Bir bölgenin yerel hava akışını, kara-deniz meltemi, şehir ısı adası gibi farklı yüzeylerin termal özelliklerinin neden olduğu mezo-ölçek sirkulasyonlar etkilemektedir. Bu çalışmada, hem stratejik açıdan önemli olan hem de Türkiye’nin en kalabalık şehri olan İstanbul ve çevresi için yerel hava akışı incelenmiştir. Şehrin morfolojisinin ve şehirleşmenin yerel hava akışına yapmış olduğu katkıyı göreceli olarak incelemek için duyarlılık testleri, atmosferik-meteorolojik sayısal model yardımı ile gerçekleştirilmiştir. Dinamik modelleme, duyarlılık testlerinin gerçekleştirilmesi sırasında kullanılan yöntemdir. Bu çalışmada, üç boyutlu, hidrostatik olmayan mezo-ölçek model Operational Multi-scale Environment model with Grid Adaptivity (OMEGA) kullanılmıştır. İstanbul Boğazı’nın etkisi ve hafif yükseltilerden oluşan şehir topoğrafyasının meydana getireceği etki de bu çalışmada incelenmiştir. Duyarlılık testleri kurgulanırken, mümkün olabilecek en basit durumdan mevcut duruma kadar yapılan çeşitli değişikliklerle akış özellikleri incelenmiştir. Topoğrafyanın olmadığı ve tek bir arazi örtüsünün kullanıldığı bölge üzerindeki mevcut su yüzeylerinin etkisinin de izole edildiği test en basit durum olarak düşünülmüştür. Daha sonra İstanbul Boğazı ilave edilerek boğaz etkisi incelenmiştir. Büyük ölçek hava akışlarının etkisini anlamak için ise farklı jeostrofik rüzgâr yönü ile benzeşim yapılmıştır. Bu testlere topoğrafya ve mevcut arazi örtüsü de ilave edilerek simülasyonlar gerçekleştirilmiştir. Çalışmada, Boğaz’ın kanal etkisi yarattığı ve topoğrafyanın varolan akışı kuvvetlendirdiği gözlenmiştir. Boğaz boyunca yer alan yerleşimin ise deniz melteminin kıyıdan içeriye dogru ilerlemesini engellediği belirlenmiştir. Ayrıca sinoptik ölçekli rüzgârın yön değişiminin yerel akışı etkilediği de elde edilen önemli bir sonuçtur. Anahtar Kelimeler: Karadeniz meltemi, şehir ısı adası, dinamik modelleme.Atmospheric motion is affected by wide range of scales from a few milimeters to thousands of kilometers. Therefore, it is crucial to understand the behavior of the atmospheric flows on these scales. Although all of these scales play a role on the transport and dispersion of the pollutants, mesoscale atmospheric processes are significant over the traveling time and distance for short and long range transport and dispersion of pollutants. Thermally and terrain induced circulations contribute to the transport and dispersion pattern of the pollutants via terrain characteristics and the surface inhomogeneities. Besides, growing urbanization modifies the mesoscale circulation characteristics. The response of the local scale circulation to the other scales or its interaction with them influences the characteristics of pollutant transport by means of the modified mesoscale circulation. Therefore, understanding mechanism of the development and evolution of small scale flow features and their interaction with large scale flow features is one of the desires. Mesoscale circulations such as sea-land breeze, urban heat island are controls our local atmosphere. Theoretically, the mechanism behind the development of the sea-land breeze circulation has been known since 1800s. Thermally-induced circulations, sea-land breeze and urban heat island, occur depending on the difference on the heating and cooling of the underlying surface. Heat capacity difference between land and water triggers the onset of the sea-land breeze. Similar to the sea-land breeze circulation, urban heat island circulation occurs due to the heat capacity difference between the urban and rural areas. Although mechanisms of the thermally-induced circulations are known, the interaction between other mesoscale features (such as mountain-valley wind) or large scale circulations is important and it changes by the properties of the location. Since the shorelines are preferred as a residential area, it is unavoidable to encounter the interaction between the sea-land breeze and the urban heat island. In addition to these circulations, topography makes the contribution to the local flow. The present research is concentrated on the mesoscale circulations, sea-land breeze and urban heat island. Since the dynamic downscaling is very useful tool while studying sensitivity studies, the idealized numerical simulations are performed using a fully non-hydrostatic three-dimensional numerical model OMEGA with its unstructured grid. The unstructured grid enables us to resolve land and water boundaries better than the other mesoscale models which use structured grid. The more detailed information about the land cover benefits us to study mesoscale circulation. It should be kept in mind that it is not possible to solve any small properties of the domain even using unstructured grid. Uniform initial flow is utilized at horizontal and vertical directions. The study is focused on Istanbul since it is the longest and the most crowded city in Turkey and also the wide range of data availability for direct comparison. Since each feature of a region affects the local flow, relative effects of geographic features on the local circulation are exemplified by means of the idealized simulations. Simulations are conducted starting from very simple case to a complex one, which means that contribution of each feature is added one-by-one to the domain. For instance, as a simple case, surface is set to the flat, and a uniform land-use and land cover map is used. Then after, effect of the presence of a strait is investigated adding the Bosphorus strait to the domain. Topographic effect is also analyzed including model resolved topography. Finally, current land-use types and land-water properties are used with different large scale flow directions to investigate effect of the urbanization. Although studies in the literature have showed that smooth topography has no influence on the circulation, topography-induced effect is observed over the hilly topography of Istanbul. Results show that the Bosphorus strait channeled the air flow that is blowing from the north through the Bosphorus. This channeled flow is enhanced when the topography is added to the domain. Significant contribution to the local flow comes from urbanization by arranging the sea-land breeze existence or penetration over the domain. The presence of the urban area causes a convergence over the urbanized area by hindering the inland penetration of the sea breeze. Results indicate that the flow over Istanbul converges at a point depending on the surface properties which are utilized in the sensitivity simulation. The southwesterly flow direction prevents the existence of the land breeze that is observed under the northeasterly flow condition. Keywords: Sea-land breeze, urban heat island, dynamic modeling
