Vertical Energy and Momentum Fluxes in the Centre of Athens, Greece During a Heatwave Period (Thermopolis 2009 Campaign)

Abstract

The atmospheric energy budget in the centre of Athens, Greece was determined during the Thermopolis 2009 campaign in order to investigate the development of the urban heat island. Heatwaves during summer are a common occurrence in this large conurbation. Micrometeorological data from a tower were acquired in a densely built central district, and net all-wave radiation, sensible heat, latent heat and momentum flux densities were derived by the eddy-covariance method and also estimated using Monin-Obukhov similarity relationships. Under the prevailing hot and dry conditions, sensible heat-flux density was on average five times larger than the latent heat-flux density. The anthropogenic contribution to the energy budget was also determined on the basis of the acquired data.

Integrating Urban Form, Function, and Energy Fluxes in a Heat Exposure Indicator in View of Intra-Urban Heat Island Assessment and Climate Change Adaptation

“…A change of 2 K of the term results in a deviation of ~20% for the estimated QH, which is an uncertainty of ~30 W m −2 in the case The validation of the modeled Q H and Q E values was performed using the turbulent fluxes, which were calculated employing the EC method from the available flux tower measurements during July 2009. The Q H values estimated by the EC techniques ranged here from slightly positive nocturnal values to an average peak of ∼150 W m −2 at midday, which is in accordance with the results presented in [79]. In addition, relatively low Q E (EC) values were found, with a daily average maximum value of approximately 30 W m −2 .…”

“…In addition, relatively low Q E (EC) values were found, with a daily average maximum value of approximately 30 W m −2 . For the detailed presentation and analysis of the measured energy and momentum fluxes at the Thermopolis campaign site, the reader is referred to the corresponding research papers [79,87]. To expand the synchronous remote sensing imagery and EC estimates availability in this study, along with the midday MODIS-TERRA, nighttime MODIS-TERRA (∼23:00 local time), and early afternoon MODIS-AQUA (∼15:00 local time), thermal images were used that were preprocessed as in [55] (imagery with a sensor viewing angle over 55 • were not included).…”

“…With increasing distance from the city center core, the anthropogenic emissions are becoming lower in magnitude (Figure 9c). For Patission Street (Thermopolis site), an estimation for Q F of approximately 50 W m −2 has been previously derived in [79]-for a coincident period under similar meteorological conditions (July 2009)-using the assumption that during the daylight hours, anthropogenic heat emissions are included in the sum of turbulent fluxes and upwelling longwave radiation, and subsequently employing a fluxes time lag between normal working days and Sunday. Despite the differences in methodologies in [79] and in this study, a reasonable level of agreement is found, with estimated daytime values of ∼40 W m −2 (Figure 9c).…”

“…For Patission Street (Thermopolis site), an estimation for Q F of approximately 50 W m −2 has been previously derived in [79]-for a coincident period under similar meteorological conditions (July 2009)-using the assumption that during the daylight hours, anthropogenic heat emissions are included in the sum of turbulent fluxes and upwelling longwave radiation, and subsequently employing a fluxes time lag between normal working days and Sunday. Despite the differences in methodologies in [79] and in this study, a reasonable level of agreement is found, with estimated daytime values of ∼40 W m −2 (Figure 9c). The temporal profile shown in this figure presents differences from the profile in Figure 9a, due to the stronger relative contributions of vehicular, residential, and metabolic emissions in the area compared to the commercial buildings' Q F component.…”

“…Eddy covariance (EC) is a micrometeorological technique that allows the direct measurement of vertical turbulent fluxes, using the high-frequency data of the vertical velocity component, along with the scalar quantities of the corresponding atmospheric parameters. Employing this method, the validation of Q H and Q E was performed using flux tower measurements from the European Space Agency (ESA) Thermopolis campaign that took place at the Athens city center in July 2009 (tower measurements conducted from 18 to 27 July) [78,79]. The fast-response measurements of wind, temperature, and humidity, as performed by the Democritus University of Thrace (DUTH) and provided for this work by ESA (https://earth.esa.int/web/guest/campaigns), were preprocessed using standard eddy covariance flux corrections [79] employing the processing code from [80].…”