Investigation of local flow features in Istanbul. Part I: high-resolution sensitivity simulations

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (215) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 1.12/joc.4248 Investigation of local flow features in Istanbul. Part I: high-resolution sensitivity simulations Yasemin Ezber, a * Omer Lutfi Sen, a Zafer Boybeyi b and Mehmet Karaca a a Eurasia Institute of Earth Sciences, Istanbul Technical University, Turkey b Department of Atmospheric, Oceanic and Earth Sciences, College of Science, George Mason University, Fairfax, VA, USA ATRACT: The three-dimensional non-hydrostatic mesoscale model, OMEGA (Operational Multiscale Environment model with Grid Adaptivity), is utilized to investigate the thermally driven local flows and their interaction with each other over the province of Istanbul, Turkey. Idealized case simulations are conducted in order to describe the contribution of sea land breezes and urban heat island (UHI) circulation to the local flow over the city. The city of Istanbul is located between two water bodies, Black Sea in the north and Sea of Marmara in the south. A convergence zone is observed at about LST over the region due to the merge of two sea breezes that develop at both sides and advance towards inland areas. Investigation further indicates that the other geographic features of the city have substantial effects on the local flow. The Bosphorus Strait, which connects Black Sea to Sea of Marmara and divides the city into two parts, channels the flow. Although not significantly high and steep, the topography enhances the flow strength. Besides, urbanized regions in the south, by generating UHI circulation, prevent the inland penetration of the sea breeze that develops in the south. The numerical simulations reveal that the large-scale wind direction controls the inland penetration of the sea breeze. One of the outstanding features of the local flows is that all hypothetical simulations including the Bosphorus indicate a strong fan-like airflow in the immediate south of the strait. The flow is more intense in the northerly prevailing wind case. It is also found that stronger large-scale flows (e.g. 2.5 and 1 m s 1 ) do not allow the formation of a sea breeze over the region. A warmer sea surface than land surface (about 2 C) makes changes in the duration and extension of the land sea breeze circulation. The duration of a land breeze circulation is long, and the southerly sea breeze is onset close to the south coast under the northeasterly flow conditions. KEY WORDS sea land breeze; urban heat island circulation; idealized case simulation; mesoscale model Received 16 April 213; Revised 25 November 214; Accepted 5 December Introduction In fair-weather conditions when large-scale flows are weak, and skies are clear, terrain heterogeneities and landscape diversity generate mesoscale circulations, such as sea land breezes and mountain-valley winds, which often dominate the local circulation pattern. Difference of the heat capacity between land and water bodies is the main reason of the onset of the thermally driven diurnal circulation known as sea land breeze circulation. Large-scale flows affect the development of a sea breeze. Satisfactorily strong offshore geostrophic winds can help generate a fully developed sea breeze, while onshore geostrophic winds tend to suppress the sea breeze perturbation (Estoque, 1962; Bechtold et al., 1991; Arritt, 1993; Simpson, 1994). Offshore flows (in the opposite direction to the sea breeze progress direction) advect the warm air over the land towards the sea, which produces a strong horizontal temperature gradient in a thick atmospheric layer and a strong pressure gradient in the surface layer (Estoque, 1962; Zhong and Takle, 1993). However, under the onshore * Correspondence to: Y. Ezber, Eurasia Institute of Earth Sciences, Istanbul Technical University, Maslak, Istanbul, Turkey. yaseminezber@gmail.com large-scale flow conditions, temperature increase in the atmosphere is hindered by the advection that weakens the pressure gradient and the sea breeze circulation (Estoque, 1962). Researchers have investigated the factors causing the diurnal rotation of the sea land breeze winds as well as the effects of geographic location and topography on the circulation for the different regions of the world. The main factors are identified as the Coriolis effect, mesoscale pressure gradient and frictional force (Atkinson, 1981; Garratt and Physick, 1985; Zhong and Takle, 1993). Zhong and Takle (1993) implied that the rotation results from the imbalance between the Coriolis force, mesoscale and large-scale pressure gradients, horizontal and vertical advection. Geographic location affects the development and life span of the sea land breeze in terms of the solar insolation rate and the magnitude of the Coriolis force. The sea breeze circulation is usually stronger in summer days as solar insolation is much higher in summer than in winter. Hills can accentuate the sea breeze effect, and mountains and valleys can completely change the sea breeze direction (Atkinson, 1981). Mahrer and Pielke (1977) found that combination of the mountain circulation and the sea breeze in case of zero prevailing wind conditions enhanced the circulation both in day and night compared with their separate effects. 215 Royal Meteorological Society

2 Y. EZBER et al. In case of opposing flows, topography is of the greatest importance, and the convergence developed as a result of the differential heating (e.g. sea breeze or slope flows) is enhanced by the surrounding orography (Abbs, 1986). Coastal irregularity can influence the sea land breeze circulation by means of shape of the shoreline. McPherson (197) states that the any large indentation (such as a bay) produces an inland distortion of the sea breeze convergence zone. He also indicates that the Coriolis acceleration and the component of the pressure gradient force normal to the bay sides act in opposite direction over the west and same direction over the east side of the bay. In a similar manner to the sea land breeze mechanism, the heat capacity difference of urban rural areas results in another important circulation known as urban heat island (UHI) the air over the urban canopy is warmer than the surrounding suburban or rural areas. UHI characteristics show variation in time and space depending on meteorological conditions, morphology of the urban areas and different urban characteristics (Oke, 1987). Populated cities located near water bodies are exposed to both sea land breeze circulation and UHI effects. Martilli (23) reports that an urban region accelerates development of the sea breeze in the morning whereas it slows the penetration of the sea breeze front. Yoshikado and Kondo (1989) suggested that the sea breeze front structure was strengthened by differences in thermal characteristics between the urban and rural areas. Characteristics of the urban area define the UHI intensity thereby affect the penetration and the intensity of the sea breeze. Yoshikado (1994) points out that the strength of the interaction between the sea breeze and UHI is controlled by the size of the urban area, its distance from the sea and the intensity of the UHI. Kusaka et al. () suggest that the sea breeze front stalls over the city as a result of the UHI effect. Ohashi and Kida (22) states that coupled sea breeze and UHI circulation prevents the further inland progress of the sea breeze. Thus, although the onset and evolution of the sea breeze and its interaction with UHI circulation have similar characteristics, each city has their peculiar circulation depending on its physiography, location (distance from the seas) and size. Freitas et al. (27) numerically shows the possibility for UHI to accelerate and retard the sea breeze propagation over a city. They describe three phases: (1) UHI accelerates the sea breeze front towards the city centre; (2) when the sea breeze front reaches the urban area, it stalls over the city centre for a couple of hours; and (3) UHI dissipation allows the sea breeze to progress beyond the city centre. Mountain-valley circulation could be another important circulation influencing the local weather patterns over coastal cities (Nitis et al., 25). Mountains can prevent the inland penetration of the sea breeze as a barrier, and can intensify the sea breeze associated with mountain-valley wind system via up-slope or down-slope wind (Ookouchi et al., 1978). Distance from the sea, height of mountains and their orientation influence the sea breeze characteristics and hence the air pollutants transport. Pollutants disperse into a deeper layer in the atmosphere rather than transport over to the sea during the night-time because the presence of the city produces weaker land breeze. In the morning, pollutants over the sea are brought back to the city by the sea breeze, and the presence of the city accelerates the recirculation processes (Martilli, 23). Istanbul is one of the most crowded cities in the world with approximately 13 million inhabitants. The province of Istanbul is located between 28 1 E E longitudes and 4 28 N 33 N latitudes. Its bridge-like territory between Asian and European parts of Turkey is divided by a narrow strait, called the Bosphorus ( 1km wide and 31 km long), which connects the Black Sea in the north to the Marmara Sea in the south. Istanbul has a hilly topography rising to about 35 m height above the sea level at most. The slopes overlooking the Bosphorus strait usually rise gently along the southern and mid sections (looking like a valley). In the north, however, they rise sharply (looking like a canyon). The province of Istanbul has an island chain with hills in the Marmara Sea. Most of the residential districts in Istanbul are located along the coast of the Marmara Sea and the Bosphorus. The central and northern parts of the province are mostly covered by forest and bush. There are also several water reservoirs in the province to supply water for domestic use. All together, the province of Istanbul depicts a picture of a relatively complex region with some remarkable contrasting features in landscape, land sea boundary and topography. Considering the fact that the city of Istanbul has been growing and expanding rapidly during the recent decades, it becomes critical to study the thermally driven mesoscale circulations over the city and its interaction with some of the surface characteristics of Istanbul province. This is important not only for understanding of the dynamics of the local circulation over a relatively complex region but also for attaining knowledge about the air quality of the city where millions of people reside. To achieve these objectives, we carried out several sensitivity simulations using a mesoscale model capable of resolving the basic features of the province of Istanbul. The numerical model and experimental design are described in Sections 2 and 3, respectively. Section 4 gives the results of the experiments and Section 5 provides the conclusions of this study. 2. Numerical model The numerical model used in this study is the Operational Multiscale Environment model with Grid Adaptivity (OMEGA). It is a fully non-hydrostatic, three-dimensional prognostic atmospheric model with an adaptive grid, which is unstructured triangular prism referenced to a rotating Cartesian coordinate system in horizontal and terrain following in vertical grid structure (Figure 1). The model uses a finite-volume flux-based numerical advection algorithm derived from Smolarkiewicz (1984). OMEGA has a detailed physical model for the planetary boundary layer (PBL) with a 2.5 level Mellor and Yamada (1974) closure scheme, which uses a 1.5-order turbulent

3 LOCAL FLOW FEATURES IN ISTANBUL: SENSITIVITY SIMULATIONS Figure 1. The vertical levels of the OMEGA grid are constructed by extending radials through each surface vertex. A set of stretchable layers exists near the surface with constant thickness layers above (Bacon, 23). kinetic energy closure scheme. In the PBL scheme, the revised formulas by Beljaars and Holtslag (1991) are used for the surface layer. The land surface model in OMEGA is based on the scheme proposed by Noilhan and Planton (1989). The implemented scheme allows for two layers in soil and a single vegetation canopy. The parameters used to describe the main physical processes in a soil-vegetation system are divided into two classes: primary parameters to be specified by spatial distribution, and values of secondary parameters can be associated with the values of the primary parameters. A modified Kuo (Kuo, 1965; Anthes, 1977) scheme is used for cumulus parameterization. The explicit cloud microphysics is active over the whole domain. If cells are larger than 1 km 2, the cumulus parameterization calculates the sub-grid convection. An extensive bulk-water microphysics package derived from Lin et al. (1983) is used in OMEGA. It models the shortwave absorption by water vapour and longwave emissivities of water vapour and carbon dioxide using the computationally efficient technique of Sasamori (1972). Urban parameterization in OMEGA is fundamentally based on Brown and Williams (1998) and Brown (). In order to account for urban influences, drag and turbulence production into the flow equations, the surface energy budget and heat equation are modified. These parameterizations endeavour to account for the area-average effect of drag, turbulence production, heating and surface energy budget modification induced by buildings and urban land use. As lack of spatial resolution does not allow mesoscale models to simulate the fluid dynamics and thermodynamics in and around urban structures directly, urban canopy parameterizations are used to approximate the drag, heating, radiation attenuation and enhanced turbulent mixing produced by sub-grid scale urban elements. Urban scheme includes the following parameter values: urban fraction is.9, between-building fraction is.6, building height is 25 m, urban albedo is.18, urban emissivity is.85, roof albedo is.2 and roof emissivity is.9. OMEGA uses an Optimum Interpolation (OI) analysis scheme (Daley, 1991) to create initial and boundary conditions. The model uses one-degree horizontal resolution global soil type database created from the Global Ecosystem Database. The source data set consists of separate fields containing the amounts of clay, sand and silt in 12 soil types. Land-use/land-cover characteristics in the model are based on 1 km AVHRR data spanning April 1992 through March However, during the simulations, especially for urban investigation runs, we modified its land-use map according to the surface cover maps available in the Municipality of Istanbul in order to represent the current land-use distribution correctly (Figure 2). An important feature of OMEGA is its unstructured grid. The unstructured grid of the model provides flexibility to facilitate the gridding of arbitrary surfaces and volumes in three dimensions (e.g. to terrain features). In particular, unstructured grid cells in the horizontal dimension can increase local resolution statically to better capture the important physical features of atmospheric flow. This grid feature provides smooth transition from high resolution where needed to low resolution elsewhere (details of the grid structure can be found in Appendix). The variable resolution and adaptive nature of the OMEGA grid can, for example, adapt to urban plumes, thus resolving the large-scale dynamics as well as the local scale circulations associated with fine scale representation of urban outflow (Bacon, 23). The OMEGA model evaluation studies give the details of the model performances for different aspects such as aerosol and gas hazard prediction, dispersion, hurricane forecast and local circulation (Boybeyi et al., 1994, 21; Bacon et al.,, 28; Gopalakrishnan et al., 22). 3. Experimental design A triple-nested grid domain is used in the model simulations (cf., Figure 3). The coarse domain (Figure 3) is located between N and E, mainly covering Eastern Europe, central and eastern parts of Mediterranean Sea, the Anatolian Peninsula and the Black Sea. The innermost domain (Figure 3(b)) focuses primarily on the province of Istanbul. Horizontal grid resolution ranges from 6 km in the coarse domain to.56 km in the innermost domain. Thirty-five layers are

4 Y. EZBER et al. Figure 2. Model modified land-use types for the simulations. used to represent the atmosphere vertically. The mesh, which is kept static during the simulations, comprises grid cells. The model was set up to derive the initial and boundary conditions from the standard atmosphere whose base temperature was prescribed as 2 C (for the standard atmospheric condition, the model default temperature value is 288 K or 15 C) to reflect summer conditions. In addition, a.25 m s 1 northeasterly wind (default is ms 1 for the standard atmosphere) was set uniformly at horizontal and vertical directions to reflect the prevailing wind direction in Istanbul. In one sensitivity simulation, however, we prescribed southwesterly winds with the same magnitude to investigate the effects of the second dominant wind direction in Istanbul. The model was set to run 24 h for a summer day (July 23). It is known that there is a gradual increase in the sea surface temperature (about 1 C) from the Black Sea to the Marmara Sea. When the standard atmospheric conditions are used, SST is set constant by default. Instead of assigning a constant SST, we used the SST climatology in order to represent the gradual change in the temperature conditions along the strait. We also initialized the land grids warmer than the nearby sea grids (by about 2 C). Urban scheme in the model prescribes 2 C ground temperature difference between urban and rural areas. Therefore, the urban grids are approximately 4 C warmer than its nearby sea grids. An observation-based sea surface temperature distribution was also considered by running the model for real-case simulations for both winter and summer, and the results of this study are given on a follow-up separate paper (Part II, Y. Ezber, 214; personal communication). The model is capable of capturing the main circulation features and the distribution and evolution of temperature over the province of Istanbul in these real-case simulations. Using the OMEGA model with the same initial and boundary conditions described above for the hypothetical cases, we performed five simulations with different landscape configurations (see Figure 4). The first simulation includes a relatively simple landscape configuration (i.e. flat topography at sea level, no strait, no islands, land grids are covered by forest). The second simulation includes only the strait over the landscape of the first one, the third simulation includes the strait plus actual topography, the fourth simulation includes the strait plus actual topography plus the islands, and plus the actual land cover land use (including inland water surfaces). In all these simulations, we used northeasterly winds (as initial and boundary conditions). In a fifth simulation, we changed only the wind direction (to southwesterly) over the configuration of the fourth simulation described above. Hereafter, these simulations are called f 1, f 2, t1, u1 andu2, respectively. The reason for starting with a very simple landscape configuration is to better observe the onset and subsequent phases of the sea breeze development over a narrow land between two seas. Then, we wanted to explore how it is modified with the addition of a different surface feature to this landscape configuration at a time. The addition is made in a cumulative manner, and the order of the features added is the strait first, topography second, islands third and urbanized area fourth. With the fourth one, the landscape becomes the closest to the present-time conditions. It should be noted that the aim of this study is not to understand the effects of individual features on the development of the thermally driven circulation over the province of Istanbul, but to explore how the land sea configuration in Istanbul province (essentially the dominant cause) initiates and sustains a sea breeze circulation and how this circulation is manipulated by the other features using experimental setups including very simple, intermediate and present-time landscape configurations. However, if the order of the included features changes, it is possible to observe different flows over the province (except u1 and u2 cases as they already have all the possible features in their configurations).

5 LOCAL FLOW FEATURES IN ISTANBUL: SENSITIVITY SIMULATIONS Figure 3. Mesh structure of the OMEGA simulation domain. The results are illustrated via time-longitude/latitude and longitude/latitude altitude plots for three transects; one crosses the Asian side longitudinally, one closely follows the strait, and the last one crosses the urban areas at both sides of the city latitudinally (Figure 5). 4. Simulation results 4.1. Along meridional land transect The thermally driven mesoscale circulation over the province is investigated primarily through the differences in the potential temperature, wind speed and direction, vertical velocity and PBL height. Figures 6 and 7, thus, illustrate, respectively, the evolution of the hourly potential 215 Royal Meteorological Society temperature and wind speed/direction along the transect in the Asian side in a 24-h time slice for the aforementioned simulations. Land starts warming at about 7 LST (UTC + 2) in the morning (Figure 6) for the simplest sensitivity case, f 1. Wind blows from north for the early morning simulation hours as the initial wind direction is set northeasterly (Figure 7). After 6 LST, with rising sun, surface temperatures over both the northern and southern parts of the region increase in time. Temperature increase over the land results in increasing temperature difference between land and sea that triggers the sea breeze flow over both sides of the transect (at about 8 LST). Northerly and southerly sea breezes start to develop in the north from the Black Sea and south from Int. J. Climatol. (215)

6 Y. EZBER et al. Simulation background NE SW Strait Topography Island Urban.25 m s 1.25 m s 1 Yes No No No No No Yes Yes No No No No Yes Yes Yes No No No Yes Yes Yes Yes Yes No No Yes Yes Yes Yes Yes Figure 4. Sensitivity simulation description. Figure 5. Transect locations over the domain. Symbols are specified grids where the convergence occurs over the transects.

7 LOCAL FLOW FEATURES IN ISTANBUL: SENSITIVITY SIMULATIONS Latitude 4.9 (b) 4.9 (c) (d) 4.9 (e) 4.9 (h) K Figure 6. Temporal evolution of the potential temperature along the land transect for idealized cases f 1, (b) f 2, (c) t1, (d) u1 and(e) u2 (black solid lines show where lands are located, the upper one shows mainland and the lower one shows island). Acronyms on y-axes in the plots indicate the side of the transect ( is for the Marmara Sea side, and is for the Black Sea side). the Marmara Sea, respectively. With the further warming of the land from 9 to LST, sea breeze fronts progress towards the inland areas. The temperature over land reaches a maximum at about 11 LST, thus, resulting in the maximum temperature difference between sea and land. In response to this, relatively strong sea breezes at both sides advance and create convergence over a land section that is closer to the Black Sea coast in the transect (Figure 7). Positive vertical velocity at 11 LST (see Figure 8) is also an indication of the convergence of two sea breezes. Advection of cooler air over seas by the sea breezes reduces the temperatures over the land from about LST on local time. The cooling of the land surface temperatures results in the dampening of the temperature differences between land and seas, thus, the weakening of the sea breezes. This could be referred as a negative feedback phase in the sea breeze circulation. After this brief cooler period, warming of the land resumes and continues in response to the diurnal cycle of the incoming solar radiation. Surface temperature peaks at about 14 LST over the land. During the rest of the day, it decreases gradually. Northerly winds usually prevail along the transect for the period between 12 and 14 LST. After 14 LST, another sea breeze circulation forms over the southern sections of the transect, though not as strong as that formed in the morning. Northerly and southerly winds converge again at about 15 LST. The weak convergence in this case takes place at a point closer to the Sea of Marmara. Inclusion of the strait to the flat topography (f 2) produces a fairly similar potential temperature evolution along the transect as in the first case (Figure 6(b)). However, the first temperature maximum takes place at LST, an hour earlier compared to the f 1 case. The cooling after LST is somewhat weaker and the second warming after 12 LST is relatively stronger than those in the f 1 case. These differences are primarily caused by the thermal differences between land and water surface, which initiate sea breezes along the strait, and aerodynamic change (i.e. roughness change from land with forest cover to water surface), which affects the movement of air over the transect. The topography is added to the landscape configuration by switching to the default topographic data in the model (called t1 case). The diurnal change of potential temperature indicates that the heating in the morning in this case is not as strong as those in the flat cases. The cooling period in this case is nearly subtle. It seems that, after the morning warming, the temperatures do not change much until 12 LST when the warming resumes. The convergence of the sea breezes occurs at about LST (Figures 6(c) and

8 Y. EZBER et al. Latitude 4.9 (b) (c) (d) 4.9 (e) 4.9 (h) m s Figure 7. Temporal evolution of v-component of the wind (m s 1 ) along the land transect for idealized cases f 1, (b) f 2, (c) t1, (d) u1 and(e)u2 (black solid lines show where lands are located, the upper one shows mainland and the lower one shows island). Latitude 4.9 HOUR (b) 4.9 (c) 4.9 (d) 4.9 (e) 4.9 (h) m 2 s Figure 8. Temporal evolution of divergence (m 2 s 2 ) along the land transect for idealized cases f 1, (b) f 2, (c) t1, (d) u1 and(e)u2. Negative divergence values give the convergence along the transect (black solid lines show where lands are located, the upper one shows mainland and the lower one shows island).

9 LOCAL FLOW FEATURES IN ISTANBUL: SENSITIVITY SIMULATIONS 8(c)) as in the f 2 case; however, the breezes are comparatively weak in t1 case at that time (Figure 7(c)). Maximum heating is observed in the afternoon, but the comparatively stronger northerly sea breeze sweeps heated air over the land towards the southern part of the transect. Late afternoon cooling is observed a bit earlier than the flat cases. The northerly wind prevails after 17 LST and it switches to the southerly after 21 LST. Last two sensitivity simulations contain all realistic surface features, including urban areas, in the domain. Large-scale wind direction (i.e. geostrophic wind) is the only difference between these two cases. The fourth sensitivity case whose large-scale winds are northerly (u1) yields a daily pattern for the potential temperature that is broadly similar to that in the t1 case. However, the urban areas, which are mostly located in the southern parts of the transect, produce some noticeable differences in the evolution of the potential temperature. The t1 case shows a clear advection of the heat towards south in the afternoon. Such advection is not that clear for the u1 case. Maximum warming occurs at 14 LST over the mainland and at 15 LST over the islands (shown in Figure 6(d)). Sea breeze starts at 7 LST in the north and 6 LST in the south of the transect (Figure 7(d)). Flow is not as strong as that yielded in the t1 simulation for the morning hours. In addition, convergence due to the merged sea breezes is also not as intense as those in previous cases, however, it is observed at the same hour (at LST). Observed northerly flow is weaker than those observed in t1. On the other hand, the southerly flow at 15 LST is stronger than the previous cases. After 17 LST, similar potential temperature and flow characteristics as the previous cases are observed in the u1 case (Figures 6(d), 7(d) and 8(d)). In the last sensitivity case, the wind direction is southwesterly (u2). Comparison of potential temperatures indicates that the u2 case generates higher potential temperature values than the u1 case, especially in the afternoon. Sea breezes start after 6 LST in the south of the transect and after 7 LST in the north. Similar to the u1 case, convergence by the merged sea breezes is observed at LST (Figure 8(e)), and northerly winds, as seen in almost all cases, are dominant along the transect after 11 LST (Figure 7(e)). A comparatively strong warming along the transect occurs between 13 and 17 LST (Figure 6(e)). Northerly winds blow during the afternoon and the strong southerly flow is not observed around 15 LST in the u2 case in contrast to the u1 case. In order to understand the temporal evolution of the horizontal wind, a grid, affected by the convergence of the two breezes, is specified over the land transect (Figure 9). Start of the strong negative values for the v-component of the wind, which show northerly flow, varies from case to case. They are observed at 113 LST in the f 1 case (Figure 9), at 13 LST in the f 2 case (Figure 9(b)) and at 83 LST in the t1 case (Figures 9(c)). Negative velocity values, which show northerly flow, are seen at 13 LST in the u1 case, while it is at 9 LST in the u2 case (Figure 9(d) and (e)). Winds generally stay strong. However, winds in the u2 case are usually weaker than those in the other cases at all levels. Time history of the vertical velocity indicates that all sensitivity simulations have strong upward motion in the morning hours (Figure 1). The flat case (f 1) has strong positive vertical velocity from surface to the upper levels at 11 LST (Figure 1). However, vertical velocity is strong in the upper atmospheric levels in the f 2 and t1 cases. Relatively weak negative vertical velocities Height (m) (d) (b) (c) (e) m s (h) Figure 9. Temporal evolution of v-component of the horizontal wind (m s 1 ) for the specified grid in all cases f 1, (b) f 2, (c) t1, (d) u1and(e)u2 (for the land transect).

10 Y. EZBER et al. Height (m) (b) (c) (d) m s (e) (h) Figure 1. Temporal evolution of vertical velocity (m s 1 ) for the specified grid in all cases f 1, (b) f 2, (c) t1, (d) u1 and(e)u2 (for the land transect). are observed at the surface in these cases at LST (Figure 1(b) and (c)). Last two simulations yield strong upward motion at LST, up to m in the atmosphere (Figure 1(d) and (e)). The temporal evolution of the vertical velocity indicates that strong vertical velocity values are observed when the convergence occurs due to the merge of two sea breezes. PBL heights of all cases are also investigated to improve our understanding of the differences in the vertical structure over the specified grid. PBL height starts to increase at 7 LST for all sensitivity simulations, and it reaches its maximum in the morning (Figure 11). The time of the maximum PBL height is different for flat and non-flat cases. Namely, maximum PBL height is observed at 11 LST in the flat cases (f 1andf 2) shown in Figure 1 and (b), while it occurs at LST in the rest of the cases. There are secondary peaks in the PBL heights in all cases, and they are observed in the afternoon. The time of the second maxima is 13 LST in the f 2, t1 andu2 cases (Figure 11(b), (c) and (e)), while it is 12 LST in u1 and LST in f 1. PBL height decreases in the late afternoon and evening, becoming minimum at LST for all cases except f 1 whose minimum occurs 1 h later. The onset time of the convergence occurring as a result of the merged sea breezes can be determined from the time history of the v-component of the wind and PBL height. Vertical extent of the sea breezes can be determined from the v-component of the wind (cf., Figure 12). The convergence is mostly observed at LST in most cases except for the simplest one (f 1), and we plot the cross-section of the v-component of the wind through the land transect for this time. Figure 12 illustrates the positions of two sea breezes along the transect in the f 1 case. They are relatively shallow and still close the shorelines. Addition of the strait makes the strongest parts of the sea breezes closer to each other at this time (Figure 12(b)). Moreover, they prevail at a relatively thicker atmospheric layer. Inclusion of the real topography and islands strengthens, especially, the northerly sea breeze (Figure 12(c)). The convergence occurs a little further to the south compared with the flat cases. In the cases with the realistic topography and landscape, the convergence zones are well defined, with the fronts of the sea breezes positioned even closer to each other (Figure 12(d) and (e)). The geostrophic wind direction causes a few small differences between these two simulations. In the case of northerly wind, the convergence takes place a bit further to the north compared with that in the case of southerly wind. The winds in the northerly sea breeze, especially those closer to the front, are stronger in the u2 case than in the u1 case. In conclusion, it could be said that the thermally driven local circulation over the province of Istanbul has three relatively distinct phases. The first phase is the morning warming over the land and the initiation and convergence

11 LOCAL FLOW FEATURES IN ISTANBUL: SENSITIVITY SIMULATIONS PBL height (m) (b) 5 15 (c) 5 15 (d) 5 15 (e) 5 (h) Figure 11. Time history of the PBL height (m) for the specified grid in all cases f 1, (b) f 2, (c) t1, (d) u1and(e)u2 (for the land transect). (b) m s Height (m) (c) (d) (e) C Latitude D Figure 12. Vertical changes of the v-component of the wind over the land transect at LST f 1, (b) f 2, (c) t1, (d) u1 and(e)u2.

12 Y. EZBER et al. Latitude (b) (c) (d) (e).5 (h) K Figure 13. Temporal evolution of the potential temperature along the strait transect for idealized cases f 1, (b) f 2, (c) t1, (d) u1and(e)u2. of the northerly and southerly sea breezes. The second phase is the subsequent cooling (or no warming) as a result of the inland advection of the cooler maritime air over the land. The last phase is the afternoon warming and the dominance of the northerly sea breeze, which sweeps the warmed air over the land towards south. The last phase gradually dissipates in response to the diminishing solar energy. These phases are more or less observed at all six sensitivity cases, implying that the major driver is the relative positioning of the land and sea bodies. The local landscape features such as topography, strait, urban areas, islands and the direction of the prevailing wind seem to primarily affect the strength (and timing) of the local circulation and associated features over the land Along strait transect Second transect is taken along the strait (Figure 5). The first sensitivity case does not, however, include the strait. Therefore, it yields a similar diurnal temperature pattern as is observed in the land transect. The land surface starts warming gradually after 7 LST in the morning (Figure 13). Two sea breeze circulations form over the edges of the transect at about 8 LST in the north and 9 LST in the south (Figure 14). Approximately in the middle of the transect, warmer temperatures are observed due to the progressive sea breezes from the north and south of the region. This is also the convergence region as the sea breezes merge there at about 11 LST. This is also the time for the maximum temperature observed at the convergence region (Figure 15). The sea breezes sweep the warmer air in front of their fronts towards the centre, but at the same time, they bring cool maritime air over the land behind their fronts. This process weakens the strength of the sea breezes by reducing the temperature difference between land and sea. After the temperature maximum at about 11 LST, a cooling period is observed especially over the southern two thirds of the transect. This period is then followed by a relatively fast warming period. The whole transect comes under the influence of the northerly flow that sweeps the warmer air over the land towards the southern sections of the transect where the maximum temperatures occur. The temperatures start to decrease gradually at about 14 LST in the north and about 16 LST in the south. The second sensitivity case, f 2, is to understand the effects of the addition of the strait to this simple landscape configuration. The transect follows the zigzags of the strait; therefore, the potential temperature fluctuates along the transect. Although temperature is lower than those in the f 1 simulation, diurnal temperature cycle has a similar pattern (Figure 13), i.e. warming in the morning followed by a brief cooler period and another warm period in the afternoon. Warming starts at about 8 LST in the morning, and it seems that the presence of the strait delays

13 LOCAL FLOW FEATURES IN ISTANBUL: SENSITIVITY SIMULATIONS Latitude (b) (c) (d) (e) (h) m s Figure 14. Temporal evolution of v-component of the wind (m s 1 ) along the strait transect for idealized cases f 1, (b) f 2, (c) t1, (d) u1and(e)u2. Latitude (b) 5.5 a (c) (d) (e).5 (h) m 2 s Figure 15. Temporal evolution of divergence (m 2 s 2 ) along the strait transect for idealized cases f 1, (b) f 2, (c) t1, (d) u1 and(e)u2. Negative divergence values give the convergence along the transect (black solid lines show where lands are located, the upper one shows mainland and the lower one shows island).

14 Y. EZBER et al. the appearance of the sea breezes over both south and north sides. It is observed at around 83 LST in the south, while it appears at around 93 LST in the north (Figure 13(b)). A convergence occurs at about LST (Figures 13(b) and 15(b)). However, this convergence is not as strong as that in the f 1 case. It also occurs an hour earlier than the f 1 case, and its location is a bit further to the south. It seems that the smoother water surface along the strait causes faster movement of the sea breezes from both sides towards the centre of the strait. Northerly flow in the afternoon is much stronger than that in the f 1 case. Especially at about 14 LST, northerly flow resulting from the change in surface (from land-forest to water) seems to be channelled along the strait (Figure 14(b)). In general, the temperatures in the t1 case (Figure 13(c)) are lower than the f 1 and f 2 cases. Warmer surface is observed over the Marmara Sea side of the transect between 13 and 18 LST. The sea breeze onsets at about 8 LST, however, the presence of the topography seems to attenuate the intensity and inland penetration of the southerly sea breeze flow (Figure 14(c)). The life span of the southerly sea breeze is longer in the t1 case (Figure 14(c)) than in the f 2 simulation (Figure 14(b)). The convergence occurs at the same time as in the f 2 case (at LST in Figure 15(b)). Northerly flow advances from about to 13 LST, and dominates much of the transect. It briefly looses its strength at about 13 LST, but regains it later on. After 13 LST, it prevails along the transect. Thus, enhanced and channelled flow through the strait is observed in the t1 case between 14 and 18 LST (Figure 14(c)). Potential temperature in the u1 case follows a similar diurnal cycle as in t1; however, it shows slightly cooler surface conditions than the previous simulations (Figure 13(c) and (d)). In terms of sea breeze characteristics, urban area in the south of the domain seems to cause differences in the flow. The presence of the urban area provides enough temperature gradient between land and water that causes the early onset of the sea breeze in the south of the domain (from the Marmara Sea), and the onset time is about 7 LST. The northerly propagation between and 13 LST observed in the t1 simulation is not seen in the urban cases (Figure 14(d) and (e)). The northerly flow is also weaker in the u1 case than the previous cases. The southerly sea breeze is comparatively strong, but urbanization seems to prevent the inland penetration of the southerly sea breeze flow by probably generating an UHI circulation in the opposite direction over the northern parts of the urban areas. The convergence, therefore, takes place closer to the southern end of the transect (Figure 15(d) and (e)). Northerly flow becomes dominant over the transect after 12 LST. Last case (u2) has southwesterly prevailing wind instead of northerly. Warming in that case starts earlier than the u1 case, and the potential temperatures are higher than those in the u1 case (Figure 13(d) and (e)). Especially the afternoon heating over the transect is apparent in Figure 13(e). Sea breeze from the Marmara Sea side starts at about 8 LST and influences the south of the domain until 11 LST. Onset time of the sea breeze in the north (the Black Sea side) is about 7 LST in the u2 case (Figure 14(e)). Similar to the previous sensitivity simulations (except f 1 case), convergence in the u2 simulation occurs at about LST. The location of the convergence is further inland compared with that in the u1 simulation. The northerly sea breeze and convergence are quite strong in the u2 simulation (Figure 15(e)). Comparison of both urban simulations (u1 and u2) indicates stronger channelled flow in u2 simulation in the afternoon. A grid on the strait transect is selected to evaluate the time evolution of v-component of horizontal wind and vertical velocity with height. Figure 16 shows that all cases have the negative horizontal wind speed in the afternoon, which means that their directions are from the north at the surface level over the Bosphorus. Positive vertical velocity values are observed at the same time when the sea breezes merge, and these values continue up to higher levels (shown in Figure 17). Each case has intense upward motion through the upper levels, and each of them is compatible with the onset time of the convergence shown in Figure 14. For example, first case illustrates convergence at 11 LST in Figure 14, and Figure 17 also gives the high vertical velocity values at that time. The other cases, which indicate the existence of the convergence at LST, have the similar behaviour as well. As done for the land transect, we investigated the sea breeze conditions over the strait at LST, which is the common convergence time for all cases except the simplest one (f 1). Figure 18 demonstrates the positions of two sea breezes and their vertical structures along the strait transect. Development of two sea breezes over both sides of the transect could be clearly observed for the simplest case (Figure 18). As mentioned earlier, they converge at about 11 LST. For the f 2andt1 cases, both northerly and southerly sea breezes advance inland and converge over the southern parts of the strait (Figure 18(b) and (c)). The vertical influence of these breezes is relatively limited, and the strongest parts lie below about 5 m. In the case of the most realistic landscape (Figure 18(d)), the most intensive flows could be observed at thicker surface layer. The convergence also takes place comparatively further to the south. With the southerly geostrophic winds (Figure 18(e)), both the strength and vertical influence of the breezes are enhanced, albeit the convergence area does not change much in the u1 case. Analysis of the strait transect reveals that the evolution of the thermally driven circulation over the strait has similar phases as identified for the land transect. That is, there is a morning warming period ceased by the inland advancement of the sea breezes, a subsequent cooling (or no warming) period that weakens the sea breezes and an afternoon warm period characterized by the northerly flow. It could be said that this broad agreement primarily happens most likely because the strait is not wide enough to completely change the circulation characteristics caused by the land sea positioning, which is arguably the primary driver of the circulation over the province of Istanbul. It does, nevertheless, make some noticeable changes. Such

15 LOCAL FLOW FEATURES IN ISTANBUL: SENSITIVITY SIMULATIONS Height (m) (b) (c) (d) (e) m s (h) Figure 16. Temporal evolution of v-component of the horizontal wind (m s 1 ) for the specified grid in all cases f 1, (b) f 2, (c) t1, (d) u1 and (e) u2 (for the strait transect). Height (m) (b) (c) (d) (e) m s (h) Figure 17. Temporal evolution of vertical velocity (m s 1 ) for the specified grid in all cases f 1, (b) f 2, (c) t1, (d) u1 and(e)u2 (for the strait transect).

16 Y. EZBER et al. (b) m s Height (m) (c) (d) (e) A B Latitude Figure 18. Vertical changes of the v-component of the wind over the strait transect at LST f 1, (b) f 2, (c) t1, (d) u1 and(e)u2. changes are mainly caused by the differences in elevation and surface characteristics such as roughness. The strait with a smoother surface results in the channelling of the flows. This is probably one of the reasons why the northerly flow could penetrate further to the south compared with that in the land case. The retardation of the southerly flow by the UHI effect may also have a role in that. In the south of the transects, the UHI causes another circulation as the temperatures over the urban areas are larger than the surrounding areas (i.e. forests in the north and sea surface in the south). Therefore, the southerly sea breeze is enforced along the southern edge of the city by the concurring flows of the UHI-induced circulation, while it is dampened along the northern edge of the city by the counter flows of the UHI-induced circulation (Figure 19). To further explore the contribution of urbanization to local circulation, a zonal cross-section (from E to F in Figure 5), which goes through the urban areas over both sides of the southern part of the province crossing off the southern end of Bosphorus, is utilized. In the cases of coastal cities, the sea breeze cannot penetrate farther inland since the UHI circulation interacts with the sea breeze (Yoshikado and Tsuchida, 1996). Moreover, urban region acts as a buffer zone between the water surface and rural region. A convergence zone occurs over the urban region as a result of the UHI-induced circulation, which helps delay the progress of the sea breeze (Figure 19). The sea breeze onset is identified by positive u-component of the wind over the eastern part, while it has negative values over the west (Figure 2 and (b)). Details of the flow features in response to the urbanization are shown using simulation outputs at 8 LST as it is the time when the sea breezes start over the province. The sea breeze is more intense over the both sides of the strait due to the larger temperature difference between sea surface and the urban areas. Presence of the convergence regions over the transect is also seen. Rising air corresponding with the convergence reaches up to m. The convergence is intense over both sides of the Bosphorus (Figure 21 and (b)). A significant feature that can be identified is the existence of the recurrent flow over the urban region (seen in Figure 2). Large-scale wind seems to affect the recurrent flow. The u1 simulation indicates a recurrent circulation over the western urban region at about 2 m, whereas southwesterly flow (in u2) distorts the recurrent circulation (Figure 2 and (b)). For this reason, it is not possible to see a clear recurrent circulation in the u2 case. Instead, the features of the see breeze circulation are still observed at all levels over the region.

17 LOCAL FLOW FEATURES IN ISTANBUL: SENSITIVITY SIMULATIONS (b) Figure 19. Onset of the sea breeze for current conditions u1 and (b) u2 simulations at 8 LST (dark grey colour shows the urban areas, grey is forest and light grey is woody savannah) m s Height (m) (b) Water City Forest City City Forest Longitude Figure 2. u-component distribution of the horizontal wind (m s 1 ) with height at 8 LST. u1 and(b)u2. Onshore background flow over the south coastline where the urban areas are located seems to hinder the inland penetration of the southerly sea breeze because it works to decrease temperature gradient. Offshore background flow over the same coastline seems to enhance inland penetration of the southerly sea breeze because the flow from non-urban region works to increase the temperature gradient. Therefore, strong convergence occurs close to the shoreline. However, the offshore flow creates the strong temperature gradient at the border of the urban area. The sensitivity simulations have revealed several distinct features about the local circulation over the province of Istanbul, but one of them is intriguing to mention here. There exists a fan-shaped air flow over the northern parts of Marmara Sea, just off the southern end of the Bosphorus Strait (Figure 22 and (b)). It is possible to see this flow in the afternoon in all sensitivity simulations except the

18 Y. EZBER et al m s Height (m) (b) Water City Forest City City Forest Longitude Figure 21. Vertical velocity changes (m s 1 ) with height at 8 LST. u1and(b)u2. f 1 case. Shape of the coastline can also contribute to such flow as the Bosphorus opens to the Marmara Sea with a concave shoreline. The channelled flow from the strait fans out, and thus, similar fan-shaped outflows appear in all sensitivity simulations except the first one. The shape of the outflow is a bit distorted if the background flow is from southwesterly. However, the northeasterly wind conditions produce more intense channelled flow along the strait, and therefore, the fan-shaped outflow is stronger and covers a larger area over Marmara Sea. 5. Conclusion This study investigates the thermally driven mesoscale circulations, such as sea breeze and UHI circulation, over the relatively complex terrain of the province of Istanbul. A series of hypothetical numerical simulations, involving landscape configurations from a relatively simple one to the realistic present-time one, are performed to explore and understand the various features of the mesoscale circulations over Istanbul and their interactions with each other and with the terrain and surface characteristics. It should be mentioned that the order of the feature inclusion is important, and if it is changed, then it may not be possible to find the exact same results due to nonlinear interactions. The hypothetical simulations suggest that there are three relatively distinct phases of the thermally induced local circulation over the province of Istanbul during the daytime of a clear summer day. The first one is associated with the morning warming over the land in response to the increase in solar heating after sunrise. It results in the initiation, inland progress and convergence of the northerly and southerly sea breezes. The second phase is associated with the subsequent cooling (or no warming), which is the result of the inland advection of the cooler maritime air over the land. It weakens the sea breezes by reducing the temperature difference between land and sea. The last phase is associated with the afternoon warming. In the afternoon, the flow over the province is dominated by the northerly sea breeze, which sweeps the warmed land air southward. The flow in the last phase gradually dissipates in response to the diminishing solar energy at the surface. These phases are more or less observed at all five sensitivity cases, implying that the major driver is the relative positioning of the land and sea bodies. The local landscape features such as topography, strait, urban areas, islands and the direction of the prevailing wind seem to primarily affect the strength (and timing) of the local circulation and associated features over the land. A few additional simulations that are not included here suggest that if the northeasterly wind speed is increased to about 2.5 m s 1

19 LOCAL FLOW FEATURES IN ISTANBUL: SENSITIVITY SIMULATIONS Figure 22. Flow structure in u1 (b)u2 cases at 15 LST (at first model vertical layer). (which is set to.25 m s 1 in the simulations included in this study), the northerly sea breeze moves faster than the light wind conditions, while the southerly sea breeze stays close to the coastline in the south (not shown). If the wind speed is increased further to 1 m s 1, the strong wind dominates the whole domain without allowing the formation of a sea breeze. When water is set 2 C warmer than land (opposite to the sensitivity simulations included in this study), changes in duration and extension of land sea breeze circulation are observed (not shown). It seems that a sea breeze starts relatively late in this case that cannot move far from the southern coastline under the effect of the northerly (light) prevailing winds. However, a sea breeze developing over the north side moves further inland due to the northerly large-scale airflow. Because the water surface is comparatively warmer, duration of the land breeze is longer. Xian and Pielke (1991) report that two sea breezes forming over the opposite coasts of a peninsula-like domain, about 1 15 km width, merge over the central part in the afternoon (between 14 and 17 LST). This study concludes that the orientation of the terrain of Istanbul with