Convective Momentum Transport Research Articles

Improvements in the Subgrid-Scale Representation of Moist Convection in a Cumulus Parameterization Scheme: The Single-Column Test and Its Impact on Seasonal Prediction

Abstract This study describes a revised approach for the subgrid-scale convective properties of a moist convection scheme in a global model and evaluates its effects on a simulated model climate. The subgrid-scale convective processes tested in this study comprise three components: 1) the random selection of cloud top, 2) the inclusion of convective momentum transport, and 3) a revised large-scale destabilization effect considering synoptic-scale forcing in the cumulus convection scheme of the National Centers for Environmental Prediction medium-range forecast model. Each component in the scheme has been evaluated within a single-column model (SCM) framework forced by the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment data. The impact of the changes in the scheme on seasonal predictions has been examined for the boreal summers of 1996, 1997, and 1999. In the SCM simulations, an experiment that includes all the modifications reproduces the typical convective heating and drying feature. The simulated surface rainfall is in good agreement with the observed precipitation. Random selection of the cloud top effectively moistens and cools the upper troposphere, and it induces drying and warming below the cloud-top level due to the cloud–radiation feedback. However, the two other components in the revised scheme do not play a significant role in the SCM simulations. On the other hand, the role of each modification component in the scheme is significant in the ensemble seasonal simulations. The random selection process of the cloud top preferentially plays an important role in the adjustment of the thermodynamic profile in a manner similar to that in the SCM framework. The inclusion of convective momentum transport in the scheme weakens the meridional circulation. The revised large-scale destabilization process plays an important role in the modulation of the meridional circulation when this process is combined with other processes; on the other hand, this process does not induce significant changes in large-scale fields by itself. Consequently, the experiment that involves all the modifications shows a significant improvement in the seasonal precipitation, thereby highlighting the importance of nonlinear interaction between the physical processes in the model and the simulated climate.

The Impact of Convective Momentum Transport on Tropical Cyclone Track Forecasts Using the Emanuel Cumulus Parameterization

Abstract The influence of convective momentum transport (CMT) on tropical cyclone (TC) track forecasts is examined in the Navy Operational Global Atmospheric Prediction System (NOGAPS) with the Emanuel cumulus parameterization. Data assimilation and medium-range forecast experiments show that for 35 tropical cyclones during August and September 2004 the inclusion of CMT in the cumulus parameterization significantly improves the TC track forecasts. The tests show that the track forecasts are very sensitive to the magnitude of the Emanuel parameterization’s convective momentum transport parameter, which controls the CMT tendency returned by the parameterization. While the overall effect of this formulation of CMT in NOGAPS data assimilation/medium-range forecasts results in the surface pressure of tropical cyclones being less intense (and more consistent with the analysis), the parameterization is not equivalent to a simple diffusion of winds in the presence of convection. This is demonstrated by two data assimilation/medium-range forecast tests in which a vertical diffusion algorithm replaces the CMT. Two additional data assimilation/medium-range forecast experiments were conducted to test whether the skill increase primarily comes from the CMT in the immediate vicinity of the tropical cyclones. The results show that the inclusion of the CMT calculation in the vicinity of the TC makes the largest contribution to the increase in forecast skill, but the general contribution of CMT away from the TC also plays an important role.

Coupling of Convective Momentum Transport with Convective Heating in Global Climate Simulations

Abstract The effects of convective momentum transport (CMT) on global climate simulations are examined in this study. Comparison between two sets of 20-yr (1979–98) integration using the NCAR Community Climate Model version 3 (CCM3) illustrates that the inclusion of CMT in the convection scheme systematically modifies the climate mean state over the equatorial region. The convective momentum tendencies slow down the equatorward flow at higher latitudes near the surface and weaken the equatorial convergence and convection. This reduces the convective heating and drying around the equator and produces an improved meridional distribution within the upward branch of the Hadley circulation. The major heating peak during the boreal winter is moved to south of the equator at about 10°S, which is closer to the heat budget residuals of the ECMWF reanalysis data. The responses of meridional wind to the reduced heating result in the secondary meridional circulation within the intertropical convergence zone.

Cloud Resolving Modeling

One of the most promising methods to test the representation of cloud processes used in climate models is to use observations together with cloud-resolving models (CRMs). CRMs use more sophisticated and realistic representations of cloud microphysical processes, and they can reasonably well resolve the time evolution, structure, and life cycles of clouds and cloud systems (with sizes ranging from about 2-200 km). CRMs also allow for explicit interaction between clouds, outgoing longwave (cooling) and incoming solar (heating) radiation, and ocean and land surface processes. Observations are required to initialize CRMs and to validate their results. This paper provides a brief discussion and review of the main characteristics of CRMs as well as some of their major applications. These include the use of CRMs to improve our understanding of: (1) convective organization, (2) cloud temperature and water vapor budgets, and convective momentum transport, (3) diurnal variation of precipitation processes, (4) radiative-convective quasi-equilibrium states, (5) cloud-chemistry interaction, (6) aerosol-precipitation interaction, and (7) improving moist processes in large-scale models. In addition, current and future developments and applications of CRMs will be presented.

FLOW RESISTANCE AND TURBULENT STRUCTURE IN OPEN CHANNEL FLOW WITH SUBMERGED VEGETATION

Vegetation colonies are often observed to grow in stripe pattern or patches within many stream channel. In this paper, the influence that interaction of longitudinal vegetation zones gave to the flow resistance in an open-channel was examined experimentally. Laboratory experiments were conducted using a two-component electromagnetic current-meter to clarify the momentum transport between main flow and flexible artificial vegetation. Results showed that the flow resistance significantly depended on the lateral distance between longitudinal vegetation zones and the reason could be explained by the difference of convective momentum transport through the interaction between longitudinal vegetation zones.

Flow Rate Limitation in Open Capillary Channel Flows

This paper reports the experimental and theoretical investigations of forced liquid flows through open capillary channels under reduced gravity conditions. An open capillary channel is a structure that establishes a liquid flow path at low Bond numbers, when the capillary pressure caused by the surface tension force dominates in comparison to the hydrostatic pressure induced by gravitational or residual accelerations. In case of steady flow through the channel, the capillary pressure of the free surface balances the pressure difference between the liquid and the surrounding constant-pressure gas phase. Because of convective and viscous momentum transport, the pressure along the flow path decreases and causes the free surface to bend inward. The maximum flow rate is achieved when the free surface collapses and gas ingestion occurs at the outlet. This critical flow rate depends on the geometry of the channel and the properties of the liquid. In this paper we present a comparison of the theoretical and experimental critical flow rates and surface profiles for convective dominated flows. For the prediction of the critical flow rate a one-dimensional theoretical model taking into account the entrance pressure loss and the frictional pressure loss in the channel is developed.

Sensitivity of Hurricane Intensity Forecast to Convective Momentum Transport Parameterization

Abstract A parameterization of the convection-induced pressure gradient force (PGF) in convective momentum transport (CMT) is tested for hurricane intensity forecasting using NCEP's operational Global Forecast System (GFS) and its nested Regional Spectral Model (RSM). In the parameterization the PGF is assumed to be proportional to the product of the cloud mass flux and vertical wind shear. Compared to control forecasts using the present operational GFS and RSM where the PGF effect in CMT is taken into account empirically, the new PGF parameterization helps increase hurricane intensity by reducing the vertical momentum exchange, giving rise to a closer comparison to the observations. In addition, the new PGF parameterization forecasts not only show more realistically organized precipitation patterns with enhanced hurricane intensity but also reduce the forecast track error. Nevertheless, the model forecasts with the new PGF parameterization still largely underpredict the observed intensity. One of the many possible reasons for the large underprediction may be the absence of hurricane initialization in the models.

Thermocapillary Migration of a Drop: An Exact Solution with Newtonian Interfacial Rheology and Stretching/Shrinkage of Interfacial Area Elements for Small Marangoni Numbers

In this paper we analyze the effects of the following phenomena associated with the thermocapillary migration of a drop. The first is the influence of Newtonian surface rheology of the interface and the second is that of the energy changes associated with stretching and shrinkage of the interfacial area elements, when the drop is in motion. The former occurs because of dissipative processes in the interfacial region, such as when surfactant molecules are adsorbed at the interface in sufficient concentration. The interface is typically modeled in this instance by ascribing to it a surface viscosity. This is a different effect from that of interfacial tension gradients arising from surfactant concentration gradients. The stretching and shrinkage of interfacial area elements leads to changes in the internal energy of these elements that affects the transport of energy in the fluids adjoining the interface. When an element on the interface is stretched, its internal energy increases because of the increase in its area. This energy is supplied by the neighboring fluids that are cooled as a consequence. Conversely, when an element on the interface shrinks, the adjoining fluids are warmed. In the case of a moving drop, elements of interfacial area are stretched in the forward half of the drop, and are shrunk in the rear half. Consequently, the temperature variation on the surface of the drop and its migration speed are modified. The analysis of the motion of a drop including these effects was first performed by LeVan in 1981, in the limit when convective transport of momentum and energy are negligible. We extend the analysis of LeVan to include the convective transport of momentum by demonstrating that an exact solution of the momentum equation is obtained for an arbitrary value of the Reynolds number. This solution is then used to calculate the slightly deformed shape of the drop from a sphere.

Thermocapillary convection due to a stationary bubble

We analyze the velocity and temperature fields due to thermocapillary convection around a gas bubble that is stationary in a liquid. A linear temperature field is imposed in the undisturbed liquid. Our interest is in the effect of convective transport of momentum and energy on the velocity and temperature fields. We assume that buoyant convection is negligible. The relevant Reynolds and Marangoni numbers are assumed to be small compared with unity. When the Reynolds and Marangoni numbers are set equal to zero, the steady velocity field far from the bubble decays inversely with distance from it. We show that this behavior of the velocity field, coupled with the linear variation of the imposed temperature far away from the bubble, leads to an ill-posedness in the problem for the perturbation temperature field at steady state for small Marangoni numbers, when the Reynolds number is zero. We also consider the effect of convective transport of momentum for small Reynolds number, when the Marangoni number is zero. In this case, the velocity field far away from the bubble decays inversely as the distance from the bubble even in the presence of inertia. It follows that, for a fixed Prandtl number, when both the Marangoni and Reynolds number are nonzero, the behavior of the velocity field far from the bubble continues to yield an ill-posed problem for the steady perturbation temperature field. Therefore, inertia cannot relieve the ill-posedness of the perturbation problem for the steady temperature field. In order to understand the origin of this ill-posedness, we analyze a related problem involving the unsteady development of the temperature field in the liquid from the moment that a point force is applied to it, and held constant subsequently. We show that including the effect of convective transport of energy by a perturbation expansion leads to a temperature field in the liquid that is always time dependent. The temperature field never achieves a steady state.

Analytic Representation of the Large-Scale Organization of Tropical Convection

The pivotal role of mesoscale organization on the large-scale coherence of tropical convection is represented by a nonlinear dynamical model. The general model consists of two interlocked systems: a mesoscale parameterization of organized convection and a two-layer model of large-scale equatorial dynamics. The lower-layer dynamics is Rossby gyre‐like, whereas outflow from organized convection maintains the upper-layer circulation. The transports of zonal momentum in the vertical and meridional directions are key processes. An archetype of the general model, in spite of being brutally simplified, represents the convective organization, momentum transport, and equatorial superrotation realized by the cloud-resolving convection parameterization or superparameterization explicit approach developed by Grabowski. The mesoscale parameterization is an analytic equivalent of the cloud-system-resolving models used in this computational approach. Finally, issues in parameterizing convective organization are discussed.

A winwin mechanism for low-drag transients in controlled two-dimensional channel flow and its implications for sustained drag reduction

A simple pressure-based feedback control strategy for wall-transpiration control of incompressible unsteady two-dimensional channel flow was recently investigated by Aamo, Krstic & Bewley (2003). Nonlinear two-dimensional channel flow simulations which implemented this control strategy resulted in flow transients with instantaneous drag far lower than that of the corresponding laminar flow. The present article examines the physical mechanism by which this very low level of instantaneous drag was attained. It then explores the possibility of achieving sustained drag reductions to below the laminar level by initiating such low-drag transients on a periodic basis. All attempts at sustaining the mean flow drag below the laminar level fail, perhaps providing indirect evidence in favour of the conjecture that the laminar state might provide a fundamental ‘performance limitation’ in such flows. Mathematical analysis of two-dimensional and three-dimensional channel-flow systems establishes a direct link between the average drag increase due to flow-field unsteadiness and a weighted space/time average of the Reynolds stress. Phenomenological justification of the conjecture is provided by a Reynolds analogy between convective momentum transport and convective heat transport. Proof of the conjecture remains an open problem.

Seasonal migration of ITCZ precipitation across the equator: Why can't GCMs simulate it?

The secondary meridional circulation induced by convective momentum transport (CMT) within the ascending branch of the Hadley circulation is a missing dynamical mechanism that can cause common failure of general circulation models (GCMs) in simulating seasonal migration of the intertropical convergence zone (ITCZ) precipitation maximum across the equator. This failure is manifested by the model bias that the precipitation peak remains north of the equator during November–March. The CMT‐induced secondary circulation, characterized by strong downward motion along the equatorial belt and upward motion south and north of the belt, tends to modify the meridional distribution of precipitation with the strongest impacts during boreal winter and spring. A 20‐year GCM simulation with the CMT parameterization successfully reproduces the observed seasonal migration of the ITCZ precipitation across the equator with the peaks near 8°N during boreal summer and near 8°S during boreal winter.

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.

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

Convective Momentum Transport Observed during the TOGA COARE IOP. Part II: Case Studies

Abstract Convective momentum transport (CMT) associated with the Madden–Julian oscillation (MJO), tropical waves, squall and nonsquall mesoscale convective systems (MCSs), and the diurnal cycle is studied by examining the momentum budget residual X = (X, Y) deduced from the objectively analyzed in situ observations during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observing period (IOP; November 1992–February 1993). Using wavelet transform, time evolution of signals of these disturbances in the time series of |X| and ITBB (an index for deep convection), averaged over the intensive flux array (IFA), is analyzed. Signals of disturbances with periods ≥1 day in |X| generally evolve in phase with those in ITBB. During the convective phase of MJO, signals in both |X| and ITBB with shorter periods are also enhanced. Frequency distribution of IFA-mean E = −v · X in the troposphere is examined. The mean E is positive, that is, kinetic energy (K) transfe...

Large‐scale organization of moist convection in idealized aquaplanet simulations

AbstractThis paper discusses results from idealized simulations using a non‐hydrostatic general circulation model with Cloud‐Resolving Convection Parameterization (CRCP, the ‘super parameterization’) pertinent to the large‐scale organization of tropical convection. The essence of tropical dynamics is the intricate balance between large‐scale processes—such as radiative transfer, large‐scale waves, monsoons, Hadley and Walker circulations—and the convective dynamics. Traditional approaches to this problem consider either large‐scale models using convection parameterization or cloud‐resolving models in which large‐scale effects are prescribed. The CRCP merges the two approaches. It uses a 2D cloud‐resolving model to represent the impact of cloud‐scale processes—such as convective motions, precipitation formation and fallout, interaction of clouds with radiative and surface processes—in every column of a large‐scale or global model. The global CRCP model is applied to the idealized problem of a rotating constant‐SST aquaplanet in convective–radiative equilibrium. The aquaplanet has the size and rate of rotation of the Earth. The global CRCP simulations feature pronounced large‐scale organization of convection within the equatorial waveguide. Prominent equatorial ‘super cloud clusters’ spontaneously develop in CRCP global simulations and bear a strong resemblance to the Madden–Julian Oscillation observed in the terrestrial tropics. Model results suggest that convective transport of zonal momentum, supposedly due to the impact of organized convection, plays a significant role in the large‐scale organization of tropical deep convection. Copyright © 2002 John Wiley & Sons, Ltd.

Convective Momentum Transport Observed during the TOGA COARE IOP. Part I: General Features

Abstract The momentum budget residual X = (X, Y) is estimated with objectively analyzed soundings taken during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intense observing period (IOP; November 1992–February 1993) to study the effects of convective momentum transport (CMT) over the western Pacific warm pool. The time series of X and Y exhibit multiscale temporal behavior, showing modulations by the Madden–Julian oscillation (MJO) and other disturbances. The power spectra of X, Y, and ITBB (an index of convective activity) are remarkably similar, showing peaks near 10, 4–5, and 2 days, and at the diurnal period, suggesting a link between deep cumulus convection and the acceleration–deceleration of the large-scale horizontal motion, via CMT, which is being modulated by various atmospheric disturbances. The temporal behavior of X and Y can be described as fractals from 1/4 to ∼20 and from 1/4 to ∼16 days, respectively. Their fractal characteristics are ...

Cumulus Ensembles in Shear: Implications for Parameterization

Abstract A systematic numerical investigation is conducted into the role of ambient shear on the macrophysical properties of tropical cumulus ensembles maintained by convective available potential energy generated by constant surface fluxes of temperature and moisture and large-scale advective cooling and moistening. The effects of five distinct idealized wind profiles on the organization of convection, and quantities relevant to the parameterization of convection and convectively generated clouds, are examined in a series of 6-day two-dimensional cloud-resolving simulations. Lower-tropospheric shear affects the mesoscale organization of convection through interaction with evaporatively driven downdraft outflows (convective triggering), while shear in mid-to-upper levels determines the amount of stratiform cloud and whether the convective transport of momentum is upgradient or downgradient. Shear significantly affects the convective heating and drying, momentum transport, mass fluxes, and cloud fraction. ...

Convective Momentum Transport over the Tropical Pacific: Budget Estimates

Abstract ECMWF and NCEP–NCAR reanalyses are used to calculate convective momentum transport (CMT) as a momentum budget residual over several tropical oceanic convective regions, including the TOGA COARE Intensive Flux Array (IFA). Sources of uncertainty are quantified through intercomparison, and methods for minimizing uncertainty are discussed. The authors examine how the environmental shear of the time-mean, and time-varying, flow in these regions is modified by CMT, and compare the budget results with some parameterizations. The zonal residual is a significant term in the time-mean reanalysis budgets in all regions below 850 mb. Since the subcloud turbulent layer typically does not extend above 940 mb, this suggests a possibly important role for CMT by shallow convection. In the rest of the troposphere, the residuals are small, but a slight tendency for downgradient transport of zonal momentum, and smoothing of the environmental wind profile, is suggested. The main budget uncertainties associated with ...

An Updated Climatology of the Troposphere–Stratosphere Configuration of the Met Office's Unified Model

Abstract In this paper results are presented from an improved version of the troposphere–stratosphere configuration of the Met Office Unified Model (UM). The new version incorporates a number of changes, including new radiation and orographic gravity wave parameterization schemes, an interannually varying sea surface temperature and sea ice climatology, and the inclusion of convective momentum transport. The UM climatology is compared with assimilated data and with results from a previous version of the UM. It is shown that the model cold biases in the January winter stratosphere and the January and July summer stratosphere are reduced, chiefly because the new radiation scheme is more accurate. The separation between subtropical and polar night jets in July is also better simulated. In addition, in the current version stratospheric planetary wave amplitudes in southern winter are less than half those in northern winter, which is in much better agreement with observations than the previous model version. D...