Investigation of local flow features in Istanbul. Part II: high-resolution real case simulations

This paper reports the second part of a study that aims to understand the thermally driven local flows and their interaction with each other over the province of Istanbul. The first part of the research evaluates a series of sensitivity simulations involving perturbations to the landscape. The present one includes an analysis of the real case simulations with an evaluation of the model performance. The second part also uses the same meso-scale model (OMEGA) with the same model configuration as in the first part. The model is able to reproduce the large-scale fields. Comparison of model-estimated fields with the radiosonde measurements over Istanbul also yields satisfactory results. The model is also able to capture the diurnal change of surface meteorological parameters such as temperature and wind speed, but there are over-and underestimations. The real case simulations indicate a clear cycle of land and sea breeze circulations in both summer and winter. In agreement with the sensitivity cases, a convergence zone occurs over the province due to the merge of two sea breezes in both seasons. In summer case, the convergence takes place at 1100 LST, and in winter case, it occurs at 1500 LST. Development of a channeled flow along the Bosphorus is also observed in the real case simulations. Return flows are observed above 1 km height during the summer case and above 300m height during the winter case. The northerly flow induced by the urban heat island effect prevents the inland penetration of the southerly sea breeze over the province. Thus, the northerly sea breeze moves faster, surmounts the southerly one over the south of the province, where the urban area is located, and dominates the airflow over Istanbul, especially in the afternoon.