Increasing summertime air temperature deteriorates human health especially in cities where the warming tendency is exacerbated by urban heat island. Human-biometeorological studies shed light on the primary role of radiation conditions in the development of summertime heat stress. However, only a limited number of field investigations have been conducted up to now. Based on a 26-hour long complex radiation measurement, this study presents the evolved differences within a medium-sized rectangular square in Szeged, Hungary. Besides assessing the impact of woody vegetation and façade orientation on the radiation heat load, different modeling software programs (ENVI-met, SOLWEIG, and RayMan) are evaluated in reproducing mean radiant temperature (
Regional climate change is expected to bring rising air temperature values and to increase the frequency, length, and severity of heat waves in Central Europe, and thus in Hungary too [
Researchers in the field of urban human-biometeorology demonstrated that radiation heat load, quantified usually as mean radiant temperature (
Numerical models are popular and easily obtainable alternatives of the time- and resource-consuming onsite investigations to determine
According to the above mentioned, this study intends to contribute to the urban human-biometeorological knowledge by conducting a detailed analysis of the evolved radiation conditions (radiation flux densities from six main directions) and the resulted
The field measurements were conducted in the city of Szeged (46.3°N, 20.1°E), the southeastern regional center of Hungary with an urbanized area of 40 km2 [
The medium-sized rectangular Bartók Square (Figure
Bartók Square of Szeged, illustrated by an aerial image, site photos, and an object elevation map.
Five measurement sites were selected to investigate the radiation load on pedestrians that either walk on the sidewalks surrounding the square or linger under the mature shade trees in the central area (Figure P1, P2, P3, and P4 are located near to buildings encircling the square. The nearest façades to these points are located at SSW, WNW, NNE, and ESE sides, respectively, at ca. 1.3 m distance. P5 is in the middle of the square, under a 10-meter tall
Survey points in the Bartók Square with their fish-eye photographs.
Two human-biometeorological stations were used to record one-minute averages of all atmospheric parameters influencing human thermal comfort (Figure
One of the human-biometeorological stations used in this study (photo taken at P3 site).
Typically, in the first position, the arm of the net radiometers pointed to the south, while the sensors were faced upwards and downwards. This means that in this position, the two pyranometers and two pyrgeometers measured
Mean radiant temperature (
The 26-hour field campaign was conducted on two consecutive late-summer days with clear sky conditions (Figure
Background weather parameters (yellow: global radiation, red: air temperature) during the field measurements (10-min average data were obtained from the inner-city weather station of Szeged, 0.9 km away from the survey site).
Three numerical simulation models were assessed in their ability to reproduce radiation conditions in complex urban environments: ENVI-met (Version 4.0 Preview III), SOLWEIG (Version 2015a Beta), and RayMan Pro (Version 3.1 Beta). The study also utilized MATLAB and MS Excel for the analysis of the results.
The digital models of the square were developed utilizing (i) the GIS map of the city, (ii) the recent urban tree inventory of Szeged, based on a comprehensive field survey conducted by the Department of Climatology and Landscape Ecology, the University of Szeged [
In the case of ENVI-met, the 116 × 151 model area had a 3-meter horizontal resolution. Besides Bartók Square, the model domain encompassed the eight adjacent urban blocks as well. The vertical resolution utilized the telescopic setup. Here, the lowest four grids were set to 0.5 meter, while from 2 meter the height of each consequent grid increased by 20%. The top of the 3D model was at 105 m with the tallest building being 38 m. The model trees were selected from the software’s predefined, species-specific, and three-dimensional tree catalogue by adjusting their physical shape and size only to match the surveyed values. The materials assigned to the ground surfaces were as follows: gravel asphalt to roads, sandy loam soil to urban blocks, and concrete pavement to paved surfaces within the square. The albedo of the gravel asphalt and the concrete surfaces was set to 0.25 and 0.35, respectively. The albedo of roofs and walls was set to 0.35, uniformly. In terms of atmospheric conditions, a simple model forcing was applied with air temperature and relative humidity values taken from the nearby urban weather station. In order to match the measured maximum global radiation values a solar adjustment factor of 0.98 was applied.
In the case of SOLWEIG, the digital surface models (DSMs) of buildings and tree canopies were derived from the city’s GIS map using 1 meter resolution. The 477 × 424 digital model encompassed several streets and urban blocks around the square. Based on a long-term tree shade survey in Szeged [
Similarly to SOLWEIG, the files describing the three-dimensional physical environment in RayMan Pro were obtained from the city’s GIS map. The process of generating digital models for RayMan requires the “Shp to Obs” plugin, which converts the coordinates of the observation points and that of the adjacent buildings and trees to the required format. The derived five digital models encompass 200 m × 200 m areas describing the surroundings of the observation points. In the same way as the other two models, the input weather data were obtained from the nearby urban weather station. In the simulations, the “reduction of global radiation (
The model evaluations were based on the 15-minute
Then statistical evaluation of the utilized models was also implemented by calculating three parameters recommended by [
In (
As illustrated by Figure
Short- (
Due to its NNE exposure, P1 received direct solar radiation only for a brief period (see
Due to its ESE exposure and lack of shading from trees, P2 received direct solar radiation for a long period (Figure
P3, with its SSW exposure and without any trees to provide shade, received the greatest amount of solar radiation for the longest period (Figure
Likewise, in the case of P4, we would expect a high irradiation load due to its WNW exposure (mainly because the adjacent building provided shade only until 13:00). However, due to the presence of a row of mature trees along the street shading the sidewalk during most of the afternoon, this location is characterized not only by the most obstructed sky view but also by the least amount of direct solar income (Figure
In the case of P5, located in the middle of the square and shaded by mature park trees,
At each site, exposure to direct solar radiation, meaning high
Figure
Sum of the short- (
At night, in the absence of solar radiation, the radiation budget consists of longwave components only. Although
Figure
Figure
Air temperature (
For most of the day, there is little difference between
Figure
Deviation of the modeled
In general, extreme deviations (i.e., peaks and valleys) are the outcome of the mismatch between observed and modeled times when a given observation point becomes irradiated or shaded. A good example for this kind of error is the graph of P2. Here, ENVI-met’s error curve dips at 8:00 (indicating that the place is still shaded according to the model), but it rebounces by 9:00 in the morning. Similarly, when the observation point becomes shaded in the afternoon at around 14:00, each model still indicates the presence of direct radiation and hence significantly overestimates the actual
Besides the model-based errors (due to model inaccuracies and coarse model resolutions) other modeling error trends can also be deduced from the results: First, all models underestimate nighttime Second, for those daylight hours when the survey points were shaded by buildings for a long time Third, all models underestimate the daytime
Table
Summary of the statistical analysis.
Six-directional measurements | ||||
---|---|---|---|---|
ENVI-met | SOLWEIG | RayMan | ||
MAE | P1 | 6.68 | 5.00 | 3.13 |
P2 | 8.27 | 6.03 | 8.09 | |
P3 | 7.85 | 9.58 | 11.61 | |
P4 | 6.45 | 3.74 | 4.40 | |
P5 | 6.67 | 4.58 | 8.58 | |
|
||||
RMSE | P1 | 7.67 | 5.92 | 3.82 |
P2 | 9.94 | 7.15 | 9.71 | |
P3 | 8.56 | 12.06 | 14.91 | |
P4 | 6.94 | 4.80 | 6.23 | |
P5 | 8.90 | 7.09 | 10.58 | |
|
||||
IA | P1 | 0.89 | 0.92 | 0.96 |
P2 | 0.93 | 0.96 | 0.91 | |
P3 | 0.96 | 0.90 | 0.83 | |
P4 | 0.80 | 0.84 | 0.67 | |
P5 | 0.82 | 0.81 | 0.78 |
For IA, values close to 1.0 indicate better model performance. The highest index of agreements was achieved by SOLWEIG in P2 and P4 locations, whereas ENVI-met excelled at P3 and P5 points. In the case of P1, RayMan’s results were closest to the measured values. In terms of IA, models performed better in the case of the survey points without tree shade. At P1, P2, and P3 the IA values were generally close to or above 0.9, while in the presence of tree shade (P4, P5) IA values were always below 0.85. The lowest IA (0.67) was obtained in the case of P4 by simulations with the RayMan. A comparatively good IA of 0.96 was achieved by RayMan at P1, by SOLWEIG at P2, and by ENVI-met at P3.
Taking into account all of the abovementioned analyses (graphical analyses of Figure
Figure
Data extracted from previous model validation studies (
With an urban population of around 75%, and especially in the light of the aging population in European countries, reduction of the adverse effects of heat waves and maintenance of comfortable conditions within cities are extremely important issues for urban planning and landscape design. Human-biometeorological studies, conducted in Central European cities, shed light on the leading role of radiation conditions in the development of summertime heat stress. Up till now, however, only a limited number of field investigations aiming to map at fine spatial and temporal resolution the variations of heat stress owing to the landscape design have been conducted. Nonetheless, their results demonstrate the potential of climate-conscious and climate-adaptive urban planning and can be used to validate the results of numerical simulations.
Our complex study comprising field measurements and numerical simulations was undertaken to investigate radiative conditions and their modeled reproduction at a complex urban environment over a 26-hour period in Szeged, Hungary. The field measurement data obtained from five measurement points were compared and the performance of three commonly available microclimate models in reproducing
The measurements confirmed that on clear summer days
The numerical model assessment found that models generally underestimate the nighttime
Our study demonstrates that detailed field measurements can enhance our understanding of microclimatic conditions at a fine-scale, which, in turn, can be used by landscape designers and architects for climate-responsive urban design. Recent planning directives of the European Commission (EC) gave priority to nature-based solutions (NBS) and hence to renaturing cities [
There are no conflicts of interest related to this paper.
The presented analyses were conducted within the frame of the Nature4Cities project, which has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant agreement no. 730468. This research was supported by the EU-funded Hungarian Grant EFOP-3.6.1-16-2016-00008. The authors would like to express their gratitude for those who supported the field measurements, especially Gábor Horváth, Márton Kiss, Ágnes Takács, and Zsuzsa Győri.