UCP2: UHI characteristics II : vertical and horizontal structure
About the relation between urban topsoil moisture and local air temperature
1Meteorological Institute, CEN, Universität Hamburg, Germany; 2Institute of Soil Science, CEN, Universität Hamburg, Germany
The climate in cities differs from that in the surrounding area due to modified surfaces. As surface sealing ratio, vegetation and building material are known to be relevant for the intensity of the microclimatic modification – what about topsoil moisture content and availability?
Soil acts as a storage and transmitter for water. According to water availability at the surface - dependant from soil physical properties and water refill from above or below - it may have different impact intensities on local climate through distinct evaporation. To find out to which extend soil hydrological characteristics and water replenishment limit the local cooling effect of soils in the urban environment is a main issue of the project HUSCO (Hamburg Urban Soil Climate Observatory). Hereby, focus lies on the impact of soil properties, urban land-use and groundwater table depth on the climate relevance of soils.
Four years record of ongoing measurements in the city of Hamburg, Germany, is evaluated. The data is provided by atmospheric and pedologic measurement sites, located within six urban districts: the city core, four suburban districts, featuring different mean groundwater table depths (> 5 m below surface / < 2.5 m below surface), and one industrial area.
It is found that groundwater table depth, soil substrate and vegetation types influence the temporal evolution of volumetric water content and water tension within the upper horizons of suburban soils. Soil hydrological processes show characteristic patterns at each measurement site, including topsoil moisture variability. Yet, differences between distinct urban land use types are visible only according to differences in the prevailing soil texture.
Air temperature (Ta) anomalies of the suburban sites from the inner city site are analysed for several periods and seasons. During daytime a significant annual mean deviation is found above unsealed, vegetated surfaces from a sealed site during days with a turbulent mixing induced by wind speed = 2 m/s and partly cloudy sky. For days matching these criteria, differences in the Ta span, i.e. increase of Ta during the day, is observed. About a fifth of the variance of the diurnal Ta span is found to be explained by topsoil water content for selected relevant days.
In this contribution the observed relation between topsoil moisture and air temperature increase during daytime at suburban sites will be presented after describing the local conditions. Case studies complement the statistical analyses.
Investigating the impact of anthropogenic heat on urban climate using a top-down methodology
University of Hamburg, Germany
Since urbanization will continue in the future and the temperatures and also the heat stress are projected to increase, the anthropogenic influence on the urban climate was investigated more intensively during the last years. Having in mind the indirect and direct effects caused by buildings and sealed surfaces, we are focusing on the anthropogenic heat emissions and its contribution to the urban climate.
One way to estimate the anthropogenic heat are the so-called inventory approaches following Sailor (2011). Sailor (2004) developed an inventory-based generic methodology for US cities. Our contribution is a generic methodology, which aims for the determination of the anthropogenic heat releases over the German region based on energy budget reports, i.e. reports provided by the State authorities with a yearly update cycle. Spatial and temporal distributions of anthropogenic heat release are determined by mapping energy consumption in different SNAP classes on high-resolution landuse data and SNAP-based emission inventory (Kuenen et al., 2010). The resulting anthropogenic fluxes are compared and evaluated with the well-established LUCY data base. It is demonstrated that our approach offers some more details about the sources of anthropogenic heat which cannot be resolved by the existing, coarse LUCY data base (e.g. the heat releases at some large industrial sites).
In order to quantify the direct impact of such anthropogenic emissions on the urban climate, mesoscale simulations were performed for the Hamburg Metropolitan region using a nonhydrostatic model and a realistic atmospheric forcing. The anthropogenic heat sources were incorporated on a computational grid with 250 m horizontal resolution. Focusing on the seasonal effect of anthropogenic heat, urban climate was investigated by performing not only one specific weather situation with calm winds, but simulating 19 different weather situations characterizing the typical weather patterns for the Hamburg city in summer (using a dynamical downscaling of ERA analyses).
Results show that the anthropogenic heat emissions have a direct impact on the temperature in urban areas: Taking a mean over the whole summer season the night-time temperature at 10 meter height is increased by about half a degree at specific hot spots in the city. For specific weather types the increase in temperature due to anthropogenic heat can be much higher. This result highlights the importance of anthropogenic heat for the urban heat island but also for future adaptation and urban planning measures.
Effects of Urban Form and Atmospheric Stability on Local Microclimate
1UFRJ, Brazil; 2Glasgow Caledonian University, UK
Although proper urban planning options could help minimize the effects of UHI, studies to adapt the built environment to climate changes in urban areas are rare, particularly in the context of cool climate cities where urban warming is typically not seen as a current problem. This will change as the background climate continues to warm. While the exploration of the influence of urban form on local microclimate is helpful, it is necessary to untangle this effect from background atmospheric conditions that lead to such effects. The present paper evaluates the effect of urban morphology (as measured by the Sky View Factor – SVF) on local climate according to atmospheric conditions exemplified by atmospheric stability (modified Pasquill-Gifford-Turner [PGT] classification system) in a cold climate city. The aim is to highlight their combined importance and to make preliminary investigations on the local warming effect of urban morphology under specific atmospheric stability classes. Forty-nine locations were selected in the city centre, on the basis of SVF to represent a wide variety of urban forms (narrow streets, neighbourhood green spaces, urban parks, typical street canyons and public squares) and seven of these were assigned as locations for fixed weather stations. Thirty one temperature measurement campaigns were made during spring and summer 2013, using a ‘traverse’ method on a ‘Meteobike’ and on foot. The locations were chosen to represent a variety of urban formation (narrow streets, neighborhood greenspaces, urban parks, uniform and non-uniform street canyons and public squares). The visualization of local temperature variations was accomplished using Arc-Map tool from Arc-GIS package. The present work indicates that the maximum intra-urban temperature differences (i.e. temperature difference between the coolest and the warmest spots in a given urban region) is strongly correlated with atmospheric stability. It appears that atmospheric stability has larger effect on intra-urban temperature variations than urban morphology in a cold climate city. The combined effect of the two provides interesting variations in local temperatures that may have urban planning implications, especially as the background climate continues to warm.
Urban Heat Island of Arctic cities
1Lomonosov Moscow State University, Faculty of Geography, Russian Federation; 2A.M. Obukhov Institute of Atmospheric Physics of Russian Academy of Sciences; 3WMO (World Meteorological Organization)
Actual progress in Urban climatology in XXI century covers new regions and new cities. New studies shows us a big amount of Urban Heat Island varieties all around the world. This effect is well-known in modern climatology due to its influence on different economic features and urban air quality (Oke, 1987). Also UHI characteristics differs in different climate zones, for example in summer in Mediterranean and subtropical monsoon climate types it leads to growing energy consumption due to AC systems using (Ohashi et al, 2007). But there is only a few papers about UHI (Magee et al, 1999) in high latitudes, for the cities over the Polar Circle and especially about behavior of the heat islands during the polar night, while anthropogenic heat is the main source of thermal energy. The main goal of this study is to mitigate this lack of information about climatology of UHI formation in big cities (with population exceeding 50 000) of Arctic zone.
In this work, we consider the results of experimental research of the UHI of 4 biggest Arctic Cities (Murmansk, Norilsk, Apatity and Vorkuta), which were obtained during the expedition of Russian Geographic Society in 2013-2014. During the project we used a different measurements techniques:
1. Installation of two automatic weather stations (AWS) in rural zone and city center
2. Installation of small temperature sensors (iButton) network in the city and suburbs
3. Regular car-based temperature sounding of the city with AWS.
4. Using MTP-5 microwave temperature profiler.
This investigations allowed to collect unique data about UHI in high latitudes. Analysis of the collected data showed the existence of UHI with the difference between city center and surrounding landscape up to few degrees Celcius.
The most interesting results we received in Norilsk and Apatity. Tha last one can be determined now as the ideal city for Arctic Urban climate issues. Its topography and building density allow UHI to reach about 5-7 degrees during nighttime in January with well observed warm city core. In Norilsk the negative correlation of the UHI power with air temperature was determined and its intensity can vary in different sinoptic conditions.
In Murmansk - the largest city north of the Arctic Circle ( about 300 000 inhabitants), UHI is neutralized by Kola Bay influence.
The reported study was supported by RGS (Russian Geographical Society ), research project No.27/2013-NZ
1. Magee N., Curtis J., Wendler G., The Urban Heat Island Effect at Fairbanks, Alaska// Theor. Appl. Climatol.
Vertical range of urban ‘heat island’ in Moscow
1Lomonosov Moscow State University, Faculty of Geography, Russian Federation; 2Central Aerological Observatory, Russian Federation; 3Institute of Experimental Meteorology, RPA 'Typhoon', Russian Federation; 4Russian Radio Television Network, Russian Federation
The urban ‘heat island’ vertical range in Moscow region has been studied using collected data of in situ measurements of air temperature T by sensors on TV tower, high meteorological mast and radiosondes. The tower is situated at Ostankino district inside Moscow close to the city centre and has a height of 540 m; mast of 310 m height is located in Obninsk on a distance about 100 km to the South from Moscow; radiosondes are launched in Dolgoprudny on 5 km to the North from the city margin twice a day and measure T with a spatial resolution of 100 m up to the height of 1 km.
As one knows the ‘cross-over’ effect is an intersection between nocturnal profiles of T above the city centre and suburbs as a result of higher intensity of the surface inversion outside a city than inside it. Hence, from some level at night the air temperature above city, especially its central part, is less than at rural zone at the same height. Vice versa, in the afternoon the air temperature above a city is higher due to more super-adiabatic vertical gradient of T in the urban ground air layer. Evidently the vertical range of the ‘heat island’ is equal to the lowest level at which T becomes the same inside and outside a city both at night and at midday. Our data represent conditions of nearly the centre of Moscow (TV tower), city margin (radiosondes) and rural zone (mast). All measurements of the air temperature which were made simultaneously at three locations have been collected for the period from 2000 to 2013 at four heights: the ground, 100-128 m, 300-305 m and 500-503 m. Preliminary results demonstrate that on the ground level (2 m) the air temperature is maximal in Ostankino and minimal in Obninsk both in the afternoon (9.2 and 8.2 ÝC correspondingly) and at night (5.0 and 3.1 ÝC). In Dolgoprudny (close to city margin) average values of T are intermediate between city centre and rural zone: 8.6 ÝC at midday and 3.4 ÝC at night. On the level of 500 m the air temperature in Ostankino is in average 0.1 ÝC less at night and 0.2-0.3 ÝC more at midday than in Dolgoprudny. Thus, vertical range of the urban ‘heat island’ seems to be close to 500 m at night and is a bit more than 500 m (the highest level which is available for comparison) in the midday.
Besides, a dynamics of the air temperature in the lower troposphere above Moscow region (in the air layer up to 4 km height) during last two decades (since 1991 till 2013) was studied in details by the data of radiosondes and high mast. A tendency to some deceleration of current climate warming has been detected at the recent time.
Urban Heat Island in the Lyon metropolitan areas.
University of Lyon, France
Lyon metropolitan area is located in the centre-east of France, in the Rhone-Alps region. Two rivers flow through and converge at the southern side of the city. In the lower part its altitude is only 170 m, but three hills with altitudes around 250-300 meters surround the city. Lyon metropolitan area is the second in France with 1 300 000 citizens and an important urban growth (for 2030 1 450 000 citizens are predicted). The last study of Lyon Urban Agency shows that, between 2000 and 2010, the city lost 4 percent of its non-urban surface, due to the extension of infrastructures and economic activity areas into the peri-urban area.
Since 2009 Greater Lyon takes part in climate protection strategy and the Urban Heat Island (UHI) is one of the key issues of its climate change adaptation policies. The heat wave in 2003 proved that extreme climate events provoke an increase of mortality in urban area. One essential question for the Greater Lyon is how land cover impacts urban climate and population comfort and health. Urban climate modeling is essential to enhance the knowledge of this local phenomenon as there are very few measurements and no previous studies available to describe the Greater Lyon UHI.
The aim of the present study is to estimate the Urban Heat Island event.
It also presents the impact of data land cover resolution on temperature distribution in Lyon metropolitan area obtained from climate modeling.
Due to the lack of meteorological measurements in the urban area in this study simulation results issued from Meso NH model are used in order to characterize the UHI in Lyon metropolitan area. Meso NH is coupled with a surface model, Surfex, that integrates an urban component TEB.
For the high resolution modeling, the son grid is based on an accurate surface database that includes a description of the urban fraction at a 250m resolution. Outside this grid, ECOCLIMAP is used as land surface reference.
To obtain the Greater Lyon land cover characteristics a new database is created. Three land cover bases are combined: the Greater Lyon local urban plan, SPOT Thema and an urban historical database (N. Ferrand, 2010). The urban historical database gives information for each parcel of Lyon metropolitan area for 5 periods: 1950, 1975, 1990, 2000 and 2010. The basic design principles are the same that are used in Spot Thema with the 3 levels of typology to identify the process of urbanization. Land cover changes and dating of the buildings in the Greater Lyon can be obtained from this historical database. The study in Greater Paris MUSCADE (A. Lemonsu et al., 2012) provides information about the materials of each building regarding its date of construction and its use. A local adaptation of this material typology is completed with architects to take into account the main architectural differences between Lyon and Paris.