Integrating the Urban Canopy Layer in a Lagrangian Particle Dispersion Model

<p>Traditional Lagrangian particle dispersion models reflect particles at the zero-plane displacement height and therefore cannot properly take near-ground effects into account. In this study, we investigate whether including the urban canopy layer improves the performance of such a Lagrangian particle dispersion model. Here, spatially averaged flow and turbulence profiles throughout the urban canopy are constructed based on data from the literature (mostly from wind tunnel and numerical modeling studies).</p><p>We apply a first-order approach to test to what degree the explicit inclusion of the urban canopy changes the simulated concentration distributions. In a comprehensive sensitivity study, we show that most of the parameters introduced to describe the turbulence and flow profiles in the canopy have a relatively minor impact on the dispersion (and hence concentration distribution) – despite their inherent uncertainty. In particular, concentration fields are more sensitive to previously existing parameters of the model. One exception is a parameter describing the mean canopy wind speed profile, to which the model is sensitive.</p><p>When compared to data from the BUBBLE tracer experiment, the results show that the inclusion of the urban canopy layer slightly improves the modelled concentration values. The improvement is minor and might likely differ when comparing with other field experiments. However, the key point here is that the increased complexity and added capability of near-ground concentration simulation did not fundamentally change the model performance.</p><p>Ultimately, inclusion of the urban canopy layer will allow the model to be used as the dispersion core for an urban footprint model with footprint estimates near the ground.</p>

Influence of Urban Canopy Layer on a heavy rainfall over Beijing

<p>Urban canopy layer (UCL) is generally considered in numerical study of urban meteorology. The weather research and forecasting Model (WRF) coupled with urban canopy layer scheme is used to simulate a heavy rainfall case in Beijing. Comparative analysis is applied for the case between coupled simulation and non coupled simulation and therefore exhibits the effect of the UCL on the rainfall. Sensitive experiments are performed to investigate anthropogenic heat source and urban area extension to affect the precipitation. The results show that the coupled UCL model has captured the rainfall characteristics at the regional scale. The coupled simulation has improved accuracy of the rainfall area, the peak value and the rainfall duration compared to the non coupled simulation. The main effect achieves as longer duriation of the ascending motions and enhancement of the layers unstabilities. Although the intensity of the vertical motion has a little reduction, the time of the motion has increased 2 hours in a day. Sensitive experiments present an obvious influence on precipitation intensity, precipitation centralization and heat island effect. The precipitation center moves toward the urban center, the accumulated rainfall increases 78.5 mm and the center moves by distance 13 km when anthropogenic heat source is perturbed to double. Urban area extension induces increase of the precipitation area and intensity due to high humidity and ascending motion. The experment also reveals shift of the island heat effect.</p>

Study on the Change Characters of the Urban Spatial Light Pollution from Ground to Zenith

As the problem of light pollution becomes more serious, more and more scholars pay attention to this issue and carry out related research. In the perspective of cities, the measurements of light pollution mainly focus on the brightness of the sky or artificial lighting on the ground. However, there is lack of research on the whole urban space. With the two-dimensional brightness analyses, this paper processes the changes of the light environment of the whole urban space into image quantization. It gets the 3D and 2D light environment changes of luminance distribution, color temperature distribution and chromaticity in the three space layers, the ground layer, the urban canopy layer and the sky layer, from dusk with natural light to night with artificial lighting completely. It is found that the brightness difference between the light environments among the three city levels gradually reduces with the measuring time, and the final values maintain at 0.11~0.25 cd/m2. In the ground layer, the light environment is mainly affected by the lighting facilities, and vegetation can prevent the light from scattering up. The light environment of the urban canopy layer is the brightest in the whole city space and has the largest influence on the sky layer. The color concentrates in the range of yellow and red. The color temperature near the ground distributes in 3000K~15000K, and near the sky distributes in 2300K~2700K which is warmer than the natural night sky. The sky brightness of Dalian city is about 951 times than the natural night sky. 

Absolute moisture content in mid-latitude urban canopy layer

This part of the study on absolute moisture content in the mid-latitude urban canopy layer first gives a comparison on intra-urban relative and absolute humidity patterns showing an example based on a long dataset. The comparison clearly demonstrates the usefulness of the utilization of absolute measure opposite to the temperature dependent relative one. This supports the earlier statements found in the literature albeit these statements are based on only case studies or short datasets. Then a short overview follows which presents the main results of studies about urban absolute moisture content. These studies focused mainly on urban-rural and less on intra-urban humidity differences. The scale differences are used for the grouping of studies based on the number of available measurement sites as well as their spatial distribution and density in the investigated urban regions.

Absolute moisture content in mid-latitude urban canopy layer

This study gives a comprehensive picture on the air humidity observation and mapping in urban canopy layer in Szeged, Hungary, analyzing three-year long vapor pressure dataset (e) calculated from observations of a 22-station urban network. The analysis was divided into two directions, namely the urban-rural and intra-urban ones where the latter was partly based on the local climate zone approach. (i) The general features of the annual and diurnal variations of urban-rural absolute humidity difference in cities with mid-latitude climates are also detectable in the case of Szeged. (ii) In the annual and seasonal e means there is no clear zone sequence that would follow the differences in the compactness or building height of the zones and even the built-up versus land cover distinction. (iii) The highest e values and their differences among stations appear in summer, while the lowest ones in winter and the values of transitional seasons are between them. In certain cases the intra-zone differences can exceed the inter-zone ones since the effect of microscale environment is essential. The decisive factors are the permeability of the surface and the vegetation cover. (iv) The diurnal course of the e pattern in normalized 4-hour time steps does not show a regular shape, the patterns are mosaic-like: in all time steps the driest and wettest areas are mainly in the north-western and south-eastern parts, respectively.