Stability of an air–water mixing layer: focus on the confinement effect

The shear instability occurring at the interface between a slow water layer and a fast air stream is a complex phenomenon driven by momentum and viscosity differences across the interface, velocity gradients as well as by injector geometries. Simulating such an instability under experimental conditions is numerically challenging and few studies exist in the literature. This work aims at filling a part of this gap by presenting a study of the convergence between two-dimensional simulations, linear theory and experiments, in regimes where the instability is triggered by the confinement, i.e. finite thicknesses of gas and liquid streams. It is found that very good agreement between the three approaches is obtained. Moreover, using simulations and linear theory, we explore in detail the effects of confinement on the stability of the flow and on the transition between absolute and convective instability regimes, which is shown to depend on the length scale of the confinement as well as on the dynamic pressure ratio. In the absolute regime under study, the interfacial wave frequency is found to be inversely proportional to the smallest injector size (liquid or gas).

Large Eddy Simulation and Dynamic Mode Decomposition of Turbulent Mixing Layers

Turbulent mixing layers are canonical flow in nature and engineering, and deserve comprehensive studies under various conditions using different methods. In this paper, turbulent mixing layers are investigated using large eddy simulation and dynamic mode decomposition. The accuracy of the computations is verified and validated. Standard dynamic mode decomposition is utilized to flow decomposition, reconstruction and prediction. It was found that the dominant-mode selection criterion based on mode amplitude is more suitable for turbulent mixing layer flow compared with the other three criteria based on singular value, modal energy and integral modal amplitude, respectively. For the mixing layer with random disturbance, the standard dynamic mode decomposition method could accurately reconstruct and predict the region before instability happens, but is not qualified in the regions after that, which implies that improved dynamic mode decomposition methods need to be utilized or developed for the future dynamic mode decomposition of turbulent mixing layers.

Dynamic interactions between city and atmospheric boundary layer – urbisphere Berlin campaign

<p class="western">In order to better understand dynamic interactions between a city and the regional atmospheric boundary layer, the '<em>urbisphere </em>Berlin campaign' is being conducted during 2021-2022 in Germany within the ERC Synergy <em>urbisphere</em> grant. <em>urbisphere</em> aims to enhance understanding, forecasting, and projecting feedbacks between climate change and drivers of urban transformation. One foci is the development of the next generation of urban climate <span lang="en-GB">simulations</span> with dynamic atmosphere-urban <span lang="en-GB">feedbacks</span>.</p> <p class="western">A <span lang="en-GB">key aspect</span> of <em>urbisphere</em> are comprehensive measurement campaigns in different cities. These involve undertaking high-quality research <span lang="en-GB">observations</span> on urban effects for observation-based studies as well as for model development and evaluation. The <span lang="en-GB">Berlin</span> campaign is investigating the dynamics of the atmospheric boundary layer within and beyond the city, and how the atmosphere dynamically responds to urban surface forcings, emissions, and human activity cycles <span lang="en-GB">from</span> diurnal to an annual cycle. <span lang="en-GB">A</span> dense network of ground-based remote sensing instruments (e.g. automatic lidars and ceilometers, doppler-wind lidars) for mixing-layer height detection within the city and along a rural-urban-rural transect, scintillometer paths <span lang="en-GB">for</span> spatially averaged information on turbulent sensible heat flux, and radiation measurements for quantification of the influence of urban emissions, aerosols and clouds on downwelling radiative fluxes is deployed. Altogether, the additional observations supplement the existing Urban Climate Observatory (UCO) in Berlin to allow for a comprehensive and spatially detailed understanding of city-atmosphere interactions, and the effect of cities on downwind regions. This contribution provides an overview of the measurement campaign and gives first insights into collected data.</p>

Diurnal evolution of negative atmospheric ions above the boreal forest: From ground level to the free troposphere

At SMEAR II research station in Hyytiälä, located in the Finnish boreal forest, the process of new particle formation and the role of ions has been investigated for almost 20 years near the ground and at canopy level. However, above SMEAR II, the vertical distribution and diurnal variation of these different atmospheric ions are poorly characterized. In this study, we assess the atmospheric ion composition in the stable boundary layer, residual layer, mixing layer and free troposphere, and the 5 evolution of these atmospheric ions due to photochemistry and turbulent mixing through the day. To measure the vertical profile of atmospheric ions, we developed a tailored setup for online mass spectrometric measurements, capable of being deployed in a Cessna 172 with minimal modifications. Simultaneously, instruments dedicated to aerosol properties measured in a second Cessna. We conducted a total of 16 measurement flights in May 2017, during the spring, which is the most active new particle formation season. A flight day typically consisted of three distinct flights through the day (dawn, morning and afternoon) to 10 observe the diurnal variation and at different altitudes (from 100 m to 3200 m above ground), and to capture the boundary layer development from stable boundary layer, residual layer to mixing layer, and the free troposphere. Our observations showed that the ion composition is distinctly different in each layer and depends on the air mass origin and time of the day. Before sunrise, the layers are separated from each other and have their own ion chemistry. We observed that the ions present within the stable layer are of the same composition as the ions measured at the canopy level. During daytime when the mixing layer evolved and the compounds are vertically mixed, we observed that highly oxidised organic molecules are distributed to the top of the boundary layer. The ion composition in the residual layer varies with each day, showing similarities with either the stable boundary layer or the free troposphere. Finally, within the free troposphere, we detected a variety of carboxylic acids and ions that are likely containing halogens, originating from the Arctic Sea.