1. Introduction
Numerous studies suggest that urbanization changes in surface energy budget [
1,
2,
3]. The land surface energy budget can be described as [
4]:
where α is surface albedo, S
d is downward surface insulation. LW
d is downward longwave radiation. ε is surface emissivity. σ is the Steven Boltzmann constant. T
skin is the land surface skin temperature. SH, LE are the sensible and latent heat fluxes. G is the ground heat flux. Urbanization, by changing atmosphere as well as land surface, modifies all the terms in Equation (1): First, urban has high aerosols load in the urban atmosphere, which reduces the surface insulation. Previous research [
5] reported that the reduction of surface insulation, so-called aerosol direct effect [
6], could be up to 20–40 Wm
−2 for a normally polluted sky for New York City and may be about 100 Wm
−2 for the dense polluted city (Beijing) in summer. In addition, certain types of aerosols (
cf. soot) absorb and emit longwave radiation and thus change LW
d in Equation (1). Furthermore, the clouds microphysical properties (cloud droplet size, cloud top albedo,
etc) are changed due to aerosol’s indirect effect [
7], which may further affect cloud and even rainfall formation. Therefore, cloud cover changes the LW
d term in Equation (1). Second, besides the atmosphere conditions, land surface parameters are also modified by urbanization. Jin
et al. [
8] showed that urbanization changes surface albedo, surface emissivity as well as skin temperature. As a result, SH, LE and G, which are function of land surface temperature, are expected to be changed by urbanization. All the above mentioned changes are considered as the urban impact on climate system.
Urban impact varies with the location, size, population density, and traffic pattern of a city [
2]. Therefore, assessing urban physical properties from the most advanced observations is the first step to understand urban impacts. This paper focuses on Shanghai, China, which is the biggest city in East Asia, with over 20 million people. Shanghai is located on China’s southeastern coast at the Yangtze River delta and covers areas over 7,000 km
2. With the rapid economic growth in China, Shanghai has rapidly developed over the past decades and the urban area of Shanghai has now expanded more than 5,000 km
2.
Figure 1 is the Landsat measurements revealing the dramatic spatial growth of Shanghai from 1989 to 2005. As a result of that, air pollution becomes one of important environmental issues in Shanghai. In addition, Shanghai has subtropical climate and therefore has four seasons: winter, spring, summer, and fall, with cold but dry winters and warm but humid summers. Extreme heat in warm summer is observed due to urban heat island effect (UHI).
By comparing the differences between the Shanghai city and the nearby rural areas, this paper sheds light on the quantitative changes an urban system induced to this area. This paper analyzes land albedo change, atmosphere aerosol and water vapor changes over Shanghai and rural regions. The National Aeronautics and Space Administration (NASA) Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing observations, together with the AErosol RObotic NETwork (AERONET) ground observations are used to identify the spatial and temporal features of aerosol load, albedo, skin temperature, and land cover in Shanghai and nearby rural regions.
Figure 1.
The growth of urban regions over Shanghai, China. (a) is based on Landsat TM for year 1989, and (b) is for Year 2008 from Landsat ETM+. Rural is generally blue, water is black, urban is gold. Data is using Near-to-mid-infrared spectrum.
Figure 1.
The growth of urban regions over Shanghai, China. (a) is based on Landsat TM for year 1989, and (b) is for Year 2008 from Landsat ETM+. Rural is generally blue, water is black, urban is gold. Data is using Near-to-mid-infrared spectrum.
Besides aerosol, urban heat island effect (UHI) is the most significant land surface feature in urban system. UHI means that the surface temperature over urban is higher than that over rural regions [
1]. Using satellite observed land skin temperature, Jin
et al. [
8] reported that urban surface temperature is higher than the surrounding rural regions for Beijing area. In this paper, UHI from satellite-retrieved skin temperature (T
skin) data is assessed for Shanghai and its rural regions. Again, a clear UHI signal is observed for Shanghai.
Section 2 describes the data sets and variables analyzed.
Section 3 presents the new results, followed by a discussion on the urban land and atmosphere interactions in terms of the change in the land surface energy budget (
Section 4).
2. Data
Monthly observations of skin temperature (T
skin), albedo, and land cover data measured by MODIS Terra [
9,
10,
11,
12] from March 2000 to December 2008 are used to analyze the climatology of skin temperature and how it relates to surface properties. Details of MODIS skin temperature collection 5 process and validation can be found at Wan [
9], or the user’s guide at (
http://www.icess.ucsb.edu/modis/LstUsrGuide/MODIS_LST_products_Users_guide_C5.pdf). The MODIS data (Collection 5) is at 0.05° latitude/longitude grid, or so-called Climate Model Grid (CMG) resolution in MODIS. T
skin data is only retrieved on clear days. The daytime and nighttime T
skin values in MOD11C3 are averaged from the daytime and nighttime LST values in the daily T
skin product (MOD11C1) in clear-days only. The daytime T
skin in MOD11C1 is around 10:30 AM local solar time and the nighttime T
skin is around 10:30 PM local solar time. The observation time may differ by 90 minutes in low latitude regions or up to several hours in high latitude regions. Local solar time is equal to Coordinated Universal Time (UTC) + longitude (in degrees)/15.
Aerosol optical thickness (AOT) observations from MODIS Collection 5 [
6,
13] are also used in this paper to identify the aerosol load in one atmospheric column. In addition, water vapor and cloud observations from MODIS [
14,
15] are also used to examine the connections between surface temperature and atmospheric conditions. MODIS atmosphere data are at 1° by 1° resolution. Aerosol measurements are only available for clear-day conditions and are sampled into monthly data.
AERONET data from the Taihu Site (31°25′15″N, 120°12′54″E) are analyzed to show the diurnal, seasonal, and interannual variations of aerosol optical thickness (AOT) in this region [
16]. Hourly data of AOT with level 2.0 quality assured [
17] available from 2005 to 2008 to show the diurnal, seasonal, and interannual patterns of AOT. Taihu site is close to Shanghai. Since near-surface wind is from Shanghai to Taihu (see
Figure 2 below), AOT of Taihu is largely transported from Shanghai. Because AERONET does not have site over Shanghai, using nearby Taihu-site observed AOT to represent Shanghai is valid.
Figure 2.
Shanghai Land Cover. The land cover is defined by MODIS as (1). Evergreen Needleleaf Forest, (2). Evergreen Broadleaf Forest, (3). Deciduous Needleleaf Forest, (4). Deciduous Broadleaf Forest, (5). Mixed Forest, (6). Closed Shrubland, (7). Open Shrubland, (8). Woody Savannas, (9). Savannas, (10). Grassland, (11). Permanent Wetland, (12). Cropland, (13). Urbanand Built-up, (14). Cropland/Natural Vegetation Mosaic, (15). Snow and Ice, (16). Barren or Sparsely Vegetated. In addition, 0 is water. Both studies Shanghai region and the selected region are outlined by rectangular boxes.
Figure 2.
Shanghai Land Cover. The land cover is defined by MODIS as (1). Evergreen Needleleaf Forest, (2). Evergreen Broadleaf Forest, (3). Deciduous Needleleaf Forest, (4). Deciduous Broadleaf Forest, (5). Mixed Forest, (6). Closed Shrubland, (7). Open Shrubland, (8). Woody Savannas, (9). Savannas, (10). Grassland, (11). Permanent Wetland, (12). Cropland, (13). Urbanand Built-up, (14). Cropland/Natural Vegetation Mosaic, (15). Snow and Ice, (16). Barren or Sparsely Vegetated. In addition, 0 is water. Both studies Shanghai region and the selected region are outlined by rectangular boxes.
Monthly reanalysis outputs from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-40) are analyzed to reveal the regional wind circulation at 925 mb. ERA-40 is a new reanalysis project conducted at ECMWF to cover the period from mid-1957 to 2001 including the earlier ECMWF reanalysis ERA-15, 1979–1993. The reanalysis data this study uses is monthly surface wind at 925mb 2.5° by 2.5°. The data are freely available at
http://www.ecmwf.int/research/era/do/get/era-40.
4. Final Discussion—Land–Atmosphere Interactions
Satellite observations, together with ground observations, reveal that urban is a unique climate system and is the extreme case of human-activity induced changes. By constructing buildings, parking lots, and roads, human being replace the natural vegetation-covered surface with water-proof concrete. Consequently, surface albedo (
Figure 5) and surface emissivity (not shown, but see [
8]
Figure 9(b) and [
19],
Figure 5) are reduced. Meanwhile, vegetation cover is dramatically decreased in urban regions, and no soil moisture available from urban water-proof surfaces. All these directly contribute to the UHI phenomenon for urban system. As shown in Equation 1, reducing albedo leads to more solar radiation being kept at the urban surface instead of being reflected back to atmosphere; reducing surface emissivity leads to more thermal radiation kept at the surface instead of emitted into atmosphere; less soil moisture at the urban surface would reduce the Bowen ratio, namely, more energy is used to heat up the surface in terms of sensible heat flux instead of being redistributed into latent heat flux, and reduction in trees also reduce surface evapotranspiration. Simply put, the land surface physical properties and physical processes are dramatically changed by urbanization and result in UHI.
Why urban has low albedo? Here are some possible reasons: First, the dense building regions has canyon effect, which not only affects momentum and also results in the so-called “slope effect”, namely, the building walls cause multiple reflection for solar photons, and therefore, eventually, fewer solar photons can be reflected back to atmosphere. This is part of the reason of urban low surface albedo. Another reason may be that the concrete material has low albedo than the natural surface in broadband albedo. Nevertheless, urban has high heterogeneity, and therefore urban surface abedo may not be uniformly lower than natural rural regions. This is because urban dry surface may also lead to a high surface albedo (wet surface is low albedo, an example for this is that wet sands look darker than dry sands).
Urbanization modifies atmosphere conditions, in terms of aerosol load, cloud, and water vapor. The land surface energy budget (Equation 1) can clearly disclose that the land-atmosphere interaction is very critical for an urban system. First, urban regions, especially for those big cities, distinguish with high urban aerosol load. Aerosols directly scatter or absorb solar radiation (aerosol direct effect) and thus reduce surface insolation [
20]. This aerosol-reduce-surface insolation mechanism is a competing process for the land surface UHI processes and cools urban surface. Meanwhile, aerosols serve as cloud condensation nucli [
7] and affect the microphysical properties of clouds (namely, aerosol’s indirect effect). Jin and Shepherd [
20] reported that over East Asia, more aerosols are related to smaller cloud droplet size for both water and ice clouds. In this study, the differences of cloud and water vapor between Shanghai and urban regions would suggest that urban has different clouds conditions, which is directly or indirectly related to urban aerosols.