UCP6: Radiation processes
Urban-rural differences in longwave radiation – Lódz case study
University o Lodz, Poland
The influence of the city on shortwave radiation is well established. It is well accepted that the solar radiation is reduced about 10% in the case of annual totals, about 20% for monthly totals and more than 30% for selected days. In contrary, the city role in modification of longwave radiation is poorly documented, even if general processes leading to alternation of this radiation by a city are well understood. The pollution in urban atmosphere increases downward longwave radiation, Ldown, about 6-10%, but this estimations based on a few available publications only. Here will we present a comparison of data based on three years measurements at two sites in central Poland. The urban site is located in the centre of Lódz (population 710 000), and the rural point is about 50 km east, at typical Polish agricultural area. The large distance between sites reduces probability of the city influences on rural site, but needs a careful data selection to exclude influence of synoptic variability of cloudiness. The preliminary results show that under favourable weather conditions Ldown can be about 8-14% lower at rural site. In the same time the upward longwave radiation in the centre of Lódz is about 10-20% higher.
Spatial variability, horizontal anisotropy and diurnal evolution of measured infra-red fluxes in a city neighborhood of Toulouse
1CEREA, France; 2Meteo France, CNRM, France
In flat and uniform terrain, the infra-red (IR) fluxes are horizontally homogeneous and isotropic, depending only on the vertical direction. The diurnal evolution, with ground cooling at night and heating during the day is very uniform. This can be modeled with the “plane-parallel” approximation where the fluxes depend only locally on the vertical coordinate and that is the basis of a number of radiative models.
In urban area, however, local variations of the urban fabric, such as big buildings, little houses, parks …, lead to both a horizontal variation of the upward IR fluxes and to an anisotropy of their horizontal component. For example in a northern hemisphere mid-latitude city, a southward facing wall receiving the sunlight will radiate more than its north facing counter part and this will evolve throughout the day and will depend on the local building layout. At night this will tend to equilibrate with the neighboring buildings. To take these effects into account the radiative model needs to be three dimensional.
In order to document and quantify these effects experimentally and in the framework of the EUREQUA project (Environmental improvement of neighborhood, sponsored by French ANR), we have deployed an IR imager accompanying a mobile meteorological measurement system and sound recordings. These mobile systems were walked in the neighborhood of Toulouse approximately every three hours during three days. At each of the 9 stop points for each hourly walk, we took one ground picture and eight horizontal pictures one for each direction. The imager records simultaneously a visible picture, an IR picture and a text file containing the IR measurements that can be later reprocessed. The images then needs to be manually tagged to each stop point and direction. These two parts have been the most labor intensive of the data collection. For image processing, we have then developed a python script to plot histograms, group images and compute simple statistics, such as the mean for each image that we will discuss here.
For each hourly walk we have collected around 100 images (x2 with the visible ones) that amounts to 2500-3000 for one Intensive Observing Period (IOP) of 3 days, which has been repeated in January, April and June in Toulouse (the project also did 2 IOP in Paris and Marseilles but they will not be discussed here).
If we first look at spatial variability of the ground IR fluxes, we notice that the variability is very small in early morning (6h UTC; all stop points around 400 W/m2 for June) but that it is maximal at 15h UTC (3h time resolution). At this time, some stop points can be around 500 W/m2 whereas as others can be close to 700 W/m2. To characterize anisotropy, based on each set of 8 horizontal pictures, we have computed the mean, minimum and maximum. Again in early morning, when the mean horizontal fluxes are minimal, all three values are very close (+- 20 W/m2). At its maximum, the anisotropy measured can reach above 100 W/m2. These measurements will be used in the future for the validation of our 3D IR radiative scheme.
Investigation of the effect of different sealed surfaces on local climate and thermal stress
1University of Natural Resources and Life Sciences, Austria; 2Smartminerals Gmbh, Vienna, Austria
Local climate is driven by the interaction between energy balance and energy transported by advected air. Short-wave and long-wave radiation are major components in this interaction. Huge differences in temperature (~10°C) between sunlit and shadowed surfaces may result from the radiation balance. Hence adjusting the grade of reflection of surfaces is an efficient way to influence this range of temperature. While reflectivity is growing with the amount of reflected radiation the absorbed radiation is transformed into thermal energy heating the affected body and giving off heat to the air.
In urban areas the specific geometry of the building structure leads to a larger surface area, thus the absorbable amount of solar radiation is higher. On the contrary undeveloped areas do not heat up like urban areas because of the higher amount of shadow and the higher capacity of evapotranspiration from vegetation. On hot summer days when the heat exchange is on a low level, buildings begin to heat up and act as a thermal storage system, leading to the well-known “heat island” effect.
Climate warming at global- and urban-scale enhance this effect, therefore using different materials for buildings or streets can be considered as an effective method to influence urban microclimate. Santamouris et al. investigated the influence of albedo of asphalt materials on air temperature. They found a decrease of surface temperature of 12 °C and of air temperature of 1.9°C - compared to a conventional asphalt surface - above an asphalt surface with a reflection of 47% in the visible and 71% in the infrared spectral range.
The goal of the present study is the comparison of two urban energy balance models (TEB and EnviMet) and their output with respect to different building and road surfaces. The models are used to simulate the air temperature of the local climate for an urban canyon in Vienna. In a next step thermal stress indices (UTCI, PMV) are calculated based on the simulations. Input parameters are taken from routine measurements of the radiation balance, of the ground and of the air temperature and humidity at different heights above the ground and from measurements of the SW and LW optical properties (albedo, emissivity) from/above 6 different types of sealed surfaces. During this measurement campaign the above mentioned components were measured over a duration of 4 months above 2 conventional asphalt surfaces, one conventional concrete and three newly developed concrete surface with increased reflectances. Measured albedo values amounted to 0.12‘0.02 for the asphalt surfaces and to maximum values of 0.56 for concrete.