Vertical Mixing Profiles Research Articles

Probing the Extent of Vertical Mixing in Brown Dwarf Atmospheres with Disequilibrium Chemistry

Evidence of disequilibrium chemistry due to vertical mixing in the atmospheres of many T- and Y-dwarfs has been inferred due to enhanced mixing ratios of CO and reduced NH3. Atmospheric models of planets and brown dwarfs typically parameterize this vertical mixing phenomenon with the vertical eddy diffusion coefficient, K zz . While K zz can perhaps be approximated in the convective regions in the atmosphere with mixing length theory, in radiative regions, the strength of vertical mixing is uncertain by many orders of magnitude. With a new grid of self-consistent 1D model atmospheres from T eff of 400–1000 K, computed with a new radiative-convective equilibrium python code PICASO 3.0, we aim to assess how molecular abundances and corresponding spectra can be used as a probe of depth-dependent K zz . At a given surface gravity, we find nonmonotonic behavior in the CO abundance as a function of T eff, as chemical abundances are sometimes quenched in either of two potential atmospheric convective zones, or quenched in either of two possible radiative zones. The temperature structure and chemical quenching behavior also change with gravity. We compare our models with available near-infrared and M-band spectroscopy of several T- and Y-dwarfs and assess their atmospheric vertical mixing profiles. We also compare to color–magnitude diagrams and make predictions for James Webb Space Telescope spectra. This work yields new constraints, and points the way to significant future gains, in determining K zz , a fundamental atmospheric parameter in substellar atmospheres, with significant implications for chemistry and cloud modeling.

Spatially Resolved Modeling of Optical Albedos for a Sample of Six Hot Jupiters

Abstract Optical secondary eclipse measurements made by Kepler reveal a diverse set of geometric albedos for hot Jupiters with equilibrium temperatures between 1550 and 1700 K. The presence or absence of high-altitude condensates, such as Mg2SiO4, Fe, Al2O3, and TiO2, can significantly alter optical albedos, but these clouds are expected to be confined to localized regions in the atmospheres of these tidally locked planets. Here, we present 3D general circulation models and corresponding cloud and albedo maps for six hot Jupiters with measured optical albedos in this temperature range. We find that the observed optical albedos of K2-31b and K2-107b are best matched by either cloud-free models or models with relatively compact cloud layers, while Kepler-8b’s and Kepler-17b’s optical albedos can be matched by moderately extended (f sed = 0.1) parametric cloud models. HATS-11b has a high optical albedo, corresponding to models with bright Mg2SiO4 clouds extending to very low pressures (f sed = 0.03). We are unable to reproduce Kepler-7b’s high albedo, as our models predict that the dayside will be dominated by dark Al2O3 clouds at most longitudes. We compare our parametric cloud model with a microphysical cloud model. We find that even after accounting for the 3D thermal structure, no single cloud model can explain the full range of observed albedos within the sample. We conclude that a better knowledge of the vertical mixing profiles, cloud radiative feedback, cloud condensate properties, and atmospheric metallicities is needed in order to explain the unexpected diversity of albedos in this temperature range.

Modeling Water-Mass Distributions in the Modern and LGM Ocean: Circulation Change and Isopycnal and Diapycnal Mixing

AbstractPaleoproxy observations suggest that deep-ocean water-mass distributions were different at the Last Glacial Maximum than they are today. However, even modern deep-ocean water-mass distributions are not completely explained by observations of the modern ocean circulation. This paper investigates two processes that influence deep-ocean water-mass distributions: 1) interior downwelling caused by vertical mixing that increases in the downward direction and 2) isopycnal mixing. Passive tracers are used to assess how changes in the circulation and in the isopycnal-mixing coefficient impact deep-ocean water-mass distributions in an idealized two-basin model. We compare two circulations, one in which the upper cell of the overturning reaches to 4000-m depth and one in which it shoals to 2500-m depth. Previous work suggests that in the latter case the upper cell and the abyssal cell of the overturning are separate structures. Nonetheless, high concentrations of North Atlantic Water (NAW) are found in our model’s abyssal cell: these tracers are advected into the abyssal cell by interior downwelling caused by our vertical mixing profile, which increases in the downward direction. Further experiments suggest that the NAW concentration in the deep South Atlantic Ocean and in the deep Pacific Ocean is influenced by the isopycnal-mixing coefficient in the top 2000 m of the Southern Ocean. Both the strength and the vertical profile of isopycnal mixing are important for setting deep-ocean tracer concentrations. A 1D advection–diffusion model elucidates how NAW concentration depends on advective and diffusive processes.

Effects of Turbulent Hyporheic Mixing on Reach‐Scale Transport

AbstractTurbulence causes rapid mixing of solutes and fine particles between open channel flow and coarse‐grained streambeds. Turbulent mixing is known to control hyporheic exchange fluxes and the distribution of vertical mixing rates in the streambed, but it is unclear how turbulent mixing ultimately influences mass transport at the reach scale. We used a particle‐tracking model to simulate local‐ and reach‐scale solute transport for a stream with coarse‐grained sediments. Simulations were first used to determine profiles of vertical mixing rates that best described solute concentration profiles measured within a coarse granular bed in flume experiments. These vertical mixing profiles were then used to simulate a pulse solute injection to show the effects of turbulent hyporheic exchange on reach‐scale solute transport. Experimentally measured concentrations were best described by simulations with a nonmonotonic mixing profile, with highest mixing at the sediment–water interface and exponential decay into the bed. Reach‐scale simulations show that this enhanced interfacial mixing couples in‐stream and hyporheic solute transport. Coupling produces an interval of exponential decay in breakthrough curves and delays the onset of power law tailing. High streamwise velocities in the hyporheic zone reduce mass recovery in the water column and cause breakthrough curves to exhibit steeper power law slopes than predictions from mobile‐immobile modeling theory. These results demonstrate that transport models must consider the spatial variability of streamwise velocity and vertical mixing for both the stream and the hyporheic zone, and new analytical theory is needed to describe reach‐scale transport when high streamwise velocities are present in the hyporheic zone.

Sensitivity of phytoplankton distributions to vertical mixing along a North Atlantic transect

Abstract. Using in situ data of upper ocean vertical mixing along a transect in the North Atlantic and a one-dimensional phytoplankton growth model, we study the sensitivity of the surface phytoplankton concentration to vertical mixing distributions. The study is divided into two parts. In the first part, the model is calibrated to the observations. The optical model parameters are determined from measurements of the light attenuation. The biological parameters are calibrated to three different reference stations with observed vertical profiles of the chlorophyll a (Chl a) concentration and the nutrient concentration. In the second part, the sensitivity of the three model calibrations to the vertical mixing is studied. Therefore measured vertical mixing profiles are applied to the model. These mixing profiles are based on the measurements along the transect and are treated as a set of possible mixing situations of the North Atlantic. Results show that shifts in vertical mixing are able to induce a transition from an upper chlorophyll maximum to a deep one and vice versa. Furthermore, a clear correlation between the surface phytoplankton concentration and the mixing induced nutrient flux is found for nutrient-limited cases. This may open up the possibility to extract characteristics of vertical mixing from satellite ocean colour data using data-assimilation methods.

Constraints on dust aerosols from the Mars Exploration Rovers using MGS overflights and Mini‐TES

Overflights of the Mars Exploration Rovers (MER) by the Mars Global Surveyor (MGS) provide a unique opportunity to examine some of the basic properties of dust aerosols, starting with one of the most fundamental, the indices of refraction (m = n + ik) in the infrared. The upward‐viewing geometry of the Miniature Thermal Emission Spectrometer (Mini‐TES) and the combined contemporaneous observations from both MER and MGS are powerful tools. Their use allows atmospheric retrievals to directly determine n and k while offering constraints for the menagerie of other radiative transfer input parameters. We exploit these coordinated observing campaigns, along additional data sources, to carry out series of radiative transfer analyses that ultimately return the set of refractive indices. We apply the resulting m to a larger sample of Mini‐TES data to both further validate our approach and retrieve several other aerosol properties, including dust optical depth, dust size, and a measure of the vertical mixing profile. We find good agreement with the empirical approach of Smith et al. (2006), in terms of both the optical depths themselves and the frequency dependence of their extinction cross section and single scattering albedo. The retrieved dust sizes vary from near 1.3 μm to 1.8 μm within the selected sample, with a precision estimated to be ≃0.1–0.2 μm. The vertical mixing profile evolves from well‐mixed to appreciably confined by LS ∼ 30°. For Spirit (MER‐A), there is an abrupt transition back to a more well‐mixed vertical profile with the onset of regional dust activity at LS ∼ 140°. We discuss the lack of a definitive detection of water ice clouds in Mini‐TES observations and the potential effects of vertical gradients in particle size distribution. Finally, as part of coordinated overflight analyses, an atmospherically corrected TES Lambert albedo map is derived and presented in Appendix A.

Predicting the vertical profiles of anchovy (Engraulis mordax) and sardine (Sardinops sagax) eggs in the California Current System

AbstractSeveral published models exist for simulating vertical profiles of pelagic fish eggs, but no one has rigorously assessed their capacity to explain observed variability. In this study, we applied a steady‐state model, with four different formulations for vertical diffusivity, to northern anchovy (Engraulis mordax) and Pacific sardine (Sardinops sagax) eggs in the California Current region. Vertical mixing profiles, based on wind speed and hydrography, were combined with estimated terminal ascent velocities of the eggs based on measurements of egg buoyancy and size, to simulate the vertical profiles of the eggs. We evaluated model performance with two data sets: (1) vertically stratified tows for both species and (2) paired samples for sardine eggs from 3‐m depth and in vertically integrated tows. We used two criteria: whether the model predicted individual observed vertical profiles (1) as well as the observed mean and (2) better than the observed mean. Model predictions made with the formulation producing the most gradual profile of vertical diffusivity provided the best match to observations from both data sets and for both species. Addition of a random error term to the terminal ascent velocity further improved prediction for anchovy eggs, but not sardine. For the paired data, model prediction of integrated abundance from abundance at 3‐m depth had significantly lower mean square error than prediction based on a linear regression of 3 m on integrated abundance. Our results support the feasibility of using data from the Continuous Underway Fish Egg Sampler quantitatively as well as qualitatively in stock assessments.

Evaluation of various vertical mixing parameterizations in a tropical Pacific Ocean GCM

Coupled general circulation models (GCMs) have had weak El Niño/Southern Oscillation variability that has been attributed to a diffuse thermocline in the modeled equatorial Pacific Ocean. Consequently, there have been many attempts to improve the thermocline by developing new or improved ocean vertical mixing schemes. This paper investigates the influence of gradient Richardson Number-based vertical mixing scheme profiles in a tropical Pacific Ocean GCM. It has been common for vertical mixing schemes to be assessed in tropical Pacific Ocean models that have a limited latitudinal domain bounded by zonal walls with sponge layers. However, recent work has shown that warm surface water can accumulate in these models and stop them from achieving the observed sharp equatorial thermocline. The present model employs a parameterized wall heat transport scheme that prevents warm surface water from accumulating. Thus we are able assess the influence of vertical mixing profiles in an ocean model that does not allow warm surface water to accumulate and influence the thermocline. In this paper we evaluate the equatorial performance of three different Richardson number ( Ri)-based vertical mixing profiles: an integer power (IP) profile based on the observations of Peters, Gregg and Toole; a form of the Pacanowski and Philander profile modified to have low background mixing; and the Max Planck Institute profile. With the accumulation of warm surface water prevented, each of these profiles is able to achieve a sharp thermocline. When compared with observations, the IP profile achieves a better upwelling velocity distribution. We also examine the influence on equatorial performance of very high mixing coefficients at low Richardson number, and of low background mixing coefficients.