Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session L20: Geophysical Fluid Dynamics: Atmospheric I |
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Chair: Marco Giometto, Columbia University Room: 146C |
Monday, November 20, 2023 8:00AM - 8:13AM |
L20.00001: Lagrangian supersaturation fluctuations at the cloud edge Johan Fries, Gaetano Sardina, Gunilla Svensson, Alain J Pumir, Bernhard Mehlig Evaporation of cloud droplets accelerates when turbulence mixes dry air into the cloud, affecting droplet-size distributions in atmospheric clouds, combustion sprays, and jets of exhaled droplets. The challenge is to model local correlations between droplet numbers, sizes, and supersaturation, which determine supersaturation fluctuations along droplet paths ({em Lagrangian} fluctuations). We derived a statistical model that accounts for these correlations. Its predictions are in quantitative agreement with results of direct numerical simulations, and it explains the key mechanisms at play. |
Monday, November 20, 2023 8:13AM - 8:26AM |
L20.00002: On the dynamics of unsaturated atmospheres in a simple model of moist convection Jeff S Oishi, Benjamin P Brown Convection represents a fundamental process in the atmosphere, and clouds represent a major uncertainy in climate modeling. These phenomena are coupled by the latent heat of condensation: moisture is not merely a passive tracer in convective dynamics but a key source of buoyancy. Here, we present linear and non-linear results using the simplified, Boussinesq "Rainy-Benard" model with the aim of drawing on the vast reservoir of knowledge for Rayleigh-Benard convection to elucidate systems with Earth-like condensation physics. We focus on systems that transition from unsaturated conditions in the lower atmosphere to saturated conditions aloft. At modest Rayleigh number of order 1012-1014, systems that are stable to dry convection but destabilized by condensation show a propensity to spontaneously form layers in both two- and three-dimensions. The strength and non-linear dynamics of these layers depend on the both the Rayleigh number and the specific latent heat released by the condensing fluid. |
Monday, November 20, 2023 8:26AM - 8:39AM |
L20.00003: Gravity Wave Interactions in the Stratocumulus-Topped Boundary Layer Arun Balakrishna, Hao Fu, Morgan E O'Neill Stratocumulus clouds are low-level clouds that greatly influence climate. Their extensive coverage over the subtropical oceans provide a net-cooling effect via reflecting solar insolation and allowing terrestrial radiation to reemit. Previous studies across a range of scales have suggested a causal link between uncertainties in climate projections to poor prediction of low-clouds as well as positing greenhouse gas induced breakup. It is important to understand the regimes that can cause the cloud deck to dissipate which may have implications for warming under a climate change framework. |
Monday, November 20, 2023 8:39AM - 8:52AM |
L20.00004: Sea surface temperature and subsidence effects on steady-state stratocumulus to shallow cumulus cloud transitions OBAIDULLAH KHAWAR, Georgios Matheou, Joao Teixeira Clouds forming in the atmospheric boundary layer play a crucial role in the Earth's energy balance. Climate projections are sensitive to the amount of low-cloud cover and small variations in stratocumulus area coverage can produce energy-balance changes comparable to those due to greenhouse gases. The objective of this study is to understand the relation between boundary layer cloudiness and large-scale circulation. We follow the canonical problem of Chung et al. (2012) and investigate statistically steady state marine boundary layers with varying subsidence and sea surface temperature. The boundary layer is modeled using large-eddy simulation. Each simulation differs with respect to the SST and subsidence, which span a typical subtropical-value parameter space. To reduce the problem parameters, the free tropospheric profiles, i.e., the lapse rate/stability above the boundary layer, depend on the large-scale subsidence. The balance of the various cloud-controlling parameters is studied using the column-integrated liquid water potential temperature equation. Established relations between atmospheric stability and boundary layer cloudiness are discussed in the context of the present statistically stationary boundary layers. |
Monday, November 20, 2023 8:52AM - 9:05AM |
L20.00005: Abstract Withdrawn
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Monday, November 20, 2023 9:05AM - 9:18AM |
L20.00006: Computational domain size effects on large-eddy simulations of precipitating shallow cumulus convection Oumaima Lamaakel, Joao Teixeira, Ravon Venters, Georgios Matheou Idealized large-eddy simulations of shallow convection often utilize horizontally periodic computational domains. The development of precipitation in shallow cumulus convection changes the spatial structure of convection and creates large-scale organization. However, the limited periodic domain constrains the horizontal variability of the atmospheric boundary layer. Small computational domains cannot capture the mesoscale boundary layer organization and artificially constrain the horizontal convection structure. The effects of the horizontal domain size on large-eddy simulations of shallow precipitating cumulus convection are investigated using four computational domains, ranging from 40 × 40 km2 to 320 × 320 km2 and fine grid resolution (40 m). The horizontal variability of the boundary layer is captured in computational domains of 160 × 160 km2 . Small LES domains (≤ 40 km) cannot reproduce the mesoscale flow features, which are about 100 km long, but the boundary layer mean profiles are similar to those of the larger domains. Turbulent fluxes, temperature and moisture variances, and horizontal length scales are converged with respect to domain size for domains equal to or larger than 160 × 160 km2 . Vertical velocity flow statistics, such as variance and spectra, are essentially identical in all domains and show minor dependence on domain size. Characteristic horizontal length scales (i.e., those relating to the mesoscale organization) of horizontal wind components, temperature and moisture reach an equilibrium after about hour 30 |
Monday, November 20, 2023 9:18AM - 9:31AM |
L20.00007: Large-Eddy Simulation of Non-Spherical Heavy Particle Transport in the Atmospheric Boundary Layer Shuolin Xiao, Qi Li, Yuanfeng Cui, Xiaowei Zhu, Janice Brahney, Natalie M Mahowald Heavy particles in the atmosphere, such as microplastic particles and coarse dust particles, often exhibit non-spherical configurations. A key instance of this is microplastic particles, where recent studies increasingly highlight the importance of the atmosphere as a vital yet incompletely understood transport pathway. Microplastic fibers (MPFs), in particular, are estimated to represent a substantial proportion of airborne and deposited microplastics within both natural and built environments. This study explores the transport trajectories of MPFs in the atmospheric boundary layer (ABL) under varying atmospheric stability conditions, employing large-eddy simulation (LES) techniques. The gravitational settling speeds of MPFs are characterized using our newly developed model, which takes into account their unique morphology and the surrounding turbulent atmosphere. Simulation outcomes indicate notable variations in the spatial and temporal distributions of MPFs, depending on the specific properties of the fibers and the prevailing atmospheric stability conditions. These findings suggest the necessity of acknowledging the impact of non-spherical particle morphology in regional and global models, especially when dealing with heavy particles such as microplastics and coarse dust particles. |
Monday, November 20, 2023 9:31AM - 9:44AM |
L20.00008: Experimental Observation of boudary layer and movement of topographically varying desert dunes with unmanned systems Kerrick Ray, Jamey D Jacob, Victoria A Natalie Desert dunes offer unique geophysical interactions, and these aeolian dominant interactions occur over a significant portion of earth surfaces. We pursued whether the movement of desert dunes can be accurately measured using unmanned aerial systems (UAS) data and a structure-from-motion as well as tracking boundary layer propagation and change in the topography during the boundary layer interaction. This volatile landscape makes an ideal setup for tracking imagery combined with the effect of atmospheric data. |
Monday, November 20, 2023 9:44AM - 9:57AM |
L20.00009: Study of Sea/Land-Breeze Condition in Atmospheric Boundary Layer in the Persian Gulf, Qatar Reza Sadr, Yuan Li The sea/land breeze in the coastal region affects the local air quality and provides relief from hot weather. The present study presents near-surface turbulence characteristics of the atmospheric boundary layer (ABL) under sea breeze conditions in the coastal region of Qatar (26.08 N, 51.36 E). Sea breeze days (SBDs) were identified based on the diurnal reversal of wind direction and the land-sea temperature difference from Sep 2015 to Aug 2016. Hodographs of the average wind speed and direction for sea breeze days show a clockwise rotation, characteristics of the Coriolis force veering in the Northern Hemisphere. The diurnal pattern of the relative humidity, sensible heat flux, and turbulent kinetic energy (TKE) during SBDs are investigated. Stability analysis shows mostly unstable and stable ABL during sea and land breeze conditions, respectively. Finally, the ABL heights during SBDs are compared with the general values of local ABL heights in previous observations. Formation of the convective internal boundary layer (CIBL) was observed during sea breeze with the height of the CIBL estimated based on the vertical velocity spectra and the distribution of TKE dissipation rate. The air pollutants can be trapped near the surface layer in this condition since the CIBL dramatically reduces the atmospheric mixing depth. |
Monday, November 20, 2023 9:57AM - 10:10AM |
L20.00010: A Mechanism for Coastal Fog Genesis at Evening Transition Thomas J Hintz, Kelly Y Huang, Sebastian Hoch, Stef L Bardoel, Sasa Gabersek, Jesus Ruiz-Plancarte, Ismail Gultepe, Eric R Pardyjak, Qing Wang, Harindra J Fernando Evening transition from a daytime/convective to a nighttime/stable atmospheric boundary layer often begets fog, in which near-surface visibility reduces below 1 km due to suspended water droplets. Despite the numerous societal impacts of fog (in safety, transportation, defense, and ecology), it remains one of the most poorly predicted meteorological phenomena due to the myriad of atmospheric processes that interact and shape its lifecycle. Here, a new mechanism for fog genesis at evening transition is identified using the comprehensive set of data collected during a local fog event through evening transition on Sable Island, Canada, as part of the Fatima (Fog and Turbulence Interactions in the Marine Atmosphere) campaign. Observations show the appearance of a low-level cloud (LLC) that resulted from the interplay between several stacked boundary layers at the ocean-land discontinuity during the rapid cooling period of evening transition. Top-down turbulent entrainment of the LLC then fills the surface layer with saturated air to form a thick fog layer. While a high-resolution model predicted the appearance of fog in this case study, it did not capture the onset time nor the flow structure, indicating the need for near-surface parametrizations that better capture flow statistics including stratification and turbulent energy transport. |
Monday, November 20, 2023 10:10AM - 10:23AM |
L20.00011: Mesoscale structure of the atmospheric boundary layer across a natural roughness transition at White Sands National Park Justin P Cooke, Douglas Jerolmack, George I Park Roughness is ubiquitous to many atmospheric boundary layer (ABL) flows, and often ABLs encounter roughness transitions. This step-change introduces a new flow scale: the internal boundary layer (IBL) which modifies transport of momentum, heat, and particles/pollutants. Many well-known models for predicting the growth of the IBL exist, but not for the flow structure within. We deploy wall-modeled large eddy simulation to simulate the natural roughness change of the dune field at White Sands National Park in New Mexico, with flow conditions at Reτ ∼ O(106). We validate our simulations against available field observation data and show evolution of mean velocities and boundary stress, τb, after the step-change. We also provide a prediction of IBL thickness, δIBL, and compare a power-law fit to existing models. Lastly, we investigate the mesoscale interaction of large- and small-scales within the IBL via amplitude modulation analysis. Our results support the existence of δIBL, but show existing models built on δIBL fail to predict τb, as they do not address streamwise roughness heterogeneity in their current form, and our results suggest models for sediment flux would fail as well. |
Monday, November 20, 2023 10:23AM - 10:36AM |
L20.00012: Coherent Structures in Unsteady Flow over Urban Canopies Marco G Giometto, Weiyi Li This study examines structural changes in coherent structures populating the boundary layer within and above an array of surface-mounted cuboids induced by flow unsteadiness, and the impact of such changes on momentum transport. We focus on the non-stationarity induced by a pulsatile pressure gradient. A large-eddy simulation of flow over urban-like roughness subjected to a pulsatile pressure gradient is carried out, and results are analyzed and compared against those from a constant pressure gradient case in terms of first- and second-order statistics, the relative importance of each quadrant, two-point velocity correlations, and the instantaneous flow structures. Additionally, the technique of conditional averaging is employed to extract the characteristic eddy structures during the pulsatile cycle. Throughout the pulsatile cycle, hairpin packets, comprising organized hairpin vortices aligned in the streamwise direction, maintain their orientation but experience a substantial change in their extent. When the local shear rate increases, the hairpin vortex structures are intensified, and shrink in size due to the self-induction. As a consequence, the amount of ejection events between the hairpin legs reduces, but the strength of each event becomes stronger. Such a process is reversed with the decreasing shear rate. |
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