On the determination of the spatial energy balance of a megacity on the example of Cairo, Egypt

Abstract

This research deals with different aspects of the spatial urban energy balance on the example of the megacity Cairo, Egypt. The energy balance and its single terms were measured in situ during a field campaign in Cairo at three different locations (urban, suburban agricultural and suburban desert) from November 2007 to February 2008. The net radiation and the heat fluxes showed distinct variations between the three stations, representing part of the spatial diversity of the area. The net radiation was highest at the suburban-agricultural location; lowest values were recorded at the suburban-desert station. The urban station ranged in between. The soil heat flux was only measured at the two suburban sites and proved to be highly dependent on the storage term. While the urban and the suburban-desert station had comparable turbulent heat fluxes, the suburban-agricultural station stand out with a low sensible but very high latent heat flux. Cairo acted as a nocturnal heat island - comparing the urban with the two suburban stations. During the day however, the suburban-desert temperatures topped the urban temperatures. The spatial diversity was also captured using various remote sensing approaches using ASTER satellite data. The strong heterogeneity of the area of interest proved to be the major challenge for the different approaches. The estimation of the net radiation was dependent on a accurate atmospheric correction, which was complicated by the heavy, but spatially varying air pollution over the megacity. The determination of the ground heat flux was done using empirical equations. Some of the used approaches proved to be applicable even in this extreme environment. One promising, as simple approach for the turbulent heat fluxes (S-SEBI: Simplified Surface Energy Balance Index) was not usable in the area due to observed high variations in surface temperatures in the desert. Two other approaches (LUMPS: Local-Scale Urban Meteorological Parameterization Scheme and ARM: Aerodynamic Resistance Method) could be used to deduct turbulent heat fluxes in a satisfactory range. However, the spatial analysis showed that more research is needed to represent turbulent fluxes in such a heterogeneous area. Besides this, a small study on the estimation of aerodynamic resistance to heat using morphometric methods was conducted. The study showed that the aerodynamic resistance to heat can be estimated successfully from a digital surface model, knowing surface specific empirical parameters. Besides the energy balance research, also the CO2 flux and concentrations were analysed. The CO2 flux showed a clear weekly dependence on the traffic, but generally fluxes were low considering the strong emissions induced by the old cars and the heavy traffic of Cairo. This result might be due to the spatial distance of the measurement to the streets. Two in-depth studies about the urban albedo were conducted additionally to the flux research, analysing the dependence of the satellite measured albedo on the sun’s position, atmospheric scattering, housing density and viewing angle.