Impact of Urbanization on the Simulation of Extreme Rainfall in the City of Jeddah, Saudi Arabia
Thang M. Luong, Hari P. Dasari, and Ibrahim Hoteit
Summary:
The city of Jeddah, Saudi Arabia has experienced massive urbanization over the years and is considered to be a "heat island", characterized by hot climate. However, extreme rainfall events still occur in the city which is primarily caused by mesoscale convective systems and its association with the Red Sea Trough (RST). The orographic features of the region also enhances these convective activities which triggers strong rainfall that often leads to flood damages. In this paper, they studied about the effects of urbanization on the extreme rainfall patterns in Jeddah over the years. They incorporated Urban Canopy Model (UCM) and the natural land cover for two experiments, one in urban scheme, and one with its presettlement and natural state. Ten rainfall events were subjected in the experiment but this paper focused on the event that happened on November 26, 2009 which is one of the most severe weather events that occurred in the city. All ten experiment where chosen as they were all associated with RST. It was concluded that the urban area of Jeddah exhibits a nocturnal heat-island effects which leads to a deeper PBL, stronger baroclinicity, and more unstable atmosphere.
Problem and Statement of Purpose
This research is about the effects of urbanization on the simulation of extreme rainfall events in Jeddah, as the city has been subjected to urbanization for the past decades. It was recognized that urbanized cities absorb more solar energy from the sun and thus, have more sensible heat. Many documented studies have proved that urbanization enhances precipitation in cities around the world. The focus of the study is to evaluate how changes in land cover impact severe thunderstorm events and to explain the physical mechanisms behind them. Before the event happened on November 2009, the intensification of Arabian anti-cyclone provided warm moist air to the region, creating a favorable environment for an extreme weather event. The intrusion of a mid-latitude upper-level trough generated cyclone flow and cold air advection and the intensification of the subtropical jet due to mid-latitude forcing further enhanced upper-level divergence that triggered the event. This resulted in an estimated damage if about $1 Billion and caused 161 fatalities. The objective of the study is to observe how changes in land use affect thunderstorm formation and its physical characteristics.
Review of Related Literature
The impact of urbanization to precipitation has been proved in numerous studies. A study about the precipitation in Tokyo examined how urbanization affects precipitation climatology in the said area. It showed that a robust increase in precipitation in metropolitan area and reduction in inland areas are caused by urbanization. They are characterized by increase in surface sensible heat flux which destabilizes the atmosphere (Kusaka et. al 2019, 2014) A similar study investigated the effect of urban areas on precipitation over the cities of western countries like Indonesia and Malaysia using a convection-permitting model which is also used in this study. It was found out that the presence of these cities enhances precipitation by over 30% which is mainly due to the intensification of diurnal cycle (Argüeso et al. 2016). Another study was conducted about the impact of land cover change in storm events in Sydney, Australia and the results show that the area has influence on the triggering mechanisms of the storm. These are some of the studies which greatly influenced the basic idea of this study, which is the impact of urbanization to precipitation (Gero and Pitman, 2006).
Hypothesis or Research Question
Does the heat island in Jeddah City locally affect the thunderstorms? This was the question of interest in the study, which are testable through these experiments which are simulations of the selected rainfall events occured in the city.
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