The Vertical Profile of UHI
Any change in the surface energy budget and in the surface temperatures produces effects on urban heat island (UHI) that can be noticed also in its vertical profile. The difference of temperatures between urban core and rural site origins different momentum at different heights in urban canopy layer (Oke, 2006). The vertical difference of temperatures at different heights is mostly visible during nights (Landsberg, 1981). A quite common phenomenon is the crossover effect of temperature in the sky layers. It is the steeper increase of the air temperature over rural sites than urban ones that determines at a variable height a crossover point in which the rural air temperature is higher than the urban one.
The increase in temperature at different heights is mainly due to the more stable climatic condition on rural areas, while urban areas are characterized by a relative instability (Santamouris, 2006).
Typically, the vertical profile of the urban heat island is influenced by wind especially during nights. Summers, in 1964, expressed the magnitude of the UHI in Montreal as inversely dependent on wind speed (Landsberg, 1981). However, Landsberg (1981) affirmed that since every urban area is characterized by a different morphology and a different climatology is not possible to generalize the results obtained for other cities.
Oke (1976) shows the relation between the UHI and the wind speed in Vancouver. By his study it results a residual, although small, UHI also under high velocity wind conditions. In contrast, in correspondence of weak wind speed it is possible verify a stronger UHI.
Wind speed in an urban context is heavily influence by the urban morphology (Shahgedanova, Burt, & Davies, 1997; Montavez, Rodriguez, & Jimenez, 2000; Gaffin, et al., 2007).
At the beginning of the twentieth century, Kremsen focused his research on the decrease in wind speed in urban areas. He observed in Berlin during a decade a wind speed drop of 24% (Landsberg, 1981). Almost the same decrease was observed also in New York City. In Maryland it has been observed that wind speed was about 70% weaker than that recorded in the airport (Landsberg, 1981). Many authors have attributed the wind speed decrease to the increase in urbanization (e.g., Bacci & Maugeri, 1992; Brunetti, Mangianti, Maugeri, & Nanni, 2000).
The wind speed not only varies in the urban pattern in dependence on its density, but it also varies with the height according to the urban roughness.
Taylor found the expression of the vertical wind profile in a neutrally stratified atmosphere (Landsberg, 1981):
Equation 1 Vertical wind profile
Where k is the von Kàrmàn’s constant, its value is about 0.4; h is the height of the measurement; z0 is the superficial roughness; u* is the friction velocity given by:
ū* = τ/ρ
Equation 2 Friction velocity expression
τ is the surface shearing stress and ρ is the atmospheric density. z0 can be expressed as follows (Landsberg, 1981):
Equation 3 Urban superficial roughness
Where h average is the mean height of buildings; A is the ratio between the cross section beaten by the wind and the area of buildings in that area (Landsberg, 1981).
Nakamura and Oke (1988) suggest a simplified expression for wind velocity in street canyons:
ūcanyon = p ūroof
Equation 4 Mean canyon wind speed
Where ūcanyon is the horizontal mean wind speed measured in the street canyon center at a height of 0.06H; p is a factor depending on H/W and ūroof is the wind speed at a height of 1/2H above the roof level. For a wind velocity up to 5 m s-1 Nakamura and Oke found an aspect ratio (H/W) of about 1, p ≈ 2/3.
Canyon effect at micro scale can channel wind and maximize its velocity; this is due to a wrong design of street dimensions. Oke (1988) found the mathematical correlation between the wind velocity and H/W ratio of a street canyon. The high decrease in the wind, or, on the contrary the high increase in the wind speed can be a hazard for population. Oke (1988) found that a ratio H/W of about 0.65 can ensure the best comfort for people. If the high wind velocity can be a hazard for population, on the other hand, the excessive decrease in wind speed can provoke the stagnation of pollutants with effects on human health.
The urban canyons geometry also plays a crucial role in the energy balance. The canyon surfaces are important because by their characteristics depend the amount of energy absorbed and re-radiated. Street canyon characterized by a high ratio between height and width provokes the trapping of solar radiations and the increase in temperatures especially during night (Santamouris, 2006). Although, typically, just a little amount of solar radiation reaches the canyon surface, the emitted radiation depends on the sky view factor (SVF).
The sky view factor is defined as the “openness of a site within an urban setting” (Grimmond, 2007). Such a factor plays an important role in the thermal behavior of the street canyon in the urban environment. In the urban canyons both the pavement and the building façades are involved in the thermal balance. During the day, the surface temperature mainly depends on the solar radiation reaching the canyon, thus depends also on the aspect ratio (H/W) and on orientation, and depends also on the thermal and optic characteristics of the building materials. At night, the façade temperature is governed by the radiative balance. Its value depends on the SVF.
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* Rearranged text from: Susca, T. (2011). Evaluation of the Surface Albedo in a LCA Multi-scale Approach. The Case Study of Green, White and Black Roofs in New York City. Ph.D. Thesis