The urban heat island effect is a well-known phenomenon, i.e. cities are usually several degrees warmer at night than the surrounding country side. Surprisingly, a significant amount of studies have observed a cooler city than their surroundings in the morning or during the entire day, i.e. the urban cool island. However, an explanation for the phenomenon so far has been lacking. In this research we use a conceptual boundary-layer model to explain the physical mechanism behind the urban cool island. In the morning the rural boundary-layer is shallower than the urban boundary layer, because the urban boundary layer often remains unstable or neutral during the night. Hence, the provided solar heating in the morning is distributed in a shallower layer in the countryside, resulting in a rapid heating. The urban boundary layer takes longer to warm up because the volume of air to be heated is much larger. Whereas, the rural boundary layer has less air heat, and warms faster. This causes the rural mixed layer to be warmer than the urban environment at the start of the day. The intensity of the formed urban cool island depends on many parameters, including the difference in stability of the urban and rural boundary layer, the stability of the free troposphere and surface heat flux dependent on the local climate zone. The conceptual model is initialised and validated with observations from the BUBBLE campaign in Basel, Switzerland.

This study investigates interactive effects from the Beijing urban area on tem- perature, humidity, wind speed and direction, and precipitation by use of hourly automatic weather station data from June to August 2008-12. Results show the Beijing summer urban heat island (UHI) as a multi-center distribution (corresponding to underlying land-use features), with stronger nighttime than daytime values (aver- ages of 1.7 vs. 0.8°C, respectively). Specific humidity was lower in urban Beijing than in surrounding non-urban areas, and this urban dry island is stronger during day than night (maximum of -2.4 vs. -1.9 g kg-1). Wind direction is affected by both a mountain- valley breeze circulation and by urbanization. Morning low-level flows converged into the strong UHI, but afternoon and evening southerly winds were bifurcated by an urban building-barrier induced divergence. Summer thunderstorms also thus bifurcated and bypassed the urban center, due to the building-barrier effect during both daytime and nighttime weak-UHI (<1.25°C) periods. This produced a regional- normalized rainfall (NR) minimum in the urban-center and directly-down- wind of the urban area (of up to -35%), with max values along its downwind lateral edges (of >15%). Strong-UHIs (>1.25°C), however, induced or enhanced thunder- storm-formation (again day and night) which produced an NR maximum in the most urbanized area of up 75%.

Understanding the dynamic and the thermal impacts of urbanization is essential for improving our parameterization of urban flow and urban fluxes. This study investigates the urban-rural contrast in terms of surface temperature (TSK) and roughness length (z0) using Large Eddy Simulations (LES) with the Weather Research and Forecast (WRF). In addition, the impacts of small-scale urban heterogeneities in terms of surface temperature and roughness length are also examined. The WRF-LES is first modified in order to use prescribed TSK, in addition to prescribed sensible heat flux, as the surface boundary conditions. Numerical simulations are then conducted to examine turbulence characteristics and mesoscale circulations resulting from the urban-rural contrasts in TSK and z0 as well as their small-scale heterogeneities in urban areas. The results indicate that: (1) the urban-rural contrasts in TSK and z0 have significant but different effects on the vertical distribution of potential temperature and winds, and they interact synergistically to modify the urban boundary layer. As a result, using sensible heat flux as the surface boundary conditions cannot separate the dynamic and thermal impacts of urbanization. (2) the impacts of small-scale urban heterogeneities can be important but significantly depend on their characteristics length scales.

Urban heat island (UHI) effect acts as a hot topic in urban climate research. Meanwhile, it is an important issue related to urban environment, urban development planning and climate change. Based on the data of 2010 ‘Nanjing Summer UHI Observation’ and numerical simulation results, the temporal and spatial distribution characteristics of UHI in Nanjing were showed as the following

(1) The daily UHI intensity on typical sunny day was above 1°C in the summer of 2010 in Nanjing. UHI was stronger and more stable in the nighttime than it was in the daytime. The average UHI intensity was 1.63°C at night, and its distribution corresponded well to urban landuse status. The strong heat island existed under sunny, windless weather conditions and UHI intensity was weakened with wind speed increasing. In addition, wind direction can significantly influence on the spatial distribution of UHI. The leeward of a city can be warmed by the tail flow of the city, resulting in the extension of the thermal region to down wind direction.

(2) During the daytime, the mixed layer in urban developed faster and higher than that in suburb. Due to the strong heat island and the strong convective mixing at night, mixed layer can still exist in this period and led to a uplifting night inversion layer. Urban mixed layer height was estimated as 300m in the nighttime and 1300m in the daytime in this observation. Owing to high heat storage and strong turbulent transport of urban underlying surface, heat island took shape in urban boundary layer, with UHI intensity decreasing with height. Heat island in boundary layer extended up to 900m in the daytime and maintained at 300m in the nighttime.

(3) Simulation results showed that, in the summer when the weather was fine, heat island took shape in boundary layer all day in Nanjing. At 02:00 p.m., sensible heat flux reached 350W/m2 in city, twice as that in suburbs. In comparison, the soil heat flux reached 200W/m2 in city as four times that of the suburbs. Urban underlying surface stored a large mount of heat, providing the heat source of UHI. At the same time, TKE in city reached the higher value of 1.2m2/s2 in 200-700m, twice as that in suburbs. Strong turbulent motion in the daytime promoted the development of urban mixed layer, and UHI in boundary layer formed within the height 700m, which was higher than that at night. At 02:00 a.m., sensible heat flux and TKE decreased significantly both in urban and suburban, when urban soil heat flux was up to 30-35W/m2, twice as that in suburbs. Urban surface continuously released heat storage, to maintain UHI in the nighttime.

Keywords: urban heat island; urban boundary layer; urban underlying surface; Nanjing summer; WRF mode

Urban heat island (UHI) is a phenomenon created by changes in the surface characteristics caused by the urbanization process: waterproof soil, high buildings, channelization of rivers, deforestation, anthropogenic heat, etc. These changes cause different values for albedo, heat conductivity and capacity, surface roughness, hydraulic conductivity and capacity and atmospheric transmissivity and emissivity when compared to the surrounding non-urbanized areas. Therefore, higher air temperatures may be found over the city, creating UHI, and the intensity of this phenomenon is evaluated by the difference in temperature between the city and the non-urban surrounding areas. In high altitudes, the anthropogenic heat plays an important role in generating and sustaining the UHI at night and during the winter, however, in tropical latitudes, the UHI is less intense and its maximum is during summer daytime. These difference occurs because at lower latitudes the heat island is strongly modulated by the solar radiation and the difference in the water content in the soil between urban and rural areas. Also the influence of the anthropogenic heat is smaller. The Metropolitan Region of São Paulo (MRSP) is formed by 39 cities, including São Paulo (23° 32' 56? S, 46° 38' 20? W), the largest city in the South Hemisphere, and has a population of nearly 21 millions of inhabitants in an area of 7,946.84 km2. It has a complex topography, including the Cantareira Mountains at the northern part of the city, three large river valleys (Tietê, Pinheiros and Tamanduateí) and the Serra do Mar Mountains at the southern part. The climate is usually dry and cool during winter and humid and warm during summer. It is located at an average altitude of 760 meters above sea level and is approximately 70 km distant from the ocean, however it is influenced by sea-breeze circulation 50 % of the days of the year, in average. The surface is very complex, presenting parks, water bodies, and familiar agriculture, residences, industries, highways, high buildings, and three airports. The UHI intensity in the city of São Paulo was estimated based on observations in 2004 and resulted between 2.6 (July) to 5.5 °C (September), with maximum intensity between 14 and 16h. Like in other low latitude cities, the UHI presented strong correlation to the net solar radiation at the surface. The summer of the last year (Dec/2013 to Jan/2014) presented an unusual synoptic condition, with a warm and dry air mass over the MRSP. Therefore, cloud cover and precipitation were lower than the climatological averages and temperature and net solar radiation at the surface were higher. The present work aimed to better understand the air temperature field and verify the influence of this particular phenomenon on the UHI of the MRSP using the available observations. The analysis was based on the datasets of three meteorological stations, Mirante de Santana (urban), USP (suburban) and EACH (suburban), 13 air quality monitoring stations (urban sites), and three micrometeorological stations IAG (suburban), Secretaria da Fazenda (urban) and Itutinga (rural). The preliminary results showed that the summer of 2014 presented a more intense UHI and reinforced the influence of the net solar radiation. However, the air temperature in the MRSP presented a complex field, that can produce complexes UHI phenomenon and atmospheric circulation.