On Urban Precipitation in the Pearl River Delta Region

Abstract

In the research leading to this thesis the complex behaviour of precipitation was investigated over urban environments and their direct vicinity. The Pearl River Delta Region as one of South China’s largest urban accumulations with Hong Kong located at its edge served as an ideal study area. The sensitivity of rainfall to anthropogenic sensible heat fluxes, urban extent and large scale variability have been studied in a series of partially idealized real-case scenarios and mechanisms potentially leading up to previously observed changes could be derived from the results. Urban precipitation scaling depends on the investigated forcing physics in complex ways and is discussed in detail in the thesis. Anthropogenic heat flux is contributing to heavier precipitation more than light precipitation through convective processes and increased buoyancy and turbulence. This mechanism only applies for sufficiently large urban areas and sufficient amounts of anthropogenic heat release. In smaller urban clusters, the initiated buoyancy is not resulting in local precipitation increases but rather downstream increases, dependent on advection length scales in comparison to buoyant (convection penetration) length scales and spatial extent of the urban cluster. Changes in the large scale moisture fields affect the precipitation in a more complex way in that the ratio of amounts of light and heavy precipitation is affected by background moisture adjustments. The relative importance of local convection and advection effects, as well as atmospheric stability parameters are affected by the background moisture changes. After taking into account these findings, one may conclude that observed changes in precipitation climate do not necessarily depend on global climate change, but show signatures of local forcing. In the final part of the thesis a discussion of the relevance of the study and its relation to past and future studies concludes, that the study of micro climatic forcing behaviour is essential to understand the larger scale changes which have been observed in the past century.

Date of Award	22 Dec 2016
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chung Leung Johnny CHAN (Supervisor) & Chi Yung Francis TAM (Supervisor)