This study evaluated the summertime microclimate variations at three scales against building density, ground cover and tree canopy characteristics. The three scales of microclimate data analyzed were, fixed station data (seven stations), site traverse data (six traverses) and a city-wide traverse data-set.(Figure 1)

Urban climate modifications were first analyzed from a historical perspective to ascertain whether the urban heat island (UHI) has been a long term phenomenon in Ann Arbor. Indeed, the historical summer climate trends indicate a growing heat island problem (Figure 2).

Of the data classification types employed by the present study, stable nighttime and unstable daytime conditions dominated the study period.

Their preponderance indicates that the urban design suggestions by the study should be particularly applicable to stable nights and unstable days.

The “normal” climate pattern in Ann Arbor during summer is one of frequent thunderstorms followed by fair, warm weather. This was generally the case during the study period, although precipitation was below normal. The daily maximum and minimum temperatures on most of the study days fell between the 33^rd and 66^th percentile of the historical values. Therefore, we can conclude that urban microclimate modification has been a historical reality in Ann Arbor and the study obtained a representative picture of this fact.

Summertime heat island at the city-wide scale correlated well with ground cover characteristics (Figure 3). Green cover alone explained about three fourths of microclimate variations. The influences of buildings and vegetation maturity were apparent when the focus was on fixed stations. Fixed station data revealed that there was a downtown-centered UHI at night (Figure 4). The magnitude of the temperature differences decreased as background climate became hotter. But hotter conditions lead to larger intra-urban thermal comfort differences than the cooler nights. This finding highlights the need to distinguish between temperature island and “thermal comfort island”. The former peaks at clear calm nights while the latter reaches its maximum under overcast or hazy summer conditions.

During the day a mix of cool and heat islands was recorded. The more open residential sites were the warmest at day and the coolest at night. Under very hot conditions, the soft ground cover at the more open residential sites did not significantly improve daytime cooling. This suggests that shading was more central to daytime cooling than ground cover.

Extensive tree canopy produced some cooling during the day, but the cooling provided by building shade at the high-density sites also did equally well. In addition, extensive tree canopy cover resulted in warm microclimate at night.

The importance of shading to daytime microclimate was further accentuated by the analysis of data at the street canyon scale (Figure 5). The warming influence of obstruction to nighttime sky was highlighted by the Cluster Thermal Time Constant (CTTC) model simulations for the downtown site. Simulation scenarios that lead to increase in street canyon shading did depress the daytime temperature but also elevated the nighttime UHI. A combination of thermal property improvements and moderate increase in building density fared well (Figure 6).

Thus, improving shading while ensuring adequate sky view appears to be the basic requirement of urban design strategies for mitigating the negative impacts of summertime UHIs. However, improvement to thermal properties is beneficial at the city-wide scale. This dichotomy indicates that urban design strategies have to be different for the downtown core and the suburban residential locations.

The findings of the study in terms of the five research questions can be summarized as following:

1. High-density developments can create comfortable outdoor spaces during the day. However, the sky view factor (SVF) needs to be high for nighttime heat loss. Site NW4 exemplifies these patterns.

2. Climate and thermal comfort variations at site CRL indicate that the utility of extensive tree cover is questionable. The daytime cooling associated with extensive tree canopy was not very different from cooling by building shade. Furthermore, extensive tree-cover inhibited nighttime comfort due to the reduced SVF.

3. A comparison of mature vegetation-covered site JNR and young vegetation-covered site KBR did not clearly reveal the effects of mature vegetation cover. It appears that an air temperature improvement is associated with mature vegetation cover, but the same cannot be said about the thermal comfort.

4. Site traverse data presented in Figure 30 clearly show the importance of shading to daytime thermal comfort. The cooling associated with building shade more than offset the effects of soft ground cover.

5. The CTTC model simulations indicated that the UHI at the downtown site can be minimized by building massing that leads to street-level shading during the day without reducing the SVF.

Based on these findings, the study proposed six design strategies for the mitigation of Ann Arbor UHI. Four of these strategies were applicable to the downtown core. Employing these strategies will result in substantial green cover increase in the downtown district while street-level thermal comfort is enhanced by arcades and suitable building massing. The building massing is such that tall buildings are on the south-side of city blocks while green area is in the center and to the north-side (Figures 7).

Two other design strategies aim at enhancing the thermal properties of the high density residential areas and multi-family residential neighborhoods. The design strategy for the former proposes a 50% increase in tree cover. Suggestions for multi-family residential areas include a courtyard built-form with grass-covered inner lawns to allow for building-shaded, soft ground-covered commons.

There are two shortcomings in the present study. First, the data were collected during one year only. Conclusions drawn from multi-year data would be more authentic than a single season data. Unfortunately, multi-year data collection was not possible due to time and resource constraints. However, results from a pilot study conducted during the summer of 1995 generally agreed with the current findings (Emmanuel, 1997).

The second shortcoming relates to the representative-ness of the fixed stations. Finding representative points in a heterogeneous urban terrain is impossible (Pease et al., 1976: 558). By definition, land cover characteristics at heterogeneous urban terrain is so varied that the question of representativity is a moot one. A consolation is that the magnitude of the UHI recorded during the city-wide traverse matches the intra-urban differences among the fixed stations. This suggests that the thermal characteristics at the selected sites were at least as varied as the entire urban terrain in Ann Arbor.

The present study was based on the premise that the UHI effects need to be mitigated. However, some researchers claim that the phenomenon of UHI is beneficial during the winter and should not be eliminated in cities dominated by the heating season. For example, Bründl and Höppe (1984) stated that the heat island phenomenon could help reduce heating energy use in downtown Munich. Tuller (1980) found that the temperature elevation in downtown Christ Church, New Zealand, enables one to wear less clothing insulation in the city center than in the suburbs.

However, when the data are closely scrutinized, the advantages of UHIs during winter do not hold. For example, the Bründl and Höppe, (1984) study cited above compared the heating degree days (HDD) at a central city location with a peripheral residential site in Munich. The study shows that winter HDDs at the city center were almost exactly equal to that at the peripheral site. The summertime HDDs were somewhat lower at the city center site (27 HDDs less). However, summertime heating load is inconsequential in Munich and the energy savings will therefore be minimal.

The insignificance of the “advantage” of winter UHI was indirectly confirmed by a city-wide traverse carried out in Ann Arbor during February. The maximum UHI recorded during the winter traverse was less than 3^oF. The heavily vegetated areas of the city were almost as warm as the downtown itself (Average temperatures were 22.5^oF and 23^oF respectively).

The outdoor comfort during winter is a function of wind control (Arens and Bosselman, 1989). The Westerly winter winds combined with East-West running streets in downtown Ann Arbor create strong eddies that could lead to severe cold stress. It is likely that the warmth created by the urban geometry at the downtown is more than offset by this wind discomfort. Concentrating on summer UHI mitigation therefore is a correct approach as long as it does not compromise the winter comfort.

The city-wide analysis by the present study had access only to a coarse land cover data based on census tracts. A better approach is to use remote sensing techniques to gather representative urban land cover data. This can be compared with detailed ground climate data at street-canyon scale. The combination of these two methods will yield fine-resolution urban land cover data that could be more satisfactorily linked to intra-urban climate variations. However, the use of surface temperatures derived from remote sensing is inappropriate for urban thermal comfort research. Remotely sensed data emphasizes horizontal surfaces as opposed to the vertical surfaces “seen” by ground-level monitors (cf. Roth et al., 1989). This in turn overestimates the effects of rooftops and tree tops whereas street level comfort is largely a function of walls, below canopy surfaces, ground cover and their radiative properties.

Another improvement to the present study would be the simulation of the effects of vegetation cover and anthropogenic heat input to the street level climate. Very recently, improvements to the CTTC model have been suggested to take these factors into account (Elnahas and Williamson, 1997). However, the error term associated with the exclusion of anthropogenic heat input remains too small to be significant. This may be due to difficulties in quantifying the anthropogenic heat input to street canyons.

Finally, urban design alternatives for the mitigation of the negative effects of UHIs need to be developed for cities in other climate zones as well. This need is particularly urgent in the tropics where even minor microclimate modifications are keenly felt by the inhabitants (Nieuwolt, 1981). Furthermore, the near-future urbanization will be intense in the tropics. As such, improvements to the outdoor climate in the tropics will benefit a large segment of humanity.

In conclusion, we can state that there are indeed conflicting design requirements at the city-wide scale and the street canyon scale. Even within the street canyons, the need is to both increase shading as well as increase the sky view. While these dichotomies seem insurmountable at first, a possible approach is to concentrate on improving thermal comfort at the street-level. In the end, street design is what urban design is all about.

Remember, if you will, that between 25 to 35 percent of the developed land in cities is made up of public rights-of-way. Moreover, we change these with considerable regularity. We seem always to be tinkering with them. In large measure street constitutes the public realm. If we can design streets well, we will have designed a large part of our urban environment directly and indirectly designed most of the city. (Jacobs, 1995: 191)

At the same time, city-wide greening should remain a long-term goal. A combined strategy of street architecture that creates shade and a city-wide greening campaign could minimize the negative effects of summertime UHIs.

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