Assessment of Urbanization Impact On Heavy Precipitation in Istanbul, Turkey

Cities have undergone a substantial increase in urbanization over the past decades. Whether the change in land-use type and the consequent Urban Heat Island (UHI) affects the extreme precipitation was of interest and has been under investigation for various developing cities. This study pursued a similar purpose and investigated the impact of urbanization on a heavy precipitation incident that took place in Istanbul on 18 July 2017. Two particular land-use scenarios were used to simulate the event by Weather Research and Forecasting Model (WRF). First, the control simulation (WRF-urban) was performed using the default CORINE 2018 land-use dataset. Subsequently, the test simulation (WRF-nourban) was implemented by replacing the urbanized land-use type of Istanbul with the most dominant land use category of arid cultivated area. Comparison of the WRF-urban simulation with station observations and satellite data reveal that the WRF captured the heavy precipitation event reasonably well over Istanbul.  Results showed that urbanization has a notable impact on both the magnitude and timing of heavy rainfall. Event day total precipitation amount decreased considerably over urbanized regions of Istanbul on the control run. Although the start time and location of the incident reasonably matched for both runs, the control run with urbanization advanced the rainfall quicker over Istanbul, and the heavy precipitation event took place about 1 hour earlier than the test run without urbanization. Another pronounced distinction between the simulations with and without urbanization is detected over the north of Istanbul as the maximum daily total precipitation line slightly shifted northwest on the WRF-urban run compared to the WRF-nourban run. This result indicates that urban-areas may have a substantial effect on the direction of the airflow. Analysis of both vertical cross-sections and sensible heat fluxes on the city revealed that urbanized areas increased the atmospheric instability, thus caused heavier precipitation.