Shallow Convective Layer Research Articles

Constraints on the Long-term Existence of Dilute Cores in Giant Planets

Post-Cassini ring seismology analysis suggests the existence of a stable stratification inside Saturn that extends from the center to ∼60% of its radius, in what is recognized today as Saturn’s dilute core. Similarly, gravity measurements on Jupiter suggest the existence of a dilute core of weekly constrained radial extent. These cores are likely in a double-diffusive regime, which prompts the question of their long-term stability. Indeed, previous direct numerical simulation (DNS) studies in triply periodic domains have shown that, in some regimes, double-diffusive convection tends to spontaneously form shallow convective layers, which coarsen until the region becomes fully convective. In this paper, we study the conditions for layering in double-diffusive convection using different boundary conditions, in which temperature and composition fluxes are fixed at the domain boundaries. We run a suite of DNSs varying microscopic diffusivities of the fluid and the strength of the initial stratification. We find that convective layers still form as a result of the previously discovered γ-instability, which takes place whenever the local stratification drops below a critical threshold that only depends on the fluid diffusivities. We also find that the layers grow once formed, eventually occupying the entire domain. Our work thus recovers the results of previous studies, despite the new boundary conditions, suggesting that this behavior is universal. The existence of Saturn’s stably stratified core, today, therefore suggests that this threshold has never been reached, which places a new constraint on scenarios for the planet’s formation and evolution.

The Implications for Radiative Cloud Forcing via the Link Between Shallow Convection and Planetary Boundary Layer Mixing

AbstractIn this study, the role of shallow convection and its links with planetary boundary layer (PBL) mixing is analyzed using a global atmospheric model. All of the simulations are conducted with the Korean Integrated Model (KIM), which is a next‐generation operational global model for Korea Meteorological Administration. The simulation results show that the direct effect of shallow convection enhances additional vertical mixing inside the shallow convective layer, whereas its indirect effect reduces vertical mixing inside the PBL by changing the PBL height, thus vertical mixing strength. It is also found that the impact of shallow convection on low‐level clouds is tightly related to the overlapping between PBL and shallowing convection. When the PBL height is located in the higher (lower) part of the shallow convective layer, the mixing across the top of the PBL becomes smaller (larger). Therefore, the diffusivity profile of KIM shallow convection scheme is modified according to the overlapping pattern of PBL mixing and shallow convection. The systematic bias of downward solar radiation at the surface of KIM is improved with the revised diffusivity profile of the shallow convection scheme proposed in this study.

Metal pollution of low-mass Population III stars through accretion of interstellar objects like ‘Oumuamua

Abstract We calculate the accretion mass of interstellar objects (ISOs) like ‘Oumuamua on to low-mass Population III stars (Pop. III survivors), and estimate the surface pollution of Pop. III survivors. The ISO number density estimated from the discovery of ‘Oumuamua is so high (∼0.2 au−3) that Pop. III survivors have chances of colliding with ISOs ≳ 105 times per 1 Gyr. ‘Oumuamua itself would be sublimated near Pop. III survivors, since its size is small; ∼100 m. However, ISOs with size ≳3 km would reach the Pop. III survivor surfaces. Supposing an ISO cumulative number density with size larger than D is n∝D−α, Pop. III survivors can accrete ISO mass ≳10−16 M⊙, or ISO iron mass ≳10−17 M⊙, if α < 4. This iron mass is larger than the accretion mass of interstellar medium (ISM) by several orders of magnitude. Taking into account material mixing in the convection zone of Pop. III survivors, we find that their surface pollution is typically [Fe/H] ≲ −8 in most cases; however, the surface pollution of Pop. III survivors with 0.8 M⊙ can be [Fe/H] ≳ −6 because of the very shallow convective layer. If we consider Pop. III survivors located at the Galactocentric distance of 8 kpc, the dependence of the metal pollution is as follows. If α > 4, Pop. III survivors have no chance at colliding with ISOs with D ≳ 3 km, and remain metal-free. If 3 < α < 4, Pop. III survivors would be most polluted by ISOs up to [Fe/H] ∼ −7. If α < 3 up to D ∼ 10 km, Pop. III survivors could hide in metal-poor stars so far discovered. Pop. III survivors would be more polluted with decreasing Galactocentric distance. Although the metal pollution depends on α and the Galactocentric distance, we first show the importance of ISOs for the metal pollution of Pop. III survivors.

Baroclinic, Kelvin and inertia-gravity waves in the barostrat instability experiment

The differentially heated rotating annulus is a laboratory experiment historically designed for modelling large-scale features of the mid-latitude atmosphere. In the present study, we investigate a modified version of the classic baroclinic experiment in which a juxtaposition of convective and motionless stratified layers is created by introducing a vertical salt stratification. The thermal convective motions are suppressed in a central region at mid-depth of the rotating tank, therefore double-diffusive convection rolls can develop only in thin layers located at top and bottom, where the salt stratification is weakest. For high enough rotation rates, the baroclinic instability destabilises the flow in the top and the bottom shallow convective layers, generating cyclonic and anticyclonic eddies separated by the stable stratified layer. Thanks to this alternation of layers resembling the convective and radiative layers of stars, the planetary’s atmospheric troposphere and stratosphere or turbulent layers at the sea surface above stratified waters, this new laboratory setup is of interest for both astrophysics and geophysical sciences. More specifically, it allows to study the exchange of momentum and energy between the layers, primarily by the propagation of internal gravity waves (IGW). PIV velocity maps are used to describe the wavy flow pattern at different heights. Using a co-rotating laser and camera, the wave field is well resolved and different wave types can be found: baroclinic waves, Kelvin and Poincaré type waves. The signature of small-scale IGW can also be observed attached to the baroclinic jet. The baroclinic waves occur at the thin convectively active layer at the surface and the bottom of the tank, though decoupled they show different manifestation of nonlinear interactions. The inertial Kelvin and Poincaré waves seem to be mechanically forced. The small-scale wave trains attached to the meandering jet point to an imbalance of the large-scale flow. For the first time, the simultaneous occurrence of different wave types is reported in detail for a differentially heated rotating annulus experiment.

The Atmospheric Boundary Layer during Wintertime Persistent Inversions in the Grenoble Valleys

This study addresses the atmospheric boundary layer dynamics in the Grenoble valleys during persistent inversions, for 5 months during the 2006-2007 winter. During a persistent inversion, the boundary layer contains a layer with a positive vertical temperature gradient over a few days. Temperature data recorded on the valley sidewalls are first used. A bulk measure of the boundary layer stability, based upon the temperature difference between the valley top and the valley bottom, is introduced and a criterion is proposed to detect persistent inversions. We show that this criterion is equivalently expressed in terms of the heat deficit inside the boundary layer. Nine episodes are detected and coincide with the PM10-polluted periods of the 2006-2007 winter. Secondly, the five strongest and longest persistent inversions are simulated using the MesoNH model. Focus is made on the stagnation stage of the episode, during which the inversion exhibits a diurnal cycle that does not significantly evolve from day to day. Whatever the episode, the inversion develops from the ground over a height of about 1200 m, with a nighttime temperature strength of about 20 K.The boundary-layer dynamics within the inversion layer are fully decoupled from the (anticyclonic, weak) synoptic flow, independent from the synoptic-wind direction and similar whatever the episode. This implies that these dynamics are controlled by thermal winds and solely depends upon the geometry of the topography and upon the radiative cooling of the ground. Finally, a two-day high-resolution simulation is made for the strongest case, representative of any persistent inversion. The flow pattern displays a well-defined spatial structure, with a vertical layering resulting from the superposition of the down-valley winds flowing from the different valleys surrounding Grenoble. This pattern persists all day long over a shallow convective layer of about 50 m forming above the ground during the reduced daytime period. Within this shallow layer, convection triggers a weak up-valley wind. Ventilation and stagnation areas in the surface layer are also computed, providing insights for air quality studies. The main characteristics of these persistent inversions are comparable to the most extreme wintertime inversions recorded in the Grand Canyon, Arizona.

The differential rotation of G dwarfs

A series of stellar models of spectral type G is computed to study the rotation laws resulting from mean-field equations. The rotation laws of the slowly rotating Sun, the fast rotating MOST stars epsilon Eri and kappa1 Cet and the rapid rotators R58 and LQ Lup can easily be reproduced. We also find that differences in the depth of the convection zone cause large differences in the surface rotation law and that the extreme surface shear of HD 171488 can only be explained with a artificially shallow convection layer. We also check the thermal wind equilibrium in fast-rotating G dwarfs and find that the polar subrotation (dOmega/dz<0) is due to the barocline effect and that the equatorial superrotation (dOmega/dr>0) is due to the Lambda effect as part of the Reynolds stresses. In the bulk of the convection zones where the meridional flow is slow and smooth the thermal wind equilibrium actually holds between the centrifugal and the pressure forces. It does not hold, however, in the bounding shear layers including the equatorial region where the Reynolds stresses dominate.

Meteorological assessment of the surface temperatures on Titan: constraints on the surface type

The latitudinal profile of near-surface air temperature on Titan retrieved by Voyager 1 has been difficult to understand and raised several speculations about possible exotic processes that might be occurring near Titan's surface, while the thermal properties of the surface itself are unknown. This study systematically investigates the seasonal and spatial variation of the surface temperature and air temperature in the lower troposphere by a 3-dimensional general circulation model for different putative surface types (porous icy regolith, rock–ice mixture, hydrocarbon lakes). For any viable surface type the surface temperature is unlikely to be constant through the year and should more or less vary seasonally and even diurnally, most likely by a few K. Recent observations of tropospheric clouds may be evidence of seasonal variation of the surface temperature and the model predicts in the case of solid surface the development of a convective layer with superadiabatic lapse rates near the surface exactly at those latitudes and seasons where clouds have been identified. The latitudinal profile of the surface temperature retrieved from Voyager 1 infrared spectra can be explained without invoking exotic effects, provided the thermal inertia of the surface is relatively small and/or the surface albedo is low. A dominance of water ice (high thermal inertia and high albedo) at the surface is unfavorable to reproduce the observation. The latitudinal gradient of the surface temperature is particularly large at the hydrocarbon lake surface due to low albedo and small surface drag. Local anomalies of the surface albedo or surface thermal inertia are likely to cause substantial inhomogeneities of the surface temperature. Quasi-permanent accumulation of stratospheric haze at both poles would create a perennial equator-to-pole contrast of the surface temperature, but also a substantially lower global-mean surface temperature due to an enhanced anti-greenhouse effect in summer. The air temperature in the lower troposphere exhibits a tiny latitudinal gradient and a pole-to-pole gradient due to the presence of a pole-to-pole Hadley circulation, indicating that the temperature within the planetary boundary layer may exhibit a vertical profile characteristic of season, location and scenario. There may be a shallow near-surface inversion layer in cold seasons and a shallow convective layer in warm seasons.

Limits on Chromospheres and Convection among the Main‐Sequence A Stars

In deeply convective stars, the nonthermal energy required to heat the chromosphere ultimately is supplied by turbulent magnetoconvection. Because the early and middle A stars have very shallow convective layers, they are not expected to produce enough magnetoconvective power to sustain luminous chromospheres or hot coronae. Here we describe a search for chromospheric emission in the far-ultraviolet (905–1185 A ˚ ) spectra of seven main-sequence A stars, based on observations from the Far Ultraviolet Spectroscopic Explorer (FUSE) telescope. Our survey spans the interval in effective temperature along the main sequence over which powerful subsurface convection zones and hence chromospheric emission are expected to vanish. The presence or absence of high-temperature emissions in our FUSE spectra therefore can be used to identify the locus for the transition from convective to radiative envelopes—a change in stellar structure that is difficult to assess by other means. We present our observations and analysis of the subcoronal emission lines of C iii �� 977, 1175 and O vi �� 1032, 1037, which bracket a range in formation temperatures from 50,000 to 300,000 K. To supplement our FUSE observations, we also report Goddard High Resolution Spectrograph measurements of Si iii � 1206 and H i Ly �� 1215, obtained from archival observations of the Hubble Space Telescope, as well as X-ray measurements from previous ROSAT survey and pointed observations. We detected C iii and O vi emission features in the FUSE spectra of the coolest stars of our sample, at Teff d8200 K. When normalized to the bolometric luminosities, the detected emission-line fluxes are comparable to solar values. We detected none of the hotter stars in our survey at Teff � 8300 K. Upper limits on the normalized flux in some instances approach 40 times less than solar. Within an uncertainty in the effective temperature scale of up to several hundred kelvins, our FUSE observations indicate that the transition between convective and radiative stellar envelopes takes place at, or very near, the point along the main sequence where stellar structure models predict and, moreover, that the changeover occurs very abruptly, over a temperature interval no greater than � 100 K in width. Our FUSE sample also includes two binary stars. In both cases, the narrow UV line profiles we have observed suggest that the high-temperature emission is most likely associated with the late-type companions rather than the A stars themselves. Subject headings: stars: activity — stars: chromospheres — stars: late-type — ultraviolet: stars

Role of Gravity Waves in Triggering Deep Convection during TOGA COARE

The role of gravity waves in the initiation of convection over oceanic regions during the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) experiment is investigated. First, an autocorrelation method is applied to infrared temperature observations of convective events from satellite images. It reveals that new deep convective cells often occur a few hours after a previous intense event at a typical distance of a few hundred kilometers. Such fast moving modes (faster than 15 m s 21) are interpreted as the trace of gravity waves excited by previous convection and contributing to trigger further convection. Second, the specific case of 11‐12 December 1992, during which an active squall line is generated after the collapse of a previous mesoscale convective system (MCS) nearby, is analyzed with a nonhydrostatic model. The triggering of the second MCS is well reproduced explicitly, owing to the use of the two-way interactive grid nesting. The convective source appears to emit pulses of gravity waves on a wide range of small scales. On the contrary, the troposphere response to the convective source exhibits a spectral simplicity. A slowly evolving mode, characterized by an ascent in the PBL and a compensating subsidence in the free troposphere, favors shallow convection and inhibits deep convection, respectively. Traveling modes propagating away from the convective source are characterized by a fast mode (;50 m s21) and a slower mode (; 25 ms 21) associated with the convective and stratiform development of the source, respectively, in agreement with previous studies. A budget analysis reveals the different factors leading to the deep convection triggering. First, an active PBL characterized by strong surface fluxes and mean ascent, initiates shallow convection, lasting about 2 h, inhibited above by the subsiding motions maintaining a dry layer. Second, horizontal advection of moist and cooler air at midlevels, and detrainment from cumuli, contribute to destroy the dry air layer capping the shallow convective layer. Finally, vertical advection induced by the gravity waves passage modulates the vapor and temperature evolution. Ascending phases favor moistening and cooling, whereas subsiding phases stop these effects, delaying the deep convection onset. The triggering occurs after a strong subsidence, when the ascent phases of deep and shallow modes are combined.

Thessaloniki '91 field measurement campaign—I. Wind field and atmospheric boundary layer structure over greater Thessaloniki area, under light background flow

In this study, the major characteristics of the sea and land breeze circulation cells over the Greater Thessaloniki Area (GTA) are presented. During the Thessaloniki '91 Field Measurement Campaign (15 September–13 October) the local flows were examined under different conditions of background wind using a network of ten surface measuring stations, two tethered balloons and a high-resolution monostatic Sodar. The measurements showed that the sea breeze developing over the area maintained two circulation cells. The first was generated over the Thessaloniki Bay during the morning hours and caused sea breeze spreading with the wind direction perpendicular to the long convoluted shoreline at moderate wind speed. The second circulation cell was established over the entire Thermaikos Gulf at noon and was maintained with a southerly wind direction. During the night, a land breeze developed and the wind pattern was influenced by various drainage flows resulting from the surrounding complex topography. The measurements also confirmed the influence of the heat island effect and the creation of a shallow convection layer, during the night which gave rise to layering aloft. Finally, tethersonde ascents revealed the depth of the sea breeze, which was found to range from 300 m to more than 500 m, depending on the prevailing synoptic flow.

Non-linear Convection with Hexagonal Planform

Observations of the solar surface indicate a cellular structure, photospheric granulation, which, as suggested by Wilson and others, could arise from a shallow convection layer. The basic observed granulation pattern has been likened to a hexagonal cell type structure.