Department of Meteorology An Analysis of Sea Breezes on the South Coast of England and Evidence of Pre- and Post-Frontal Waves at the Surface. GILLEAN J KEITH A dissertation submitted in partial fulfilment of the requirement for the degree of Master in Science in Applied Meteorology. 16 th August 2004
ACKNOWLEDGEMENTS I would like to thank my supervisor Dr R Plant for his patience and guidance throughout the dissertation process. In addition I would like to thank my family and friends for their support and encouragement throughout this year. Thanks to Simon for reading through my work, drawing pretty pictures and being the comma fairy!
ABSTRACT This study investigates sea breeze formation on th e South coast of England using high temporal scale data taken from Chimet, a weather recording station based at the entrance to Chichester Harbour. A program is created to detect sea breeze fronts in the data for 2003. The program detects the fronts on the basis that there will be a change in wind direction, wind speed, gustiness and temperature as the front crosses the Chimet recording station. Two scales of analysis are considered. Firstly the synoptic scale is studied and effects of prevailing winds a nd differentials between sea surface temperature and air temperature were investigated. Onshore winds before the sea breeze cause less distinctive fronts which tend to cross Chimet after 1200UTC. Fronts formed on days of opposing synoptic winds are more distinctive due to greater density differences across the front and cross Chimet before 1300UTC. Effects of temperature differences were not clear due to the Sea surface temperature data used which was found to be unreliable. Smaller scale waves can be identified in surface parameters on certain sea breeze days. Wind speed gives the best indicator of the waves followed by pressure and temperature. The wavelengths tend to increase from an initial 5km and then decrease as the waves disintegrate. The waves may be formed by a Kelvin Helmholtz instability at the top of the sea breeze head where the greatest shear stress is found and billows propagate away from the front. The instability is a result of denser sea air underneath dryer less dense air resulting in mixing at the point of maximum shear to try to redress the imbalance. The mixing can act to destroy the front or slow down the inland propagation. Oscillations in surface parameters were most visible on days when there was little change in wind direc tion as a result of the sea breeze front. There is support for this result from the gravity current laboratory experiments conducted by Simpson (1994). Alternatively the waves may have worked themselves out before reaching Chimet. Numerical simulation stu dies have dominated the study of Kelvin Helmholtz billows at sea breeze fronts and more observational studies are required to compare results. The investigation of wave-like features and synoptic conditions is a new area and one that requires further study to advance knowledge of turbulence within the sea breeze.
CONTENTS Page 1. Introduction 1 2. Literature Review 4 2.1 Synoptic Scale 4 2.2 Circulation Cell Scale 6 2.3 Front & Head Scale 8 2.4 Wave Scale 11 2.5 Pollution and the Sea Breeze 19 2.6 Forecasting Sea Breezes 20 3. Aims 22 4. Methodology 23 4.1 Location 23 4.2 Evidence of Sea Breezes in the Chimet Data 25 4.3 Criteria for Sea Breeze Identification 28 4.3.1 Points about the Criteria 30 4.4 Non-Sea Breeze Fronts 31 4.5 Identification of Oscillations 32 5. Analysis 34 5.1 Total number of Sea Breezes 34 5.2 Temperature Effects on Sea Breeze Development 35 5.2.1 Annual Variations 35 5.2.2 Individual Days 37 5.3 Synoptic Conditions 39 5.3.1 Onshore Winds 39 5.3.2 Seasonality 41 5.3.3 Timing with Onshore and Offshore Winds 43 5.4 Looking at Fronts in Detail over Time and Distance 46 4.4.1 Examples of Oscillations in Surface Parameters 47 4.4.2 Links between Days with and without Oscillations 54 6. Discussion 56 6.1 Large Synoptic Scale Effects 56 6.2 Smaller Scale Wave-like Features 58 6.3 Relationships between Synoptic Scale Findings and 61 Wave Scale Findings 7. Conclusions 64 8. References 66 Appendix 1 70
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