Convective Characteristics Research Articles

“Gray Zone” Simulations Using a Three-Dimensional Planetary Boundary Layer Parameterization in the Weather Research and Forecasting Model

Abstract Generating accurate weather forecasts of planetary boundary layer (PBL) properties is challenging in many geographical regions, oftentimes due to complex topography or horizontal variability in, for example, land characteristics. While recent advances in high-performance computing platforms have led to an increase in the spatial resolution of numerical weather prediction (NWP) models, the horizontal gridcell spacing (Δx) of many regional-scale NWP models currently fall within or are beginning to approach the gray zone (i.e., Δx ≈ 100–1000 m). At these gridcell spacings, three-dimensional (3D) effects are important, as the most energetic turbulent eddies are neither fully parameterized (as in traditional mesoscale simulations) nor fully resolved [as in traditional large-eddy simulations (LES)]. In light of this modeling challenge, we have implemented a 3D PBL parameterization for high-resolution mesoscale simulations using the Weather Research and Forecasting Model. The PBL scheme, which is based on the algebraic model developed by Mellor and Yamada, accounts for the 3D effects of turbulence by calculating explicitly the momentum, heat, and moisture flux divergences in addition to the turbulent kinetic energy. In this study, we present results from idealized simulations in the gray zone that illustrate the benefit of using a fully consistent turbulence closure framework under convective conditions. While the 3D PBL scheme reproduces the evolution of convective features more appropriately than the traditional 1D PBL scheme, we highlight the need to improve the turbulent length scale formulation. Significance Statement The spatial resolution of weather models continues to increase at a rapid rate in accordance with the enhancement of computing power. As a result, smaller-scale atmospheric features become more explicitly resolved. However, most numerical models still ignore the impact of horizontal weather variations on boundary layer flows, which becomes more important at these smaller spatial scales. To address this issue, we have implemented a new modeling approach, using fundamental principles, which accounts for horizontal variability. Our results show that including three-dimensional effects of turbulence is necessary to achieve realistic boundary layer characteristics. This novel technique may be useful for many applications including complex terrain flows, pollutant dispersion, and surface–atmosphere interaction studies.

Plume‐Slab Interactions Can Shut Off Subduction

AbstractMantle plumes are typically considered secondary features of mantle convection, yet their surface effects over Earth's evolution may have been significant. We use 2‐D convection models to show that mantle plumes can in fact cause the termination of a subduction zone. This extreme case of plume‐slab interaction is found when the slab is readily weakened, for example, by damage‐type rheology, and the subducting slab is young. We posit that this mechanism may be relevant, particularly for the early Earth, and a subdued version of these plume‐slab interactions may remain relevant for modern subduction zones. Such core‐mantle boundary–surface interactions may be behind some of the complexity of tomographically imaged mantle structures, for example, in South America. More generally, plume “talk back” to subduction zones may make plate tectonics more episodic.

Oscillatory natural convection of Al2O3-water nanofluid near its density maximum in a narrow horizontal annulus

In order to reveal the oscillatory characteristics of the natural convection of nanofluid, this paper presents a comprehensive numerical work on the oscillatory natural convection of Al2O3-water nanofluid near its density maximum in a narrow horizontal annulus. The influences of nanoparticle volume fraction, Rayleigh number and density inversion parameter on the flow pattern evolution and heat transfer ability are analyzed. Results show that the density inversion of the working fluid affects the velocity and temperature distributions of the natural convection of Al2O3-water nanofluid significantly. Adding nanoparticles is beneficial to stabilizing the flow of nanofluid and keeping the symmetry of the flow structure. The convective heat transfer is enhanced with the increase of the volume fraction of nanoparticles. Furthermore, the evolution of the flow regime with the Rayleigh number at different volume fractions of nanoparticles is summarized.

Transition of Large‐Scale Environmental Conditions and Characteristics of Four Rainfall Types Observed by S‐PolKa During the MJO‐1 Active Phase of DYNAMO/CINDY/AMIE

AbstractAnalyses of National Center for Atmospheric Research (NCAR) S‐PolKa dual‐polarization radar data and ERA5 reanalysis fields indicate gradual changes in convection characteristics and large‐scale environmental conditions during a central Indian Ocean Madden‐Julian Oscillation (MJO) event observed by the DYNAMO/CINDY/AMIE field campaigns in late October 2011 (MJO‐1). Examination of four rainfall types (isolated convective cores, convective, mixed, and stratiform) reveals a transition of convection characteristics (i.e., areal coverage and depth) between distinct 5‐day environmental periods at the beginning and end of this ∼2‐week MJO active phase. A shift toward less frequent rainfall covering less of the radar domain for all four rainfall types occurs when large‐scale lower‐tropospheric dry air advects into the region with the westerly wind burst (WWB). Drier and warmer lower‐free‐tropospheric conditions associated with the WWB contribute to increased large‐scale surface‐based convective inhibition (CIN), surface‐based convective available potential energy (CAPE), and cloud base heights. A thermodynamic budget analysis indicates reduced surface heat fluxes contribute to the increased surface‐based CAPE. Greater CAPE at the end of MJO‐1 coincides with deeper 50‐dBZ convective echoes, while decreased 10‐dBZ depth for all rainfall types corresponds in time with WWB‐related dry‐air advection. Increased (decreased) reflectivity values in the lower‐level vertical reflectivity distribution of convective (stratiform) precipitation indicate increased (decreased) convective (stratiform) intensities when the WWB is present. The opposite depth changes for convective echoes and opposite shifts in convective and stratiform precipitation intensities underscore how the WWB can have differing impacts at different reflectivity thresholds and stages of the deep convection lifecycle.

Heat transport and the aspects of retardation time phenomenon in the flow of highly viscoelastic nanofluid with a Newtonian heating agent

This study investigates the convective thermal and solutal characteristics in the stagnation point flow of Burgers nanofluid accelerated due to stretching cylinder. The thermal features of the flow are examined by employing Fourier's law in addition to the effects of heat source/sink and Joule heating. The essential feature of this research is to utilize the activation energy and binary chemical reaction effects to investigate the convective mass transform phenomenon. Moreover, the process of heat as well as mass convection from the surface to the fluid is also observed by employing convective thermal and concentration boundary conditions. The dimensionless similarity transformations are invoked to attain the ordinary differential equations (ODE's) from governed partial differential equations (PDE's). The numerical solutions are carried out by utilizing bvp midrich numerical procedure. The section on physical analysis is proposed to elaborate on the outcomes of this study physically. The graphical illustration of physical parameters is also depicted and physically analyzed. The rate of mass transfer is depreciated for the augmenting scales of reaction rate and temperature difference constraints. Moreover, the escalating magnitudes of thermal Biot number drive up the thermal energy transport in the flow.

Heat transfer characteristics of mixed convection in packed beds

An experimental study on mixed convective heat transfer in a packed bed was carried out. By controlling the temperatures of the fluid and solid and mass flow and flow directions of the fluid, the formation and distribution of the temperature field in the packed bed were analyzed. In addition, the fluid flow and heat transfer process in the fluid–solid system under the coupling effect of natural and forced convection were studied. A modified heat transfer correlation (NuF= 0.345/ε·(2 + 1.033Reh0.6Pr1/3)) of forced convection in a packed bed was proposed. It was found that natural convection had a significant influence on the overall mixed convective heat transfer. An aiding flow occurred when the fluid with a higher temperature flowed downward in the packed bed, and an opposing flow occurred when the fluid flowed upward. As Ri increased, the strengthening effect of natural convection became more significant. Based on this result, the heat transfer correlation (NuM2=NuF2+NuN2) for the mixed convective heat transfer in the packed bed was obtained.

Study on the Time-Varying Characteristics of In-Situ State Temperature Field of Drilled Work Piece Based on Unsteady Heat Conduction

In-Situ state heat conduction in drilling workpiece results in time-varying temperature field, which affects the final machining accuracy. From the joint action of many factors, such as geometry, material, and thermal convection characteristics, it is difficult to accurately model and describe the time-varying characteristics of the temperature field. Therefore, the time-varying model of the In-Situ state temperature field is analyzed in this paper, which lays a foundation for establishing a high-precision multi-physical field model. First, following the geometric characteristics of the drilling hole and the three-dimensional heat transfer mechanism, the heat transfer model of the drilled part is established, and the relationship between the temperature field and the physical parameters of the workpiece is characterized. Then, combining radial and axial unsteady heat conduction mechanisms, the dimensionless temperature calculation method is adopted to realize the construction of a temperature time-varying model. After that, the model parameters are revised according to the In-Situ state evolution process of the checkpoints. Finally, the experiments are carried out to calculate the credibility of the model. The results show that the prediction model is consistent with the experimental data, and the credibility of non-room temperature dynamic interval is 99.23%, which can effectively reflect the real temperature field conduction process.

High-resolution outgoing long wave radiation data (2014–2020) of INSAT-3D Imager and its comparison with Clouds and Earth’s Radiant Energy System (CERES) data

As a proxy of convection INSAT-3D satellite-derived product Outgoing Long Wave Radiation (OLR) data is available in both high temporal and spatial ranges over 40°N–40°S & 35°E–135°E. Daily gridded data set of 7 years of data (2014–2020) has been generated at 10 km × 10 km resolution and the same is compared with Clouds and Earth’s Radiant Energy System (CERES) instrument data taken from CERES as reference. Almost all the INSAT-3D data set generated is Global Space-based Inter-Calibration System (GSICS) corrected. The spatiotemporal consistency of the data set was statistically analyzed and found to be reasonably good agreement having a bias of ∼±5–6 W/m2 over above said domain.This inter-comparison is essential to get confidence in the data sets and release it further in the public domain for any scientific study. Again, this data set will be very useful in diagnosing the variations of convection at different scales (daily, weekly, monthly, annual, seasonal, intra-seasonal, etc.) & an important repository of Daily Climate Data Records (DCDR) for future studies. The specified domain of the present study is affected throughout the year with variable (weak, moderate, intense, or severe) spatiotemporal Inter-Tropical Convergence Zone (ITCZ) convection streams due to different types of weather activities (winter, pre-monsoon, monsoon, and post-monsoon) throughout the year. To visualize the importance of this high-resolution OLR data set a case study of Cyclone Amphan and Vayu is presented. The extremely severe intense convection (OLR departure −112 W/m2 with INSAT-3D new data set whereas −104 W/m2 in CERES data) was observed in both the data sets on 18th May-2020 at 13.7–16°N & 86.2–86.8°E during the super cyclonic stage of Amphan. A similar type of variation in the OLR has been noticed for Vayu Cyclone (OLR departure −94 W/m2 with INSAT-3D new data set whereas −86 W/m2 in CERES data). This information is very useful in impact-based forecasting and further future actions for disaster managers/decision-makers. The localized convective features during cyclone activity over the Indian Ocean region both in the Arabian Sea and the Bay of Bengal are well captured with this new data set and the difference in OLR of INSAT -3D and CERES -9 W/m2 and -12 W/m2 respectively.

An accurate and stable numerical method for option hedge parameters

We propose an unconditionally stable numerical algorithm, which uses the Feynman-Kac formula of the Black-Scholes equation to obtain accurate option prices and hedge parameters. We discretize the asset and time using uniform grid points. We approximate the option values by piecewise quadratic polynomials for each time step and integrate them analytically over the log-normal distribution. The piecewise quadratic approximation gives the third-order convergence in the asset direction, and the analytic integration reduces truncation error in the time direction. The estimation errors are propagated backward in time following the convection and diffusion characteristics of the Black-Scholes equation, which assures the unconditional stability of our method. The vectorized code implementation reduces the time complexity. The convergence test shows that our approach outperforms the Crank-Nicolson scheme of the finite difference method in both time and asset directions, and the stability test verifies that our method is stable as the Crank-Nicolson. Furthermore, we show that our algorithm reduces the price errors and hedge parameter errors by more than 50% from the benchmark.

Numerical study on long-term passive heat removal of EPRHR cooling water tank (CWT) using heat pipe heat exchanger

The EPRHR in the HPR1000 adopts natural circulation to meet the heat removal requirements of the containment for 72-hours under accident conditions. To reduce the evaporation depletion of water in CWT and enhances the passive cooling capability of the EPRHR, a long-term passive cooling scheme based on a heat pipe heat exchanger is proposed for CWT in this paper. Experimental study and numerical simulation are used to study the natural convection characteristics in the CWT. The results show that the scheme can take away 9 MW heat load as the average water temperature (Tave) is below 90 °C. The heat removal ability of the tube bundle is weakened row by row and the maximum difference of heat removal ability between tube bundles is 23%. The position of the tube bundles affects the Tave, and when the dimensionless layout factor (γ) = 3/4, Tave (346.34 K) is the smallest.

Thermo-hydraulic characteristics of mixed convection of transition flow in a conduit at different tilts

In this study, numerical investigation was carried out on combined free and forced convection for transition flow of water in a conduit that has a mechanical component, which is a three-sided-polygon-perforations-tranverse-axes tape, inserted in its interior. The purpose of the investigation is to find out the thermo-hydraulic characteristics of mixed convection of transition flow in the tube, and compare them with those of forced convection. Fluent software was used to perform the numerical investigation for the transition flow at Reynolds number (Re) in the range 2,150≤Re≤3,700 for the tube bent at sundry tilts (λ) in the range〖 15〗^o≤λ≤〖 90〗^o. The results indicated that the inside temperature and surface temperature of the tube whose fluid flow is forced convection are lower than the case of the tube whose fluid flow is mixed convection, and that the temperatures (tube’s inside temperature, dimensionless outlet temperature, and surface temperature) increase as the tube tilt increases. Not only that, as the tube tilt increases, the dimensionless outlet temperature increases, but the outlet velocity decreases. The Nusselt number in the tube whose fluid flow is forced convection is lower than the case of mixed convection. For the tube with flow tilts of 15^o,30^o,〖 60〗^o, and 90^o, the Nusselt number are 4.01 to 6.10%, 4.60 to 7.41%, 5.32 to 8.71%, and 6.31 to 10.12%, respectively, greater than the case of the tube whose fluid flow is forced convection, for the Reynolds number considered. The validation of the numerical work with an experimental work confirms the correctness of the numerical work.

Modelling stellar convective transport with plumes – I. Non-equilibrium turbulence effect in double-averaging formulation

ABSTRACT Plumes in a convective flow are considered to be relevant to the turbulent transport in convection. The effective mass, momentum, and heat transports in the convective turbulence are investigated in the framework of time–space double averaging procedure, where a field quantity is decomposed into three parts: the spatiotemporal mean (spatial average of the time-averaged) field, the dispersion or coherent fluctuation, and the random or incoherent fluctuation. With this framework, turbulent correlations in the mean-field equations are divided into the dispersion/coherent and random/incoherent correlation part. By reckoning the plume as the coherent fluctuation, a transport model for the convective turbulence is constructed with the aid of the non-equilibrium effect, in which the change of turbulence characteristics along the mean stream is taken into account for the modelling of the turbulent transport coefficients. In this work, for the first time, change of turbulence properties along plume motions is incorporated into the expression of the turbulent transport coefficients. This non-equilibrium model is applied to a stellar convective flow. One of the prominent characteristics of a surface cooling-driven convection, the enhanced and localized turbulent mass flux below the surface layer, which cannot be reproduced at all by the usual eddy-diffusivity model with mixing length theory (MLT), is well reproduced by the present model. Our results show that the incorporation of plume motion into turbulent transport model is an important and very relevant extension of mean-field theory beyond the heuristic gradient transport model with MLT.

Numerical study on the flow and heat transfer characteristics of natural convection in CFETR cryostat

The cryostat in the China Fusion Engineering Testing Reactor (CFETR) provides vacuum and low temperature operating conditions for the superconducting magnet system. A high pressure in the cryostat can increase the Rayleigh number (Ra) and cause the natural convection within the continuous hypothesis. A three-dimensional model, using incompressible ideal gas model and realizable k-ε model, is built to simulate the natural convection between the superconducting magnet system and the thermal shield within the temperature range of 4.5-80 K. The distributions of velocity field and heat flux on different surfaces of superconducting magnet system are obtained under different Rayleigh number conditions. The results show that the areas with large natural convection velocity mainly locate near the Central Solenoid (CS) coils and at the bottom of vacuum chamber. When the Rayleigh number ranges from 104 to 1010, the maximum velocity of helium gas decreases, while the heat flux on walls of superconducting magnets increases rapidly, and the heat flux on walls of PF-1, CS and TF coils is more affected by pressure, which needs to be considered in designing the safety system.

A preliminary study of 4He convective heat switches

Heat switches are key components in sub-kelvin refrigeration. It is widely used in cryogenic systems, such as controlling thermodynamic cycle, coupling/decoupling redundant cryocoolers, accelerating the cooling of cryogenic components and so on. Convective heat switch features simple structure, no moving parts and great thermal conductance, which is mainly used to precool cryogenic components. In order to accelerate the pre-cooling of a test platform to 4K, the passive convective heat switch is investigated in this paper, which consists of a circuit comprising two identical stainless steel tubes and two copper cavities respectively connected to the cold and warm stages. The ON/OFF state of the switch is realized by the presence/absence of a gravity-driven convective helium-4 gas loop. Several convective heat switches with different cavity structures and charge pressures are designed and assembled. The preliminary experimental results on thermal conductance are presented. With a typical dimension of a cylindrical cavity with a diameter of 28mm and a height of 16.8mm, a stainless steel tube with a diameter of 9.6mm and a length of 72mm, thermal conductance of 40-50mW/K in ON state has been obtained.

Investigation of natural convection characteristics in the molding chamber of a 3-D printer cooled by thermoelectric cooling modules

Refrigeration using thermoelectric effect is a hot spot and frontier issue at present for its advantages of simple structure, no refrigerant, and no moving parts. This paper aims at characterizing the natural convection characteristics of air in the molding chamber of a 3-D printer that is cooled by thermoelectric cooling modules (TECMs). The distributions of the temperature and velocity field in the molding chamber are obtained, and the influences of the number of the TECMs and their arrangement modes on the magnitude and the spatial difference of temperature and velocity are analyzed. It is found that the Case 2 arrangement mode that all the TECMs are located on the top of the chamber is characterized as the optimal mode with the lowest final average air temperature and the smallest temperature non-uniformity coefficient of the molding chamber. Based on the optimal arrangement mode, a computational formula that correlates the final average air temperature of molding chamber with the volume of the molding chamber and the number of the TECMs is proposed. The new correlation can reproduce all of the 104 data points within error band of ±1.0 °C, among which 76.9% of the data can be predicted within error band of ±0.5 °C. The conclusions of this paper can provide a reference for the application of 3-D printing technology and the design of temperature control scheme of low-temperature molding chamber.

Microphysical Characteristics of Extreme-Rainfall Convection over the Pearl River Delta Region, South China from Polarimetric Radar Data during the Pre-summer Rainy Season

Microphysical Characteristics of Extreme-Rainfall Convection over the Pearl River Delta Region, South China from Polarimetric Radar Data during the Pre-summer Rainy Season

GPM-CO observations of Medicane Ianos: Comparative analysis of precipitation structure between development and mature phase

The present work exploits the satellite measurements from the NASA/JAXA Global Precipitation Measurement Core Observatory (GPM-CO) active and passive microwave sensors to analyze the precipitation structure of the most intense Mediterranean Hurricane (Medicane) on record, named Ianos, which swept across the Ionian Sea between 14 and 18 September 2020. Two GPM-CO overpasses, available during Ianos development and tropical-like cyclone (mature) phase, are analyzed in detail. GPM Microwave Imager measurements are used to carry out a comparative analysis of the medicane precipitation structure and microphysics processes between the two phases. The GPM-CO Dual-frequency Precipitation Radar overpass, available for the first time during a medicane mature phase, provides key measurements and products to analyze the 3D precipitation structure in the rainbands, offering further evidence of the main precipitation microphysics processes inferred from the passive microwave measurement analysis. Substantial difference in the rainband precipitation structure is observed, with deeper convection and stronger updraft features during development than at the mature phase, when also shallow precipitation/warm rain processes are observed in the inner region around the medicane eye. These features play a key role to explain the substantial drop in lightning activity during Ianos mature phase. Graupel-ice electrification process is inhibited due to the combined effect of strong horizontal wind and the observed limited growth of graupel. The study demonstrates the value of the GPM-CO not only to characterize Medicane Ianos precipitation structure and microphysics processes with unprecedented detail, but also to provide evidence of its exceptional intensity, tropical-like characteristics during its mature phase, and its similarities with tropical cyclones.

Natural convection characteristics of honeycomb fin with different hole cells for battery phase-change material cooling systems

As a nature production the lightweight honeycomb is used as a fin to increase the contact area with the PCM and improve its overall thermal conductivity. In this paper the effects of different honeycomb core shapes, sizes, and arrangements on melting under natural convection were investigated to determine the optimal honeycomb cell parameters. In this study, transient simulations based on VOF and enthalpy-porosity methods are developed, and the results show that compared with heat conduction, the melting time under natural convection conditions can be reduced. By analyzing different shapes, it is discovered that the melting effects of triangles, trapezoids, rectangles, and rhombuses are better than that of hexagons, the triangular melting time is reduced by 23.1%. The orthogonal experiments show that the ranking order of the effects on melting time and natural convection is Ra > Shape > LHR. Furthermore, the optimal honeycomb hole core parameters are obtained from the analysis of a single factor. Compared with the hexagonal honeycomb fin, a battery's PCM with an optimal triangular honeycomb fin under natural convection exhibits a 23.3% increase in terms of temperature decrease.

A playground for compressible natural convection with a nearly uniform density

In the quest to understand the basic universal features of compressible convection, one would like to disentangle genuine consequences of compression from spatial variations of transport properties. For instance, one may choose to consider a fluid with uniform dynamic viscosity, but, then, compressible effects will generate a density gradient and consequently the kinematic viscosity will not be uniform. In the present work, we consider a very peculiar equation of state, whereby entropy is solely dependent on density, so that a nearly isentropic fluid domain is nearly isochoric. Within this class of equations of state, there is a thermal adiabatic gradient and a key property of compressible convection is still present, namely its capacity to viscously dissipate a large fraction of the thermal energy involved, of the order of the well-named dissipation number. In the anelastic approximation, under the assumption of an infinite Prandtl number, the number of governing parameters can be brought down to two, the Rayleigh number and the dissipation number. This framework is proposed as a playground for compressible convection, an opportunity to extend the vast corpus of theoretical analyses on the Oberbeck–Boussinesq equations regarding stability, bifurcations or the determination of upper bounds for the turbulent heat transfer. Here, in a two-dimensional geometry, we concentrate on the structure of numerical solutions. For all Rayleigh numbers, a change in the vertical temperature profile is observed in the range of dissipation number between $0$ and less than $0.4$ , associated with the weakening of ascending plumes. For larger dissipation numbers, the heat flux dependence on this number is found to be well predicted by Malkus's model of critical layers. For dissipation numbers of order unity, and large Rayleigh numbers, dissipation becomes related to the entropy heat flux at each depth, so that the vertical dissipation profile can be predicted, and so does the total ratio of dissipation to convective heat flux.

The sensitivity of the West African monsoon circulation to intraseasonal soil moisture feedbacks

AbstractIntraseasonal soil moisture variability has the potential to feed back onto the West Africa monsoon circulation through its influence on surface turbulent fluxes and planetary boundary‐layer characteristics. Using satellite observations and an atmospheric reanalysis, we investigate intraseasonal soil moisture–atmosphere feedbacks triggered by large‐scale dynamics within the West African monsoon. Surprisingly, even though the surface response across the Sahel to strong convection is short‐lived (days) and precipitation accumulations are spatially and temporally heterogeneous, a coherent regional‐scale surface response to intraseasonal variability is observed. This surface response then feeds back onto the West African monsoon circulation. For example, during a dry intraseasonal event, Sahelian surface soil moisture significantly decreases, which elevates surface temperatures by 1.5 °C and shifts the monsoon circulation southward by approximately 1.5° latitude. Also, during a wet event the surface moistens and cools which leads to a northward monsoon shift. Alongside a low‐level wind response, the African Easterly Jet (AEJ) also responds to surface changes due to variations in the meridional temperature gradient. For example, an increased temperature gradient during a dry event intensifies and shifts the AEJ southward. The combined response of low‐level monsoon westerlies and the AEJ impacts low‐level shear and characteristics of strong convection. Elevated low‐level shear during a dry event promotes an intensification of deep convection across southern West Africa. This study provides new insight into the sensitivity of the West African monsoon circulation to intraseasonal soil moisture feedbacks and encourages similar research in other regions. An improved understanding and model representation of soil moisture–atmosphere feedbacks has the potential to improve forecasts beyond daily time‐scales and enhance early warning systems.