Convection Field Research Articles

Integrated water vapour observations in the Caribbean arc from a network of ground-based GNSS receivers during EUREC<sup>4</sup>A

Abstract. Ground-based Global Navigation Satellite System (GNSS) measurements from nearly 50 stations distributed over the Caribbean arc have been analysed for the period 1 January–29 February 2020 in the framework of the EUREC4A (Elucidate the Couplings Between Clouds, Convection and Circulation) field campaign. The aim of this effort is to deliver high-quality integrated water vapour (IWV) estimates to investigate the moisture environment of mesoscale cloud patterns in the trade winds and their feedback on the large-scale circulation and energy budget. This paper describes the GNSS data processing procedures and assesses the quality of the GNSS IWV retrievals from four operational streams and one reprocessed research stream which is the main data set used for offline scientific applications. The uncertainties associated with each of the data sets, including the zenith tropospheric delay (ZTD)-to-IWV conversion methods and auxiliary data, are quantified and discussed. The IWV estimates from the reprocessed data set are compared to the Vaisala RS41 radiosonde measurements operated from the Barbados Cloud Observatory (BCO) and to the measurements from the operational radiosonde station at Grantley Adams International Airport (GAIA), Bridgetown, Barbados. A significant dry bias is found in the GAIA humidity observations with respect to the BCO sondes (−2.9 kg m−2) and the GNSS results (−1.2 kg m−2). A systematic bias between the BCO sondes and GNSS is also observed (1.7 kg m−2), where the Vaisala RS41 measurements are moister than the GNSS retrievals. The IWV estimates from a collocated microwave radiometer agree with the BCO soundings after an instrumental update on 27 January, while they exhibit a dry bias compared to the soundings and to GNSS before that date. IWV estimates from the ECMWF fifth-generation reanalysis (ERA5) are overall close to the GAIA observations, probably due to the assimilation of these observations in the reanalysis. However, during several events where strong peaks in IWV occurred, ERA5 is shown to significantly underestimate the GNSS-derived IWV peaks. Two successive peaks are observed on 22 January and 23–24 January which were associated with heavy rain and deep moist layers extending from the surface up to altitudes of 3.5 and 5 km, respectively. ERA5 significantly underestimates the moisture content in the upper part of these layers. The origins of the various moisture biases are currently being investigated. We classified the cloud organization for five representative GNSS stations across the Caribbean arc using visible satellite images. A statistically significant link was found between the cloud patterns and the local IWV observations from the GNSS sites as well as the larger-scale IWV patterns from the ECMWF ERA5 reanalysis. The reprocessed ZTD and IWV data sets from 49 GNSS stations used in this study are available from the French data and service centre for atmosphere (AERIS) (https://doi.org/10.25326/79; Bock, 2020b).

Recent advances in the 3D kinematic Babcock–Leighton solar dynamo modeling

In this review, we explain recent progress made in the Babcock-Leighton dynamo models for the Sun, which have been most successful to explain various properties of the solar cycle. In general, these models are 2D axisymmetric and the mean-field dynamo equations are solved in the meriodional plane of the Sun. Various physical processes (e.g., magnetic buoyancy and Babcock-Leighton mechanism) involved in these models are inherently 3D process and could not be modeled properly in a 2D framework. After pointing out limitations of 2D models (e.g., Mean-field Babcock-Leighton dynamo models and Surface Flux Transport models), we describe recently developed next-generation 3D dynamo models that implement more sophisticated flux emergence algorithm of buoyant flux tube rise through the convection zone and capture Babcock-Leighton process more realistically than previous 2D models. The detailed results from these 3D dynamo models including surface flux transport counterpart are presented. We explain the cycle irregularities that are reproduced in 3D dynamo models by introducing scattering around the tilt angle only. Some results by assimilating observed photospheric convective velocity fields into the 3D models are also discussed, pointing out the wide opportunity that these 3D models hold to deliver.

Helioseismological determination of the subsurface spatial spectrum of solar convection: Demonstration using numerical simulations

Context. Understanding convection is important in stellar physics, for example, when it is an input in stellar evolution models. Helioseismic estimates of convective flow amplitudes in deeper regions of the solar interior disagree by orders of magnitude among themselves and with simulations. Aims. We aim to assess the validity of an existing upper limit of solar convective flow amplitudes at a depth of 0.96 solar radii obtained using time-distance helioseismology and several simplifying assumptions. Methods. We generated synthetic observations for convective flow fields from a magnetohydrodynamic simulation (MURaM) using travel-time sensitivity functions and a noise model. We compared the estimates of the flow amplitude with the actual value of the flow. Results. For the scales of interest (ℓ < 100), we find that the current procedure for obtaining an upper limit gives the correct order of magnitude of the flow for the given flow fields. We also show that this estimate is not an upper limit in a strict sense because it underestimates the flow amplitude at the largest scales by a factor of about two because the scale dependence of the signal-to-noise ratio has to be taken into account. After correcting for this and after taking the dependence of the measurements on direction in Fourier space into account, we show that the obtained estimate is indeed an upper limit. Conclusions. We conclude that time-distance helioseismology is able to correctly estimate the order of magnitude (or an upper limit) of solar convective flows in the deeper interior when the vertical correlation function of the different flow components is known and the scale dependence of the signal-to-noise ratio is taken into account. We suggest that future work should include information from different target depths to better separate the effect of near-surface flows from those at greater depths. In addition, the measurements are sensitive to all three flow directions, which should be taken into account.

Influence of applied magnetic field on nonlinear mixed convective nanoliquid flow past a permeable rough cone

The concept of magnetohydrodynamic nonlinear mixed convective nanoliquid flow over a vertical rough cone finds applications in science and engineering fields, especially in aerospace and thermal engineering as well as in other designing and manufacturing processes. In the present investigation, the cone surface roughness, which arises during the manufacturing processes, is modelled using sine wave forms. A uniform magnetic field is applied to analyse its impact on the transport characteristics of the flow. The study of such flow problems involving physical parameters such as nonlinear convection, permeable roughness, nanoparticles and external magnetic field is an innovative approach. The flow problem is modelled using nonlinear coupled partial differential equations which are transformed into dimensionless form by using Mangler’s non-similar transformations. The resulting equations are solved by utilizing the quasilinearization technique in combination with the implicit finite difference scheme. Effects of the different physical parameters on profiles and gradients are analysed by the graphical representations. Findings show that the higher values of the nonlinear mixed convection parameter will increase nanofluid velocity and magnitude of the coefficient of surface friction. Prominent sinusoidal oscillations are found in the surface gradients away from the apex of the cone due to the surface roughness effects.

Study of double-diffusive convection in a rotating couple-stress ferromagnetic fluid in the presence of varying gravitational field and horizontal magnetic field saturating in a porous medium

The results of double-diffusive convection on the liquid coatings of ferromagnetic liquids is studied by researchers by various theoretical and hydrodynamics concepts of hydrodynamics. However this study is done in presence of varying gravitational field. The study of such couple-stress ferromagnetic fluids in presence of fluctuating gravitational field is still not explored. As we know that gravitational field varies on different points on the earth; for this reason the study of couple-stress ferromagnetic liquids in presence of variable gravity fields is highly required. In this paper we studied the impact of medium permeability, stable solute gradient, couple-stress, rotation, magnetic field, and magnetization of a couple-stress ferromagnetic fluid in presence of variable gravitational field. For this study a coating of couple-stress ferromagnetic fluid saturating in a porous medium when fluid layer is heated and soluted. We examined the results of the magnetic field on couple-stress ferromagnetic fluids, its thermosolutal instability during rotation. A linearized concept and normal mode technique are used to acquire the dispersion relation. For the case of stationary convection, medium permeability, couple-stress, and magnetic field have both stabilizing and destabilizing results under certain conditions. A stable solute gradient has a stabilizing result on the system. Also, the rotation has a stabilizing result for λ>0 and destabilizing result for λ 0 and λ<0. The critical Rayleigh number for the onset of instability is decided numerically and graphically also. The principle of exchange of stabilities is observed to keep genuine in the absence of rotation, magnetic field, and stable solute gradient under certain conditions.

Description of mesoscale pattern formation in shallow convective cloud fields by using time-dependent Ginzburg-Landau and Swift-Hohenberg stochastic equations.

The time-dependent Ginzburg-Landau (or Allen-Cahn) equation and the Swift-Hohenberg equation, both added with a stochastic term, are proposed to describe cloud pattern formation and cloud regime phase transitions of shallow convective clouds organized in mesoscale systems. The starting point is the Hottovy-Stechmann linear spatiotemporal stochastic model for tropical precipitation, used to describe the dynamics of water vapor and tropical convection. By taking into account that shallow stratiform clouds are close to a self-organized criticality and that water vapor content is the order parameter, it is observed that sources must have nonlinear terms in the equation to include the dynamical feedback due to precipitation and evaporation. The nonlinear terms are derived by using the known mean field of the Ising model, as the Hottovy-Stechmann linear model presents the same probability distribution. The inclusion of this nonlinearity leads to a kind of time-dependent Ginzburg-Landau stochastic equation, originally used to describe superconductivity phases. By performing numerical simulations, pattern formation is observed. These patterns are better compared with real satellite observations than the pure linear model. This is done by comparing the spatial Fourier transform of real and numerical cloud fields. However, for highly ordered cellular convective phases, considered as a form of Rayleigh-Bénard convection in moist atmospheric air, the Ginzburg-Landau model does not allow us to reproduce such patterns. Therefore, a change in the form of the small-scale flux convergence term in the equation for moist atmospheric air is proposed. This allows us to derive a Swift-Hohenberg equation. In the case of closed cellular and roll convection, the resulting patterns are much more organized than the ones obtained from the Ginzburg-Landau equation and better reproduce satellite observations as, for example, horizontal convective fields.

High-resolution in situ observations of atmospheric thermodynamics using dropsondes during the Organization of Tropical East Pacific Convection (OTREC) field campaign

Abstract. The Organization of Tropical East Pacific Convection (OTREC) field campaign investigated the dynamical structure of convection in the tropical east Pacific and Caribbean. One of the central data sets for this field campaign is the thermodynamic structure of the atmosphere measured by dropsondes released from the NSF/NCAR G-V research aircraft. Between 7 August and 2 October 2019, 648 dropsondes were successfully released from 22 research flights. Soundings were launched in a grid pattern with a typical spacing of 1∘ longitude and 1.2∘ latitude and provided profiles of pressure, temperature, humidity, and winds between the surface and on average 13.3 km. Of these soundings, 636 provided complete vertical profiles of all parameters with a nominal vertical resolution between 6 to 12 m from the surface to almost flight altitude. OTREC deployed the new NRD41 dropsonde, which is the most advanced model that has been developed at NCAR. Here, we describe the data set, the processing of the measurements, and general statistics of all dropsonde observations. The data set is available at https://doi.org/10.26023/EHRT-TN96-9W04 (UCAR/NCAR and Vömel, 2019).

Towards a Mechanistic Understanding of Precipitation Over the Far Eastern Tropical Pacific and Western Colombia, One of the Rainiest Spots on Earth

AbstractAccording to Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellite precipitation composites, a broad maritime area over the far eastern tropical Pacific and western Colombia houses one of the rainiest spots on Earth. This study aims to present a suite of mechanistic drivers that help create such a world‐record‐breaking rainy spot. Previous research has shown that this oceanic and nearly continental precipitation maximum has a strong early morning precipitation peak and a high density of mesoscale convective systems. We examined new and unique observational evidence highlighting the role of both dynamical and thermodynamical drivers in the activation and duration of organized convection. Results showed the existence of a rather large combination of mechanisms, including: (1) dynamics of the Choco (ChocoJet) and Caribbean Low‐Level Jets along their confluence zone, including the Panama semi‐permanent low; (2) ChocoJet deceleration offshore is favored by land breeze, enhancing the nighttime and early morning low‐level convergence; (3) a wind sheared environment that conforms to the long‐lived squall line theory; (4) action of mid‐level gravity waves, which further support the strong diurnal variability; and (5) mesoscale convective vortices related to subsidence in the stratiform region and top‐heavy mass flux profiles. This study emphasizes the multiscale circulation and thermodynamics mechanisms associated with the formation of one of the rainiest spots on Earth and showcases new observations gathered during the Organization of Tropical East Pacific Convection field campaign (OTREC; August–September, 2019) that support the outlined mechanisms.

Conjugate Heat Transfer Model Based on SIMPLE and Coupled Energy and Heat Equations

In this study, a numerical weak coupling strategy for the modeling of a conjugate heat transfer phenomenon is considered. Where the incompressible Navier-Stokes equations are solved using the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) as a first step, and then the heat conduction equation for solid is solved in a second step considering the convective velocity field resulting from the first step. A finite-difference approach is used for both discretized time and spatial operators. In this paper, a two-dimensional simulation case study of a steady uniform streamwise flow around heated rectangular and triangle solids is presented. The simulation is forward in time until the steady-state regime is reached as the residuals converge and tend to zero. The spatial analysis of the temperature is obtained through the numerical resolution of the incompressible Navier-Stokes energy equation and the heat diffusion equation for the fluid and solid media, respectively. The results show the temperature, velocity, and pressure fields in the space domain. The code is written in MATLAB®, and the flow chart of the method is provided. It was noted that the convection was more dominant than the diffusion.

Fluid flow and heat transfer over corrugated backward facing step channel

The convective heat transfer and flow field characteristics through a backward-facing step (BFS) channel combined with a corrugated wall is investigated numerically and experimentally. Uniform heat flux is applied on 200 mm of the downstream wall, while the rest of the walls are considered isolated surfaces. The range of Reynolds number (Re) of the flow was between 5000 and 20,000. The governing equations were solved using the RNG k−ε turbulent model. The effects of wavelength and amplitude height of the corrugated wall on the friction factor and Nusselt number are studied. The results indicated that the fluid flow in the BFS channel combined with the corrugated wall significantly enhanced the heat transfer with increased friction. The average increase in the heat transfer rate and friction factor in the experiment is 40.7% and 46.2% respectively. The simulation results are comparable to the experimental ones. According to the simulations, the channel with an amplitude height of 4 mm and a wavelength of 20 mm has the highest heat transfer enhancement, reaching a performance evaluation criterion factor of 1.33 at 5000 Re.

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Laminar Flow over a Square Cylinder Undergoing Combined Rotational and Transverse Oscillations

This work numerically investigates the effects of combined rotational and transverse oscillations of a square cylinder on the flow field and force coefficients. The primary non-dimensional parameters that were varied are frequency ratio fR (0.5, 0.8), Re (50-200), phase difference (ϕ) between the motions and rotational amplitude (θ0) with the influence of the last three parameters being discussed in detail. The amplitude of transverse oscillations is fixed at 0.2D, where D is the cylinder width. The study has been validated using the mean drag coefficient for stationary and transversely oscillating square cylinders from literature. Output data was obtained in the form of force coefficient (Cd), vorticity and pressure contours. The governing equations for the 2dimensional model were solved from an inertial frame of reference (overset meshing) using finite volume method. The interplay between the convective field and prescribed motion has been used to explain many of the results obtained. The relative dominance of cylinder motion over the flow stream was determined using Discrete Fast Fourier Transform. The influence of Re on Cd disappears when the motions are completely out of phase (ϕ = π). In general, the Cd for low Re flows exhibited low sensitivity to change in other parameters. Direct correlation has been observed between frontal area, vortex patterns and drag coefficient

Unravelling the last milliseconds of an individual graphene nanoplatelet before impact with a Pt surface by bipolar electrochemistry.

Contactless interactions of micro/nano-particles near electrochemically or chemically active interfaces are ubiquitous in chemistry and biochemistry. Forces arising from a convective field, an electric field or chemical gradients act on different scales ranging from few microns down to few nanometers making their study difficult. Here, we correlated optical microscopy and electrochemical measurements to track at the millisecond timescale the dynamics of individual two-dimensional particles, graphene nanoplatelets (GNPs), when approaching an electrified Pt micro-interface. Our original approach takes advantage of the bipolar feedback current recorded when a conducting particle approaches an electrified surface without electrical contact and numerical simulations to access the velocity of individual GNPs. We evidenced a strong deceleration of GNPs from few tens of μm s−1 down to few μm s−1 within the last μm above the surface. This observation reveals the existence of strongly non-uniform forces between tens of and a thousand nanometers from the surface.

Study of double-diffusive convection in a rotating couple-stress ferromagnetic fluid in the presence of varying gravitational field and horizontal magnetic field saturating in a porous medium

The results of double-diffusive convection on the liquid coatings of ferromagnetic liquids is studied by researchers by various theoretical and hydrodynamics concepts of hydrodynamics. However this study is done in presence of varying gravitational field. The study of such couple-stress ferromagnetic fluids in presence of fluctuating gravitational field is still not explored. As we know that gravitational field varies on different points on the earth; for this reason the study of couple-stress ferromagnetic liquids in presence of variable gravity fields is highly required. In this paper we studied the impact of medium permeability, stable solute gradient, couple-stress, rotation, magnetic field, and magnetization of a couple-stress ferromagnetic fluid in presence of variable gravitational field. For this study a coating of couple-stress ferromagnetic fluid saturating in a porous medium when fluid layer is heated and soluted. We examined the results of the magnetic field on couple-stress ferromagnetic fluids, its thermosolutal instability during rotation. A linearized concept and normal mode technique are used to acquire the dispersion relation. For the case of stationary convection, medium permeability, couple-stress, and magnetic field have both stabilizing and destabilizing results under certain conditions. A stable solute gradient has a stabilizing result on the system. Also, the rotation has a stabilizing result for λ&gt;0 and destabilizing result for λ&lt;0 the system. It is likewise observed that in the absence of a stable solute gradient, magnetization has a stabilizing result on the system for both λ&gt;0 and λ&lt;0. The critical Rayleigh number for the onset of instability is decided numerically and graphically also. The principle of exchange of stabilities is observed to keep genuine in the absence of rotation, magnetic field, and stable solute gradient under certain conditions.

Cahn–Hilliard–Brinkman systems for tumour growth

&lt;p style='text-indent:20px;'&gt;A phase field model for tumour growth is introduced that is based on a Brinkman law for convective velocity fields. The model couples a convective Cahn–Hilliard equation for the evolution of the tumour to a reaction-diffusion-advection equation for a nutrient and to a Brinkman–Stokes type law for the fluid velocity. The model is derived from basic thermodynamical principles, sharp interface limits are derived by matched asymptotics and an existence theory is presented for the case of a mobility which degenerates in one phase leading to a degenerate parabolic equation of fourth order. Finally numerical results describe qualitative features of the solutions and illustrate instabilities in certain situations.&lt;/p&gt;

Shape optimization in thermal convection field considering a slight compressibility

Shape optimization in thermal convection field considering a slight compressibility

Shape design in unsteady forced thermal convection field considering pulsating flow

Shape design in unsteady forced thermal convection field considering pulsating flow

Natural Thermal Convection Field Measurement Technology Based on Background Oriented Schlieren Technology

Background oriented schlieren technology is developed from the combination of schlieren technology and PIV technology. It is an emerging non-contact flow field quantitative measurement method. The development and principle of background oriented schlieren technology are introduced in detail. A set of background oriented schlieren imaging system suitable for natural thermal convection field is built. The quantitative measurement research is carried out on the natural thermal convection field above the candle flame, and the density field is calculated. Distorted vector displacement map. The results show that the background oriented schlieren technology can be better applied to the quantitative measurement of natural thermal convection field, and it has broad development prospects.

Optical Visualization of the Formation Behavior of Magnetic Particle Clusters in a Forced Convection Field

Optical Visualization of the Formation Behavior of Magnetic Particle Clusters in a Forced Convection Field

The Distribution of Hydrodynamic Fields in Stationary Convection with Free Boundary

The paper considers a stationary flow of an incompressible fluid in a flat layer caused by an inhomogeneous source that induces heating or cooling of the lower boundary (there are four types of heating). Regularities in the distribution of hydrodynamic velocity fields between the types of heating are revealed, estimates of the parameters of dimensionless similarity numbers for each type of heating corresponding to counterflows and stagnant zones are obtained.