“…With temperature decrease, the rest of upslope flow that formed in the last day disappears and the third phase commences to form. Similar structure has been seen for Athens (Flocast et al 1998). Figures 3-8 also demonstrate two maximum centers for katabatic flow related to two jets.…”
In this paper monthly trends of vertical wind profiles within and above an urban area with complex topography (Tehran) have been investigated using data from a Sodar, a 100-m and two 28-m towers and some surface stations. It includes the occurrence, evolution, dissipation time, peak time and maximum height for katabatic-anabatic winds due to topography, return flow and urban circulation. Vertical distributions of wind and the heat and momentum fluxes up to 600 m were also considered.The vertical wind profiles which have diurnal and seasonal variations show southwesterly daily anabatic wind and northeasterly nocturnal katabatic wind. Daily vertical wind profile structure which has two layers and two jets contains a decrease approximately at 300-400 m which may be due to the return flow and a daily thermal convective flow in the urban convergent terrain. At night the nocturnal wind profile structure, in the majority of months, has two layers and sometimes three layers containing northeasterly flows. In two layers structure, a decrease can be seen between two layers. In three layers structure, the middle layer has a different direction that indicates the return flow and urban circulation more clearly. Katabatic flows also develop in successive phases varying from 1 to 3 phases in different months. Investigation on surface wind demonstrates that in cold months a drainage flow from a valley located in the west of the station can affect wind speed and direction especially at evening transition time. These flow patterns can influence the way air pollutants disperse in this area.
“…With temperature decrease, the rest of upslope flow that formed in the last day disappears and the third phase commences to form. Similar structure has been seen for Athens (Flocast et al 1998). Figures 3-8 also demonstrate two maximum centers for katabatic flow related to two jets.…”
In this paper monthly trends of vertical wind profiles within and above an urban area with complex topography (Tehran) have been investigated using data from a Sodar, a 100-m and two 28-m towers and some surface stations. It includes the occurrence, evolution, dissipation time, peak time and maximum height for katabatic-anabatic winds due to topography, return flow and urban circulation. Vertical distributions of wind and the heat and momentum fluxes up to 600 m were also considered.The vertical wind profiles which have diurnal and seasonal variations show southwesterly daily anabatic wind and northeasterly nocturnal katabatic wind. Daily vertical wind profile structure which has two layers and two jets contains a decrease approximately at 300-400 m which may be due to the return flow and a daily thermal convective flow in the urban convergent terrain. At night the nocturnal wind profile structure, in the majority of months, has two layers and sometimes three layers containing northeasterly flows. In two layers structure, a decrease can be seen between two layers. In three layers structure, the middle layer has a different direction that indicates the return flow and urban circulation more clearly. Katabatic flows also develop in successive phases varying from 1 to 3 phases in different months. Investigation on surface wind demonstrates that in cold months a drainage flow from a valley located in the west of the station can affect wind speed and direction especially at evening transition time. These flow patterns can influence the way air pollutants disperse in this area.
“…During the day the station is exposed to advected pollution from the greater area due to mixing within the Athens valley subject to dominant meteorological conditions. An increase in particle number concentration during the night is observed, often in the absence of aerosol particle sources, due to the lowering of the nocturnal boundary layer height (NBLH) 13 , 14 , while occasional katabatic wind from Mount Hymettus also has an influence 15 . Figure 1 The Demokritos Atmospheric Aerosol Measurement station in Ag.…”
Air quality in urban areas and megacities is dependent on emissions, physicochemical process and atmospheric conditions in a complex manner. The impact on air quality metrics of the COVID-19 lockdown measures was evaluated during two periods in Athens, Greece. The first period involved stoppage of educational and recreational activities and the second severe restrictions to all but necessary transport and workplace activities. Fresh traffic emissions and their aerosol products in terms of ultrafine nuclei particles and nitrates showed the most significant reduction especially during the 2nd period (40–50%). Carbonaceous aerosol both from fossil fuel emissions and biomass burning, as well as aging ultrafine and accumulation mode particles showed an increase of 10–20% of average before showing a decline (5 to 30%). It is found that removal of small nuclei and Aitken modes increased growth rates and migration of condensable species to larger particles maintaining aerosol volume.
“…We employ the level of 6 m for the classification, since at 3 and 10 m the events showing different features cannot be so clearly detached. Flocas et al (1998) for instance studied katabatic flows at a similar height (7 m), since the influence of the large-scale wind was minimised at this level. Table 6.2: Classification of katabatic types according to their maximum 10-min averaged wind speed at 6 m from the onset until 2400 UTC.…”
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.
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