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1. Entrainment Rates and Their Synoptic Dependence on Wind Speed Aloft in California's Central Valley

   https://doi.org/10.1007/s10546-022-00770-1  

   Caputi, Dani J.; Trousdell, Justin; Mehrotra, Shobhit; Conley, Stephen; Alexander, G. Aaron; Oldroyd, Holly J.; Faloona, Ian (March 2023, Boundary-Layer Meteorology)

   Abstract Daytime atmospheric boundary layer (ABL) dynamics—including potential temperature budgets, water vapour budgets, and entrainment rates—are presented from in situ flight data taken on six afternoons near Fresno in the San Joaquin Valley (SJV) of California during July/August 2016. The flights took place as a part of the California Baseline Ozone Transport Study aimed at investigating transport pathways of air entering the Central Valley from offshore and mixing down to the surface. Midday entrainment velocity estimates ranged from 0.8 to 5.4 cm s −1 and were derived from a combination of continuously determined ABL heights during each flight and model-derived subsidence rates, which averaged -2.0 cm s −1 in the flight region. A strong correlation was found between entrainment velocity (normalized by the convective velocity scale) and an inverse bulk ABL Richardson number, suggesting that wind shear at the ABL top plays a significant role in driving entrainment. Similarly, we found a strong correlation between the entrainment efficiency (the ratio of entrainment to surface heat fluxes with an average of 0.23 ± 0.15) and the wind speed at the ABL top. We explore the synoptic conditions that generate higher winds near the ABL top and propose that warm anomalies in the southern Sierra Nevada mountains promote increased entrainment. Additionally, a method is outlined to estimate turbulence kinetic energy, convective velocity scale ( w * ), and the surface sensible heat flux in the ABL from a slow, airborne wind measurement system using mixed-layer similarity theory. 

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2. The Influence of Heterogeneous Surface Heating on Organized Vertical Motions within and above a Sheared, Unstable Atmospheric Boundary Layer

   https://doi.org/10.1175/JAS-D-24-0116.1  

   Warner, Garett R_T; Pan, Ying; Peters, John (September 2025, Journal of the Atmospheric Sciences)

   Bou-Zeid, Elie (Ed.)

   Abstract Large-eddy simulation (LES) runs are performed to understand the influence of a one-dimensional (1D) surface heating heterogeneity on organized vertical motions within and above the atmospheric boundary layer (ABL). Two knowledge gaps are of interest: (i) how updrafts develop in the low free troposphere and (ii) what parameters control updraft location and strength within the ABL? LES runs are performed for a sheared, unstable ABL driven by geostrophic winds of the same magnitude but in various directions relative to a 1D surface-heat-flux heterogeneity. Quasi-steady-state LES results are phase-averaged over time and the horizontal dimension perpendicular to the surface-heat-flux gradient to quantify secondary circulations. Regarding the first knowledge gap, the results show that organized vertical motions in the low free troposphere can be modeled as two-dimensional (2D), stationary gravity waves, whose amplitudes depend on ABL updraft strength and instability development within the free troposphere. For the second gap, the results show that organized updrafts within the ABL may form above warm surfaces or downwind of warm-to-cool transitions. These different locations are well explained by both the relative contributions of horizontal and vertical velocities to the phase-averaged vorticity fluctuations tied to secondary circulations, and the relative importance of horizontal advection and turbulent transport in the phase-averaged internal energy fluctuation equation. The main balances associated with each updraft location are used to propose empirical models of updraft strength, and it is shown that the presence of sufficiently strong organized vertical motions can potentially change parameters used by atmospheric models that do not resolve ABL turbulence. Significance StatementThe purpose of this study is to better understand how heterogeneous surface heating affects updraft location and strength in the lowest kilometers of the atmosphere. We focus on horizontal heterogeneity scales comparable to the most frequently observed cloud size, a necessary step toward the parameterization of cloud shadow effects in weather and climate models. The results show that persistent updrafts may form above either warm or cool surfaces, with their location depending on the relative importance of terms in certain budget equations. Near-surface updrafts become stronger as the background mean wind becomes more perpendicular to the surface-heat-flux gradient, but their potential to influence clouds peaks when the background mean wind is neither parallel nor perpendicular to the surface-heat-flux gradient. 

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3. The winter central Arctic surface energy budget: A model evaluation using observations from the MOSAiC campaign

   https://doi.org/10.1525/elementa.2022.00104  

   Solomon, Amy; Shupe, Matthew D.; Svensson, Gunilla; Barton, Neil P.; Batrak, Yurii; Bazile, Eric; Day, Jonathan J.; Doyle, James D.; Frank, Helmut P.; Keeley, Sarah; et al (January 2023, Elem Sci Anth)

   This study evaluates the simulation of wintertime (15 October, 2019, to 15 March, 2020) statistics of the central Arctic near-surface atmosphere and surface energy budget observed during the MOSAiC campaign with short-term forecasts from 7 state-of-the-art operational and experimental forecast systems. Five of these systems are fully coupled ocean-sea ice-atmosphere models. Forecast systems need to simultaneously simulate the impact of radiative effects, turbulence, and precipitation processes on the surface energy budget and near-surface atmospheric conditions in order to produce useful forecasts of the Arctic system. This study focuses on processes unique to the Arctic, such as, the representation of liquid-bearing clouds at cold temperatures and the representation of a persistent stable boundary layer. It is found that contemporary models still struggle to maintain liquid water in clouds at cold temperatures. Given the simple balance between net longwave radiation, sensible heat flux, and conductive ground flux in the wintertime Arctic surface energy balance, a bias in one of these components manifests as a compensating bias in other terms. This study highlights the different manifestations of model bias and the potential implications on other terms. Three general types of challenges are found within the models evaluated: representing the radiative impact of clouds, representing the interaction of atmospheric heat fluxes with sub-surface fluxes (i.e., snow and ice properties), and representing the relationship between stability and turbulent heat fluxes. 

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4. The characteristics of atmospheric boundary layer height over the Arctic Ocean during MOSAiC

   https://doi.org/10.5194/acp-23-8683-2023  

   Peng, Shijie; Yang, Qinghua; Shupe, Matthew D.; Xi, Xingya; Han, Bo; Chen, Dake; Dahlke, Sandro; Liu, Changwei (January 2023, Atmospheric Chemistry and Physics)

   Abstract. The important roles that the atmospheric boundary layer (ABL) plays in the central Arctic climate system have been recognized, but the atmosphericboundary layer height (ABLH), defined as the layer of continuous turbulence adjacent to the surface, has rarely been investigated. Using ayear-round radiosonde dataset during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, we improve aRichardson-number-based algorithm that takes cloud effects into consideration and subsequently analyze the characteristics and variability of the ABLH over theArctic Ocean. The results reveal that the annual cycle is clearly characterized by a distinct peak in May and two respective minima in January and July. Thisannual variation in the ABLH is primarily controlled by the evolution of the ABL thermal structure. Temperature inversions in the winter and summer areintensified by seasonal radiative cooling and warm-air advection with the surface temperature constrained by melting, respectively, leading to the lowABLH at these times. Meteorological and turbulence variables also play a significant role in ABLH variation, including the near-surface potentialtemperature gradient, friction velocity, and turbulent kinetic energy (TKE) dissipation rate. In addition, the MOSAiC ABLH is more suppressed than the ABLH during the SurfaceHeat Budget of the Arctic Ocean (SHEBA) experiment in the summer, which indicates that there is large variability in the Arctic ABL structure during thesummer melting season. 

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5. Thermodynamic and kinematic drivers of atmospheric boundary layer stability in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC)

   https://doi.org/10.5194/acp-23-13087-2023  

   Jozef, Gina C.; Cassano, John J.; Dahlke, Sandro; Dice, Mckenzie; Cox, Christopher J.; de Boer, Gijs (January 2023, Atmospheric Chemistry and Physics)

   Abstract. Observations collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) provide a detailed description of the impact of thermodynamic and kinematic forcings on atmospheric boundary layer (ABL) stability in the central Arctic. This study reveals that the Arctic ABL is stable and near-neutral with similar frequencies, and strong stability is the most persistent of all stability regimes. MOSAiC radiosonde observations, in conjunction with observations from additional measurement platforms, including a 10 m meteorological tower, ceilometer, microwave radiometer, and radiation station, provide insight into the relationships between atmospheric stability and various atmospheric thermodynamic and kinematic forcings of ABL turbulence and how these relationships differ by season. We found that stronger stability largely occurs in low-wind (i.e., wind speeds are slow), low-radiation (i.e., surface radiative fluxes are minimal) environments; a very shallow mixed ABL forms in low-wind, high-radiation environments; weak stability occurs in high-wind, moderate-radiation environments; and a near-neutral ABL forms in high-wind, high-radiation environments. Surface pressure (a proxy for synoptic staging) partially explains the observed wind speeds for different stability regimes. Cloud frequency and atmospheric moisture contribute to the observed surface radiation budget. Unique to summer, stronger stability may also form when moist air is advected from over the warmer open ocean to over the colder sea ice surface, which decouples the colder near-surface atmosphere from the advected layer, and is identifiable through observations of fog and atmospheric moisture. 

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