Vertical Transport Of Horizontal Momentum Research Articles

Insights into convective momentum transport and its parametrization from idealized simulations of organized convection

Deep convection is a multiscale process that influences the budgets of heat, moisture and momentum significantly. In global climate models, the thermodynamic effects of convection are normally treated by parametrization schemes, with a separate formulation for convective momentum transport (CMT). The schemes for current thermodynamic and momentum parametrizations are based on upright entraining plume models that do not account for vertically tilted mesoscale circulations, which characterize organized convection in sheared environments. The associated countergradient vertical transport of horizontal momentum fundamentally affects dynamical interactions between the convection and the mean flow. This study examines the CMT properties of simulated idealized mesoscale convective systems, including their sensitivity to horizontal resolution, domain size and lateral boundary conditions. It is found that, even for large domains, the horizontal gradient terms are important, especially the mesoscale pressure gradients, which are neglected in CMT parametrizations. A nonlinear analytic model provides a dynamical foundation for the effects of convective organization, including the role of the horizontal pressure gradient. It is found that a small computational domain affects the convective organization adversely by generating artificially large compensating subsidence and an unrealistic evolution of CMT. Finally, analyses of the cross‐updraught/downdraught pressure gradients expose significant uncertainties in their representation in contemporary CMT parametrization schemes.

The Impact of Dimensionality on Barotropic Processes during KWAJEX

Abstract In this study, the 2D and 3D cloud-resolving model simulations of the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX) are compared to study the impact of dimensionality on barotropic processes during tropical convective development. Barotropic conversion of perturbation kinetic energy is associated with vertical transport of horizontal momentum under vertical shear of background horizontal winds. The similarities in both 2D and 3D model simulations show that 1) vertical wind shear is a necessary condition for barotropic conversion, but it does not control the barotropic conversion; 2) the evolution of barotropic conversion is related to that of the vertical transport of horizontal momentum; and 3) the tendency of vertical transport of horizontal momentum is mainly determined by the covariance between horizontal wind and the cloud hydrometeor component of buoyancy. The differences between the 2D and 3D model simulations reveal that 1) the barotropic conversion has shorter time scales and a larger contribution in the 2D model simulation than in the 3D model simulation and 2) kinetic energy is generally converted from the mean circulations to perturbation circulations in the 3D model simulation. In contrast, more kinetic energy is transferred from perturbation circulations to the mean circulations in the 2D model simulation. The same large-scale vertical velocity may account for the similarities, whereas the inclusion of meridional winds in the 3D model simulation may be responsible for the differences in barotropic conversion between the 2D and 3D model simulations.

Numerical Archetypal Parameterization for Mesoscale Convective Systems

Abstract Vertical shear commonly organizes atmospheric convection into coherent multiscale structures. The associated countergradient vertical transport of horizontal momentum by organized convection can enhance the wind shear and transport kinetic energy upscale. However, organized convection and its upscale effects are not represented by traditional mass-flux-based parameterizations. The present paper sets the archetypal dynamical models, originally formulated by the second author, into a parameterization context by utilizing a nonhydrostatic anelastic model with segmentally constant approximation (NAM–SCA). Using a two-dimensional framework as a starting point, NAM–SCA spontaneously generates propagating tropical squall lines in a sheared environment. High numerical efficiency is achieved through a novel compression methodology. The numerically generated archetypes produce vertical profiles of convective momentum transport that are consistent with the analytic archetype.

The role of urban boundary layer investigated with high-resolution models and ground-based observations in Rome area: a step towards understanding parameterization potentialities

Abstract. The urban forcing on thermodynamical conditions can greatly influence the local evolution of the atmospheric boundary layer. Heat stored in an urban environment can produce noteworthy mesoscale perturbations of the lower atmosphere. The new generation of high-resolution numerical weather prediction models (NWP) is nowadays often applied also to urban areas. An accurate representation of cities is key role because of the cities' influence on wind, temperature and water vapor content of the planetary boundary layer (PBL). The Advanced Weather Research and Forecasting model WRF (ARW) has been used to reproduce the circulation in the urban area of Rome. A sensitivity study is performed using different PBL and surface schemes. The significant role of the surface forcing in the PBL evolution has been investigated by comparing model results with observations coming from many instruments (lidar, sodar, sonic anemometer and surface stations). The impact of different urban canopy models (UCMs) on the forecast has also been investigated. One meteorological event will be presented, chosen as statistically relevant for the area of interest. The WRF-ARW model shows a tendency to overestimate the vertical transport of horizontal momentum from upper levels to low atmosphere if strong large-scale forcing occurs. This overestimation is partially corrected by a local PBL scheme coupled with an advanced UCM. Moreover, a general underestimation of vertical motions has been verified.

Convective Momentum Transport in a Simple Multicloud Model for Organized Convection

Abstract Convective momentum transport (CMT) is the process of vertical transport of horizontal momentum by convection onto the environmental flow. The significance of CMT from mesoscale to synoptic- and planetary-scale organized cumulus convection has been established by various theoretical and observational studies. A new strategy mimicking the effect of unresolved mesoscale circulation based on the weak temperature gradient (WTG) approximation with a Gaussian profile to redistribute the heating due to parameterized cumulus convection at the subgrid scale is adopted here to construct a CMT parameterization for general circulation models (GCMs). Two main regimes of CMT are considered: an upscale squall-line regime and a downscale non-squall-line regime. An exponential probability distribution is used to select which of these two effects is active, conditional on the state of the large-scale shear. The shear itself is used as a measure of the persistence of mesoscale organized circulation due to the presence or not of tilted deep convective heating with lagged stratiform anvils. The CMT model is tested in the simple case of the multicloud model of Khouider and Majda, used here as a toy GCM. Numerical simulations are performed here for the simple case without rotation, in a parameter regime where the multicloud model exhibits packets of convectively coupled gravity waves moving in one direction, at 17 m s−1, and planetary-scale wave envelopes moving in the opposite direction, at 4–6 m s−1, reminiscent of the Madden–Julian oscillation (MJO) and the associated embedded synoptic-scale superclusters. The results herein show that the inclusion of CMT intensifies both the synoptic-scale convectively coupled waves and the manifestation of planetary-scale waves in the multicloud model. This provides evidence that the present CMT model captures the essence of the physical mechanism through which kinetic energy is transferred from the subgrid-scale mesoscale circulation to the large-scale/resolved motion. Sensitivity simulations showed that two key parameters for the CMT parameterization are the relative strength of the parameterized stratiform anvils and the dimensional threshold used in the exponential distribution for the cumulus friction and the upscale CMT forcing resulting from organized subgrid mesoscale circulation.

Ocean Winds and Turbulent Air-Sea Fluxes Inferred From Remote Sensing

Air-sea turbulent fluxes determine the exchange of momentum, heat, freshwater, and gas between the atmosphere and ocean. These exchange processes are critical to a broad range of research questions spanning length scales from meters to thousands of kilometers and time scales from hours to decades. Examples are discussed (section 2). The estimation of surface turbulent fluxes from satellite is challenging and fraught with considerable errors (section 3); however, recent developments in retrievals (section 3) will greatly reduce these errors. Goals for the future observing system are summarized in section 4. Surface fluxes are defined as the rate per unit area at which something (e.g., momentum, energy, moisture, or CO Z ) is transferred across the air/sea interface. Wind- and buoyancy-driven surface fluxes are called surface turbulent fluxes because the mixing and transport are due to turbulence. Examples of nonturbulent processes are radiative fluxes (e.g., solar radiation) and precipitation (Schmitt et al., 2010). Turbulent fluxes are strongly dependent on wind speed; therefore, observations of wind speed are critical for the calculation of all turbulent surface fluxes. Wind stress, the vertical transport of horizontal momentum, also depends on wind direction. Stress is very important for many ocean processes, including upper ocean currents (Dohan and Maximenko, 2010) and deep ocean currents (Lee et al., 2010). On short time scales, this horizontal transport is usually small compared to surface fluxes. For long-term processes, transport can be very important but again is usually small compared to surface fluxes.

Double-diffusive interleaving on horizontal gradients

Results are presented from laboratory experiments on lateral intrusive flows driven by horizontal gradients of properties in double-diffusive systems. These are variations on the classical experiments in which salt stratified fluid was separated from a sugar stratified fluid by a removable barrier. With the removal of this barrier, lateral intrusions of sugar solution into the salt solution alternated in the vertical with salt intrusions into the sugar solution. In our present study, the fluid has continuous horizontal gradients of sugar, density-compensated by opposing horizontal gradients of salt, rather than the discontinuity of properties in the classical experiments.Observations of these new experiments include: shadowgraphs, PTV (particle tracking velocimetry) showing the structure of the flow, and PIV (particle-image velocimetry) from which velocity vector fields on a vertical plane were obtained; from these, mean flows, Reynolds stresses, and associated momentum fluxes were computed.These experiments have been conducted for three regimes of vertical stratification: (i) Salt-finger favourable (‘hot and salty’ above); (ii) Diffusive convection favourable (‘hot and salty’ below); (iii) Doubly stable (‘cold and salty’ below).One of our main conclusions concerns the driving mechanism for the horizontal intrusive flow. Away from the front or ‘nose’ of the intrusions, the lateral flows are no longer tilted but are horizontal. Here, finger layers alternate in the vertical with convecting layers. In these convecting layers, vertical transport of horizontal momentum by the Reynolds stress plays a major role in maintaining the lateral motion against viscous dissipation.

Convective Momentum Transport and Perturbation Pressure Field from a Cloud-Resolving Model Simulation

This study uses a 2D cloud-resolving model to investigate the vertical transport of horizontal momentum and to understand the role of a convection-generated perturbation pressure field in the momentum transport by convective systems during part of the Tropical Ocean and Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) Intensive Observation Period. It shows that convective updrafts transport a significant amount of momentum vertically. This transport is downgradient in the easterly wind regime, but upgradient during a westerly wind burst. The differences in convective momentum transport between easterly and westerly wind regimes are examined. The perturbation pressure gradient accounts for an important part of the apparent momentum source. In general it is opposite in sign to the product of cloud mass flux and the vertical wind shear, with smaller magnitude. Examination of the dynamic forcing to the pressure field demonstrates that the linear forcing representing the interaction between the convective updrafts and the large-scale wind shear is the dominant term, while the nonlinear forcing is of secondary importance. Thus, parameterization schemes taking into account the linear interaction between the convective updrafts and the large-scale wind shear can capture the essential features of the perturbation pressure field. The parameterization scheme for momentum transport by Zhang and Cho is evaluated using the model simulation data. The parameterized pressure gradient force using the scheme is in excellent agreement with the simulated one. The parameterized apparent momentum source is also in good agreement with the model simulation. Other parameterization methods for the pressure gradient are also discussed.

Gravity Wave–driven Flows in the Solar Tachocline. II. Stationary Flows

The effects of gravity waves on the mean radial differential rotation profile in the solar tachocline are studied, including the effect of a uniform, toroidal magnetic field. Vertical transport of horizontal momentum arises from the radiative damping of inwardly traveling waves that are generated by low-frequency, convective fluid motions. By considering two-wave and one-wave interactions, the radiatively damped gravity waves are shown to accentuate the shear in the mean radial differential rotation. In the presence of a strong horizontal magnetic field, internal gravity waves become nearly Alfvenic and cannot propagate downward through the tachocline. For a magnetic field that is weak enough to permit wave propagation, the mean shear profile is shown to be smoother than that obtained in the case of purely hydrodynamic waves. The implications of our results for gravity-wave forcing of the internal solar rotation are discussed.

Influence of upper‐tropospheric inertial stability on the convective transport of momentum

AbstractThis study verifies the apparent high positive correlation between environmental and convection‐relative inertial stability, and the transport of momentum by deep convection. We evaluate the hypothesis that convection structurally aligns itself to best access the region of lowest upper‐tropospheric inertial stability, while forming the lowest convective updraught‐relative inertial stability.A numerical experiment evaluates variations in the structure, behaviour and momentum‐transport characteristics of deep convection across the tropical‐plume genesis region in the eastern Pacific. This unique region forms as a precursor to tropical‐plume genesis, and possesses spatially varying very‐low upper‐tropospheric inertial stability (the potential vorticity locally reaches zero, or becomes slightly negative, in the northern hemisphere).Our results show that the vertical transport of horizontal momentum is predictable, given knowledge of the inertial‐stability fields immediately surrounding the convection. It is conjectured that the preferred mode of convective organization (squall‐line dominated versus more three‐dimensional) near the intertropical convergence zone is a function of the ambient inertial stability. Therefore, as the convection organizes itself, it selects the most efficient access to regions of lowest inertial stability to enable its upper‐tropospheric outflow. Copyright © 2003 Royal Meteorological Society

Nonlinear Vertical Scale Selection in Equatorial Inertial Instability

Abstract The zonal flows of the earth's equatorial stratosphere and mesosphere are prone to inertial instability when the horizontal shear Λ ≠ 0 at the equator. However, it is not clear why the vertical wavelength of the observed structures is much greater than that predicted by the simple linear theory, based on a vertical scale selection by molecular diffusion. Here, a nonlinear mechanism is described that can lead to upscaling from structures with short vertical wavelengths to long vertical wavelengths. The mechanism is dependent upon a secondary Kelvin–Helmholtz instability that develops as the inertial instability grows. Numerical simulations on an equatorial β plane, employing a simple parameterization of the vertical transport of horizontal momentum due to the secondary instability, are used to assess the likely degree of upscaling in a zonally symmetric system. For a fluid with buoyancy frequency N, it is shown that upscaling toward the buoyancy cutoff wavenumber 4Nβ/Λ2 can occur, even when diffus...

Impact of ice microphysics on multiscale organization of tropical convection in two-dimensional cloud-resolving simulations

This paper documents the impact of ice microphysics on the multiscale organization of tropical convection as simulated in two-dimensional cloud-resolving model simulations. Results from simulations which apply warm-rain microphysics are compared with results from a simulation which includes ice processes presented previously in this journal. In these simulations, moist convection and large-scale dynamics interact on an unprecedented range of spatial and temporal scales, albeit within a limited two-dimensional dynamical framework. In general, the large-scale organization of convection is similar in all simulations, although the scale and propagation speed of convectively-coup led gravity waves are different between the warm-rain and ice simulations. The most striking differences are on the mesoscale. The warm-rain simulations feature mesoscale convective systems with a reduced stratiform component and shorter life cycle than mesoscale systems in the simulation with ice microphysics. These impacts are consistent with previous modelling studies of the impact of ice microphysics on organized convection. It is hypothesized that the shorter life cycle of mesoscale convective systems in warm-rain simulations influences scale selection of the large-scale convectively-coup led gravity waves. Moreover, the vertical transport of horizontal momentum is affected as well. To the author's knowledge this is the first time the impact of cloud microphysics on mesoscale convective systems is shown to affect the coupling between deep convection and large-scale perturbations in the tropics. Copyright © 2003 Royal Meteorological Society

A Tropical Squall Line Observed during TOGA COARE: Extended Comparisons between Simulations and Doppler Radar Data and the Role of Midlevel Wind Shear

Abstract Results from a three-dimensional cloud model are extensively compared with airborne Doppler radar data in the case of a tropical oceanic squall line observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The comparison is based on the precipitation patterns, the dynamical and thermodynamical distributions, and the vertical transport of horizontal momentum. The model simulates the evolution of the mesoscale convective system (MCS) frontal convective line from a quasi-linear to a broken pattern. The area located south of the “break,” which designates the region where the MCS leading edge reorientates from the N–S to the E–W direction, is composed of a pronounced bow-shaped structure with two vortices located on both sides of a strong rear inflow. The vertical circulation is characterized by a jump updraft and an overturning downdraft. Both structures exhibit a vertical, intense updraft in the break zone, whereas the jump updraft is more sloped and less in...

An Observational Study of Wind Profiles in the Baroclinic Convective Mixed Layer

A comprehensive planetary boundary-layer (PBL) and synoptic data set is used to isolate the mechanisms that determine the vertical shear of the horizontal wind in the convective mixed layer. To do this, we compare a fair-weather convective PBL with no vertical shear through the mixed layer (10 March 1992), with a day with substantial vertical shear in the north-south wind component (27 February). The approach involves evaluating the terms of the budget equations for the two components of the vertical shear of the horizontal wind; namely: the time-rate-of-change or time-tendency term, differential advection, the Coriolis terms (a thermal wind term and a shear term), and the second derivative of the vertical transport of horizontal momentum with respect to height (turbulent-transport term). The data, gathered during the 1992 STorm-scale Operational and Research Meteorology (STORM) Fronts Experiments Systems Test (FEST) field experiment, are from gust-probe aircraft horizontal legs and soundings, 915-MHz wind profilers, a 5-cm Doppler radar, radiosondes, and surface Portable Automated Mesonet (PAM) stations in a roughly 50 × 50 km boundary-layer array in north-eastern Kansas, nested in a mesoscale-to-synoptic array of radiosondes and surface data.

Impact of Mesoscale Momentum Transport on Large-Scale Tropical Dynamics: Linear Analysis of the Shallow-Water Analog

Abstract The vertical transport of horizontal momentum by organized convection is a prominent process, yet its impact on the large-scale atmospheric circulation has not even been qualitatively assessed. In order to examine this problem in a simple framework the authors incorporate a nonlinear dynamical model of convective momentum flux into a linear model of the large-scale tropical atmosphere. This model has previously been used to investigate the WISHE (wind-induced surface-heat exchange) instability. In order to implement the dynamically determined fluxes as a parameterization, a closure assumption is required to relate the relevant mesoscale parameters to the large-scale variables. The most straightforward method is to relate the low-level large-scale pressure (pL) to the mesoscale pressure perturbation (pM), which is linked to the mesoscale momentum flux by the dynamical model. The mesoscale momentum transport under this closure reduces the effective pressure gradient in the large-scale momentum equa...

Parametrization of momentum transport by convection. II: Tests in single‐column and general circulation models

AbstractDiagnostics derived from cloud‐resolving‐model simulations in part I of this study, relating to the vertical transport of horizontal momentum by convection, are used to develop a parametrization of convective momentum‐transports for deep convection based upon the mass‐flux convection‐scheme discussed by Gregory and Rowntree. the importance of cloud pressure‐gradients to the flow within the cloud is emphasised, and a simple method of representing their effect is suggested. the scheme is able to reproduce the fluxes derived from the cloud‐resolving model studies where cloud organization by the flow is unimportant. Inclusion of the parametrization in a version of the Meteorological Office Unified Model demonstrates that convective momentum‐transports play a large role in the momentum balance of the atmosphere. Generally, simulation of the mean atmospheric circulation by the Unified Model is improved by the inclusion of such transports.

Parametrization of momentum transport by convection. II: Tests in single-column and general circulation models

Diagnostics derived from cloud-resolving-model simulations in part I of this study, relating to the vertical transport of horizontal momentum by convection, are used to develop a parametrization of convective momentum-transports for deep convection based upon the mass-flux convection-scheme discussed by Gregory and Rowntree. the importance of cloud pressure-gradients to the flow within the cloud is emphasised, and a simple method of representing their effect is suggested. the scheme is able to reproduce the fluxes derived from the cloud-resolving model studies where cloud organization by the flow is unimportant. Inclusion of the parametrization in a version of the Meteorological Office Unified Model demonstrates that convective momentum-transports play a large role in the momentum balance of the atmosphere. Generally, simulation of the mean atmospheric circulation by the Unified Model is improved by the inclusion of such transports.

Dynamic Excitation of Internal Gravity Waves in the Equatorial Oceans

Abstract It is proposed that shear instability of the upper flank of the equatorial undercurrent may generate, under a broad range of conditions, downward propagating internal gravity waves (IGW) of large amplitude. The generation mechanism is shown to require only that the background stratification is weak where the shear is large (i.e., in the mixing region) and that the stratification is sufficiently large in the far field (i.e., near the thermocline). In a series of studies, the generation of IGW from unstable shear flows is examined. Linear theory is used to predict under what circumstances the generation of IGW may be large, and fully nonlinear simulations restricted to two dimensions are employed to provide estimates of the degree of vertical mixing and of the vertical transport of horizontal momentum by IGW. In particular, the simulations demonstrate that, when large amplitude IGW are generated by shear instability, the mean flow itself is significantly decelerated in the mixing region. The moment...

Internal Gravity Wave Emission into the Middle Atmosphere from a Model Tropospheric Jet

A mechanism is investigated whereby large amplitude internal gravity waves (IGWs) may be excited by the tropospheric jet stream when this is driven to parallel shear instability following a rapid external forcing of the mean flow a circumstance that might be realized, for example, through ageostrophic effects in the process of baroclinic wave development. A series of mean states are examined, first on the basis of linear theory, to determine the characteristics of the most unstable normal mode, which is expected to dominate the initial stages of flow evolution. Two-dimensional nonlinear numerical simulations of stratified, incompressible, Boussinesq flow in a periodic channel are also performed. On the basis of linear theory, the authors show that it is possible to assess whether IGWs will be strongly excited by examining whether the initial instability satisfies an easily calculable criterion that has been previously termed the “penetration condition.” In cases in which the penetration condition is satisfied, large amplitude IGWs with the same horizontal phase speed and wavelength as the most unstable mode of linear theory are shown in the nonlinear simulations to radiate freely into the stratosphere, and the process of stabilization of the mean state is thereafter significantly influenced by the extraction and upward vertical transport of horizontal momentum by IGWs away from the mixing region. In cases in which the penetration condition is not satisfied only very weak internal wave emission is observed. These small amplitude waves evidently develop through nonlinear mechanisms. On the basis of model calculations performed using initial conditions intended to simulate realizable midlatitude zonal flows, the authors demonstrate that the vertical flux of horizontal momentum delivered into the stratosphere by such sheer instability could reach levels comparable to the fluxes associated with topograhically forced internal waves that develop during severe down-slope windstorm events. This raises the question as to whether there may be circumstances in which the action of gravity wave drag on the general circulation could be affected by the process of emission rather than by the process of absorption. The basis of all current IGW drag parameterization schemes employed in large-scale models is that momentum is being deposited into the mean flow by IGW breaking. The authors question the universal validity of this assumption.

The Role of Internal Gravity Waves in the Equatorial Current System

Using a two-dimensional nonhydrostatic model, experiments were performed to investigate the formation and maintenance of internal waves in the equatorial Pacific Ocean. The simulations show that internal waves are generated in the surface mixed layer by a type of Kelvin–Helmholtz instability that is dependent on both the flow Reynolds number (i.e., shear strength) and Richardson number. Because of the Richardson number dependence, the simulated internal waves exhibit a diurnal cycle, following the daily stability change in the mixed layer. The diurnal cycle is not evident when the wind stress is eastward because of a decreased mixed layer shear and corresponding Reynolds number. The amplitude, wavelength, frequency, and diurnal variability of the simulated waves are in agreement with high-resolution thermistor chain measurements. Linear theory shows that the horizontal wavelength of the internal waves depends on both the thermocline stratification and the strength of the Equatorial Undercurrent. The simulations show that internal waves can provide an efficient mechanism for the vertical transport of horizontal momentum. In the surface mixed layer, the internal waves gain westerly momentum at the expense of the background flow. In some cases, this momentum is transferred back to the mean flow at a critical level resulting in a deceleration below the undercurrent core. Otherwise, the waves tend to decrease the current velocity above the undercurrent core.