Urban energy exchanges monitoring from space

Nektarios Chrysoulakis, Sue Grimmond, Christian Feigenwinter, Fredrik Lindberg, Jean-Philippe Gastellu-Etchegorry, Mattia Marconcini, Zina Mitraka, Stavros Stagakis, Ben Crawford, Frans Olofson, Lucas Landier, William Morrison (+1 others)
2018 Scientific Reports  
One important challenge facing the urbanization and global environmental change community is to understand the relation between urban form, energy use and carbon emissions. Missing from the current literature are scientific assessments that evaluate the impacts of different urban spatial units on energy fluxes; yet, this type of analysis is needed by urban planners, who recognize that local scale zoning affects energy consumption and local climate. Satellite-based estimation of urban energy
more » ... es at neighbourhood scale is still a challenge. Here we show the potential of the current satellite missions to retrieve urban energy budget fluxes, supported by meteorological observations and evaluated by direct flux measurements. We found an agreement within 5% between satellite and in-situ derived net all-wave radiation; and identified that wall facet fraction and urban materials type are the most important parameters for estimating heat storage of the urban canopy. The satellite approaches were found to underestimate measured turbulent heat fluxes, with sensible heat flux being most sensitive to surface temperature variation (−64.1, +69.3 W m −2 for ±2 K perturbation). They also underestimate anthropogenic heat fluxes. However, reasonable spatial patterns are obtained for the latter allowing hot-spots to be identified, therefore supporting both urban planning and urban climate modelling. The Urban Energy Balance (UEB) needs to account for the 3D nature of cities, quantifying the fluxes into, out of and the storage change within the control volume (Fig. 1) . In the last 15 years, significant advances in understanding urban processes have benefited from enhanced computational capacity, improved resolution of satellite sensors and increased ability to couple advance urban surface parameterization schemes with atmospheric models 1-9 . Earth Observation (EO) has been widely used to study the Urban Heat Island (UHI) phenomenon, but to a lesser extent to quantify heat fluxes 10-15 . Recent studies 16,17 have investigated the potential of EO to derive turbulent heat fluxes and identify and analyse the associated uncertainties. Both Earth system science and urban planning communities need spatially disaggregated UEB data at local scale 18-20 (neighbourhood, e.g., order (100 m × 100 m) or larger). However, such information is practically impossible to derive for extensive areas by in-situ flux measurements and EO-based estimation of spatio-temporal patterns of different UEB components is challenging. Therefore, the question arises whether EO can provide reliable estimates of UEB at the times of satellite acquisitions. Here we address this by investigating the potential of the current satellite missions to retrieve UEB fluxes at local scale, supported by meteorological observations. Recognising the range of city forms, size and settings, we explored three locations: London (UK), Basel (Switzerland) and Heraklion (Greece). Results and Discussion EO data from various sources were used to extract urban surface morphology and characteristics. Surface cover and material type were derived using advanced machine learning and fusion techniques and detailed spectral un-mixing approaches 21 (e.g., Fig. 2a ). Using EO-derived high resolution Digital Surface Models (DSM), surface roughness parameters (such as plan area index, frontal area index, roughness length and zero-displacement height) were calculated by morphometric analysis 22 . Examples of the morphometric analysis results are given in Supplementary Fig. S1 . Satellite-derived thermal radiance at 1 km × 1 km was downscaled 23 to 100 m × 100 m. Although uncertainties of the downscaling method exist, as Mitraka et al. 24 noted, the error of downscaled
doi:10.1038/s41598-018-29873-x pmid:30065389 pmcid:PMC6068159 fatcat:ebc2rhtcgbgwvnoyvppeeyeura