convective layer
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Abstract Most massive stars are born in binaries close enough for mass transfer episodes. These modify the appearance, structure, and future evolution of both stars. We compute the evolution of a 100-day-period binary, consisting initially of a 25 M ⊙ star and a 17 M ⊙ star, which experiences stable mass transfer. We focus on the impact of mass accretion on the surface composition, internal rotation, and structure of the accretor. To anchor our models, we show that our accretor broadly reproduces the properties of ζ Ophiuchi, which has long been proposed to have accreted mass before being ejected as a runaway star when the companion exploded. We compare our accretor to models of single rotating stars and find that the later and stronger spin-up provided by mass accretion produces significant differences. Specifically, the core of the accretor retains higher spin at the end of the main sequence, and a convective layer develops that changes its density profile. Moreover, the surface of the accretor star is polluted by CNO-processed material donated by the companion. Our models show effects of mass accretion in binaries that are not captured in single rotating stellar models. This possibly impacts the further evolution (either in a binary or as single stars), the final collapse, and the resulting spin of the compact object.

Anabatic flows are common phenomena in the presence of sloping terrains, which significantly affect the dynamics and the exchange of mass and momentum in the low-atmosphere. Despite this, very few studies in the literature have tackled this topic. The present contribution addresses this gap by utilising high-resolved large-eddy simulations for investigating an anabatic flow in a simplified configuration, commonly used in laboratory experiments. The purpose is to analyse the complex thermo-fluid dynamics and the turbulent structures arising from the anabatic flow near the slope. In such a flow, three main dynamic layers are identified and reported: the conductive layer close to the surface, the convective layer where the most energetic motion develops, and the outer region, which is almost unperturbed. The analysis of instantaneous fields reveals the presence of thermal plumes, which are stable turbulent structures enhancing vertical transport and mixing of momentum and temperature. Such structures are generated by thermal instabilities in the conductive layer that trigger the rise of the plumes above them. Their evolution along the slope is described, identifying three regions responsible for the plumes generation, stabilisation, and merging. To the best of the authors’ knowledge, this is the first numerical experiment describing the along-slope behaviour of the thermal plumes in the convective layer.

Nondipolar magnetic fields exhibited at Uranus and Neptune may be derived from a unique geometry of their icy mantle with a thin convective layer on top of a stratified nonconvective layer. The presence of superionic H2O and NH3 has been thought as an explanation to stabilize such nonconvective regions. However, a lack of experimental data on the physical properties of those superionic phases has prevented the clarification of this matter. Here, our Brillouin measurements for NH3 show a two-stage reduction in longitudinal wave velocity (Vp) by ∼9% and ∼20% relative to the molecular solid in the temperature range of 1,500 K and 2,000 K above 47 GPa. While the first Vp reduction observed at the boundary to the superionic α phase was most likely due to the onset of the hydrogen diffusion, the further one was likely attributed to the transition to another superionic phase, denoted γ phase, exhibiting the higher diffusivity. The reduction rate of Vp in the superionic γ phase, comparable to that of the liquid, implies that this phase elastically behaves almost like a liquid. Our measurements show that superionic NH3 becomes convective and cannot contribute to the internal stratification.

<p>A decrease in the ice-period on lakes against the background of climate warming improves its oxygen regime in the cold half of the year by reducing the winter anoxia. A decrease in the thickness of the snow-ice cover can contribute to an increase in under-ice irradiation, which can provoke an earlier onset of spring under-ice convection and activation of algal blooms. Do these processes affect the oxygen content in ice-covered lakes? This study examines the variability of dissolved oxygen, water temperature, currents, chlorophyll "a" and under-ice irradiation according to field measurements carried out in 2007-2020 during spring under-ice convection in a small Lake Vendyurskoe (northwestern Russia). Field data were obtained at autonomous stations with an interval of one minute. Measurements of temperature and dissolved oxygen (RBR TR- and DO-sensors) were carried out from October to May, covering the entire ice-period, while measurements of currents (ADCP), solar radiation fluxes («Star-shaped pyranometer»  «Theodor Friderich & Co, Meteorologishe Gerate und Systeme»), and chlorophyll "a" (BBE Moldaenke) were carried out for 3-12 days from late March to the third decade of April in different years. The thickness of the snow-ice cover was also measured. Analysis of the data showed that in 2007-2020 the thickness of the snow-ice cover of Lake Vendyurskoe in spring (late March – mid-April) varied significantly from 35 to 70 cm, depending on weather conditions. The under-ice solar radiation fluxes varied from close to zero to more than 150 W/m2. The duration of spring under-ice convection ranged from two to seven weeks. Chlorophyll "a" was fairly uniformly distributed within the convective layer, even below the photic zone. We assume the dual role of convective currents in the development of subglacial plankton: ascending currents facilitate the entry of algal cells and nutrients into the photic zone, activating photosynthesis, while descending currents carry them out of it, suppressing photosynthesis. With well-developed convection, oscillations of dissolved oxygen were recorded with a daily frequency, reaching 1 mgO2/L in the upper part of the convective layer. Presumably, an increase in the content of dissolved oxygen is associated with a daytime increase in photosynthesis against the background of an increase in under-ice radiation, and a decrease is associated with the destruction of organic matter. Convective currents also affect the vertical distribution of dissolved oxygen, involving the oxygen-depleted bottom waters in mixing, which leads to a certain decrease in oxygen concentrations in the convective layer. The total amount of oxygen in the water column during the period of spring under-ice convection can increase by 10% due to the photosynthesis of phytoplankton. Oxygen fluctuations from minutes to hours were identified, which can be caused by seiche activity, the convective cells, advective transport, and the dynamics of internal waves. The results obtained in this work will contribute to a better understanding of the variability of oxygen in ice-covered lakes, caused by the total impact of biological and hydrophysical processes. The study was supported by an RFBR grant 18-05-60921.</p>

<p>More than 6000 profiles from profiling floats in the Black Sea over the 2005-2020 period were used to study the ventilation of this basin from the top to the very bottom. In the upper layers and in the main pycnocline, water masses show a strong interannual variability following intermittent events of cold water formation. The density ratio decreased three times during the last 15 years, revealing the decreasing role of temperature in the vertical layering of the Black Sea halocline. The deep transition layer (DTL) between 700 and 1700 m acts as an interface between the baroclinic layer and the largest bottom convective layer (BCL) of the world oceans. On top of DTL are the warm intermediate layer (WIL) and deep cold intermediate layer (DCIL). They both showed strong trends in the last fifteen years due to warmer climate and intensification of warmer intrusions from Bosporus. A “salinity wave” was detected in 2005-2009 below ~1700 m, which evidenced for the first time the penetration of gravity flow from Bosporus down to the bottom. The layering of water masses was explained as resulting from the different distribution of sources of heat and salt, double duffusion and balances between the geothermal and salinity flows in the BCL.</p>

The present work had the objective to evaluate the development of the convective boundary layer in the Amazon region simulated by a high resolution Large Eddy Simulation model (named PALM model), for days representative for rainy and dry seasons. The study used data from the GOAmazon Project 2014/2015 (Green Ocean Amazon). Using data from radiosondes and Ceilometer as truth values, they were compared with the simulations performed through the PALM model. The results showed that, in general, the convective boundary layer cycle for the Amazon region was well represented by PALM model. It´s outputs has showed an overestimation of ≈ 35 m in a rainy day and an underestimation of ≈ 20 m in a dry day, both in development phase of the convective layer at late morning. It was also observed that the latent heat flux profile was higher than the sensible heat in the atmosphere, because it is a region with a lot of humidity, with the boundary layer responding rapidly to the maximum surface forcing.

Abstract. The atmospheric mixing layer height (MLH) determines the space in which pollutants diffuse and is thus conducive to the estimation of the pollutant concentration near the surface. The study evaluates the capability of lidar to describe the evolution of the atmospheric mixing layer and then presents a long-term observed climatology of the MLH diurnal cycle. Detection of the mixing layer heights (MLHL and MLHL′) using the wavelet method based on lidar observations was conducted from January 2013 to December 2018 in the Beijing urban area. The two dataset results are compared with radiosonde as case studies and statistical forms. MLHL shows good performance in calculating the convective layer height in the daytime and the residual layer height at night. While MLHL′ has the potential to describe the stable layer height at night, the performance is limited due to the high range gate of lidar. A nearly 6-year climatology for the diurnal cycle of the MLH is calculated for convective and stable conditions using the dataset of MLHL from lidar. The daily maximum MLHL characteristics of seasonal change in Beijing indicate that it is low in winter (1.404±0.751 km) and autumn (1.445±0.837 km) and high in spring (1.647±0.754 km) and summer (1.526±0.581 km). A significant phenomenon is found from 2014 to 2018: the magnitude of the diurnal cycle of MLHL increases year by year, with peak values of 1.291±0.646 km, 1.435±0.755 km, 1.577±0.739 km, 1.597±0.701 km and 1.629±0.751 km, respectively. It may partly benefit from the improvement of air quality. As to converting the column optical depth to surface pollution, the calculated PM2.5 using MLHL data from lidar shows better accuracy than that from radiosonde compared with observational PM2.5. Additionally, the accuracy of calculated PM2.5 using MLHL shows a diurnal cycle in the daytime, with the peak at 14:00 LST. The study provides a significant dataset of MLHL based on measurements and could be an effective reference for atmospheric models of surface air pollution calculation and analysis.

Abstract. The atmospheric mixing layer height (MLH) determines the volume available for the dispersion of pollutants and thus contributes to the assessment of the pollutant concentration near the surface. The study evaluates the capability of lidar to describe the evolution of atmospheric mixing layer and then presents a long term observed climatology of MLH diurnal cycle. A system for automatic detection of the mixing layer height based on two wavelet methods (MLH and MLH') applied to lidar observations was operated from January 2013 to December 2018 in the Beijing urban area. The two dataset results are compared with radiosonde as case studies and statistical form. MLH shows good performance to calculate the convective layer height at daytime and the residual layer height at night. While MLH' has the potential to describe the stable layer height as radiosonde at night, the performance is limited due to the high range gate of lidar. A nearly six year climatology for diurnal cycle of MLH is calculated for convective and stable conditions using the dataset of MLH from lidar. The MLH characteristics of seasonal change in Beijing indicate that it is low in winter and autumn, and high in spring and summer. A significant phenomenon is found that from 2013 to 2018, the diurnal cycle of MLH increase year by year. It may partly benefit from the improvement of air quality. As to converting the column optical depth to the surface pollution, MLH from lidar shows better accuracy than that from radiosonde. Additionally, the accuracy with lidar MLH shows a diurnal cycle, with the peak at time of 14:00 LST. The study provides a significant dataset of MLH based on measurement and could be an effective reference to atmospheric models for surface air pollution calculation and analysis.

The paper discusses approaches to the construction of integral models of the convective boundary layer (CBL), based on the concept of complete mixing. To test analytic bulk models and the basic hypotheses of similarity, we use the results of eddy modeling (LES – Large Eddy Simulation). The empirical constants of the CBL integral models obtained according to the LES data for the case of free convection, are in good agreement with the previously published data of laboratory experiments. It is also shown that the flow of kinetic energy from the upper boundary of the CPS, carried out by gravitational waves, is small compared with other components of the balance of turbulent kinetic energy (TKE) in the convective layer. Parametrization of TKE generation for the case of sheared convective boundary layer in terms of the friction velocity and the average wind velocity in the CBL derived; resulting dimensionless constants are obtained from LES data. The results of the work allow us to formulate an integral model of the shear KPS suitable for practical use.