Chiu, Tsz Fung Clinton;
(2025)
On the interactions between urban heat islands and convective cloud formation.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Understanding the mechanisms that govern the exchange of mass and en- ergy within the land-atmosphere-cloud system remains an open subject of research in the field of urban climatology and meteorology. Urban ar- eas significantly influence atmospheric boundary layer (ABL) processes, leading to increased air and surface temperatures, known as the urban heat island (UHI) effect, and associated changes in atmospheric humidity, termed the urban dry island (UDI) effect. This thesis aims to investigate the impacts of urban modifications on the surface energy budget, diurnal evolution of temperature and humidity in the ABL, and the interaction between UHIs, UDIs, and the initiation of convective clouds. To this purpose, a zero-order model of the ABL is developed and used to quantify urban-rural differences in surface energy fluxes and ABL potential temperature and humidity. By investigating the crossing be- tween ABL height and the lifting condensation level (LCL) as a proxy for convective cloud triggering, the model demonstrates that urban areas, being generally warmer and drier, increase both ABL and LCL heights. The response of the ABL-LCL intersection to surface conditions is non- linear, revealing a range of atmospheric conditions where urbanization can influence convective cloud formation. Model results compared well with measurements of energy fluxes, atmospheric conditions (tempera- ture, humidity), and initiation of convective cloud formation in Basel, Switzerland. To broaden the scope of analysis and clarify how urban areas around the globe influence energy and water budgets, ultimately triggering convective cloud formation, observational meteorological and remote sensing data from 28 large cities worldwide are also analyzed. Our results show a moderately strong correlation between UHI intensity and urban cloud cover occurrence. Ground observations indicate that UHI enhances convective cloud formation and elevates the cloud base over urban areas compared to rural surroundings. Our global analysis also demonstrates a strong positive correlation between UHI and UDI effects, highlighting the interplay among surface temperature, humidity, and changes in cloud cover. Finally, the limitations of this work are dis- cussed, highlighting the need for detailed urban climate simulations to assess the role of topography and land surface heterogeneity on urban- cloud interactions.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | On the interactions between urban heat islands and convective cloud formation |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Civil, Environ and Geomatic Eng |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10204309 |
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