conditioning condenser units, there is also an indirect regional cooling associated with increasing vegetative cover. This paper models the regional cooling impacts of urban vegetation augmentation through a series of meteorological simulations. Numerical experiments were conducted for a hypothetical city located at various latitudes (25-45°N) and subjected to several background climate conditions. Simulations were conducted for one day from each month of the year to determine seasonal variability of the impacts of vegetation on urban climates. To provide a simple and useful index of the climatic impact of urban vegetation, cooling and heating degree days were calculated for each simulation. Comparison of baseline degree days for six modeled cities at various latitudes across the United States with the corresponding historical climate data indicate that the modeling approach was successful in reproducing the general temperature profile characteristics of each city. Simulation results indicate that the regional climate can be significantly cooled through the planting of urban vegetation. For regions of low-to-moderate ambient humidity, increasing the vegetative fraction of the core of a hypothetical city by less than 0.065 resulted in an estimated 3-5% decrease in summertime cooling loads. it is believed that this effect could be doubled by application of a more ambitious program. This energy saving is due to the indirect regional cooling effects of vegetation, and does not include the direct energy savings associated with shading of individual buildings. The wintertime energy costs associated with vegetative augmentation were found to be smaller than the summertime savings, and may be negligible in the case of deciduous vegetation.