Vegetation Modification

Trees, rooftop gardens, and other urban vegetation cool the urban environment in two ways. First, vegetation uses moisture available to its root system to convert liquid water to vapor water - a process referred to as evapotranspiration. The result is a cooling of vegetative surfaces and the air which comes into contact with these surfaces. The second mode of vegetative cooling is the direct effect of shading surfaces. In MIST vegetation modification is assumed to be applied uniformly across the entire city. Vegetation fractional increase is limited in MIST to the range of -0.5 to +0.5, where negative values reflect a reduction in vegetative cover. Users should refer to the resource links at the left for more information on potential for albedo modification.

Vegetation ismost effective when planted in strategic locations around buildings. Researchers have found that planting deciduous species to the west and east is typically most effective for cooling buildings, especially if these trees shade windows, air conditioning condenser units, and part of the building’s roof. Planting trees to the south must be done carefully. Depending on tree and building height - and the distance between tree and building - trees may block useful solar energy in the winter, when the sun is low in the sky, without providing much shade during the summer when the sun is high in the sky. Also, proper planting is important for providing shelter from winter winds without reducing overall wind speeds that can help alleviate summertime heat islands. Planting evergreen species to the north is one approach to help block winter winds.

Shading pavements in parking lots and streets is also an effective way to cool a community. Trees can be planted around the perimeters or in medians inside parking lots or along the length of streets. Some cities have ordinances requiring street and parking lot landscaping. These ordinances are intended not only to beautify these areas but to make them cooler. Finally, strategically placed trees also can benefit playgrounds, schoolyards, ball fields, and similar areas.

Green roofs or rooftop gardens are an emerging option for shading and landscaping urban areas. A green roof sits over an existing, structurally reinforced roof. Sloped roofs can be covered with low growing, horizontally spreading root ground covers, while flat roofs can often support more intensive gardens including those with fully developed trees. The initial cost of a green roof is relatively high. The benefits of green roofing, however, are broad and include: building energy savings, heat island mitigation, storm-water management, biodiversity, and aesthetic beauty.

Lastly, vegetated pavements, such as grasspavers, are becoming more common and can provide multiple benefits, similar to those associated with green roofs.

 

Advanced users seeking more detailed information on this and other topics related to the scientific and modeling underpinnings of the MIST software tool should read the detailed model description document that can be downloaded from the MIST website.