YUKITAKA OHASHI. National Institute of Advanced Industrial Science and Technology, Ibaraki, Japan HIDEJI KIDA

Transcription

1 119 Local Circulations Developed in the Vicinity of Both Coastal and Inland Urban Areas. Part II: Effects of Urban and Mountain Areas on Moisture Transport YUKITAKA OHASHI National Institute of Advanced Industrial Science and Technology, Ibaraki, Japan HIDEJI KIDA Graduate School of Science, Kyoto University, Kyoto, Japan (Manuscript received 7 March 2003, in final form 28 July 2003) ABSTRACT The moisture distribution near the ground surface in and around the Japanese cities of Osaka and Kyoto was investigated. From the analysis of observed data, the atmosphere over the suburban areas between coastal Osaka and inland Kyoto was drier than that over Osaka and Kyoto during the daytime hours. This feature differs from results in previous studies and from expectations based on urban and suburban surface heat budgets. To understand the drying mechanism, numerical experiments were performed, using a simplified geometrical model consisting of a straight coastline, a square urban area on the coast, a square inland urban area, and a plateau mountain surrounding the urban areas. The following main results were obtained. First, suburban drying during the daytime was mainly caused by a valley circulation that developed over the surrounding mountain area. In addition, the two heat island circulations that developed over the two urban areas also caused suburban drying. As a consequence, the coexistence of mountain and urban areas caused more notable suburban drying. Second, the amount of suburban drying was greatest when the urban distance was km, which is roughly equal to the actual distance between the Osaka and Kyoto urban areas. Last, temporal changes in moisture and those of suspended particulate matter, SO 2, and NO x concentrations decreased before the arrival of the sea-breeze front. Thus, it is argued that moisture and pollutants were transported by the two heat island circulations that developed over Osaka and Kyoto and by the valley circulations and then were modified by the sea-breeze circulation. 1. Introduction It is well known that the climate of an urban area is quite different from that of the surrounding suburbs. This difference results from the fact that the surface heat budgets over the two areas differ from each other, which is mainly caused by artificial surfaces (e.g., concrete and asphalt) and high-rise buildings in urban areas. The heat island phenomenon has been so far mentioned as one specific to the urban climate (e.g., Bornstein 1968 and Oke 1976, for the observational research; Myrup 1969 and Atwater 1972, for the numerical research). This phenomenon appears mainly to be due to an excessive transfer of sensible heat from the urban surface to the atmosphere, as well as to the emission of anthropogenic heat through human activities. Additionally, a drying of the urban atmosphere occurs because the urban surface has less evaporation than nonurban surfaces. This means a decrease not only of the relative Corresponding author address: Yukitaka Ohashi, Research Center for Life Cycle Assessment, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki , Japan. oohashi-y@aist.go.jp humidity because of the higher temperatures, but also of the absolute humidity (e.g., Aida and Yaji 1979; Sakakibara 2001). Thus, urban areas tend to be warmer and drier than suburban areas (e.g., Oke 1988; Lemonsu and Masson 2002). An exception occurs in desert regions, for which the urban areas can be moister than suburban areas because of canals and irrigation (Britter and Hanna 2003). In coastal regions, a sea-breeze circulation (SBC) develops during the daytime. Subsequently, the SBC front gradually penetrates into inland regions. Observations and model simulations have revealed that the SBC front arrives at inland locations several tens of kilometers from the coastline by sunset (e.g., Pearson 1973; Simpson et al. 1977; Holland and McBride 1989; Sha et al. 1991). Consequently, the SBC front transports pollutants emitted from the coast region into inland regions (e.g., Kondo and Gambo 1979; Kurita and Ueda 1986). When the urban area is located near the coast, the urban atmosphere is influenced by the SBC during daytime hours (e.g., Takano 1977; Patrinos and Kistler 1977, for lake breezes; Savijarvi 1985; Kanda et al. 2001). Meanwhile, the structure of SBC can be deformed, or the behavior changed, by the urban areas as 2004 American Meteorological Society

2 120 JOURNAL OF APPLIED METEOROLOGY VOLUME 43 FIG. 1. Vertical cross sections of wind vectors and vertical projections of the simulated Lagrangian particles (pollutants) at 1300 LST: the case without inland urban area B for (a) wind and (c) pollutants, and the case with inland urban area B for (b) wind and (d) pollutants. Dark and light dots are particles continuously emitted from the urban A and B areas, respectively. After Ohashi and Kida (2002b). studied by Yoshikado (1992) and Martilli (2003) for one urban area and by Ohashi and Kida (2002b) for two urban areas. These studies placed importance on the interactions between the SBC and the heat island circulation (HIC) and clarified that they exerted an influence on the heat and pollutant distributions, not only over the coastal urban area, but also over inland suburban or urban regions. Those results implied that urban effects on the atmosphere extended to regions far from the urban area. Such interactions between the SBC and the heat island phenomenon have been confirmed by many observational and simulation studies (e.g., Kitada et al. 1998; Yoshikado and Kondo 1989; Yoshikado 1990; Kusaka et al. 2000; Ohashi and Kida 2001). From observational and 2D model experimental results, Yoshikado and Kondo (1989) and Yoshikado (1992) clarified that the deformation of the SBC, which penetrates inland, was due to the Tokyo metropolitan area (see left-hand side in Fig. 2) in Japan. Interactions between the SBC and the heat island phenomenon caused both the vertical wind speed of the SBC front and the depth of the SBC to increase, as compared with regions without an urban area. Furthermore, it was pointed out that a landward flow could be found in the upper levels ( 1 km) over the suburban area ahead of the SBC front; it appeared as if the sea-breeze layer rose over the suburban area. Figure 1a shows how the SBC and HIC can interact and develop over a seaside urban area. This landward flow can transport pollutants to inland upper levels prior to the transport by the SBC front. On the other hand, Ohashi and Kida (2002b) numerically experimented by adding another urban area inland to that given in the previously mentioned experiment of Yoshikado (1992), based on the assumption of the Osaka and Kyoto regions in Japan. Consequently, the local circulations were more deformed by the existence of the inland urban area, as compared with those that appeared in Yoshikado s experiments; the flow inland, which appeared in the upper levels, connected with the HIC that developed over the inland urban area, as can be seen in Fig. 1b. This flow was named chain flow (Ohashi and Kida 2002b) after its shape. Pollutants, which are continuously emitted from surface sources over the urban area, are diffused upward by the SBC and HIC, as shown in Figs. 1c and 1d. When an inland urban area was present in Ohashi and Kida s experiments, pollutants in the upper levels transported to lower levels by the chain flow, as compared with those that appeared in Yoshikado s experiments. The wind speed of the chain flow was found to be 2 3 m s 1 less than that of the SBC. Ohashi and Kida (2002a) subsequently used a 3D mesoscale model including quasi-real geographical and land use data to simulate the local circulations that developed over the Osaka and Kyoto regions (Fig. 2) in Japan. These regions are near 35 N, 135 E. This district

3 121 FIG. 2. The Osaka plain and the surrounding area. The contour intervals are 100 m. The shading indicates the coverage by large buildings (e.g., skyscrapers, high-rise apartment complexes). The dashed rectangle marks the region modeled in Fig. 3. The dot dash rectangle indicates the measurement region drawn in Fig. 4. has two large urban areas: Osaka, which is on the coast, and Kyoto, which is about 40 km inland from the center of Osaka. In the summer, Japan is entirely covered by the Pacific anticyclone; therefore, fair weather and weak synoptic wind conditions frequently occur in Osaka and Kyoto. Consequently, a well-developed SBC occurs near Osaka Bay and penetrates inland to the Kyoto basin area, while an HIC appears over the Kyoto urban area; these phenomena have been confirmed from the analysis of routine observational data (Ohashi and Kida 2002a). Additionally, the chain flow, which flowed from the coastal Osaka urban area toward inland Kyoto, was numerically simulated over this area. However, several problems remain, as follows: 1) As a practical matter, mountains surround the plain where the Osaka and Kyoto urban areas are located. The valley circulation (VC), which develops in the daytime over the mountains, probably affects the chain flow. Thus, experiments that include mountains are needed to clarify interactions between the chain flow and the VC. 2) The chain flow has not as yet been detected in meteorological observations, probably because upperlevel observations are rarely done in the Osaka and Kyoto regions. Additionally, the chain flow is so weak that it would be difficult to directly observe it. On the latter problem, there is some possibility, however, of indirectly detecting effects of the chain flow on meteorological fields, such as temperature, moisture, and pollutants. The atmosphere over the suburban area, which was between Osaka and Kyoto, was drier than that over the Osaka and Kyoto urban areas in the daytime (Ohashi and Kida 2002a). This feature differs from those reported in previous studies and that expected from urban and suburban surface heat budgets. The mechanism of such a suburban drying has not yet been clarified. Thus, by experimenting with the simplified model, we hope to elucidate this mechanism. To solve the above problems, we analyzed the meteorological data from the Osaka and Kyoto region that is shown in Fig. 2. Moreover, we used a 3D mesoscale model to simulate the local circulations that develop during the daytime. This model assumes idealized conditions, including a straight coastline, square urban areas, and a plateau mountain. In section 2, we describe the model used in this study; in section 3, we describe the daytime drying of the suburban atmosphere, using the observational data and model simulation, and we perform numerical experiments to examine the relationship between drying and the urban distance. In section 4, we briefly discuss the effects of the chain flow and mountains on the other transport, that is, the sensible heat and pollutant transport.

4 122 JOURNAL OF APPLIED METEOROLOGY VOLUME 43 TABLE 1. Specifications of the DryARD model. Governing equations Incompressible fluid and hydrostatic approximation. Coordinate and grid structure z* coordinate and Arakawa-C staggered grid system. Vertical: 30 layers at heights of 3, 10, 30, 70, 150,...,and4950 m. Horizontal: 2-km grid size. Numerical schemes and smoothing Finite-difference method. Temporal: Leapfrog solution for horizontal terms, and full implicit solution for vertical terms. Spatial: Centered difference. Time filter (Robert 1966), and fourth-order linear horizontal diffusion. Boundary conditions Top: Zero wind speed, and zero gradient for scalars. Bottom: slip, and surface temperature is diagnostically calculated from the surface heat budget. Lateral: Modified Orlanski s radiation condition (Miller and Thorpe 1981) Subgrid-scale turbulent model E- model (1.5-order closure). TKE and its dissipation rate are predicted. Parameters given by Takagi and Kitada (1994, 1998) are used. Surface fluxes Bulk method. Stable: Approximate solution with use of Beljaars and Holtslag s formula (Lee 1997). Unstable: Approximate solution with use of Dyer s formula (Lee 1997). Atmospheric radiation Shortwave: Including Rayleigh scattering (Kondo 1976; Kimura 1984). Downward longwave: Empirical formula (Kondo 1976). Soil model Multilayered model (heat conduction equation). Five layers at depths of 0.05, 0.15, 0.4, 0.9, and 1.4 m. PBL height Diagnostically calculated from the gradient Richardson number, including frictional velocity (Vogelezang and Holtslag 1996). Other Sponge layer above the height of 2450 m (Klemp and Lilly 1978). Time increment: 10 s 2. Model description We use the Dry Atmospheric Regional Demonstrations (DryARD), a mesoscale atmospheric model developed by Ohashi and Kida (2002a,b). This model s specifications are summarized in Table 1. More details concerning the model equations are described in Ohashi and Kida (2002b). Figure 3 shows the model structure used in the current FIG. 3. The model domain. The dark and light tones mark urban and mountain areas, respectively. Numerals denote distances in kilometers. study, which idealizes the dashed-rectangle region shown in Fig. 2. The total area of the horizontal domain is 160 km 80 km, with a horizontal grid interval of 2 km. In the atmospheric region, 30 layers are assigned at heights of 3, 10, 30, 70, m every 100-m interval, m every 200-m interval, and m every 400-m interval. Because the model variables are staggered on the Arakawa C-type grid, the vertical velocity component w is given at the top of each grid box, whereas the horizontal velocities and scalar variables are assigned at the midpoint of each grid box (i.e., 1.5, 6.5, 20, 50, 110 m,...). The soil is divided into five layers, assigned at depths of 0.05, 0.15, 0.40, 0.90, and 1.40 m. The sea region extends from the western edge of the model to 60 km, and the land region reaches from the coastline to 100 km, with two urban areas and a mountain. The urban A area is adjacent to the sea, and the urban B area is east of urban A. Because both urban areas are assumed to be squares (Fig. 3), their sizes are denoted by the length of their sides. The distance between the two urban areas is defined by X which is the distance in kilometers from the center of one urban area to the center of the other. A transitional region, that is, a semiurban area with one model grid, surrounds each urban area to avoid rapid variations of the surface conditions. Additionally, a mountain in the shape of a plateau with a height of 500 m, which is almost as high as that found in the Osaka plain, surrounds the two urban areas. The differences in the surface properties of the urban and suburban areas generate the HIC. For simplicity, the surface properties of the urban and suburban areas in the model, such as roughness length, moisture avail-

5 123 TABLE 2. List of model case runs. Case Coastal urban A Inland urban B Sea 16 16no 24 24no 32 32no 40 40flat 40no 40ns 40lu 40hm 48 48no 56 56no 64 64no (18 km) (18 km) Urban distance X (km) Mountain Sea 16 (16) 24 (24) 32 (32) (40) (48) 56 (56) 64 (64) (height of 1.5 times) ability, and albedo, are assumed to be constant. It is assumed that the urban and suburban areas consist of concrete and bare soil, respectively. Values of the surface and soil properties were determined using observations and the modeling results of Anthes et al. (1987) and Sugawara and Kondo (1995). The simulation was started at the model time of 0300 local solar time (LST) 29 July. Initial conditions for potential temperature and relative humidity were taken from the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) reanalysis data, at 35 N, 135 E at 0300 Japan standard time (JST), on 29 July The initial wind condition was calm, and no synoptic-scale winds were assumed during the model time of integration. For the sea surface temperature (SST), we used the 29 July 1992 Advanced Very High Resolution Radiometer (AVHRR) data over Osaka Bay, taken by the National Oceanic and Atmospheric Administration (NOAA) satellite at 1432 JST. The spatially averaged SST over Osaka Bay was 27.8 C. This SST was constant in the model because the SST diurnal variation is small. On the above date, the Osaka Kyoto plain had clear skies and calm conditions. Case runs conducted in this study are listed in Table Effects of the chain flow and mountains on latent heat transport a. Daytime drying of suburban area Figure 4 displays the temporal variations of surface water vapor pressure measured at the various Osaka and Kyoto observatories, under clear and calm conditions during daytime hours. The water vapor pressures are averaged over the 14 summer days taken from 1992 to Most of days chosen had inland penetration of the SBC front from the coastal Osaka plain to the inland Kyoto basin and the southerly heat island wind that flows into the Kyoto urban area in the daytime (Ohashi and Kida 2002a). Additionally, until the arrival of the SBC front, the valley wind that flows up the mountain was measured at the many observational sites, which were located near the mountain. The field measurements reveal that the suburban areas between the northern urban area of Osaka and Kyoto, for example, the north Suita (SuitaN) and the south Takatsuki (TakatsukiS) sites, are drier than the other regions during the daytime hours. These suburban dry areas exhibit a large amplitude in the diurnal change of water vapor pressure. From the point of view of the surface energy balance, the magnitudes of latent heat flux supplied from the Osaka and Kyoto urban areas into the atmosphere are less than those from the Suita and Takatsuki suburban areas. Therefore, the air over Osaka and Kyoto could be drier than that over Suita and Takatsuki. The air over the suburban area, however, is considerably drier than that over the urban area, as can be seen in Fig. 4. This feature suggests that local circulations affect the suburban atmosphere with different effects on the drying from those in the coastal and inland urban areas. This is discussed later. On the other hand, the calculated results (case 40; X 40 km) in Fig. 5 show a clear area of daytime drying over the suburban area between the two urban areas. This figure also shows that the SBC dominates over the Osaka urban A area and that the HIC convergence zone that develops over this area moves into the suburban area by the SBC front. Also, the HIC convergence zone occurs over the Kyoto urban B area while the upslope wind of VC occurs in the surrounding mountain areas. Using case 40 as a control run, we ran some com-

6 124 JOURNAL OF APPLIED METEOROLOGY VOLUME 43 FIG. 4. Temporal variations of mean water vapor pressure (hpa), under clear-sky and calm conditions, measured at several observational sites on the Osaka plain (after Ohashi and Kida 2002a). The region corresponds to the dot dash rectangle in Fig. 2. parison cases. For example, case 40no has none of the urban areas in case 40, and, thus, the comparison can reveal the urban effects. Case 40flat has no mountains, and, thus, the comparison can reveal the mountain effects. Also, case sea has neither urban areas nor mountains and, thus, is a simulation of the pure SBC. In Fig. 6, we show the temporal variations of the specific humidity at a height of 20 m above the ground surface for cases (a) 40, (b) 40no, and (c) 40flat. In the results for case 40, the suburban area around 1300 LST is drier than the other areas. When no urban areas exist, as in case 40no, the inland area corresponding to the urban B area, which is a bare soil surface in this case, instead of an urban surface, is the driest of the three areas: the coast corresponding to the urban A area, suburbs, and inland corresponding to the urban B areas. This results from the fact that the drying due to the VC continues to occur until the SBC arrives. Even when no

7 125 FIG. 5. Horizontal wind vectors and specific humidity (shading and contours) near the surface (z* 6.5 m) for case 40 at (a) 1200 and (b) 1330 LST. The solid squares mark the urban area. The contour interval is 0.5 g kg 1, and the shading legends are on the right. mountains exist (i.e., a flat terrain; case 40flat), the suburban area is the driest of the three areas. The suburban drying in case 40 is greater than those of cases 40no and 40flat. That is, the coexistence of mountain and urban areas causes the suburban drying to be more notable. In suburban areas around the Tokyo urban area, a moisture increase or constant moisture was measured at many inland suburban sites from the morning (Sasaki and Kimura 2001); a 7-day mean specific humidity of 1.0gkg 1 increased during the period from 0600 to 1300 JST for conditions of calm with clear skies. In contrast, our simulation result for Osaka Kyoto in Fig. 6a indicates that the specific humidity of 3.5 g kg 1 decreased over the suburban area during the corresponding period. The main difference in the geographic conditions between Tokyo and Osaka Kyoto is the area of the plain; the Tokyo plain is more than 100 km 2, whereas the Osaka and Kyoto regions are in the relatively narrow plain shown in Fig. 2. Figure 7 is a vertical cross section of the wind vectors and the specific humidity at 1330 LST for each case. In case 40 (Fig. 7c), dry air descends from upper levels to the ground surface over the suburban area between the two urban areas. In case 40no (Fig. 7b), the surface air over the entire inland region becomes dry because of the descending motion of the VC, as can be seen through the comparison with case sea (Fig. 7a) which has neither urban areas nor mountains. In case 40flat (Fig. 7d), the surface air over the suburban area between the two urban areas is drier than that over other areas, because of the descending motion of the HIC. The superimposition of these descending motions over the suburban area results in the notable drying of the area during daytime. As can be seen in Fig. 6, the air over the suburban area is the driest in the case with a mountain and two urban areas. The descending motion part of the HICs, which is over the suburban area, flows toward inland. This flow corresponds to the chain flow that was mentioned in section 1. To better understand how the SBC penetration affects the HICs, we describe the no sea case (case 40ns). Without SBC conditions, the specific humidity during the morning over the coastal urban A area does not FIG. 6. Temporal variations of the specific humidity near the surface (20 m) for (a) case 40, (b) case 40no, and (c) case 40flat. The solid, dashed, and dotted lines indicate the specific humidity within the coastal urban area (x 68 km, y 42 km), the suburban area (x 88 km, y 42 km), and the inland urban area (x 110 km, y 42 km), respectively.

8 126 JOURNAL OF APPLIED METEOROLOGY VOLUME 43 FIG. 7. Vertical cross sections of wind vectors and specific humidity at 1330 LST: (a) case sea, (b) case 40no, (c) case 40, and (d) case 40flat. The legend for the specific humidity is at the right of each figure. increase; also, the specific humidity does not rapidly increase during the afternoon over the suburban area. Thus, the SBC front transports the sea moisture first into the coastal urban area, then the suburban area, and finally the inland urban area. The chain flow is not formed in case 40ns, although the descending motion part of the HICs appears over the suburban area. The SBC penetration intensifies this descending motion and, consequently, generates the chain flow from the SBC front into the inland HIC (Ohashi and Kida 2002b). The comparison between case 40 and case 40ns suggests that, before the arrival of the SBC front, the dry upper air over the suburban area is more influential than the sea-breeze moisture transported by the chain flow into the lower levels of the suburban area. (However, as discussed later, when the urban area becomes larger than that in these cases, the sea-breeze moisture transported by the chain flow cannot be neglected.) Figure 8 shows how the moisture is transported by local circulations near the ground surface during the daytime. Whereas moisture divergence by the VC and moisture convergence by the HIC (also the SBC) coexist over the coastal A and inland B urban areas, moisture divergences due to the HIC and VC coexist over the suburban area between the two urban areas. Over the suburban area, moisture transport by the local circulations plays an important role in the drying of the lower atmosphere. In Table 3, the daytime moisture budgets at the coastal, suburban, and inland areas are indicated. The integrated latent heat (Q SL ) supplied from the ground surface, from 0600 LST to time t (1300 LST in Table 3), can be calculated by t Q (t) LH(t) dt. (1) SL 0600 Here, LH(t) is the latent heat flux at the ground surface. On the other hand, the accumulated latent heat (Q CL ) within an atmospheric column (from the ground surface to the top of the model) can be calculated by z t ih z g Q [q (z) q (z)] dz. (2) CL 0600 In the above, the symbol is the latent heat of water vaporization, is the air density, z g is the ground height, and z t is the height at the top of the model. The symbol q represents the specific humidity, and q 0600 (z) denotes the vertical profile of specific humidity at 0600 LST. Table 3 shows that values of the latent heat flux Q SL,

9 127 TABLE 3. Temporally integrated latent heat fluxes from 0600 to 1300 LST; Q SL is that supplied from the ground surface and Q CL is that within the atmospheric column. Case LST Q SL (MJ m 2 ) Q CL (MJ m 2 ) Divergence (Q SL Q CL ) (MJ m 2 ) Coastal urban A Suburban area between urban Inland urban A and B B FIG. 8. Moisture transport by local circulations near the ground surface during the daytime. supplied from the ground surface over the urban A and B areas, are less than that at the suburban area between the two urban areas (i.e., this is a typical feature of the urban climate). Nevertheless, values of the latent heat flux Q CL, within atmospheric columns over both urban areas, are greater than those over the suburban area. Consequently, the moisture divergence (Q SL Q CL ) over the suburban area is the greatest among the three areas; namely, the air over the suburban area is the driest of the three areas. The reason why the coastal urban A area does not dry is that the area is mainly affected by the moist SBC from the early morning hours. On the other hand, the reason why the inland urban B area does not dry more is that moisture divergence by the VC, which develops over the surrounding mountain, weakens because of moisture convergence by the HIC that develops over the urban B area (as shown in Fig. 8). This effect also appears over the urban A area. Therefore, interactions among the local circulations dry only the air over the suburban area in the daytime, as was found in the numerical results (e.g., Fig. 6a) and the field measurements over Osaka and Kyoto regions (Fig. 4). The degrees of suburban surface drying indicated in the observed data and numerical simulations are in qualitatively agreement with each other. The numerical results of case 40 indicate that the daytime specific humidity range is between 13 and 16.5 g kg 1. These values correspond to water vapor pressures of 20.7 and 26.3 hpa, respectively. The difference between these water vapor pressures produces a relative humidity variation of 13% for a temperature of 30 C. On the other hand, the water vapor pressure range between 21.8 and 26.4 hpa was measured at the TakatsukiS site in the daytime. The difference between these water vapor pressures produces a relative humidity variation of 11% for a temperature of 30 C. This value, estimated from the observational result, is in near agreement with that from the above standard simulation result. Without the urban areas in the model (i.e., case 40no), the maximum and minimum specific humidities are 16.5 and 14.5 g kg 1, which correspond to water vapor pressures of 26.3 and 23.2 hpa, respectively, and a relative humidity variation of 7% at 30 C. Thus, removing the urban areas from the simulation reduces the relative humidity variation by about one-half. This result implies that moisture transport by the HICs over the Osaka Kyoto plain plays an important role in the heterogeneity of surface moisture. The mountain and the urban areas also affect the SBCpenetrating speed. Comparisons between Figs. 7a and 7b reveal that the mountain hastens the SBC penetration. This phenomenon has been reported by Kondo (1990) and Ohashi and Kida (2002a) through real topography experiments and observations. The comparison between Figs. 7b and 7c indicates, on the other hand, that the urban area slows the penetrating speed. In particular, the SBC front tends to stagnate for several hours around the periphery of the urban A area. Such a stagnation has been confirmed by both observations and numerical experiments (Yoshikado and Kondo 1989; Yoshikado 1990; Kusaka et al. 2000). b. Relationship between the drying and urban distance It is of interest to investigate how the aforementioned drying is related to the distance between the coastal and inland urban areas. In Fig. 9, we indicate the temporal variations of the accumulated latent heat (defined by Eq.

10 128 JOURNAL OF APPLIED METEOROLOGY VOLUME 43 FIG. 10. Temporal variations of the minimum values of accumulated latent heat at the midpoint of the suburban area between the two urban areas. Circle dash line is for the experiments with both urban areas and mountain (case X). Triangle dash line is for experiments with no urban areas (case Xno). The solid line is the difference between case X and case Xno, which is a measure of the urban effect. The numerals denote the time (LST) of the minimum accumulated latent heat. FIG. 9. Temporal variations of accumulated latent heat for cases at the midpoint of the suburban area between the two urban areas. 2) from 0600 LST for cases ( X km). The accumulated latent heat is calculated at the midpoint of the suburban area between the two urban areas. The accumulated latent heat decreases because of moisture divergence by the local circulations until the arrival of the SBC. After this time, the accumulated latent heat rapidly increases because of the moisture of the SBC. The decrease in the accumulated latent heat from the early morning becomes greater as X decreases; this implies that the atmosphere over the suburban area is influenced by the HICs, particularly when the urban areas are close to each other. The time when the suburban area becomes the driest is delayed as X increases. Additionally, the minimum value of the accumulated latent heat tends to become greater as X increases. Figure 10 shows the temporal variations of the minimum values of the accumulated latent heat for the experiments with either the urban areas or the mountain (case X: the circle dash line) and with no urban areas (case Xno: the triangle dash line). The minimum values were calculated at the midpoint of the suburban area between the two urban areas. In case X, the minimum values of accumulated latent heat continue to decrease until X reaches 48 km, and then they increase when X exceeds 48 km. That is, the air over the suburban area is driest at X 48 km. When there were no urban areas (case Xno), the minimum values of accumulated latent heat gradually decrease with X; the peak of the minimum value does not appear in this case. The difference between the results of cases X and Xno implies a drying effect, due to the existence of urban areas (the solid line). This urban effect becomes most apparent when X is km; this urban interval is roughly equal to the actual distance between the Osaka and Kyoto urban areas. During daytime hours, the air over the plain continues to dry until the SBC arrives. An increase in X inevitably causes a delay in the arrival of the SBC. Therefore, the amount of drying (i.e., negative values of accumulated latent heat) over a certain fixed point increases as X increases. Without urban areas (i.e., case Xno), such drying occurs in the range of X km. However, with urban areas (i.e., case X), the drying becomes indistinct for X greater than 48 km, and the magnitude of accumulated latent heat approaches that of case Xno. A key finding is that the HICs work to dry the suburban area for X 48 km, and do not work to dry when X 48 km. This is attributed to the downward and divergent flow of the chain flow mentioned above, which forms over the center of the suburban area unless X exceeds 48 km. When X exceeds 48 km, the chain flow decays before it can extend to the center of the suburban area and, thus, cannot transport moisture. Kimura and Kuwagata (1995) investigated the relationship between valley width and the effect of moisture transport on accumulated latent heat, through a 2D numerical model. They found that the minimum accumulated latent heat at the midpoint of the valley appeared during the daytime hours. The valley width in their study corresponds to the urban distance in our study. Additionally, the occurrence of their minimum latent heat was delayed in time as the valley width increased. The results suggested that the increase in accumulated latent heat after the minimum was attributed to moisture transport by the back current of the VC; the delay in the time of the minimum was caused by the

11 129 FIG. 11. Temporal variations of accumulated latent heat for cases 40, 40hm, and 40lu at the midpoint of the suburban area between the two urban areas. moisture transport taking longer to travel the increasing distance from the summit of the mountain. On the other hand, the increase in accumulated latent heat after the minimum in the current experiments is attributed to moisture transport by the SBC; the delay in the minimum is because moisture transport takes longer to travel the increasing distance from the coast. c. Dependence on the urban scale and mountain height The degree of suburban drying, as was mentioned above, is related not only to the urban distance X but also to the urban scale (width) and surrounding mountain height. Figure 11 shows the temporal variations of the accumulated latent heat for a reference case (case 40), a case with the mountain 1.5 times higher (case 40hm), and a case with a larger urban area (case 40lu). The high-mountain case (case 40hm) has a lower absolute value of accumulated latent heat and an earlier minimum than that of the reference (case 40). As can be seen in Fig. 12a, which indicates the vertical cross section of wind vectors and specific humidity at 1330 LST for the high-mountain case, the SBC arrives at the inland area earlier than that for the standard case (cf., Fig. 7c). The high mountain causes an increase in the SBC-penetrating speed, because the horizontal pressure gradient between the SBC front and the inland area FIG. 12. Vertical cross sections of wind vectors and specific humidity at 1330 LST for (a) case 40hm and (b) case 40lu. The legends at the right of each figure show the specific humidity scale. ahead of the front is larger because of the strong heating of the atmosphere over the inland area (Ohashi and Kida 2002a). For the case of the large urban area (case 40lu), the same tendency as in case 40hm, mentioned above, is found in Fig. 11. As is clearly indicated in Fig. 12b, the case of large urban areas simulates a strong chain flow that develops over the suburban area between the two urban areas and transports SBC moisture to the upper levels. An extended urban area causes an intensified chain flow, which consequently can more readily transport moisture from the SBC front to inland upper levels. On the other hand, a high mountain causes both the upward flow of the SBC front and the downward flow of the chain flow to weaken. Therefore, the drying of the suburban surface air for case 40hm is smaller than that for case 40lu. 4. Effects of chain flow and mountains on the other transports a. Sensible heat The transport of sensible heat seems to differ from that of latent heat. Figure 13 shows the temporal variations of the maximum values of accumulated sensible heat. Over the suburban area, the accumulated sensible heat continues to increase, which is opposite that of the

12 130 JOURNAL OF APPLIED METEOROLOGY VOLUME 43 FIG. 13. The same as in Fig. 10, but for the maximum values of accumulated sensible heat. aforementioned accumulated latent heat. This phenomenon is attributed to sensible heat transport by the VC (e.g., Kondo et al. 1989; Kuwagata and Kimura 1995, 1997). The figure indicates that the urban area effectively increases the accumulated sensible heat over the suburban area. This result implies that the sensible heat supplied from the urban area is transported by local circulations toward the suburban area. As can be seen in Fig. 14, the atmospheric column sensible heat has the largest magnitude near the SBC front, which gradually moves inland. Therefore, the accumulated sensible heat over the objective point (i.e., the midpoint of the suburban area) in Fig. 13 reaches a maximum value just before the arrival of the SBC front. The accumulated sensible heat increases with an increase in X because the SBC front takes a longer time to arrive at the objective point that is farther away from the coast. Additionally, Fig. 13 shows that the accumulated sensible heat for the case with urban areas (case X) is, to some extent, greater than that for the case with no urban areas (case Xno). This feature results from the fact that the sensible heat supplied from the urban area is transported to the suburban area between the two urban areas. The difference in the amounts of accumulated sensible heat between cases X and Xno, that is, the urban effect, becomes indistinct as X increases. This implies that it is difficult for the chain flow to influence the center of the suburban area during the daytime; for example, the chain flow does not form over the suburban area when X is too large because the HICs have already decayed before they can connect to each other. b. Pollutants Figure 15 indicates measured concentrations of SO 2, O x,no x, and the suspended particulate matter (SPM) at the surface during 27 July 1995 over the Osaka Kyoto plain. This day had clear skies and calm conditions, and the SBC, which penetrated inland from Osaka Bay, was well developed. The SPM concentration in Fig. 15a FIG. 14. Vertical cross sections of potential temperature in case 40: (a) 1200 and (b) 1330 LST. The dashed lines indicate the position of the sea-breeze front as defined by the maximum upward velocity. The legends in the right part of the figures indicate the values of potential temperature. shows that the concentrations clearly differ among the coastal Osaka urban (Sennari) site, suburban (TakatsukiS) site, and inland Kyoto urban (Mibu) site. Over the suburban area, the SPM concentration steadily decreases until the arrival of the SBC front at 1600 LST, and then the concentration rapidly increases. This decrease in suburban concentration probably results from the same mechanism as that previously described for moisture transport; the SPM emitted from the suburban area is transported toward the surrounding mountains by the VC, while the less polluted air in the upper levels is transported toward the ground by the compensating chain flow. Over the coastal and inland urban areas, no such concentration decrease appears; however, rapid increases occur when the SBC front arrives at 1200 LST at the coastal area and 1900 LST at the inland urban area. The trends in the primary pollutants SO 2 and NO x are similar to those in SPM. The polluted air originating in the coastal urban A area could be transported by the chain flow via the SBC front into the low levels of the suburban area, as was shown in Fig. 1d. However, this transport needs to spend more time to reach at the suburban surface area than the transport of the less polluted air from the upper levels, because the polluted air is transported downward

13 131 FIG. 15. Measured surface pollutants on 27 Jul 1995: (a) SPM, (b) SO 2, (c) O x, and (d) NO x concentrations. The solid, dashed, and dotted lines indicate measured values at Sennari (the coastal Osaka urban area), TakatsukiS (the suburban area), and Mibu (the inland Kyoto urban area) sites, respectively. Refer to Fig. 4 for the site locations. by the chain flow after being transported upward by the SBC front. Hence, at the ground in the suburban area, the pollutant concentration first has a long decrease from the morning. As can be seen in Figs. 15a,b,d, pollutant concentrations measured at the suburban site gradually increase just before the rapid increases due to 579 flow of the chain flow to weaken. Therefore, the drying t he arrival of the SBC front. This gradual increase is found from 1400 to 1500 JST in SPM, from 1500 to 1600 JST in NO x, and from 1200 to 1500 JST in SO 2. In contrast, such gradual increases at the coastal urban site are not found, and only the pollutant concentrations rapidly increase when the SBC front arrives. Because the observational data are measured only at the ground level, it is difficult to determine whether this short-time increase of the suburban concentration is due to the transport of coastal urban polluted air by the chain flow before the transport by the SBC front. Unlike the other pollutants, the O x concentrations continue to increase from the early morning hours over all sites. This is likely due to O 3 production by photochemical reactions. 5. Summary and conclusions We investigated the moisture distribution near the ground surface in and surrounding the Osaka and Kyoto urban areas in Japan. From the analysis of observed data, the atmosphere over the suburban area between coastal Osaka and inland Kyoto was drier than that over the Osaka and Kyoto urban areas during the daytime hours. This feature differs from those reported in previous studies and that expected from urban and suburban surface heat budgets. To clarify the mechanism for the drying, we performed numerical experiments using a simplified geometrical model consisting of a straight coastline, two square urban areas (corresponding to the coastal Osaka and inland Kyoto urban areas), and a plateau mountain surrounding the urban areas. The following main results were obtained: 1) Suburban drying clearly appeared during the daytime hours, even with the use of a simplified model geometry. This drying was caused by a valley circulation that developed over the surrounding mountain area. However, some drying also occurred without a mountain. This drying was due to the two heat island circulations that developed over the urban areas. These heat island circulations transported the suburban moisture supplied from the ground surface toward the urban areas, while the dry upper-level air descended toward the sub-

14 132 JOURNAL OF APPLIED METEOROLOGY VOLUME 43 urban surface by the compensating flow. Consequently, the coexistence of mountain and urban areas caused the suburban drying to be more notable. 2) The suburban area was drier than the inland Kyoto area, even though the Kyoto area is surrounded by mountains. The reason for this is that moisture divergence by the valley circulation weakened because of moisture convergence by the heat island circulation that developed over the urban area. 3) The amount of suburban drying depended on the distance between the coastal and inland urban areas. Surface drying was largest when the urban distance was km, which is roughly equal to the actual distance between the Osaka and Kyoto urban areas. Therefore, the Osaka and Kyoto urban areas are located at the optimum distance for suburban drying in the daytime. 4) Over the suburban areas, measured concentrations of SPM, SO 2, and NO x decrease in a manner remarkably similar to that of the moisture before the arrival of the sea-breeze front. This similarity suggests that they are caused by the same transport mechanism. Thus, moisture and pollutants are transported by the two heat island circulations that develop over the coastal and inland urban areas. The flow generated by these heat island circulations is then deformed by the seabreeze circulation into what we termed chain flow (Ohashi and Kida 2002b). This chain flow is not newly generated; instead, it consists of independent heat island circulations. Here, we studied interactions among the sea-breeze circulation, valley circulation, and heat island circulation. In general, interactions among local circulations play important roles in the transport of heat and pollutants and produce distributions of these that differ from those caused by the individual circulations. Acknowledgments. Use was made of the GFD DEN- NOU Library to draw many of the figures. The NCEP NCAR reanalysis data were used to initialize the model profiles. The sea surface temperatures, taken from the AVHRR data from the NOAA satellite, were provided by the Marine Information Science Laboratory, Kobe University of Mercantile Marine. The meteorological data were made available by the Japan Meteorological Agency, Hyogo, Osaka, and Kyoto Prefectures along with relevant cities. We thank three anonymous reviewers for providing useful comments for the improvement of the manuscript. We deeply appreciate Dr. Yutaka Genchi of the National Institute of Advanced Industrial Science and Technology (AIST) for his help. 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