Björn Holmer

Advection caused by the urban heat island circulation as a regulating factor on the nocturnal urban heat island

Nocturnal atmospheric cooling rates were investigated in a medium sized town in Sweden. Rates were found to be very different, depending on the time of the evening:night, therefore the urban heat island (UHI) development was divided into three phases: a, differential cooling; b, transition; and c, stabilization. In phase a the UHI intensity increases by differential cooling between urban and rural areas. In phase b, after the urban heat island circulation (UHIC) starts, there is a drastic change in the rural cooling rate. The urban cooling rate is unchanged in the early evening, but about 2 h after the start of the UHIC there is a sudden increase of the cooling rate as the cool rural air reaches the city centre. The UHIC is therefore assumed to be an effective way of transporting sensible heat between the rural and urban areas. In phase c this coupling results in an equalization of the cooling rates at both the rural and the urban site from 1.5 K h 1 and 1.0 K h 1 , respectively, to 0.5 K h 1 . Once the UHIC is activated, the system is self-regulating since if one factor is changed some of the others have to change as well in order to preserve the balance. In phase c the advective flux is estimated to be 99 4W m 2 and to give a central UHI potential cooling of 0.3 K h 1 . Copyright

Urban–rural vapour pressure differences and their role in the development of urban heat islands

Data for a 4-year period from Goteborg, Sweden was used to investigate the urban–rural humidity differences during clear and calm nights. The purpose was to analyse the impact of the Urban Moisture Excess (UME) on the Urban Heat Island (UHI) with focus on the longwave radiation balance and the latent heat flux. The statistical analysis demonstrated that a hot summer with reduced precipitation increased the mean UME to 3 hPa (‘normal’ summer conditions demonstrated an UME of about 1 hPa) and during some nights the UME was as high as 7 hPa. The analysis also demonstrated a positive correlation between UME and UHI, with the maximum UHI preceding the maximum UME by between 2 and 5 h. An analysis of the nocturnal development of UME demonstrated that the UME could develop from three combinations of urban and rural vapour pressure changes. The initial level of the rural vapour pressure, and thus the conditions for dewfall proved to be important. The influence of an UME of 3 hPa on the downward longwave radiation was calculated to about 3 W m−2, which is 12% of the urban–rural radiation difference giving a weak enhancing effect of the UME on the UHI. Advection of dry air from the UHI circulation was considered to influence the urban vapour pressure change. After correction of the advective effect on the urban vapour pressure change, the urban–rural difference in the latent heat flux ranged from −5 to −25 W m−2, which demonstrates that UME also has a considerable negative feedback on the UHI development. Copyright

COOLING RATES, SKY VIEW FACTORS AND THE DEVELOPMENT OF INTRA‐URBAN AIR TEMPERATURE DIFFERENCES

Abstract Intra‐urban cooling rates were monitored for a year in the centre of Göteborg on the Swedish west coast. Five sites with different building geometry ‐ from dense canyon to open space ‐ within a radius of 300 m were analysed. Results showed two modes of cooling during the night. In the first mode, the cooling was site‐dependent ‐the open space cooled the most and the dense canyon the least. In the second mode, which began about 3–4 hours after sunset, all sites cooled at the same rate. Our hypothesis is that in the early part of the evening both radiative divergence and sensible heat dominate the cooling. However, as the evening progresses, a spatially homogeneous inversion is established which controls the radiative cooling, making the cooling independent of both surface geometry and surface type. From April to November the sites cooled rapidly around sunset and the cooling slowly decreased during the rest of the night. However, between December and March, the cooling was less intense, with an almost constant rate during the entire night. It has been suggested that this might be the result of the draining of stored solar heat during the summer and a continuous flux of anthropogenic heating during the winter.

RAINFALL CHANGE AND ITS IMPLICATIONS FOR BELG HARVEST IN SOUTH WOLLO, ETHIOPIA

Abstract Most farmers in the Ethiopian highlands depend on rain‐fed agriculture. Some areas have the favourable situation of having two rainy seasons, Belg (February‐May) and the long rainy season Kiremt (June‐October), which is the case in some parts of South Wollo. Tef (Eragrostis tef) is the staple crop that farmers prefer to grow and the 90‐day variety is suitable for growing during the short rainy season. One out of eight Ethiopians lives in tef potential areas which have Belg rainfall. The aim in this study is to analyse rainfall variation in South Wollo, in particular the area east of the small town of Hayk, and to study its effect on the ability to grow tef during the Belg season. Results from interviews showed that a more difficult farming situation had emerged during the past 40 years, largely due to deterioration of physical resources such as rainfall. Rainfall is analysed by comparing daily rainfall data from four stations in two periods, 1963 to 1982 and 1984 to 2003, for changes in totals, seasonal distribution and variability. Coefficient of variance, rainy days, rainfall intensity and dry spells were analysed. Results from this study show that there have been minor rainfall changes but greater rainfall variability. During fieldwork key informants were asked if they were able to harvest tef during the past nine years, a period that has been used in a model of rainfall influence on tef cultivation during the Belg season. In both 20–year periods, farmers were able to grow tef every second year. In spite of a greater variability in rainfall the farmers proved their knowledge and flexibility and were able to harvest tef in the same number of years during both periods.

Intra-urban air pollution in a rapidly growing Sahelian city

In this paper we analyze spatial and temporal variations of air pollution (PM(1), PM(2.5), PM(10), CO, NO(x), O(3), Toluene and Benzene) and climate in areas of different development typology in Ouagadougou, Burkina Faso. Analyses are based on measurements from fixed sites and car traverse measurements during field studies in 2007 and 2010. Large spatial and temporal variations were found, showing a generally poor air quality situation, with extreme levels of PM(10), commonly exceeding air quality guidelines of WHO. Pollution levels increase considerably with increased atmospheric stability. Important sources were transported dust and re-suspension of dust from unpaved roads, but also traffic emissions and biomass burning. The spatial variations are examined with focus on effects for variations in potential exposure depending on for example area of residence and daily activity pattern, showing that great differences are likely to exist. Ouagadougou, like most developing countries worldwide, currently experiences an extremely rapid population growth in combination with limited financial means. This is likely to create increasingly harmful air pollution situations for the rapidly growing populations of these areas, and shows an urgent need for increased understanding of the pollution situation as well as development of mitigation strategies.

Characteristics of the mean radiant temperature in high latitude cities—implications for sensitive climate planning applications

Knowledge of how the mean radiant temperature (Tmrt) is affected by factors such as location, climate and urban setting contributes to the practice of climate sensitive planning. This paper examines how Tmrt varies within an urban setting and how it is influenced by cloudiness. In addition, variations of Tmrt in three high latitude cities are investigated in order to analyse the impact of geographical context and climate conditions. Results showed large spatial variations between sunlit and shaded areas during clear weather conditions, with the highest values of Tmrt close to sunlit walls and the lowest values in the areas shaded by buildings and vegetation. As cloudiness increases, the spatial pattern is altered and the differences are reduced. The highest Tmrt under cloudy conditions is instead found in open areas where the proportion of shortwave diffuse radiation from the sky vault is high. A regional comparison between three Swedish coastal cities showed that Tmrt during summer is similar regardless of latitudinal location. On the other hand, large differences in Tmrt during winter were found. Shadows, both from buildings and vegetation are the most effective measure to reduce extreme values of Tmrt. However, extensive areas of shadow are usually not desired within outdoor urban environments at high latitude cities. One solution is to create diverse outdoor urban spaces in terms of shadow and also ventilation. This would provide individuals with access to a choice of thermal environments which they can use to assist their thermal regulation, based on personal needs and desires.

Diurnal and intra-urban particle concentrations in relation to windspeed and stability during the dry season in three African cities

The spatial and temporal variations of PM2.5, PM10 and TSP in three African cities of different sizes (Dar es Salaam, Ouagadougou and Gaborone) were investigated using portable particle counters. Three different areas (downtown, green residential and traditional residential) and a reference site were designated in each of the cities in order to detect intra-urban and temporal variability. Morning, noon and night measurements were conducted in the urban areas while observations at reference stations were made continuously over the field periods. A clear diurnal pattern in particle concentrations was found in inland Gaborone and Ouagadougou, with morning and night peaks where the latter was the highest. However, in coastal Dar es Salaam the night peak was almost absent due to delayed stabilisation of the air. Particle concentrations at the Ouagadougou reference station were extreme. The direct contribution of vehicle emissions are of secondary importance since the PM2.5/PM10 ratios are low (0.1–0.3). Much of the particles are supposed to be soil particles that are entrained in the air by daytime high windspeeds followed by nighttime subsidence as the air is stabilised and windspeed decreases. However, in all three cities, resuspension are important as areas with a network of unpaved roads showed the highest concentrations of suspended particles. Generally, the central business district had the lowest concentrations of particulate matter.

WINTER LAND BREEZE IN A HIGH LATITUDE COMPLEX COASTAL AREA

The winter land breeze (WLB) was studied in an area with complex topography on the Swedish West Coast (58°N). Methods used were mapping directions of smoke plumes, temperature soundings by tethered balloon and helicopter, three years of monitoring temperature and wind at three levels on a suspension bridge and on a mast, and meteorological observations from three stations located at different distances from the coast. To initiate the development of the WLB, the necessary temperature difference between sea and land was 5°C, but the magnitude of the difference appeared to have little influence. The direction of the typical WLB was at an oblique angle to the coast. The WLB started with a sudden wind shift as a result of frictional decoupling from the prevailing synoptic wind caused by increased stability. The flow, with a depth of about 100 m, developed within the nocturnal inversion at the level of the plateau-shaped mountains and overrode the cold air in the valleys and followed the uplifted inversion over...

Using Land Cover, Population, and Night Light Data for Assessing Local Temperature Differences in Mainz, Germany

Urban areas are believed to affect temperature readings, thereby biasing the estimation of twentiethcentury warming at regional to global scales. The precise effect of changes in the surroundings of meteorological stations, particularly gradual changes due to urban growth, is difficult to determine. In this paper, data from 10 temperature stations within 15km of the city of Mainz (Germany) over a period of 842 days are examined to assess the connection between temperature and the properties of the station surroundings, considering (i) built/paved area surface coverage, (ii) population, and (iii) night light intensity. These properties were examined in circles with increasing radii from the stations to identify the most influential source areas. Daily maximum temperatures Tmax, as well as daily average temperatures, are shown to be significantly influenced by elevation and were adjusted before the analysis of anthropogenic surroundings, whereas daily minimum temperatures Tmin were not. Significant correlations (p , 0.1) between temperature and all examined properties of station surroundings up to 1000m are found, but the effects are diminished at larger distance. Other factors, such as slope and topographic position (e.g., hollows), were important, especially to Tmin. Therefore, properties of station surroundings up to 1000 m from the stations are most suitable for the assessment of potential urban influence on Tmax and Tmin in the temperate zone of central Europe.

Erratic rainfall and its consequences for the cultivation of teff in two adjacent areas in South Wollo, Ethiopia

The authors analysed rainfall variability at Combolcha and Hayk in Ethiopia during the short rainy season, the Belg (February–May), in the period 1964–1996 and compared the results with similar data for the period 1997–2012. These towns are located 30 km apart in South Wollo. They have similar in topography and surrounding land use but differ in the amount of rainfall they receive. Daily rainfall data from local weather stations were used as input in a rainfall model for the cultivation of teff to analyse the number of years when it a harvest was possible. After 1996, Belg rainfall declined by 25–30% at both sites. The decline was greatest in February; Combolcha received one-third of its earlier rainfall. The teff model showed that the sowing date changed from February to March in both study areas. Between 1964 and 1996, lack of rain reduced the years with a harvest to every second year in Hayk and every third year in Combolcha. In the period 1997–2012, the number of years with a harvest was slightly reduced in Hayk, whereas in Combolcha it declined to one in every five years. Thus, both sites suffered from the diminished Belg rainfall but the consequences were worse for Combolcha.