High-Resolution Climatic Characterization of Air Temperature

Bulletin of Atmospheric Science and Technology https://doi.org/10.1007/s42865-021-00038-5 RESEARCH ARTICLE High‑resolution climatic characterization of air temperature in the urban canopy layer Enea Montoli1 · Giuseppe Frustaci1 · Cristina Lavecchia1 · Samantha Pilati1 Received: 29 April 2021 / Accepted: 27 July 2021 © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2021 Abstract Urbanized environments are of greater relevance because of the high and still rapidly increasing percentage of the world population living in and around cities and as the pre- ferred location of human activities of every type. For this reason, much attention is paid to the urban climate worldwide. Among the UN 2030 17 Sustainable Development Goals, at least one concerns resilient cities and climate action. The WMO supports these goals pro- moting safe, healthy, and resilient cities by developing specially tailored integrated urban weather, climate, and environmental services. An unavoidable basis for that is an improved observational capability of urban weather and climate, as well as high-resolution modeling. For both the former and the latter, and of primary importance for the latter, urban meteorological surface networks are undoubtedly a very useful basis. Nevertheless, they are often unft for detailed urban climatological studies and they are generally unable to describe the air temperature feld in the urban canopy layer (UCL) with a spatial resolution which is sufcient to satisfy the requirements set by several professional activities and especially for local adaptation measures to climate change. On the other hand, remote sensing data from space ofer a much higher spatial resolution of the surface characteristics, although the frequency is still relatively lower. A useful climatological variable from space is, for instance, the land surface temperature (LST), one of the WMO Essential Climate Vari- ables (ECV). So often used to describe the Surface Urban Heat Islands (S-UHI), LST has no simple correlation with UCL air temperature, which is the most crucial variable for planning and management purposes in cities. In this work, after a review of correlation and interpolation methods and some experimentation, the cokriging methodology to obtain surface air temperature is proposed. The implemented methodology uses high quality but under-sampled in situ measurements of air temperature at the top of UCL, obtained by using a dedicated urban network, and satellite-derived LST. The satellite data used are taken at medium (1 × 1 km2) resolution from Copernicus Sentinel 3 and at high resolution (30 × 30 m2) from NASA-USGS Landsat 8. This fully exportable cokriging-based methodology, which also provides a quantitative measure of the related uncertainties, was tested and used to obtain medium to high spatial resolution air temperature maps of Milan (Italy) and the larger, much populated, but also partly rural, surrounding area of about 6000 km 2. Instantaneous as well as long period mean felds of fne spatially resolved air temperature obtained by this method for selected weather types and diferent Urban Heat Island confg- urations represent an important knowledge improvement for the climatology of the urban Extended author information available on the last page of the article Vol.:(0123456789)1 3 Bulletin of Atmospheric Science and Technology and peri-urban area of Milan. It fnds application not only in more detailed urban climate studies but also in monitoring the efects of urban activities and for the assessment of adaptation and mitigation measures in the urban environment. Finally, the frst set of interactive maps of medium–high resolution UCL air temperature was produced in the framework of the locally funded ClimaMi project and made freely available to urban authorities and professionals as an improved climatological basis for present and future plans and projects to be developed in the framework of the national and international adaptation and mitigation measures. Keywords Air temperature · Land surface temperature · Urban canopy layer · Urban heat island · Cokriging 1 Introduction Conurbations are known to be inhabited by an increasingly larger fraction of the global population: this reached 56.2% at the end of 2020 and could grow up to about 70% by 2050, while in Europe, this level has already been exceeded and is now about 75% (UN, World Urbanization Prospects 20181). The urban environment’s relevance under all aspects of human activities and wellness is thus evident, even if the urbanized land surface is a small fraction of the Earth’s surface. In particular, air temperature at the level where inhabitants live and operate is undoubtedly among the frst variables to be considered. In cities, this corresponds to the temperature in the urban canopy layer (UCL), which is defned as the part of the urban atmosphere between the surface and the envelope of building tops or, better, the mean height of buildings and trees (AMS 20202). This particular sub-layer of the more general atmospheric boundary layer (ABL) is characterized by complex interactions between air and surfaces: they are generally multi-oriented in three dimensions and may have very diferent radiative properties. Furthermore, these interactions are also a function of time due to temporal variations in solar irradiance (daily and seasonally) and anthropogenic activities and can signifcantly change with urbanistic modifcations. Both the structural and physiological characteristics of a city are important in defning the continuously variable heat and radiative fuxes, which determine the temperature of the air in the canopy: it is also evident that each urbanized environment has its properties, often much diversifed as a function of regional climates, urbanistic designs, lifestyle of inhabitants, and other cultural aspects. Under this respect, each city or even each borough needs to be studied independently, and only a few rules can be asserted in a general way: for instance, the existence of the Urban Heat Island (UHI) phenomenon, which is quite related to the dimension and population of the city (Zhou et al. 2017). Nevertheless, also the UHI climatology is strongly site-dependent (Oke et al. 2017). Detailed and accurate observations are an essential part of the UCL knowledge, and much has been done during specifc measurement campaigns as BUBBLE (Rotach et al. 2005) or in testbeds as in Birmingham with BUCL (Chapman et al. 2015). The acquired data can then be used to validate meteorological models specifcally developed to describe the urban atmosphere. But, surprisingly, less is done for the urban environment from a 1 https:// popul ation. un. org/ wup/ 2 https:// gloss ary. amets oc. org/ wiki/ 1 3 Bulletin of Atmospheric Science and Technology climatologically point of view, even if the urban climate is subject to modifcations follow- ing urbanistic evolution as well as regional climate change (Milesi and Churkina 2020). Climatological knowledge implies continuous and accurate monitoring, which up to recent times has not been done systematically and coherently in urban environments (Muller et al. 2013). Furthermore, the need for a better description of the actual (and not only future) climate in cities is more and more requested in the framework of adaptation and mitigation plans developed internationally, to achieve specifc UN Sustainable Development Goals3: 11-Sustainable Cities and Communities, and 13-Climate Action. Therefore, continuous and reliable measurements are an essential contribution to resil- ience: it is of great relevance their transformation in a form (like suitable indexes) eas- ily accessible and directly usable by technical authorities, professionals, and practitioners, who directly impact projects and realizations that, especially in cities, have sensible conse- quences to local and micro-climates and immediate efects on humans and human activities. In fact, global climate change will afect cities as any other part on the Earth, but due to the large percentage of the urbanized population and the complexity and sensibility of urban interactions, the outcomes are of utmost relevance: variations in the precipitation regimes could enhance inundations, as, for instance, more frequent and intense heat waves could positively interact with UHI, severely afecting human wellness and even life. There- fore, as stated in IPCC Special Report - Global Warming of 1.5°C, the observed global warming is of great concern for cities. Specifc actions started in recent years to cope with its efects in urbanized environments both at global and at the national level: for instance, UN-Habitat’s Cities and Climate Change Initiative4 (CCCI), the C40 Cities Climate Leadership Group5 (C40), the Covenant of Mayors EU6 as part of the EU Adaption Strategy, and several National and Regional Adaptation Plans to Climate Change, which all afect cities as a relevant part. Furthermore, the IPCC has recognized the key role of cities in responding to climate change and has proposed that the seventh assessment cycle include a Special Report on Climate Change and Cities (Masson et al. 2020). Both mitigation and adaptation plans are then required and urgent. Supposing that mitigation could be achieved by energy savings adaptation implies mod- ifcation and planning of existing (and planning of new) buildings and other urban ele- ments in order to achieve more comfortable environmental conditions. In both cases, ther- mal considerations are an essential part of

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