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C. Georgakis, M. Santamouris, and G. Kaisarlis

Abstract

The intraurban temperature variation in the center of Athens, Greece, was investigated in relation to urban geometry. This paper describes two main tasks: 1) Air temperature was recorded in the center of Athens and at the Meteorological Service Station at the University of Athens. Experimental data were collected through extensive monitoring at four different heights inside five different urban canyons in the center of Athens during the summer period. A measurement uncertainty analysis was carried out to estimate critical threshold values of air temperature below which differences were not significant. 2) The correlation between urban–suburban air temperature differences was assessed, using the geometrical characteristics of each urban street canyon. Urban–rural air temperature differences were considered to be not important if they were below the threshold value of 0.3°C. It was concluded that the major factor controlling urban–suburban air temperature differences was the geometry of the urban area. Other factors were the orientation of the observational sites, the current weather conditions, and the inversion of air masses adjacent to the ground level. An increase in the value of aspect ratios leads to a decrease in the difference between air inside the canyons and at the suburban station. The air temperature profile in an open-space area was the most important defining factor for the stratification of the urban–rural air temperature differences.

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A. Synnefa, A. Dandou, M. Santamouris, M. Tombrou, and N. Soulakellis

Abstract

The mitigation of the heat island effect can be achieved by the use of cool materials that are characterized by high solar reflectance and infrared emittance values. Several types of cool materials have been tested and their optical and thermal properties reveal that these materials can be classified as “cool” with the ability to maintain lower surface temperatures. Cool materials can be used on buildings and other surfaces of the urban environment. Based on these results, a modeling study was undertaken to assess the urban heat island effect over Athens, Greece, a densely populated city, by trying to analyze the impacts of large-scale increases in surface albedo on ambient temperature. Numerical simulations were performed by the “urbanized” version of the nonhydrostatic fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5, version 3-6-1). Two scenarios of modified albedo were studied: a moderate and an extreme increase in albedo scenario. It was found that large-scale increases in albedo could lower ambient air temperatures by 2°C. Furthermore, the impact of high albedo measures on heat island magnitude was estimated by creating a spatial representation of the urban heat island effect over the modeled area. The results of this study can help to promote the adoption of high albedo measures in building energy codes and urban planning regulations.

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M. Santamouris, G. Mihalakakou, B. Psiloglou, G. Eftaxias, and D. N. Asimakopoulos

Abstract

Three methods for analyzing and modeling the global shortwave radiation reaching the earth’s surface are presented in this study. Solar radiation is a very important input for many aspects of climatology, hydrology, atmospheric sciences, and energy applications. The estimation methods consist of an atmospheric deterministic model and two data-driven intelligent methods.

The deterministic method is a broadband atmospheric model, developed for predicting the global and diffuse solar radiation incident on the earth’s surface. The intelligent data-driven methods are a new neural network approach in which the hourly values of global radiation for several years are calculated and a new fuzzy logic method based on fuzzy sets theory. The two data-driven models, calculating the global solar radiation on a horizontal surface, are based on measured data of several meteorological parameters such as the air temperature, the relative humidity, and the sunshine duration.

The three methods are tested and compared using various sets of solar radiation measurements. The comparison of the three methods showed that the proposed intelligent techniques can be successfully used for the estimation of global solar radiation during the warm period of the year, while during the cold period the atmospheric deterministic model gives better estimations.

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