Convective Momentum Transport Research Articles

The Sensitivity of Momentum Transport and Severe Surface Winds to Environmental Moisture in Idealized Simulations of a Mesoscale Convective System

Analysis of a pair of three-dimensional simulations of mesoscale convective systems (MCSs) reveals a significant sensitivity of convective momentum transport (CMT), MCS motion, and the generation of severe surface winds to ambient moisture. The Weather Research and Forecasting model is used to simulate an idealized MCS, which is compared with an MCS in a drier midlevel environment. The MCS in the drier environment is smaller, moves slightly faster, and exhibits increased descent and more strongly focused areas of enhanced CMT near the surface in the trailing stratiform region relative to that in the control simulation. A marked increase in the occurrence of severe surface winds is observed between the dry midlevel simulation and the control. It is shown that the enhanced downward motion associated with decreased midlevel relative humidity affects CMT fields and contributes to an increase in the number of grid-cell occurrences of severe surface winds. The role of a descending rear-inflow jet in producing strong surface winds at locations trailing the gust front is also analyzed, and is found to be associated with low-level CMT maxima, particularly in the drier midlevel simulation.

Mechanism for northward propagation of boreal summer intraseasonal oscillation: Convective momentum transport

This study demonstrates that the momentum transport by cumulus convection plays a significant role in the organization and northward propagation of intraseasonal (ISO) convection anomalies over the Indian and western Pacific regions during boreal summer. A version of Seoul National University's atmosphere‐ocean coupled general circulation model simulates northward propagation when convective momentum transport (CMT) is implemented; the northward propagation disappears when CMT is disabled. An axially symmetric shallow water model with a parameterized CMT is used to understand the role of CMT in the northward propagation of ISO. The basic mechanism of northward propagation is the lower‐level convergence to the north of convection, which is induced by the secondary meridional circulation associated with large momentum mixing by convection in the region of large mean vertical shear. A large mean vertical shear exists in South Asian region during boreal summer.

Characterization of Momentum Transport Associated with Organized Moist Convection and Gravity Waves

Abstract Tropical convection is inherently multiscalar, involving complex fields of clouds and various regimes of convective organization ranging from small disorganized cumulus up to large organized convective clusters. In addition to being a crucial component of the atmospheric water cycle and the global heat budget, tropical convection induces vertical fluxes of horizontal momentum. There are two main contributions to the momentum transport. The first resides entirely in the troposphere and is due to ascent, descent, and organized circulations associated with precipitating convective systems. The second resides in the troposphere, stratosphere, and farther aloft and is caused by vertically propagating gravity waves. Both the convective momentum transport and the gravity wave momentum flux must be parameterized in general circulation models; yet in existing parameterizations, these two processes are treated independently. This paper examines the relationship between the convective momentum transport and convectively generated gravity wave momentum flux by utilizing idealized simulations of multiscale tropical convection in different wind shear conditions. The simulations produce convective systems with a variety of regimes of convective organization and therefore different convective momentum transport properties and gravity wave spectra. A number of important connections are identified, including a consistency in the sign of the momentum transports in the lower troposphere and stratosphere that is linked to the generation of gravity waves by tilted convective structures. These results elucidate important relationships between the convective momentum transport and the gravity wave momentum flux that will be useful for interlinking their parameterization in the future.

Thermocapillary Migration of Deformable Bubbles at Moderate to Large Marangoni Number in Microgravity

Using the level-set method and the continuum interface model, the axisymmetric thermocapillary migration of gas bubbles in an immiscible bulk liquid with a temperature gradient at moderate to large Marangoni number is simulated numerically. Constant material properties of the two phases are assumed. Steady state of the motion can always be reached. The terminal migration velocity decreases monotonously with the increase of the Marangoni number due to the wrapping of isotherms around the front surface of the bubble. Good agreements with space experimental data and previous theoretical and numerical studies in the literature are evident. Slight deformation of bubble is observed, but no distinct influence on the motion occurs. It is also found that the influence of the convective transport of heat inside bubbles cannot be neglected at finite Marangoni number, while the influence of the convective transport of momentum inside bubbles may be actually negligible.

Superior Convective Heat Transport for Laminar Boundary Layer Flow Over a Flat Plate Using Binary Gas Mixtures With Light Helium and Selected Heavier Gases

Abstract This paper addresses the laminar boundary layer flow of certain binary gas mixtures along a heated flat plate. To form the binary gas mixtures, light helium (He) is the primary gas and the heavier secondary gases are nitrogen (N2), oxygen (O2), xenon (Xe), carbon dioxide (CO2), methane (CH4), tetrafluoromethane (CF4), and sulfur hexafluoride (SF6). The central objective of this paper is to investigate the potential of this group of binary gas mixtures for heat transfer intensification. From fluid physics, two thermophysical properties, i.e., viscosity η and density ρ, influence the fluid flow, whereas four thermophysical properties, i.e., viscosity η, thermal conductivity λ, density ρ, and heat capacity at constant pressure Cp, affect the forced convective heat transfer. The heat transfer augmentation from the flat plate is pursued by stimulating the forced convection mode as a whole. In this regard, it became necessary to construct a specific correlation equation to handle binary gas mixtures owing Prandtl number Pr∊(0.1,1). Whenever there is heat transfer invigoration in forced flow, drag force accretion seems to be inevitable. A standard formula for estimating the drag force Fd exerted on the flat plate is available from the fluid dynamics literature. The descriptive equations for the heat transfer rate Qmix and drag force Fd,mix associated with the seven binary gas mixtures are channeled through the four thermophysical properties, i.e., density ρmix, viscosity ηmix, thermal conductivity λmix, and heat capacity at constant pressure Cp,mix, which depend on the molar gas composition w. Two case studies suffice to elucidate the modified convective heat and momentum transport that the binary gas mixtures bring forward. At a film temperature Tf=300 K and 1 atm, the He+SF6 mixture delivers the absolute maximum for the relative heat transfer Qmix,abs max/B=16.71 at an optimal molar gas composition wopt=0.96. When compared with the light primary He gas with a relative heat transfer rate QHe/B=12.04, the He+SF6 mixture generates a significant heat transfer enhancement of 39%. At a film temperature Tf=600 K and the same 1 atm, the relative heat transfer QHe/B for the light primary gas He comes down to 10.77. In reference to this, the He+SF6 mixture furnishes an absolute maximum heat transfer Qmix,abs max/B=18.11 at an optimal molar gas composition wopt=0.96, yielding a remarkable heat transfer enhancement of 68%. In the global context, the usage of exotic binary gas mixtures with light helium and selected heavier gases may be envisioned for special tasks in industries that demand high heat transfer rates.

Effects of Convective Processes on GCM Simulations of the Madden–Julian Oscillation

Abstract Weak temporal variability in tropical climates such as the Madden–Julian oscillation (MJO) is one of major deficiencies in general circulation models (GCMs). The uncertainties in the representation of convection and cloud processes are responsible for these deficiencies. With the improvement made to the convection scheme, the Iowa State University (ISU) GCM, which is based on a version of the NCAR Community Climate Model, is able to simulate many features of MJO as revealed by observations. In this study, four 10-yr (1979–88) ISU GCM simulations with observed sea surface temperatures are analyzed and compared to examine the effects of the revised convection closure, convection trigger condition, and convective momentum transport (CMT) on the MJO simulations. The modifications made in the convection scheme improve the simulations of amplitude, spatial distribution, eastward propagation, and horizontal and vertical structures, especially for the coherent feature of eastward-propagating convection and the precursor sign of convective center. The revised convection closure plays a key role in the improvement of the eastward propagation of MJO. The convection trigger helps produce less frequent but more vigorous moist convection and enhance the amplitude of the MJO signal. The inclusion of CMT results in a more coherent structure for the MJO deep convective center and its corresponding atmospheric variances.

New multi-scale models on mesoscales and squall lines

Squall lines are coherent turbulent traveling waves on scales of order 100 km in the atmosphere that emerge in a few hours from the interaction of strong vertical shear and moist deep convection on scales of order 10 km. They are canonical coherent structures in the tropics and middle latitudes reflecting upscale conversion of energy from moist buoyant sources to horizontal kinetic energy on larger scales. Here squall lines are introduced through high resolution numerical simulations which reveal a new self-similarity with respect to the shear amplitude. A new multi-scale model on mesoscales which allows for large vertical shears, appropriate for squall lines, is developed here through systematic multi-scale asymptotics. Mathematical and numerical formulations of the new multi-scale equations are utilized to illustrate both new mathematical and physical phenomena captured by these new models. In particular, non-hydrostatic Taylor-Goldstein equations govern the upscale transports of momentum and temperature from the order 10 km microscales to the order 100 km mesoscales; surprisingly, upright single mode convective heating without tilts can lead to significant upscale convective momentum transport from the microscales to the mesoscales due to the strong shear. The multi-scale models developed here should be especially useful for dynamic parameterizations of upscale transports as well as for new theory in three-dimensions with a transverse shear component, where contemporary theoretical understanding is meager.

Molecular-to-Large-Scale Heat Transfer With Multiphase Interfaces: Current Status and New Directions

The scientific understanding of multiphase interfaces and the associated convective mass, momentum, and heat transport across and along their boundaries, provide the fundamental underpinnings of the advancement of boiling heat transfer, two-phase flows, heat pipes, spray cooling, and droplet-film coating, among many other engineering applications. Numerous studies have tried to characterize the interfacial behavior and model their mechanistic influences either directly or implicitly via parametric experimental investigations and/or simulations. The goal of advancing our understanding as well as developing generalized, perhaps “universal,” and more accurate phenomenological or mechanistic correlations, for predicting mass, momentum, and heat transfer, continues to engage the worldwide research community. A collection of some such current investigations that are representative of both basic and applied issues in the field is presented in this special issue of the Journal of Heat Transfer.

Flow Rate Limitation of Steady Convective Dominated Open Capillary Channel Flows Through a Groove

An open capillary channel is a structure that establishes a liquid flow path when the capillary pressure caused by surface tension forces dominates in comparison to the hydrostatic pressure induced by gravitational or residual accelerations. To maintain a steady flow through the channel the capillary pressure of the free surface has to balance the pressure difference between the liquid and the surrounding constant pressure gas phase. Due to convective and viscous momentum transport the pressure along the flow path of the liquid decreases and causes the free surface to bend inwards. The maximum flow rate through the channel is reached when the free surface collapses and gas ingestion occurs near the outlet. This stability limit depends on the geometry of the channel and the properties of the liquid. In this paper we present an experimental setup which is used in the low-gravity environment of the Bremen Drop Tower. Experiments with convective dominated systems have been performed where the flow rate was increased up to the maximum value. In comparison to this we present a one-dimensional theoretical model to determine important characteristics of the flow, such as the free surface shape and the limiting flow rate. Furthermore we present an explanation for the mechanism of flow rate limitation for these flow conditions which is similar to the choking problem for compressible gas flows.

Evidence of Inward Toroidal Momentum Convection in the JET Tokamak

Experiments have been carried out on the Joint European Torus tokamak to determine the diffusive and convective momentum transport. Torque, injected by neutral beams, was modulated to create a periodic perturbation in the toroidal rotation velocity. Novel transport analysis shows the magnitude and profile shape of the momentum diffusivity are similar to those of the ion heat diffusivity. A significant inward momentum pinch, up to 20 m/s, has been found. Both results are consistent with gyrokinetic simulations. This evidence is complemented in plasmas with internal transport barriers.

A Simple Dynamical Model with Features of Convective Momentum Transport

Abstract Convective momentum transport (CMT) plays a central role in interactions across multiple space and time scales. However, because of the multiscale nature of CMT, quantifying and parameterizing its effects is often a challenge. Here a simple dynamic model with features of CMT is systematically derived and studied. The model includes interactions between a large-scale zonal mean flow and convectively coupled gravity waves, and convection is parameterized using a multicloud model. The moist convective wave–mean flow interactions shown here have several interesting features that distinguish them from other classical wave–mean flow settings. First an intraseasonal oscillation of the mean flow and convectively coupled waves (CCWs) is described. The mean flow oscillates due to both upscale and downscale CMT, and the CCWs weaken, change their propagation direction, and strengthen as the mean flow oscillates. The basic mechanisms of this oscillation are corroborated by linear stability theory with different mean flow background states. Another case is set up to imitate the westerly wind burst phase of the Madden–Julian oscillation (MJO) in the simplified dynamic model. In this case, CMT first accelerates the zonal jet with the strongest westerly wind aloft, and then there is deceleration of the winds due to CMT; this occurs on an intraseasonal time scale and is in qualitative agreement with actual observations of the MJO. Also, in this case, a multiscale envelope of convection propagates westward with smaller-scale convection propagating eastward within the envelope. The simplified dynamic model is able to produce this variety of behavior even though it has only a single horizontal direction and no Coriolis effect.

Stochastic models for convective momentum transport.

The improved parameterization of unresolved features of tropical convection is a central challenge in current computer models for long-range ensemble forecasting of weather and short-term climate change. Observations, theory, and detailed smaller-scale numerical simulations suggest that convective momentum transport (CMT) from the unresolved scales to the resolved scales is one of the major deficiencies in contemporary computer models. Here, a combination of mathematical and physical reasoning is utilized to build simple stochastic models that capture the significant intermittent upscale transports of CMT on the large scales due to organized unresolved convection from squall lines. Properties of the stochastic model for CMT are developed below in a test column model environment for the large-scale variables. The effects of CMT from the stochastic model on a large-scale convectively coupled wave in an idealized setting are presented below as a nontrivial test problem. Here, the upscale transports from stochastic effects are significant and even generate a large-scale mean flow which can interact with the convectively coupled wave.

The Impact of Convection on ENSO: From a Delayed Oscillator to a Series of Events

Abstract The NCAR Community Climate System Model, version 3 (CCSM3) exhibits persistent errors in its simulation of the El Niño–Southern Oscillation (ENSO) mode of coupled variability. The amplitude of the oscillation is too strong, the dominant 2-yr period too regular, and the width of the sea surface temperature response in the Pacific too narrow, with positive anomalies extending too far into the western Pacific. Two changes in the parameterization of deep convection result in a significant improvement to many aspects of the ENSO simulation. The inclusion of convective momentum transport (CMT) and a dilution approximation for the calculation of convective available potential energy (CAPE) are used in development integrations, and a striking improvement in ENSO characteristics is seen. An increase in the periodicity of ENSO is achieved by a reduction in the strength of the existing “short-circuited” delayed-oscillator mode. The off-equatorial response is weaker and less tropically confined, largely as a result of the CMT and an associated redistribution of zonal momentum. The Pacific east–west structure is improved in response to the presence of convective dilution and cooling provided by increased surface fluxes. The initiation of El Niño events is fundamentally different. Enhanced intraseasonal surface stress variability leads to absolute surface westerlies and a cooling–warming dipole between the Philippine Sea and western Pacific. Lag-regression analysis shows that intraseasonal variability may play a significant role in event initiation and maintenance as opposed to being a benign response to increased SSTs. Recent observational evidence appears to support such a leading relationship.

Understanding the Effects of Convective Momentum Transport on Climate Simulations: The Role of Convective Heating

Abstract A simplified general circulation model (GCM), consisting of a complete dynamical core, simple specified physics, and convective momentum transport (CMT) forcing, is used to understand the effects of CMT on climate simulations with a focus on the role of convective heating in the response of circulation to the CMT forcing. It is found that the convective heating dominates the meridional circulation response and dynamical processes dominate the zonal wind response to the CMT forcing in the tropics; the simplified model reproduces some of the key features of CMT-induced circulation changes observed in the full GCM in the tropics. These results suggest that the CMT-induced zonal and meridional circulation changes in the tropics in the full GCM are dominated by dynamical processes and the convective heating, respectively. Inclusion of the CMT in the model induces a marked change in convective heating, which negatively correlates with the change in vertical velocity, indicating the existence of CMT-induced convective heating–circulation feedback. The sensitivity experiment with the removal of mean convective heating feedback demonstrates that the convective heating affects the response of the meridional circulation to the CMT forcing through the CMT-induced convective heating–circulation feedback.

Simulation of state-dependent high-frequency atmospheric variability associated with ENSO

High-frequency atmospheric variability depends on the phase of El Nino/Southern Oscillation (ENSO). Recently, there is increasing evidence that state-dependent high-frequency atmospheric variability significantly modulates ENSO characteristics. Hence, in this study, we examine the model simulations of high-frequency atmospheric variability and, further, its dependency on the El Nino phase, using atmospheric and coupled GCMs (AGCM and CGCM). We use two versions of physical packages here—with and without convective momentum transport (CMT)—in both models. We found that the CMT simulation gives rise to a large climatological zonal wind difference over the Pacific. Also, both the climate models show a significantly improved performance in simulating the state-dependent noise when the CMT parameterization is implemented. We demonstrate that the better simulation of the state-dependent noise results from a better representation of anomalous, as well as climatological, zonal wind. Our further comparisons between the simulations, demonstrates that low-frequency wind is a crucial factor in determining the state-dependency of high-frequency wind variability. Therefore, it is suggested that the so-called state-dependent noise is directly induced by the low-frequency wind anomaly, which is caused by SST associated with ENSO.

Self-similar structure of magnetized advection-dominated accretion flows and convection-dominated accretion flows

(Abridged) We study the effects of a global magnetic field on viscously-rotating and vertically-integrated accretion disks around compact objects using a self-similar treatment. We extend Akizuki & Fukue's work (2006) by discussing a general magnetic field with three components ($r, \phi, z$) in advection-dominated accretion flows (ADAFs). We also investigate the effects of a global magnetic field on flows with convection. For these purposes, we first adopt a simple form of the kinematic viscosity $\nu=\alpha c_{s}^{2}/\Omega_{K}$ to study magnetized ADAFs. Then we consider a more realistic model of the kinematic viscosity $\nu=\alpha c_{s}H$, which makes the infall velocity increase but the sound speed and toroidal velocity decrease. We next use two methods to study magnetized flows with convection, i.e., we take the convective coefficient $\alpha_{c}$ as a free parameter to discuss the effects of convection for simplicity. We establish the $\alpha_{c}-\alpha$ relation for magnetized flows using the mixing-length theory and compare this relation with the non-magnetized case. If $\alpha_{c}$ is set as a free parameter, then $|v_{r}|$ and $c_{s}$ increase for a large toroidal magnetic field, while $|v_{r}|$ decreases but $|v_{\phi}|$ increases (or decreases) for a strong and dominated radial (or vertical) magnetic field with increasing $\alpha_{c}$. In addition, the magnetic field makes the $\alpha_{c}-\alpha$ relation be distinct from that of non-magnetized flows, and allows the $\rho\propto r^{-1}$ or $\rho\propto r^{-2}$ structure for magnetized non-accreting convection-dominated accretion flows with $\alpha+g\alpha_{c}< 0$ (where $g$ is the parameter to determine the condition of convective angular momentum transport).

Effects of Convective Momentum Transport on the Atmospheric Circulation in the Community Atmosphere Model, Version 3

Abstract Transport of momentum by convection is an important process affecting global circulation. Owing to the lack of global observations, the quantification of the impact of this process on the tropospheric climate is difficult. Here an implementation of two convective momentum transport parameterizations, presented by Schneider and Lindzen and Gregory et al., in the Community Atmosphere Model, version 3 (CAM3) is presented, and their effect on global climate is examined in detail. An analysis of the tropospheric zonal momentum budget reveals that convective momentum transport affects tropospheric climate mainly through changes to the Coriolis torque. These changes result in improvement of the representation of the Hadley circulation: in December–February, the upward branch of the circulation is weakened in the Northern Hemisphere and strengthened in the Southern Hemisphere, and the lower northerly branch is weakened. In June–August, similar improvements are noted. The inclusion of convective momentum transport in CAM3 reduces many of the model’s biases in the representation of surface winds, as well as in the representation of tropical convection. In an annual mean, the tropical easterly bias, subtropical westerly bias, and the bias in the 60°S jet are improved. Representation of convection is improved along the equatorial belt with decreased precipitation in the Indian Ocean and increased precipitation in the western Pacific. The improvements of the representation of tropospheric climate are greater with the implementation of the Schneider and Lindzen parameterization.

What Are the Sources of Mechanical Damping in Matsuno–Gill-Type Models?

Abstract The Matsuno–Gill model has been widely used to study the tropical large-scale circulations and atmosphere–ocean interactions. However, a common critique of this model is that it requires a strong equivalent linear mechanical damping to get realistic wind response and it is unclear what could provide such a strong damping above the boundary layer. This study evaluates the sources and strength of equivalent linear mechanical damping in the Walker circulation by calculating the zonal momentum budget using 15 yr (1979–93) of daily global reanalysis data. Two different reanalyses [NCEP–NCAR and 15-yr ECMWF Re-Analysis (ERA-15)] give qualitatively similar results for all major terms, including the budget residual, whose structure is consistent with its interpretation as eddy momentum flux convergence by convective momentum transport (CMT). The Walker circulation is characterized by two distinct regions: a deep convection region over the Indo-Pacific warm pool and a shallow convection region over the eastern Pacific cold tongue. These two regions are separated by a strong upper-tropospheric ridge and a strong lower-tropospheric trough in the central Pacific. The resultant pressure gradient forces on both sides require strong (approximately 5–10 days) damping to balance them because Coriolis force near the equator is too small to provide the balance. In the deep convection region, the damping is provided by CMT and advection together in both the upper and lower troposphere. In the shallow convection region, on the other hand, the damping is provided mainly by advection in the upper troposphere and by CMT in the lower troposphere. In other words, the upper-level tropical easterly jet and the low-level trade wind are both braked by CMT. These results support the use of strong damping in the Matsuno–Gill-type models but suggest that the damping rate is spatially inhomogeneous and the CMT-related damping increases with the strength of convection. Implications for GCM’s simulation of tropical mean climate are discussed.

Tropical Pacific impacts of convective momentum transport in the SNU coupled GCM

Impacts of convective momentum transport (CMT) on tropical Pacific climate are examined, using an atmospheric (AGCM) and coupled GCM (CGCM) from Seoul National University. The CMT scheme affects the surface mainly via a convection-compensating atmospheric subsidence which conveys momentum downward through most of the troposphere. AGCM simulations—with SSTs prescribed from climatological and El Nino Southern Oscillation (ENSO) conditions—show substantial changes in circulation when CMT is added, such as an eastward shift of the climatological trade winds and west Pacific convection. The CMT also alters the ENSO wind anomalies by shifting them eastward and widening them meridionally, despite only subtle changes in the precipitation anomaly patterns. During ENSO, CMT affects the low-level winds mainly via the anomalous convection acting on the climatological westerly wind shear over the central Pacific—so that an eastward shift of convection transfers more westerly momentum toward the surface than would occur without CMT. By altering the low-level circulation, the CMT further alters the precipitation, which in turn feeds back on the CMT. In the CGCM, CMT affects the simulated climatology by shifting the mean convection and trade winds eastward and warming the equatorial SST; the ENSO period and amplitude also increase. In contrast to the AGCM simulations, CMT substantially alters the El Nino precipitation anomaly patterns in the CGCM. Also discussed are possible impacts of the CMT-induced changes in climatology on the simulated ENSO.

Large Eddy Simulation of Turbulent Reacting Shear Layers Including Finite-Rate Chemistry and Detailed Diffusion Processes

Large eddy simulations (LES) of turbulent temporal shear layers with hydrogen chemistry are performed. In these simulations, approximate deconvolution is applied as an implicit subgrid-scale modeling approach to a reacting flow in combination with a steady flamelet model for the filtered heat release term. No additional heuristical or physical subgrid models are used. The formulation of the flamelet equations in physical space does not only allow to consider a detailed reaction scheme and the extinguished phase but also to take into account detailed diffusion mechanisms (Soret and Dufour effects, multicomponent diffusion coefficients). Two different levels of diffusion approximations are investigated in this work, the aim of which is twofold: Firstly, to verify approximate deconvolution as a tool for convective transport of mass, momentum and energy in gas flow, by comparing the LES results with those of a direct numerical simulation and secondly, to investigate the influence of detailed diffusion on the laminar flamelets and the LES results.