Characterization and modelling of sea breeze cooling in coastal cities

Author: Yifei Zhou

Zhou, Yifei, 2022 Characterization and modelling of sea breeze cooling in coastal cities, Flinders University, College of Science and Engineering

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With the combined effects of climate change and enhanced urban heat island, the intensity and frequency of heatwave events in cities are increasing rapidly over the world. This is deemed to lead to negative effects on environment and society of urban areas, such as restricted growth of vegetations and increased health risks for residents. Therefore, there is an increasing need of improving thermal environment in urban areas. In addition to the traditional mitigation measures such as adding vegetations and the use of high-albedo materials for building surfaces, emphasizes have been placed on the natural cooling sources, such as urban waterbodies and ventilation. The sea breeze, which is a typical mesoscale ventilation process of coastal cities, can significantly change the temperature pattern in coastal areas. However, there is limited understanding of the cooling effects of sea breezes. In this study, I propose to explore the cooling magnitudes of sea breezes and their influencing factors at different spatial scales.

Sea Breeze Cooling Capacity (SBCC) is firstly proposed with the definition being the difference of temperature between a sea breeze day and the average of non-sea breeze days. With this definition, I have displayed the temporal and spatial patterns of SBCC and calculated their associations with environmental variables (weather background, topography and urban structure) using data from the Adelaide urban heat island monitoring network in summer days during December 2010 - March 2013 (Chapter 2). Results show that the SBCC values averaged over all sea breeze days range from 733.9 to 858.7 °C·h per season (from 19.0 to 22.2 °C·h per event) for individual sites in the Adelaide Central Business District, which can primarily be explained by Frontal Area Index (FAI), Terrain Ruggedness Index (TRI), distance towards the coast and temperature prior to the sea breeze onset. Meanwhile, the variability of SBCC among all sea breeze events is significantly related to specific humidity and wind speed. Based on the relationship between urban structure and SBCC, I predicted the potential changes of mean SBCC induced by the projected change of building heights ranging from −195 to 143 °C·h per season (from −5.0 to 3.7 °C·h per event).

I have also evaluated the basic characteristics of sea breeze cooling across the metropolitan Adelaide to investigate the penetration process of this cooling effect (Chapter 3). An inland delaying trend of arrival time (9:29 - 10:36) and an advanced trend of cessation time (18:36 - 16:14) of cooling are revealed. There is also a decreasing trend of SBCC during the penetration with the spatial average SBCC of 21.3 °C·h per event. Hot sea breeze days here are defined as sea breeze days with maximum temperatures above the 75th percentile of all sea breeze days (32.8 °C). SBCC values are particularly smaller in hot sea breeze days compared to that in other sea breeze days and the contrasts increase when cooling fronts go inland. Correspondently, the penetration distance of sea breeze cooling is estimated to drop from 42 to 29 km from the period of all sea breeze days to hot sea breeze days. This difference can be explained by a higher frequency of synoptic systems with directional flows from east or south-east in hot sea breeze days of Adelaide. Overall, this result is helpful for understanding the whole sea breeze penetration process.

In the analysis of sea breezes across major Australian cities in Chapter 4, a large diversity of sea breeze frequency is found among the five cities (Perth, Adelaide, Melbourne, Brisbane and Sydney), which ranges from 17% to 56%. This contrast is significantly related to the frequency of days with anti-cyclone synoptic types. In Sydney and Brisbane, the characteristics of sea breezes are less significant compared to those of other three cities and this can be explained by complex topography of the two cities. Differences of SBCC exist among the three cities with flat surfaces (Perth, Adelaide and Melbourne). In coastal areas, frequency of days with anti-cyclone systems can explain most of the SBCC variability among these three cities.

In Chapter 5, I have estimated the quantitative effects of urban structure and wind speed on air temperature and thermal comfort in a typical sea breeze day of Adelaide based on ENVI-met. The comparison among 48 scenarios shows the critical roles of building height, canyon orientation and wind speed in shaping urban microclimate during a sea breeze event. Results show a reduction of averaged early afternoon air temperature from 27.16 °C to 26.92 °C with an increase of building height from 4 metres to 12 metres. It is also interesting to note that the mean cooling magnitude induced by increasing wind speed (from 2 m/s to 4 m/s) is negatively related to the averaged building height. With 4-metre buildings, the cooling is 0.28 °C, the corresponding value drops to 0.17 °C with 12-metre buildings. This demonstrates the contrasting aspects of building height in shaping urban thermal environment. On one hand, increasing building height is considered to cool the local environment by providing more shading areas. On the other hand, wind cooling can be weakened with higher buildings.

Keywords: Urban climate, Sea breeze, Heatwave, Urban heat island, Urban ventilation

Subject: Earth Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2022
School: College of Science and Engineering
Supervisor: Huade Guan