Using Local Climate Zone scheme for UHI assessment: Evaluation of the method using mobile measurements

Highlights

• Local Climate Zone scheme has been successfully applied in Nancy (France).

• Screen-height air temperature has been measured in 13 LCZ through mobile survey.

• Nocturnal temperature amplitude varies mostly from 0.5 °C to 2 °C for urban LCZ types.

• Recurrent thermal inhomogeneities in LCZ can be observed both daytime and nighttime.

• Average nocturnal temperature differences between LCZ types range from 0.2 °C to 4.4 °C.

Abstract

In the context of expansion of cities and raise of climate change awareness, urban planers are looking for methods and tools in order to take into account the urban heat island phenomenon. This study analyzes the way urban fabric modifies urban climate through the utilization of a climate scheme called Local Climate Zone (LCZ). This classification has been applied in Nancy (France). Urban indicators have been calculated so as to build 13 LCZ in the Great Nancy Area. The screen-height air temperature distribution has been investigated inside these LCZ via mobile measurements. Air temperature amplitude has mainly demonstrated lower values at nighttime than in daytime in urbanized LCZ types. Recurrent microscale hotspots and coldspots have been located in LCZ presenting heterogeneous urban fabric. Two Control Sites (CS) have been built in each LCZ. The CS average temperature has revealed good likeness with the spatially averaged air temperature. Average nocturnal air temperature differences between pairs of LCZ types have been obtained. These differences vary from less than 1°°C for close LCZ types to more than 4°°C for dissimilar LCZ types.

Introduction

According to climate change scenarios, European cities are likely to experience more intense and more frequent heat waves in the twenty first century [7]. In urban context, heat waves can have significant implications on public health [16], energy consumption [9] and outdoor thermal comfort [2]. Consequently, the integration of future climate variability to urban planning strategies becomes a raising challenge for the next decades [1], [12].

Heat waves periods tend to reinforce the Urban Heat Island (UHI) phenomenon [6], [29]. The amplitude of the UHI is usually expressed as the synchronous screen-height air temperature difference between urban and rural thermal sensors [27]. Recognized causes of UHI are the decrease of wind speed, the increase of radiative trapping due to the three-dimensional morphology of the cities, the energy storage in urban materials, the decrease of evapotranspiration in relation to the reduction of vegetalized and pervious surfaces in city centers and the heat release due to human activity [13].

In order to investigate the relationships between urban fabric and local climate, scientists have developed different classifications such as the FRAISE (“Flux Ratio-Active Index Surface Exchange”) scheme [18], which defines Urban Zones that characterize Energy partitioning (UZE). UZE classification involves the Surface Energy Balance (SEB) scheme [20] and aims to distinguish urban areas in terms of how they partition incoming total downward radiation (which corresponds to solar radiation and longwave radiation). Since urban environment implies local climate modification, another relevant approach consists in incorporate urban indicators to design a climate classification. Urban Climatic Maps (UCMaps) are based on this approach. They are built using land use information, topographic information and climatic data (see Ref. [23] for a review). Once produced, these maps provide enhanced climatic understanding of the region of interest and urban planning recommendations.

In the same manner, Urban Climate Zone (UCZ) is a simple urban classification scheme which uses urban descriptors dealing with urban morphology and land use [21]. UCZ scheme classifies urban areas regarding the way they modify the wind, thermal and moisture variables. This work has been extended by Stewart who further introduced Local Climate Zone (LCZ) [26]. A Local Climate Zone is defined as an area with a minimum diameter of 400 m which demonstrates both uniform features in terms of urban morphology, land use, urban material and urban metabolism, and a characteristic screen-height temperature regime under calm and clear sky [27].

One of the major advantages of the LCZ scheme is the possibility to redefine urban heat island magnitude. As Stewart highlighted [26], the usual UHI definition can be discussed, on the one hand because the use of the adjectives “urban” and “rural” may be misleading, and on the other hand because these adjectives may be too simple to describe the complex layout of cities. Unlike the common definition, the LCZ scheme uses standardized descriptions of urban and rural landscapes and defines the UHI amplitude as the screen-height air temperature difference between two specific LCZ that are significantly different in terms of morphology and land cover. The use of UCZ and LCZ schemes has provided an overview of the air temperature distribution in Glasgow (UK) [10], Toulouse (France) [14], Nagano (Japan), Vancouver (Canada) and Uppsala (Sweden) [28]. Nocturnal difference of 2.6 °C have been observed in Toulouse between UCZ 2 and UCZ 7 whereas nocturnal difference from 2.8 °C to 5 °C have been recorded between LCZ 2 and LCZ D in Uppsala.

This work aims to apply the LCZ scheme in the Great Nancy Area (France) in order to investigate on what extent a homogeneous urban fabric leads to a specific thermal pattern under calm, clear sky. The air temperature distribution inside LCZ, the relationship between urban indicators and air temperature features, and the air temperature differences between LCZ types are particularly analyzed.