Flux measurements in Cairo. Part 1: in situ measurements and their applicability for comparison with satellite data

Abstract

AbstractCairo 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. Copyright © 2010 Royal Meteorological Society