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Observational and numerical studies of wintertime urban boundary layer
Journal of Wind Engineering and Industrial Aerodynamics 87 (2000) 243–258
Observational and numerical studies of wintertime urban boundary layer Jianguo Sanga,*, Hepin Liua, Huizhi Liub, Zhikung Zhanga a
Department of Geophysics, Peking University, Beijing 100871, China Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China b
Accepted 3 July 2000
Abstract Observational analyses and numerical simulations were carried out to study the characteristics of the winter urban boundary layer of a large city. The observations showed that with light winds the ground inversion at nighttime in urban areas was about 200 m deep. The heat island circulation, caused mainly by anthropogenic heating, induced reverse flow at the downwind part of the city. A shallow internal boundary layer with a depth of 50–80 m developed from the fringe of the city. In the daytime heavy smoke attenuated the solar radiation, and caused a cold island in the urban area, and retarded the development of a convective boundary layer. The features described above were well simulated by the numerical model. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Internal boundary layer; Heat island; Cold island; Numerical simulation
1. Introduction The atmospheric conditions in urban boundary layer (UBL) strongly influence the dispersion processes of pollutants from various surface and elevated sources. The distribution of pollutants, e.g. aerosols, in turn affects the radiative processes and therefore the energy balance in surface-UBL system. In addition, the human activities in urban area bring about a large amount of anthropogenic heat release, which is one of the important causes of heat island phenomenon. These processes
*Corresponding author. Tel: +86-10-62753326; fax: +86-10-6275-4294. E-mail address: [email protected] (J.G. Sang). 0167-6105/99/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 0 0 ) 0 0 0 4 0 - 4
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have been widely studied in field observations and numerical simulations (e.g. Refs. [1–4]). The objective of the present paper is to investigate the processes described above in wintertime UBL in a big industrial city using both observational analyses and numerical simulation. A series of extensive atmospheric experiments was carried out in Shenyang City in February, August and December 1984 in order to study the atmospheric environment in the urban area. Shenyang is a heavy-industry city in the Northeast of China with a population of 4 million. Since a large amount of coal was burned, especially in winter, as the energy source for industry and house heating, Shenyang was also a heavily polluted city. In the studies, a numerical model of the UBL was also developed to simulate the transport and diffusion of pollutants in the urban and suburban areas. The atmospheric boundary layer in the urban area can be divided into several sublayers. The lowest one is the canopy layer, where most human activities take place. In this layer the drag force exerted by buildings may reduce the wind speed. The reduction of the sky view-factor in this layer may cause interception of the longwave radiation emitted from buildings and ground surfaces. In addition, the anthropogenic heating makes the wind and temperature fields of this layer very complicated. It is difficult to deal with a numerical model for the whole urban area including all of these detailed processes. As a compromise, we developed a UBL model, in which the structures in the canopy layer were taken as roughness elements and the bulk energy budget for the canopy layer was calculated to obtain the mean temperature of the canopy layer. In the following sections the main features of a UBL in wintertime, as observed and simulated are described.
2. Observations The observations of the UBL comprised five observational stations including pilot balloon wind measurements, soundings and tethersondes, distributed in urban and suburban areas. One station was located downtown. Two stations were located at airports in the western and eastern suburbs. The meteorological measurements at these two stations represented the upwind and downwind conditions of the urban area, respectively, since the prevailing wind in this area is westerly. The two other stations were set in a park and a college campus, and represented the conditions of open area with sparse buildings. There were also a few surface temperature measurements distributed on areas with different functions such as industrial, residential and commercial, etc. Measurements of SO2 and suspended particles were taken by airplanes at several levels over the urban and rural areas. Fig. 1 shows a map of Shenyang City including the urban area and the outskirts of the city, as well as the positions of five observational stations. In this figure the symbols W, E, N, S and C represent the West, East, North, South and Central stations, respectively. The observations in December 1984 lasted from 17th to 28th. The measurements of wind and temperature fields were taken every 2 h.
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Fig. 1. Map of Shenyang City. The shaded area is the city and suburbs. Capital letters W, N, E, S and C are the positions of observational stations at western, northern, eastern, southern suburbs and downtown, respectively. The length and width of the map are 19 and 14 km, respectively.
During the observational period there was a cold front event from 21st through 23rd. On most of the other days the wind was weak, i.e., the wind speed in the lower part of the UBL was less than 3 m sÿ1. As stated by Johnson and Bornstein [5], the urban effects on the wind field are manifested by the friction of the buildings and structures if the wind is strong, while the effects of the heat island dominate the circulation in conditions of weak winds. Since space is limited we discuss herein the situation of weak wind only. 2.1. Flow fields in light wind conditions During the observation there were 6 days, i.e., 18, 19, 20, 24, 25 and 27 of December, when the wind speeds at the lower levels were less than 3 m sÿ1. In these days the features of heat island circulation in flow fields were evident, especially at nighttime. Fig. 2 shows the wind profiles at 0600 LST on 20th at the West and the East stations, respectively. This is a typical case of weak wind conditions at nighttime. At the West station, i.e., the upwind suburb, the flow in the boundary layer is weak westerly, while at the East, i.e., downwind suburb, weak easterly, i.e., reverse flow, appears below the height of 100 m. The reverse flow is caused by the pressure gradient force pointing to the urban centre because of the low-pressure buildup by the urban heat island effect. The reverse winds and convergent flow fields at the lower levels of the UBL appeared on most of the light wind days. Table 1 shows the convergent flow fields on the 6 days. In the table uW and uE are the u components of wind speed at a height of
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Fig. 2. Wind profiles observed at 0600 LST of December 20 at suburban stations. E: The eastern suburb and W: the western suburb.
Table 1 Statistics of convergent flows at the lower part of urban heat island circulationa Date
18
19
20
24
25
27
uE uW vN vS DV
1.14 2.24 0.33 ÿ0.22 ÿ0.55
ÿ2.21 ÿ0.76 1.70 1.15 ÿ0.90
ÿ0.11 0.65 0.27 0.40 ÿ0.89
1.13 2.50 ÿ1.62 ÿ2.27 ÿ0.72
0.50 2.83 1.24 0.32 ÿ1.41
ÿ1.08 1.14 1.26 0.34 ÿ1.30
a u and v are the components of wind speed at a height of 25 m averaged on 0000, 0200, 0400, 0600 and 0800 LST every day. The subscripts E, W, N and S represent East, West, North and South Stations, respectively. DV ¼ uE ÿ uW þ vN ÿ vS . The units are all m sÿ1.
25 m averaged from 0000 to 0800 LST at West and East stations, respectively, while vN and vS the v components at North and South stations, respectively, DV ¼ uE ÿ uW þ vN ÿ vS represents the divergence of the flow field at the lower levels. DV50 means that the convergent flow dominates the lower part of the UBL. It can be seen from Table 1 that the light wind conditions at nighttime favour the heat island circulation. On all six nights the flows were convergent with convergence values of about 5 10ÿ5–10ÿ4 sÿ1 if the urban horizontal scale is assumed to be 10 km.
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2.2. Urban heat island and internal boundary layer Fig. 3 shows the temperature profiles at West, Central and East stations, respectively, at 0600 LST of December 20. They represent the temperature structures of urban nocturnal boundary layer in weak wind conditions at upwind, downtown and downwind parts of the city, respectively. It can be seen from the three profiles that the nocturnal ground inversion caused by the rural surface long-wave radiation cooling extends from upwind to downwind suburbs with the depth of 200 m remaining unchanged. As the airflow passes over the urban canopy, a thermally induced internal boundary layer (TIBL) appears. Since the internal boundary layer is caused by the urban surface heating, its depth increases with the fetch starting at the periphery of the city. At the west station the depth of the TIBL is about 20–30 m, while it increases to 50–100 m at downtown. The maximum depth of TIBL should appear at the downwind part of the city. However, this is not the case on this day. Since at East station, the reverse flow, i.e., weak easterly, occurred below the height of 100 m, the rural cold air penetrated into the warm downtown in the lower layer. Thus, the temperature profile at East presents a complicated structure. As shown in Fig. 3 the layer of the lower 25 m over the scattered buildings in the vicinity of the East station is a shallow internal boundary layer with a nearly neutral lapse rate. Above the TIBL, is an inversion with a depth of 200 m. The intensity of the inversion between the top of the TIBL and the height of 100 m is about 48C/100 m. The strong intensity can be explained as being the result of the large temperature
Fig. 3. Temperature profiles observed at 0600 LST of December 20 at suburbs and downtown. E: eastern suburb, W: western suburb and C: downtown.
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contrast between the cold advection, which comes from the eastern rural area, in the lower part, and the warm advection from the downtown in its upper part. Between the heights of 100 and 200 m the lapse rate of the profile is nearly isothermal since the layer consists of the warm plume of air from the urban area. Fig. 4 shows the temperature distribution in West–East cross-section through the Central station, diagnosed according to five soundings and the surface temperature measurements. It presents typical characteristics of a nocturnal urban boundary layer. The lower layer is the nocturnal inversion. Its depth is about 200 m and is uniform horizontally. The temperature distribution, however, is not uniform horizontally within the inversion layer. The rural air penetrates into both the western and eastern peripheries of the city to form cold wedges in the upwind and downwind parts. Because of the marked difference in temperature between the cold rural air and the warm urban surface, the internal boundary layer is distinguishable in the peripheries of the city, that is, from 0 to 8 km, and from 14 to 18 km in Fig. 4. The temperature is almost uniform in the warm core of the city centre, i.e., from 8 to 14 km in Fig. 4. However, we can still discern the isothermal layer below the height of 100 m and the inversion layer above. Because of the neutral or less stable stratification and the friction effects of the canopy, the turbulence in TIBL is more active than that above it. The pollutants emitted in the UBL are readily trapped in TIBL. 2.3. Dust dome and cold island Fig. 5 shows the vertical distribution of SO2 concentration measured by plane in a West–East cross-section through the city centre at 0700 on December 20. The urban nocturnal inversion layer and internal boundary layer play an important role in the diffusion of urban pollutants. The industrial and residential pollusion sources in Shenyang City were distributed within 200 m above the ground surface. As the depth of nocturnal inversion was 200 m, most of the pollutants were trapped in this layer. As the internal boundary layer exists, the pollutants emitted from the elevated
Fig. 4. Distribution of temperature observed at 0600 LST in the vertical xz cross-section through the Station C.
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Fig. 5. Distribution of SO2 concentration in mg mÿ3 observed at 0700 LST of December 20 in the vertical cross-section.
Fig. 6. Same as Fig. 4, but for 1200 LST.
sources readily spread down to the ground surface, while the vertical diffusion of pollutants from surface sources was suppressed by the top of the TIBL. Thus, a region of high concentration formed in the urban surface layer. In Fig. 5 most of the SO2 is concentrated below the height of 200 m and the concentration decreases rapidly with height above 200 m. The distribution of the total suspended particles (TSP) was similar to that of the SO2. The high density region of TSP with a maximum number density of 550 cmÿ3 formed a dust dome with a shape coinciding roughly with the internal boundary layer. Since the heavy smoke attenuates the solar radiation, the net radiation of the urban surface in the morning is considerably less than that of the rural surface. Thus, the urban area becomes a cold island. Fig. 6 shows the temperature distribution in the vertical cross-section at 1200 on December 20. The remarkable feature in this figure is that the temperature below 300 m in the urban area is lower than that in suburbs. The difference near the surface is about 1.58C.
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Table 2 shows the surface temperature in the urban and suburban stations in the morning in conditions of weak wind. The table indicates that as the wind in the lower layer is weak the urban area is a cold island. The temperature in downtown is 1–28C less than that in the suburbs. The maximum difference is 3.88C. There is a positive correlation between the concentration of TSP and the cold island intensity. It can be seen in Fig. 6 that there is an isothermal region near the height of 400 m, which may be taken as the top of urban mixed layer. It indicates that the mixed layer develops very slowly. In weak wind condition the interaction among the pollutants, temperature stratification and wind may induce a vicious circle: high concentration ! attenuation of solar radiation ! retardation of mixed layer development ! retardation of downward momentum transfer ! maintenance of weak wind ! maintenance of high concentration. It is not until the afternoon that the mixed layer has slowly developed to the high levels, then momentum transfer may Table 2 Temperature in urban and suburban areas in conditions of weak winda Date
17
Time
09
10
11
12
09
10
11
12
Turb Tsub DT Conc. of TSP
ÿ12.2 ÿ10.8 ÿ1.4
ÿ10.3 ÿ9.6 ÿ0.7 2.597
ÿ9.1 ÿ7.7 ÿ1.4
ÿ7.9 ÿ6.3 ÿ1.6 1.207
ÿ13.8 ÿ10.7 ÿ3.1
ÿ10.1 ÿ8.8 ÿ1.3 1.640
ÿ8.6 ÿ8.2 ÿ0.4
ÿ7.7 ÿ6.9 ÿ0.8 1.174
Date
19
Time
09
10
09
10
11
12
Turb Tsub DT Conc. of TSP
ÿ11.1 ÿ9.9 ÿ1.2
ÿ8.3 ÿ6.0 ÿ2.3 2.346
ÿ12.7 ÿ10.3 ÿ2.4
ÿ10.9 ÿ7.5 ÿ3.4 3.118
ÿ6.9 ÿ5.8 ÿ1.1
Date
23
Time
09
10
09
10
11
12
Turb Tsub DT Conc. of TSP
ÿ13.9 ÿ13.3 ÿ0.6
ÿ12.2 ÿ11.3 ÿ0.9
ÿ17.9 ÿ17.0 ÿ0.9
ÿ16.8 ÿ15.1 ÿ1.7 1.439
Date
25
Time
09
10
11
12
09
10
11
12
Turb Tsub DT Conc. of TSP
ÿ15.8 ÿ12.0 ÿ3.8
ÿ11.3 ÿ9.7 ÿ1.6 1.672
ÿ9.0 ÿ7.6 ÿ1.4
ÿ7.5 ÿ6.5 ÿ1.0 1.605
ÿ19.2 ÿ16.3 ÿ2.9
ÿ15.9 ÿ14.0 ÿ1.9
ÿ13.4 ÿ12.4 ÿ1.0
ÿ11.8 ÿ11.3 ÿ0.5 >
a
18
20 11
12
24 11
12
27
Turb is the surface temperature (8C) at downtown station, Tsub the averaged surface temperature at four suburban stations. DT=TurbÿTsub, conce. of TSP the concentration of TSP with unit of mg mÿ3.
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Fig. 7. Distribution of artificial heat flux in Wmÿ2 estimated according to the coal consumptions averaged in December 1984.
reach the ground surface and finally the condition of high concentration and weak winds can be destroyed. 2.4. Estimation of artificial waste heat in urban area In the 1980s the main energy source in urban area of China was from coal burning. The heat release from transportation might be neglected. Thus, the artificial heat emission in the city can be estimated according to the coal consumptions. Statistic of amount of coal consumptions was made in the grid of 1 1 km every month, e.g. December 1984. Since most of the industrial facilities and house heating boilers operated continuously, the average artificial heat flux was calculated by multiplying the average amount of coal burning with the fuel discharge ratio. Fig. 7 shows the distribution of the artificial heat flux estimated. The west part of the city (refer to Fig. 1) was the industrial district. The maximum flux was released from a steel mill and coking plant. The central part of the city was a commerical and residential area with buildings of 4–6 storey. The second largest flux occurred in this area. The eastern part was the old town with densely distributed low-rise buildings and houses. The southern part, close to the river, consisted of campuses with scattered buildings and more open spaces. The artificial heat flux here was less than that in other parts of the city.
3. Numerical simulation The numerical modelling system, used to simulate the UBL, includes a hydrostatic atmospheric dynamic model, a canopy layer wind parameterization scheme, and an urban canopy layer energy budget model.
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3.1. Dynamic model The three-dimensional momentum, hydrostatic, continuity, thermodynamic and turbulent energy equations are written as follows [6,7]: du @p ¼ÿy þ f v þ Fu ; ð1Þ dt @x dv @p ¼ÿy ÿ fu þ Fv ; dt @y
ð2Þ
@p g ¼ÿ ; @z y
ð3Þ
@u @v @w þ þ ¼ 0; @x @y @z
ð4Þ
dy ¼ Fy ; dt
ð5Þ
de ¼ Fe þ Kz dt
"
@u @x
2 2 # @v g @y Be3=2 ; ÿ þ ÿ Kz l @y y @z
ð6Þ
where p ¼ Cp ðp=p0 ÞR=Cp is the Exner function, P0=1000 hPa is a reference pressure, 02 Þ is the turbulent kinetic energy and the other symbols have their e ¼ 12ðu02 þ v02 þ w common meanings. The total derivatives d @ @ @ @ ¼ þu þv þw ð7Þ dt @t @x @y @z and the turbulence terms can be indicated as @ @n Kz ; Fj ¼ @z @z
ð8Þ
where j can be u, v, y, or e. Following Yamada [8], we express the vertical diffusivities Kz as, Kz ¼ Sle1=2
ð9Þ
where l is the turbulent mixing length, S is a nondimensional function of Richardson number. The horizontal grid points are 25 25 with an interval of 1 km. In the vertical the grids are set at 0, 40, 100, 200, 300, 500, 750, 1000, 1500, 2000, 3000 and 4000 m. 3.2. Wind profile in canopy layer The first grid above the ground surface in the model is at 40 m. The buildings and structures are taken as roughness elements with the lengths ranging from 0.1 m in the outer suburbs to 1.8 m in the downtown. The roughness length is assumed to be z0 ¼ h=10, where h is the mean depth of the canopy layer.
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In order to calculate the transport of pollutants in the canopy layer we should have the wind speed inside and outside the canopy layer interpolated according to wind speed u40 and v40 at z=40 m. Four fine grids: 2, 10, 20 and 30 m are set below the first grid of 40 m. If z5h, the power law is used uðzÞ ¼ u40 ½ðz ÿ d0 Þ=ð40 ÿ d0 Þp
ð10Þ
or if z5h, the exponential law is assumed uðzÞ ¼ uh exp½ÿgð1 ÿ z=hÞ;
ð11Þ
where uh is the wind speed at the top of the canopy layer, uh ¼ u40 ½ðh ÿ d0 Þ=ð40 ÿ d0 Þp . The profile for v is similar to Eqs. (10) and (11). In the formula above, d0 is the height of zero-plane displacement, d0 ¼ 7z0 , and g is a coefficient equal to or slightly larger than 1. The power law index, p, is a function of atmospheric stability and roughness. An empirical formula is suggested ð12Þ p ¼ a expðb log z0 Þ; where z0 is in meters, a and b are coefficients depending on the Obukhov length, L a ¼ 0:28 ÿ 1:35=jLj L50 ð13Þ b ¼ 0:57 þ 2=jLj and a ¼ 0:28 þ 10=L b ¼ 0:57 ÿ 15=L
L > 0:
ð14Þ
Table 3 shows some values of p with Pasquill stability category ranging from B to E and roughness length, z0, from 0.1 to 3 m. 3.3. Energy budget in canopy layer The prognostic equations for the canopy layer temperature, Tc , and the mean substrate temperature, Ts , are expressed as [9] @Tc 2p rðTc ÿ Ts Þ; ð15Þ ¼ CT ðRn ÿ H ÿ LE þ AÞ ÿ t @t @Ts 1 ¼ ðTc ÿ Ts Þ; ð16Þ @t t where Rn , H and LE are the fluxes for net radiation, sensible heat and latent heat, respectively, A is the anthropogenic heat flux, as shown in Fig. 7. t is a time constant and r is the fraction of open area, not occupied by buildings. At nighttime Rn is just the net long-wave radiation Rn ¼ L ¼ ÿesT 4c þ ea sT 4a ;
ð17Þ
where Tc is the canopy layer temperature, Ta the air temperature at z=40 m, e the canopy layer emissivity, s the Stefan-Boltzmann constant and ea the atmospheric
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Table 3 Values of p as a function of atmospheric stability and roughnessa L (m) z0 (m)
0.1 0.5 1 3
ÿ10 (B)
ÿ20 (C)
ÿ120 (D)
800 (D)
75 (E)
0.07 0.12 0.14 0.21
0.11 0.17 0.21 0.29
0.15 0.23 0.27 0.36
0.17 0.25 0.29 0.38
0.29 0.37 0.41 0.49
a The letters in the parentheses indicate the corresponding Pasquill stability categories, L the Obukhov length in meters.
emissivity, which can be empirically expressed as ea ¼ ae1=7 a ;
ð18Þ
with ea being the air water pressure in hPa, a ¼ 0:4, an empirical constant. In order to simulate the influence of pollutants on the development of a UBL in the daytime, the attenuation effects of solar radiation by aerosols should be included [10,11]. The short-wave radiation received on the surface on a clear day is given by S ¼ ð1 ÿ aS ÞðI þ DR þ Da Þ;
ð19Þ
where aS is the albedo of the surface, I is the direct radiation, DR and Da the scattered radiation by molecules (Rayleigh scattering) and suspended particles, respectively. The direct radiation is calculated by I ¼ I0 sin CTo TR TW Ta ;
ð20Þ
where I0 is the solar irradiance, defined as the flux of solar radiation passing through a plane normal to the solar beam at the top of the atmosphere, c the solar elevation angle, and To , TR , Tw and Ta are the transmissivities of the atmosphere, influenced by ozone absorption, Rayleigh scattering, water vapour absorption, as well as aerosol scattering and absorption. The scattered radiation of aerosol is expressed by Da ¼ I0 sin CTo TR TW ð1 ÿ Ta Þof ;
ð21Þ
where f is the ratio of forward scattering to the total scattering by aerosols, o is the scattering reflectivity of aerosol. The transmissivity, Ta , is determined by the optical depth of aerosol, a function of the amount and size of aerosols. The sensible heat flux H ¼ ra Cp CH VðTc ÿ Ta Þ
ð22Þ
where ra is the air density, CH=5 10ÿ3 the drag coefficient and V the resultant wind velocity at z=40 m. During cold season in urban area without snow cover the latent heat can be neglected.
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3.4. Results A case study was carried out to simulate the UBL in a weather condition similar to that of December 20, 1984. The simulation starts from 1800. The initial conditions are set same as that observed in the western suburb station in that time. The inflow boundary conditions, i.e., the temperature and wind profiles at the western boundary are input every hour according to the temporally interpolated values observed in the western station. The integration of the model lasts 24 h. Fig. 8 shows the distribution of the temperature simulated by the numerical model in the vertical cross-section, same as Fig. 4, at 0600 LST. Compared with Fig. 4 we may see that the results of the simulation agree well with that observed. In Fig. 8 the main features of the urban nocturnal boundary layer have been manifested. For examples, the depth and strength of the nocturnal inversion, the shape and position of the cold wedge at the city peripheries, and the depth and position of the internal boundary layer, which are shown in Fig. 4, are all reproduced in the simulation. Fig. 9 shows the horizontal distribution of flow field simulated at the height of 40 m at 0600 LST. Since the heat island effects induces centripetal pressure gradient, the convergent flow field appears in the lower layer of the urban area. The convergence point is located at the downwind part of the city, i.e., at about 14 km from the inflow border. Fig. 10 shows the distribution of flow field in the vertical cross-section. It can be seen in this figure that the reverse flow has occurred in a shallow layer near the ground surface at the outflow border of the city. The strength of the reverse flow is less than that of the inflow as shown in Fig. 2. However, the cold rural air, advected by the reverse flow, may penetrate about 4 km into the city from eastern border and an urban thermally internal boundary layer develops. Above the reverse flow, the air moves outside the city and the thermal plume forms as shown in Figs. 3 and 4, similar to the schematic diagram by Oke [12]. Fig. 11 shows the vertical distribution of turbulent kinetic energy simulated at 0600 LST in West–East cross-section. The active turbulence appears in the UBL, i.e., the region from 4 to 16 km in the horizontal and below the height of 200 m in the
Fig. 8. Same as Fig. 4, but for simulated temperature.
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Fig. 9. Horizontal distribution of flow field simulated at 0600 LST at the height of 40 m.
Fig. 10. Distribution of flow field simulated at 0600 LST in the vertical cross-section. The vertical velocity vectors are multiplied by 10.
vertical, with a maximum value of about 0.4 m2 sÿ2. Most of the pollutants emitted from the source within the UBL are trapped in this region. Consequently, the region of active turbulence roughly coincides with the dust dome, as shown in Fig. 5. After sunrise the urban area gradually becomes a cold island because of the attenuation of solar radiation by smoke emitted in the UBL. The numerical simulation continued to 1800 LST of December 20. The short-wave radiation was calculated by Eqs. (19)–(21), in which a constant transmissivity, Ta , that is constant optical depth of aerosols, was taken. Fig. 12 shows the distribution of temperature simulated in vertical cross-section at 1200 LST. The main features of urban cold island are all simulated. In comparison with Fig. 6, the pattern of temperature field
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Fig. 11. Distribution of turbulent kinetic energy simulated at 0600 LST in the vertical cross-section.
Fig. 12. Same as Fig. 4, but for temperature simulated at 1200 LST.
and the height of mixed layer in the simulation are similar to that in the observation. However, the intensity of the cold island simulated in Fig. 12 is stronger than that shown in Fig. 6, probably because in the calculation a too large optical depth of aerosols, which was determined according to the amount of the aerosols observed at 0700 LST of December 20, was used to cut down too much solar radiation in urban area.
4. Conclusions In weak wind condition the observation analyses indicate the following: (1) At nighttime the urban area is a heat island mainly induced by anthropogenic heating. (2) A shallow weak reverse flow may appear in the downwind suburb and the flow field in the lower part of the urban boundary layer is convergent.
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(3) The inversion height in urban area is rather uniform; the internal boundary layer is distinct and shallow in suburbs, and deeper but not clear in downtown. (4) A dust dome forms over the urban area due to the combined effects of inversion, internal boundary layer and convergent flow fields, etc., and the urban area in daytime is a cold island because of the attenuation of solar radiation by aerosols. A three-dimensional numerical model combined with parameterization scheme of urban boundary layer can be used to simulate the main features of urban heat island and cold island.
Acknowledgements This research was supported by China National Nature Science Foundation (59895410).
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Observational and numerical studies of wintertime urban boundary layer Jianguo Sanga,*, Hepin Liua, Huizhi Liub, Zhikung Zhanga a
Department of Geophysics, Peking University, Beijing 100871, China Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China b
Accepted 3 July 2000
Abstract Observational analyses and numerical simulations were carried out to study the characteristics of the winter urban boundary layer of a large city. The observations showed that with light winds the ground inversion at nighttime in urban areas was about 200 m deep. The heat island circulation, caused mainly by anthropogenic heating, induced reverse flow at the downwind part of the city. A shallow internal boundary layer with a depth of 50–80 m developed from the fringe of the city. In the daytime heavy smoke attenuated the solar radiation, and caused a cold island in the urban area, and retarded the development of a convective boundary layer. The features described above were well simulated by the numerical model. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Internal boundary layer; Heat island; Cold island; Numerical simulation
1. Introduction The atmospheric conditions in urban boundary layer (UBL) strongly influence the dispersion processes of pollutants from various surface and elevated sources. The distribution of pollutants, e.g. aerosols, in turn affects the radiative processes and therefore the energy balance in surface-UBL system. In addition, the human activities in urban area bring about a large amount of anthropogenic heat release, which is one of the important causes of heat island phenomenon. These processes
*Corresponding author. Tel: +86-10-62753326; fax: +86-10-6275-4294. E-mail address: [email protected] (J.G. Sang). 0167-6105/99/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 0 0 ) 0 0 0 4 0 - 4
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have been widely studied in field observations and numerical simulations (e.g. Refs. [1–4]). The objective of the present paper is to investigate the processes described above in wintertime UBL in a big industrial city using both observational analyses and numerical simulation. A series of extensive atmospheric experiments was carried out in Shenyang City in February, August and December 1984 in order to study the atmospheric environment in the urban area. Shenyang is a heavy-industry city in the Northeast of China with a population of 4 million. Since a large amount of coal was burned, especially in winter, as the energy source for industry and house heating, Shenyang was also a heavily polluted city. In the studies, a numerical model of the UBL was also developed to simulate the transport and diffusion of pollutants in the urban and suburban areas. The atmospheric boundary layer in the urban area can be divided into several sublayers. The lowest one is the canopy layer, where most human activities take place. In this layer the drag force exerted by buildings may reduce the wind speed. The reduction of the sky view-factor in this layer may cause interception of the longwave radiation emitted from buildings and ground surfaces. In addition, the anthropogenic heating makes the wind and temperature fields of this layer very complicated. It is difficult to deal with a numerical model for the whole urban area including all of these detailed processes. As a compromise, we developed a UBL model, in which the structures in the canopy layer were taken as roughness elements and the bulk energy budget for the canopy layer was calculated to obtain the mean temperature of the canopy layer. In the following sections the main features of a UBL in wintertime, as observed and simulated are described.
2. Observations The observations of the UBL comprised five observational stations including pilot balloon wind measurements, soundings and tethersondes, distributed in urban and suburban areas. One station was located downtown. Two stations were located at airports in the western and eastern suburbs. The meteorological measurements at these two stations represented the upwind and downwind conditions of the urban area, respectively, since the prevailing wind in this area is westerly. The two other stations were set in a park and a college campus, and represented the conditions of open area with sparse buildings. There were also a few surface temperature measurements distributed on areas with different functions such as industrial, residential and commercial, etc. Measurements of SO2 and suspended particles were taken by airplanes at several levels over the urban and rural areas. Fig. 1 shows a map of Shenyang City including the urban area and the outskirts of the city, as well as the positions of five observational stations. In this figure the symbols W, E, N, S and C represent the West, East, North, South and Central stations, respectively. The observations in December 1984 lasted from 17th to 28th. The measurements of wind and temperature fields were taken every 2 h.
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Fig. 1. Map of Shenyang City. The shaded area is the city and suburbs. Capital letters W, N, E, S and C are the positions of observational stations at western, northern, eastern, southern suburbs and downtown, respectively. The length and width of the map are 19 and 14 km, respectively.
During the observational period there was a cold front event from 21st through 23rd. On most of the other days the wind was weak, i.e., the wind speed in the lower part of the UBL was less than 3 m sÿ1. As stated by Johnson and Bornstein [5], the urban effects on the wind field are manifested by the friction of the buildings and structures if the wind is strong, while the effects of the heat island dominate the circulation in conditions of weak winds. Since space is limited we discuss herein the situation of weak wind only. 2.1. Flow fields in light wind conditions During the observation there were 6 days, i.e., 18, 19, 20, 24, 25 and 27 of December, when the wind speeds at the lower levels were less than 3 m sÿ1. In these days the features of heat island circulation in flow fields were evident, especially at nighttime. Fig. 2 shows the wind profiles at 0600 LST on 20th at the West and the East stations, respectively. This is a typical case of weak wind conditions at nighttime. At the West station, i.e., the upwind suburb, the flow in the boundary layer is weak westerly, while at the East, i.e., downwind suburb, weak easterly, i.e., reverse flow, appears below the height of 100 m. The reverse flow is caused by the pressure gradient force pointing to the urban centre because of the low-pressure buildup by the urban heat island effect. The reverse winds and convergent flow fields at the lower levels of the UBL appeared on most of the light wind days. Table 1 shows the convergent flow fields on the 6 days. In the table uW and uE are the u components of wind speed at a height of
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Fig. 2. Wind profiles observed at 0600 LST of December 20 at suburban stations. E: The eastern suburb and W: the western suburb.
Table 1 Statistics of convergent flows at the lower part of urban heat island circulationa Date
18
19
20
24
25
27
uE uW vN vS DV
1.14 2.24 0.33 ÿ0.22 ÿ0.55
ÿ2.21 ÿ0.76 1.70 1.15 ÿ0.90
ÿ0.11 0.65 0.27 0.40 ÿ0.89
1.13 2.50 ÿ1.62 ÿ2.27 ÿ0.72
0.50 2.83 1.24 0.32 ÿ1.41
ÿ1.08 1.14 1.26 0.34 ÿ1.30
a u and v are the components of wind speed at a height of 25 m averaged on 0000, 0200, 0400, 0600 and 0800 LST every day. The subscripts E, W, N and S represent East, West, North and South Stations, respectively. DV ¼ uE ÿ uW þ vN ÿ vS . The units are all m sÿ1.
25 m averaged from 0000 to 0800 LST at West and East stations, respectively, while vN and vS the v components at North and South stations, respectively, DV ¼ uE ÿ uW þ vN ÿ vS represents the divergence of the flow field at the lower levels. DV50 means that the convergent flow dominates the lower part of the UBL. It can be seen from Table 1 that the light wind conditions at nighttime favour the heat island circulation. On all six nights the flows were convergent with convergence values of about 5 10ÿ5–10ÿ4 sÿ1 if the urban horizontal scale is assumed to be 10 km.
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2.2. Urban heat island and internal boundary layer Fig. 3 shows the temperature profiles at West, Central and East stations, respectively, at 0600 LST of December 20. They represent the temperature structures of urban nocturnal boundary layer in weak wind conditions at upwind, downtown and downwind parts of the city, respectively. It can be seen from the three profiles that the nocturnal ground inversion caused by the rural surface long-wave radiation cooling extends from upwind to downwind suburbs with the depth of 200 m remaining unchanged. As the airflow passes over the urban canopy, a thermally induced internal boundary layer (TIBL) appears. Since the internal boundary layer is caused by the urban surface heating, its depth increases with the fetch starting at the periphery of the city. At the west station the depth of the TIBL is about 20–30 m, while it increases to 50–100 m at downtown. The maximum depth of TIBL should appear at the downwind part of the city. However, this is not the case on this day. Since at East station, the reverse flow, i.e., weak easterly, occurred below the height of 100 m, the rural cold air penetrated into the warm downtown in the lower layer. Thus, the temperature profile at East presents a complicated structure. As shown in Fig. 3 the layer of the lower 25 m over the scattered buildings in the vicinity of the East station is a shallow internal boundary layer with a nearly neutral lapse rate. Above the TIBL, is an inversion with a depth of 200 m. The intensity of the inversion between the top of the TIBL and the height of 100 m is about 48C/100 m. The strong intensity can be explained as being the result of the large temperature
Fig. 3. Temperature profiles observed at 0600 LST of December 20 at suburbs and downtown. E: eastern suburb, W: western suburb and C: downtown.
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contrast between the cold advection, which comes from the eastern rural area, in the lower part, and the warm advection from the downtown in its upper part. Between the heights of 100 and 200 m the lapse rate of the profile is nearly isothermal since the layer consists of the warm plume of air from the urban area. Fig. 4 shows the temperature distribution in West–East cross-section through the Central station, diagnosed according to five soundings and the surface temperature measurements. It presents typical characteristics of a nocturnal urban boundary layer. The lower layer is the nocturnal inversion. Its depth is about 200 m and is uniform horizontally. The temperature distribution, however, is not uniform horizontally within the inversion layer. The rural air penetrates into both the western and eastern peripheries of the city to form cold wedges in the upwind and downwind parts. Because of the marked difference in temperature between the cold rural air and the warm urban surface, the internal boundary layer is distinguishable in the peripheries of the city, that is, from 0 to 8 km, and from 14 to 18 km in Fig. 4. The temperature is almost uniform in the warm core of the city centre, i.e., from 8 to 14 km in Fig. 4. However, we can still discern the isothermal layer below the height of 100 m and the inversion layer above. Because of the neutral or less stable stratification and the friction effects of the canopy, the turbulence in TIBL is more active than that above it. The pollutants emitted in the UBL are readily trapped in TIBL. 2.3. Dust dome and cold island Fig. 5 shows the vertical distribution of SO2 concentration measured by plane in a West–East cross-section through the city centre at 0700 on December 20. The urban nocturnal inversion layer and internal boundary layer play an important role in the diffusion of urban pollutants. The industrial and residential pollusion sources in Shenyang City were distributed within 200 m above the ground surface. As the depth of nocturnal inversion was 200 m, most of the pollutants were trapped in this layer. As the internal boundary layer exists, the pollutants emitted from the elevated
Fig. 4. Distribution of temperature observed at 0600 LST in the vertical xz cross-section through the Station C.
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Fig. 5. Distribution of SO2 concentration in mg mÿ3 observed at 0700 LST of December 20 in the vertical cross-section.
Fig. 6. Same as Fig. 4, but for 1200 LST.
sources readily spread down to the ground surface, while the vertical diffusion of pollutants from surface sources was suppressed by the top of the TIBL. Thus, a region of high concentration formed in the urban surface layer. In Fig. 5 most of the SO2 is concentrated below the height of 200 m and the concentration decreases rapidly with height above 200 m. The distribution of the total suspended particles (TSP) was similar to that of the SO2. The high density region of TSP with a maximum number density of 550 cmÿ3 formed a dust dome with a shape coinciding roughly with the internal boundary layer. Since the heavy smoke attenuates the solar radiation, the net radiation of the urban surface in the morning is considerably less than that of the rural surface. Thus, the urban area becomes a cold island. Fig. 6 shows the temperature distribution in the vertical cross-section at 1200 on December 20. The remarkable feature in this figure is that the temperature below 300 m in the urban area is lower than that in suburbs. The difference near the surface is about 1.58C.
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Table 2 shows the surface temperature in the urban and suburban stations in the morning in conditions of weak wind. The table indicates that as the wind in the lower layer is weak the urban area is a cold island. The temperature in downtown is 1–28C less than that in the suburbs. The maximum difference is 3.88C. There is a positive correlation between the concentration of TSP and the cold island intensity. It can be seen in Fig. 6 that there is an isothermal region near the height of 400 m, which may be taken as the top of urban mixed layer. It indicates that the mixed layer develops very slowly. In weak wind condition the interaction among the pollutants, temperature stratification and wind may induce a vicious circle: high concentration ! attenuation of solar radiation ! retardation of mixed layer development ! retardation of downward momentum transfer ! maintenance of weak wind ! maintenance of high concentration. It is not until the afternoon that the mixed layer has slowly developed to the high levels, then momentum transfer may Table 2 Temperature in urban and suburban areas in conditions of weak winda Date
17
Time
09
10
11
12
09
10
11
12
Turb Tsub DT Conc. of TSP
ÿ12.2 ÿ10.8 ÿ1.4
ÿ10.3 ÿ9.6 ÿ0.7 2.597
ÿ9.1 ÿ7.7 ÿ1.4
ÿ7.9 ÿ6.3 ÿ1.6 1.207
ÿ13.8 ÿ10.7 ÿ3.1
ÿ10.1 ÿ8.8 ÿ1.3 1.640
ÿ8.6 ÿ8.2 ÿ0.4
ÿ7.7 ÿ6.9 ÿ0.8 1.174
Date
19
Time
09
10
09
10
11
12
Turb Tsub DT Conc. of TSP
ÿ11.1 ÿ9.9 ÿ1.2
ÿ8.3 ÿ6.0 ÿ2.3 2.346
ÿ12.7 ÿ10.3 ÿ2.4
ÿ10.9 ÿ7.5 ÿ3.4 3.118
ÿ6.9 ÿ5.8 ÿ1.1
Date
23
Time
09
10
09
10
11
12
Turb Tsub DT Conc. of TSP
ÿ13.9 ÿ13.3 ÿ0.6
ÿ12.2 ÿ11.3 ÿ0.9
ÿ17.9 ÿ17.0 ÿ0.9
ÿ16.8 ÿ15.1 ÿ1.7 1.439
Date
25
Time
09
10
11
12
09
10
11
12
Turb Tsub DT Conc. of TSP
ÿ15.8 ÿ12.0 ÿ3.8
ÿ11.3 ÿ9.7 ÿ1.6 1.672
ÿ9.0 ÿ7.6 ÿ1.4
ÿ7.5 ÿ6.5 ÿ1.0 1.605
ÿ19.2 ÿ16.3 ÿ2.9
ÿ15.9 ÿ14.0 ÿ1.9
ÿ13.4 ÿ12.4 ÿ1.0
ÿ11.8 ÿ11.3 ÿ0.5 >
a
18
20 11
12
24 11
12
27
Turb is the surface temperature (8C) at downtown station, Tsub the averaged surface temperature at four suburban stations. DT=TurbÿTsub, conce. of TSP the concentration of TSP with unit of mg mÿ3.
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Fig. 7. Distribution of artificial heat flux in Wmÿ2 estimated according to the coal consumptions averaged in December 1984.
reach the ground surface and finally the condition of high concentration and weak winds can be destroyed. 2.4. Estimation of artificial waste heat in urban area In the 1980s the main energy source in urban area of China was from coal burning. The heat release from transportation might be neglected. Thus, the artificial heat emission in the city can be estimated according to the coal consumptions. Statistic of amount of coal consumptions was made in the grid of 1 1 km every month, e.g. December 1984. Since most of the industrial facilities and house heating boilers operated continuously, the average artificial heat flux was calculated by multiplying the average amount of coal burning with the fuel discharge ratio. Fig. 7 shows the distribution of the artificial heat flux estimated. The west part of the city (refer to Fig. 1) was the industrial district. The maximum flux was released from a steel mill and coking plant. The central part of the city was a commerical and residential area with buildings of 4–6 storey. The second largest flux occurred in this area. The eastern part was the old town with densely distributed low-rise buildings and houses. The southern part, close to the river, consisted of campuses with scattered buildings and more open spaces. The artificial heat flux here was less than that in other parts of the city.
3. Numerical simulation The numerical modelling system, used to simulate the UBL, includes a hydrostatic atmospheric dynamic model, a canopy layer wind parameterization scheme, and an urban canopy layer energy budget model.
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3.1. Dynamic model The three-dimensional momentum, hydrostatic, continuity, thermodynamic and turbulent energy equations are written as follows [6,7]: du @p ¼ÿy þ f v þ Fu ; ð1Þ dt @x dv @p ¼ÿy ÿ fu þ Fv ; dt @y
ð2Þ
@p g ¼ÿ ; @z y
ð3Þ
@u @v @w þ þ ¼ 0; @x @y @z
ð4Þ
dy ¼ Fy ; dt
ð5Þ
de ¼ Fe þ Kz dt
"
@u @x
2 2 # @v g @y Be3=2 ; ÿ þ ÿ Kz l @y y @z
ð6Þ
where p ¼ Cp ðp=p0 ÞR=Cp is the Exner function, P0=1000 hPa is a reference pressure, 02 Þ is the turbulent kinetic energy and the other symbols have their e ¼ 12ðu02 þ v02 þ w common meanings. The total derivatives d @ @ @ @ ¼ þu þv þw ð7Þ dt @t @x @y @z and the turbulence terms can be indicated as @ @n Kz ; Fj ¼ @z @z
ð8Þ
where j can be u, v, y, or e. Following Yamada [8], we express the vertical diffusivities Kz as, Kz ¼ Sle1=2
ð9Þ
where l is the turbulent mixing length, S is a nondimensional function of Richardson number. The horizontal grid points are 25 25 with an interval of 1 km. In the vertical the grids are set at 0, 40, 100, 200, 300, 500, 750, 1000, 1500, 2000, 3000 and 4000 m. 3.2. Wind profile in canopy layer The first grid above the ground surface in the model is at 40 m. The buildings and structures are taken as roughness elements with the lengths ranging from 0.1 m in the outer suburbs to 1.8 m in the downtown. The roughness length is assumed to be z0 ¼ h=10, where h is the mean depth of the canopy layer.
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In order to calculate the transport of pollutants in the canopy layer we should have the wind speed inside and outside the canopy layer interpolated according to wind speed u40 and v40 at z=40 m. Four fine grids: 2, 10, 20 and 30 m are set below the first grid of 40 m. If z5h, the power law is used uðzÞ ¼ u40 ½ðz ÿ d0 Þ=ð40 ÿ d0 Þp
ð10Þ
or if z5h, the exponential law is assumed uðzÞ ¼ uh exp½ÿgð1 ÿ z=hÞ;
ð11Þ
where uh is the wind speed at the top of the canopy layer, uh ¼ u40 ½ðh ÿ d0 Þ=ð40 ÿ d0 Þp . The profile for v is similar to Eqs. (10) and (11). In the formula above, d0 is the height of zero-plane displacement, d0 ¼ 7z0 , and g is a coefficient equal to or slightly larger than 1. The power law index, p, is a function of atmospheric stability and roughness. An empirical formula is suggested ð12Þ p ¼ a expðb log z0 Þ; where z0 is in meters, a and b are coefficients depending on the Obukhov length, L a ¼ 0:28 ÿ 1:35=jLj L50 ð13Þ b ¼ 0:57 þ 2=jLj and a ¼ 0:28 þ 10=L b ¼ 0:57 ÿ 15=L
L > 0:
ð14Þ
Table 3 shows some values of p with Pasquill stability category ranging from B to E and roughness length, z0, from 0.1 to 3 m. 3.3. Energy budget in canopy layer The prognostic equations for the canopy layer temperature, Tc , and the mean substrate temperature, Ts , are expressed as [9] @Tc 2p rðTc ÿ Ts Þ; ð15Þ ¼ CT ðRn ÿ H ÿ LE þ AÞ ÿ t @t @Ts 1 ¼ ðTc ÿ Ts Þ; ð16Þ @t t where Rn , H and LE are the fluxes for net radiation, sensible heat and latent heat, respectively, A is the anthropogenic heat flux, as shown in Fig. 7. t is a time constant and r is the fraction of open area, not occupied by buildings. At nighttime Rn is just the net long-wave radiation Rn ¼ L ¼ ÿesT 4c þ ea sT 4a ;
ð17Þ
where Tc is the canopy layer temperature, Ta the air temperature at z=40 m, e the canopy layer emissivity, s the Stefan-Boltzmann constant and ea the atmospheric
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Table 3 Values of p as a function of atmospheric stability and roughnessa L (m) z0 (m)
0.1 0.5 1 3
ÿ10 (B)
ÿ20 (C)
ÿ120 (D)
800 (D)
75 (E)
0.07 0.12 0.14 0.21
0.11 0.17 0.21 0.29
0.15 0.23 0.27 0.36
0.17 0.25 0.29 0.38
0.29 0.37 0.41 0.49
a The letters in the parentheses indicate the corresponding Pasquill stability categories, L the Obukhov length in meters.
emissivity, which can be empirically expressed as ea ¼ ae1=7 a ;
ð18Þ
with ea being the air water pressure in hPa, a ¼ 0:4, an empirical constant. In order to simulate the influence of pollutants on the development of a UBL in the daytime, the attenuation effects of solar radiation by aerosols should be included [10,11]. The short-wave radiation received on the surface on a clear day is given by S ¼ ð1 ÿ aS ÞðI þ DR þ Da Þ;
ð19Þ
where aS is the albedo of the surface, I is the direct radiation, DR and Da the scattered radiation by molecules (Rayleigh scattering) and suspended particles, respectively. The direct radiation is calculated by I ¼ I0 sin CTo TR TW Ta ;
ð20Þ
where I0 is the solar irradiance, defined as the flux of solar radiation passing through a plane normal to the solar beam at the top of the atmosphere, c the solar elevation angle, and To , TR , Tw and Ta are the transmissivities of the atmosphere, influenced by ozone absorption, Rayleigh scattering, water vapour absorption, as well as aerosol scattering and absorption. The scattered radiation of aerosol is expressed by Da ¼ I0 sin CTo TR TW ð1 ÿ Ta Þof ;
ð21Þ
where f is the ratio of forward scattering to the total scattering by aerosols, o is the scattering reflectivity of aerosol. The transmissivity, Ta , is determined by the optical depth of aerosol, a function of the amount and size of aerosols. The sensible heat flux H ¼ ra Cp CH VðTc ÿ Ta Þ
ð22Þ
where ra is the air density, CH=5 10ÿ3 the drag coefficient and V the resultant wind velocity at z=40 m. During cold season in urban area without snow cover the latent heat can be neglected.
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3.4. Results A case study was carried out to simulate the UBL in a weather condition similar to that of December 20, 1984. The simulation starts from 1800. The initial conditions are set same as that observed in the western suburb station in that time. The inflow boundary conditions, i.e., the temperature and wind profiles at the western boundary are input every hour according to the temporally interpolated values observed in the western station. The integration of the model lasts 24 h. Fig. 8 shows the distribution of the temperature simulated by the numerical model in the vertical cross-section, same as Fig. 4, at 0600 LST. Compared with Fig. 4 we may see that the results of the simulation agree well with that observed. In Fig. 8 the main features of the urban nocturnal boundary layer have been manifested. For examples, the depth and strength of the nocturnal inversion, the shape and position of the cold wedge at the city peripheries, and the depth and position of the internal boundary layer, which are shown in Fig. 4, are all reproduced in the simulation. Fig. 9 shows the horizontal distribution of flow field simulated at the height of 40 m at 0600 LST. Since the heat island effects induces centripetal pressure gradient, the convergent flow field appears in the lower layer of the urban area. The convergence point is located at the downwind part of the city, i.e., at about 14 km from the inflow border. Fig. 10 shows the distribution of flow field in the vertical cross-section. It can be seen in this figure that the reverse flow has occurred in a shallow layer near the ground surface at the outflow border of the city. The strength of the reverse flow is less than that of the inflow as shown in Fig. 2. However, the cold rural air, advected by the reverse flow, may penetrate about 4 km into the city from eastern border and an urban thermally internal boundary layer develops. Above the reverse flow, the air moves outside the city and the thermal plume forms as shown in Figs. 3 and 4, similar to the schematic diagram by Oke [12]. Fig. 11 shows the vertical distribution of turbulent kinetic energy simulated at 0600 LST in West–East cross-section. The active turbulence appears in the UBL, i.e., the region from 4 to 16 km in the horizontal and below the height of 200 m in the
Fig. 8. Same as Fig. 4, but for simulated temperature.
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Fig. 9. Horizontal distribution of flow field simulated at 0600 LST at the height of 40 m.
Fig. 10. Distribution of flow field simulated at 0600 LST in the vertical cross-section. The vertical velocity vectors are multiplied by 10.
vertical, with a maximum value of about 0.4 m2 sÿ2. Most of the pollutants emitted from the source within the UBL are trapped in this region. Consequently, the region of active turbulence roughly coincides with the dust dome, as shown in Fig. 5. After sunrise the urban area gradually becomes a cold island because of the attenuation of solar radiation by smoke emitted in the UBL. The numerical simulation continued to 1800 LST of December 20. The short-wave radiation was calculated by Eqs. (19)–(21), in which a constant transmissivity, Ta , that is constant optical depth of aerosols, was taken. Fig. 12 shows the distribution of temperature simulated in vertical cross-section at 1200 LST. The main features of urban cold island are all simulated. In comparison with Fig. 6, the pattern of temperature field
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Fig. 11. Distribution of turbulent kinetic energy simulated at 0600 LST in the vertical cross-section.
Fig. 12. Same as Fig. 4, but for temperature simulated at 1200 LST.
and the height of mixed layer in the simulation are similar to that in the observation. However, the intensity of the cold island simulated in Fig. 12 is stronger than that shown in Fig. 6, probably because in the calculation a too large optical depth of aerosols, which was determined according to the amount of the aerosols observed at 0700 LST of December 20, was used to cut down too much solar radiation in urban area.
4. Conclusions In weak wind condition the observation analyses indicate the following: (1) At nighttime the urban area is a heat island mainly induced by anthropogenic heating. (2) A shallow weak reverse flow may appear in the downwind suburb and the flow field in the lower part of the urban boundary layer is convergent.
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(3) The inversion height in urban area is rather uniform; the internal boundary layer is distinct and shallow in suburbs, and deeper but not clear in downtown. (4) A dust dome forms over the urban area due to the combined effects of inversion, internal boundary layer and convergent flow fields, etc., and the urban area in daytime is a cold island because of the attenuation of solar radiation by aerosols. A three-dimensional numerical model combined with parameterization scheme of urban boundary layer can be used to simulate the main features of urban heat island and cold island.
Acknowledgements This research was supported by China National Nature Science Foundation (59895410).
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