EPA-NASA Urban Heat Island Cool Communities Pilot Project

By Jeff Luval et al
Huntsville, AL (October 1, 2000)- The urban landscape represents a complex heterogeneous surface that strongly influences the development of the urban heat island. In fact, doubling the canopy in Sacramento could cut “bad air” days in half. In order to assess the effectiveness of the heat island mitigation strategies, this study could be incorporated into both meteorological and air quality models. This study demonstrates the importance of vegetation in keeping cities cool, and proves that each city is unique in its albedo and surface temperature characteristics.

NASA ATLAS research aircraft over flights were flown over Sacramento, California near local solar noon on 29 and 30 June 1998 to map its surface radiative temperature urban heat island (UHI). A meteorological analysis has documented the synoptic and mesoscale weather conditions during the flights. NWS charts generally show typical summer synoptic patterns, a well-developed surface thermal trough, but only weak large scale forcing aloft. Mesoscale surface wind flow patterns at 10 m during the first flight period generally show typical summertime onshore sea breeze flow patterns, while con-current 2 m isothermal patterns show marine air intrusion into the Central Valley. Local temperature maxima over downtown Sacramento seem due to a weak UHI effect, which might have been larger in the absence of the marine air intrusion.
The corresponding albedo and surface temperature measurements indicated that Salt Lake City had the overall highest average albedo followed by Baton Rouge, and then Sacramento. However, even though Salt Lake City had the greatest albedo and the lowest solar loading, it was the warmest city, with average surface temperatures 4.8 degrees C warmer than the coolest city, Sacramento.
The main technique for “classification” of urban land surfaces is to examine using land use or by surface type, ie roof, road, vegetated, etc. and assign a albedo or temperature to that class. These “classes” are then input into some meteorological or air quality model. It is important to point out that traditional image classification techniques use only differentiation of surfaces in the visible wave lengths and not functional differentiation, i.e. how the surface functions in some respect. In a typical classification approach a key assumption is that the albedo gives a indication of how that surface may partition the sun’s energy.
This is a false assumption because the way each surface partitions energy is unique depending on material type, vegetated or non-vegetated, water status, atmospheric vapor deficits, and the relative mixtures and arrangements of the various components of that surface. The albedo is only one part of determining what the surface temperature will be and how the surface partitions energy.
Within each city, each land use has a unique “energy print” that is directly physically related to how that surface is processing energy. These “energy prints” of the land use are unique for each city; ie, the Sacramento CBD (Central Business District) skattergram is significantly different than Baton Rouge or Salt Lake City’s CBD skattergram. These results again empathize that classifications based on cover type/land use cannot be applied across a variety of cities, since they cannot represent the true energy partitioning of that surface.
Related Resources:
EPA NASA Urban Heat Island Cool Communities Pilot Project- Part 1
EPA NASA Urban Heat Island Cool Communities Pilot Project- Part 2