Thermally-driven Mesoscale Flows and their Interaction with Atmospheric Boundary Layer Turbulence

Abstract

Preface 22 2.2 Observational data 22 2.2.1 Basque coast 23 2.2.2 CESAR 25 2.2.3 La Herrería 26 2.3 Methodology 28 2.3.1 Analysis of meteorological data 29 2.3.2 WRF model 30 2.3.3 Mesoscale Selection Algorithm 33-39 3.1 Introduction 40 3.2 Observations 40 3.3 WRF model 41 3.4 Results and discussion 42 3.4.1 Observed SB characteristics 42 3.4.2 SB case study: 6 July 46 3.4.3 Anomalous case: 31 July 55 3.5 Summary and conclusions 63-65 4.1 Introduction 66 ix x Contents 4.2 Observational data 66 4.3 Local scale and mesoscale in Cabauw 68 4.3.1 Ten-year analysis of the ABL 68 4.3.2 SB phenomena 4.4 Sea breeze-turbulence interaction in the ABL 75 4.4.1 How does the ABL respond to the arrival of the SBF? 75 4.4.2 Quantifying the interaction with local turbulence 77 4.5 Impact on scalar transport 87 4.6 Summary and conclusions 90-93 5.1 Introduction 94 5.2 Method 95 5.2.1 WRF model 95 5.2.2 SB selection 95 5.3 Forcings affecting the sea-breeze detection 96 5.4 Assessment of the model performance 97 5.4.1 SB characteristics 97 5.4.2 ABL regimes 99 5.4.3 Impact on turbulence 104 5.5 Discussion and future research 106 :-107 6.1 Introduction 108 6.2 Data 109 6.2.1 Observations 109 6.2.2 Katabatic-event selection 109 6.3 Characteristics of the katabatic flows 110 6.3.1 Wind direction and intensity 110 6.3.2 Factors influencing intensity 113 6.4 Interaction between katabatics and turbulence 118 6.4.1 Turbulence regimes in the SBL 118 6.4.2 Regime transition from non-dimensional parameters 120 6.5 Analysis of representative katabatic events 123 6.5.1 Inspecting individual events 123 6.5.2 Impact of katabatic flows and turbulence on CO 2 126 6.6 Summary and conclusions 130 133 7.1 General conclusions 134 7.2 Final thought and future prospects 136 xi 139 153 155 157 159 161 xix The main conclusions of the thesis are summarised below: • The formation and development of the SB is particularly constrained by the synoptic wind. Katabatic winds are not only influenced by the synoptic wind, but also by the surface-energy balance through alterations in soil moisture. • The interaction between the thermally-driven winds and ABL turbulence occurs in a bidirectional way. Given the distinct spatial scales and influencing factors, that interconnection takes place differently for SB flows and katabatic winds. • The impact of thermally-driven flows on the variability of the CO 2 is mainly explained by changes in turbulent fluxes induced by the breeze-front passage, and horizontal advection, which can be significantly large for intense katabatic flows. • WRF reproduces the main impacts of SBs adequately, although the great overestimation of the sensible-heat flux produces sharper frontal disturbances. On the other hand, an enhanced acceleration of the AET is simulated comparing with observations.