Assessment of urban heat islands during hot weather in the U.S. Northeast and linkages to microscale thermal and radiational properties
Hardin, Aaron W.
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The urban heat island (UHI) is a well-documented phenomenon that occurs when temperatures within an urban area are greater than that of the surrounding rural area. This is due to increased sensible heat storage, anthropogenic heating, and many other factors. There have been many studies completed on understanding city-specific UHIs, yet there has been minimal research conducted on how different synoptic weather types modify the intensity of the localized UHI within the canopy layer. Increased temperatures in urban areas have been associated with a negative impact on human health by elevating personal mortality risk, exacerbating already harmful heat waves, and not allowing relief from daytime heat with higher overnight temperatures; hence, it is important to study the spatiotemporal variations in the near-surface UHI, and how different surfaces impact surface and human energy budgets. The current research incorporates findings from two studies. The first study examined four northeastern cities: Boston, Baltimore, Philadelphia, and New York. The objectives of this study were: 1) examine inter-city variations of diurnal UHI intensity under different synoptic weather types using Spatial Synoptic Classification; 2) study intra-city temperature patterns using maps and cross sectional graphs created using data from over 600 UrbaNet stations (NOAA and Earth Networks); and 3) determine the effect that wind direction has on UHI intensity. Dry weather types yielded the largest UHI intensities at night, and UHIs under the dry weather types were found to be significantly different than their moist counterparts. In Boston, Baltimore, and New York City wind direction was found to influence daytime UHI formation with daytime UHI intensity below average during onshore flow, and above average for offshore flow. The intra-city comparisons helped identify the warmest regions of each city and the underlying local climate zone of each. Understanding which regions of each city have the strongest warming potential can aid mitigation strategies in affected areas and help address urban risks and hazards to extreme risk. For the second study, a portable weather station was deployed in Lubbock, TX between the months of June and September 2014 during peak heating hours over 10 different local surface types. The objective of this study was to identify the radiational and thermal properties impacting the microclimatic conditions while an oppressive weather was present or the maximum temperature was above 100°F (37.8°C). Examples of select surfaces include artificial turf, green roof, asphalt, concrete, and grass. Data from the portable weather station were used to calculate sensible heat flux, latent heat flux, and absorbed radiation by a human (Rabs) using three methods: measurements from a net radiometer, a cylindrical radiation thermometer (CRT), and a theoretical estimation model of radiation. A sensitivity analysis was carried out to identify weather conditions within which the CRT estimation method performed optimally. It was found the CRT is most sensitive to low wind speeds and low temperature differential between the CRT and air temperature. The net radiometer and CRT offer a unique opportunity to compare methods of calculating Rabs and understand the radiational microclimate a human would be experiencing over each surface. Subsequently, this will aid in identifying the best heat island mitigation strategies to employ to avoid heat stress on the hottest and driest weather type days in Lubbock and various U.S. cities where oppressive weather types are present.