The Basel UrBan Boundary Layer Experiment (BUBBLE) was a year-long experimental effort to investigate in detail the boundary layer structure in the City of Basel, Switzerland. At several sites over different surface types (urban, suburban and rural reference) towers up to at least twice the main obstacle height provided turbulence observations at many levels. In addition, a Wind Profiler and a Lidar near the city center were profiling the entire lower troposphere. During an intensive observation period (IOP) of one month duration, several sub-studies on street canyon energetics and satellite ground truth, as well as on urban turbulence and profiling (sodar, RASS, tethered balloon) were performed. Also tracer experiments with near-roof-level release and sampling were performed. In parallel to the experimental activities within BUBBLE, a meso-scale numerical atmospheric model, which contains a surface exchange parameterization, especially designed for urban areas was evaluated and further developed. Finally, the area of the full-scale tracer experiment which also contains several sites of other special projects during the IOP (street canyon energetics, satellite ground truth) is modeled using a very detailed physical scale-model in a wind tunnel. In the present paper details of all these activities are presented together with first results.

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 fluxes 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.

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Cairo Air Pollution and Climate (CAPAC) is dedicated to the understanding of the urban energy balance in Cairo, Egypt, through measurements from space and at ground stations. The in situ measurements will provide a focussed insight into three carefully chosen microclimates (urban, suburban-agriculture, and suburban-desert) and provide at the same time ground-truth data for satellite image analysis, which will expand the acquired knowledge into the spatial domain. In situ measurements were made during a field campaign in Greater Cairo from November 2007 to February 2008. In this study, the dataset of the CAPAC measurement campaign will be presented and analysed in terms of use for a remote sensing study. Measured variables complied with our expectations. The urban area featured a distinct nocturnal heat island. During the day the choice of reference station was responsible for the magnitude of the heat island. The diurnal cycle of radiative temperature at the suburban-desert station clearly exceeded the one at the urban station, thus the urban setting seemed to have a better heat storage than the suburban-desert. The stations also determined the partitioning of the turbulent heat fluxes. While in Cairo and at the suburban-desert station most of the available energy was partitioned into the sensible heat flux, the suburban-agricultural station maintained a high latent heat flux. The radiation and soil heat flux measurements proved to be applicable for comparison with remotely sensed data. However, the analysis of the turbulent heat fluxes showed that several constraints exist: measured fluxes tend to underestimate the actual flux and directional effects complicate the interpretation. An energy balance closure and footprint modelling is necessary to compare measured fluxes with satellite image retrieved products. Finally, turbulent fluxes are time averages, which is contrary to the remote sensing principle. Consequently, a direct use is problematic.

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