1999: Forcings and Chaos: Climatic Characterization of Urban Warming

Introduction

Cities represent the most obvious man-made alteration of the environment. As a consequence, large urban centers often possess their own microclimate. The clearest manifestation of this urban climate appears in the temperature record. Through case studies, it has been determined that the centers of large cities are significantly warmer than the adjacent country sides at certain times of the day. For these periods the surface temperatures in the city may even exhibit closed isotherms. These would define the "urban heat island".

Many factors control local temperatures. In order to minimize the influence of global and regional climate anomalies, it is customary to consider the temperature difference between an urban center and its adjacent rural area when studying the urban heat island effect. Even then, urbanization is but only one of the factors controlling local temperatures. Other surface characteristics (e.g. variability of the topography, proximity of water bodies, long stretches of highways running through the city, etc.) combine to make each city's microclimate specific to its location. So specific indeed that even the rural region might have a somewhat different microclimate. In addition, temperature fluctuations due to chaotic natural variability may hide or amplify the urban heat island effect.

Of course, the best evidence for the heat island effect would come from temperature measurements taken before and after the establishment of a town. This is because the effect is defined as the difference between current temperatures and what they would be in the absence of any humans or their structures. The "urban warming" that is discussed as part of the global warming debate is essentially the temporal evolution of the urban heat island effect. The study of temperature trends associated with a town's growth has been used to characterize urban warming. Of course, the simple existence of an urban heat island is not proof that urban warming is occurring during the period of recorded temperature change.

The goal of the Forcings and Chaos' urban warming team is to determine the physical quantities that will define urban warming. The following section defines our specific objectives and the rationale of our approach.

Research Objectives

Determine the Temperature indicator(s) that most clearly show Urban Warming.

Rationale: Assume that Temperature (T) is the best proxy for describing the urban heat island effect and that trends in T can be used to study urban warming. Now, it has been observed that the largest urban heat island effect occurs a few hours after sunset and that the middle of the day experiences only small urban heat island effects. This is because the urban surface differs from the rural one in terms of heat flux balance. It absorbs more heat during daytime for release at night and has smaller heat loss through evapotranspiration.

This indicates that we need to explore the climatic trends of various T variables to determine whether urban warming can be isolated and, if so, how it manifests itself. Since T measurements a few hours after sunset are not readily available for most cities, other possible candidates are: Tmax(u), Tmax(r), Tmin(u), Tmin(r), Tmax(u)-Tmax(r), Tmin(u)-Tmin(r), Tmax(u)-Tmin(u), Tmax(r)-Tmin(r), where u=urban and r=rural.

Determine the seasonality of Urban Warming

Rationale: The urban heat island effect results from differences in radiative and heat fluxes and turbulent exchanges between the city and the rural area. Therefore, the amplitude of the effect will depend on synoptic conditions and will be largest during calm and clear days. Since discrimination of T measurements according to synoptic conditions is not readily available for most cities, one must rely on averaging to minimize variability due to weather activity. Hence, the above T variables will be averaged over seasons (i.e. summer and winter) in order to study the seasonality of urban warming.

Determine the relationship of Urban Warming to city growth

Rationale: Many attempts have been made to relate the urban heat island effect to city size. Most studies used population has a measure of size. On the one hand, population may be a poor proxy as it does not represent any real physical characteristic. On the other, it has been observed that larger cities possess larger urban heat islands. Can the urban warming trend (as represented by the above T variable(s)) be related to its city's population growth?

Research Tasks

Select a small number of North American urban and rural areas and conduct the research related to the above objectives.

Develop a lesson on the Urban Heat Island Effect.