Search Results

You are looking at 1 - 10 of 66 items for

  • Author or Editor: Mitchell W. Moncrieff x
  • All content x
Clear All Modify Search
Mitchell W. Moncrieff

Abstract

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.

Full access
Mitchell W. Moncrieff

Abstract

Simplified versions of the steady analytical models of density current developed by Moncrieff and So are shown to represent archetypes of cold-frontal rainbands (NCFRs) by making comparisons with the limited amount of published observational data that describe the phenomena. An overturning (or in a special case, stagnation) of the upper-level, system-relative flow of ahead of the rainband and vortidty within the cold air are important effects not included in conventional density current dynamics. The bands conserve mass energy and domain-averaged total momentum flux and involve a balance between inertial and pressure gradient effects. This is distinct from a semigeostrophic mechanism that requires a base-state baroclinity and an ageostrophic adjustment towards thermal-wind balance.

The theory also represents two-diimensional squall lines in the limiting cam when the convective available potential energy is negligible so squall lines could, in principle, be maintained solely by the kinetic energy of the mean flow and the work done by the pressure field. Furthermore, it is shown that squall lines with a deep inflow from ahead of the line and an anafront type of structure should be more prevalent than those having shallow inflows.

The basic dynamics of squall lines and narrow cold-frontal rainbands are formally shown to be analogous and their archetypal behavior can be represented by simple hydrodynamical models.

Full access
Jimy Dudhia and Mitchell W. Moncrieff

Abstract

A nonhydrostatic numerical mesoscale model has been applied to the study of an Oklahoma squall line with initial conditions taken from the Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) data for 7 May 1985. The model reproduced features typical of organized propagating convection occurring during spring and summer in this region, namely a squall line/mesoscale convective system containing strong right-flank convection resembling many documented cases. The alignment and motion of the system change during its development and are determined by the ambient wind at three levels, the steering level of the mature cells, the level of free convection, and the surface layer. Three persistent right-flank cells had a characteristic rightward propagation relative to the mean wind shear vector. Their propagation occurred through successive mergers of cells that had formed at a downdraft outflow convergence front and were similar to the flanking line often seen to the south of strong updraft cores.

The three-dimensional flow structure of the right-flank cells was found to center on a distinct dynamical pressure pattern that itself resulted from the interaction of the midlevel relative flow with the cyclonic vorticity in the updrafts. This low pressure on the updraft's flank extended down to low levels where it was partly responsible for directing the southward surge of downdraft air causing the convergence and flanking line. Other types of supercell propagation are speculated upon in relation to this characteristic dynamical pressure effect evident in the simulation in the neighborhood of cyclonic updrafts.

The updraft cyclonic vorticity was found to strongly influence the domain-scale circulation, particularly in the upper troposphere where it counteracted the anticyclonic production due to divergence and the Coriolis acceleration, leaving net cyclonic vorticity throughout most of the troposphere on a scale of 200 km.

Full access
Xiaoqing Wu and Mitchell W. Moncrieff

Abstract

The collective effects of organized convection the environment were estimated using a two-dimensional, two-way nested cloud-resolving numerical model with a large outer domain (4500 km). As initial conditions, the authors used an idealized environment of the onset stage of the December 1992 westerly wind burst that occurred during the Tropical Oceans Global Atmosphere Coupled Ocean-Atmosphere Response Experiment.

Two key aspects relating to convective parameterization were examined. First, the transports, sources, and sinks of heat, moisture, and momentum were derived using the model-produced dataset. In particular, the total momentum flux compares well with Moncrieff's dynamical theory. Second, the bulk energetics of the cloud system were examined using the model-produced dataset. The authors found that the shear generation of kinetic energy is comparable to the buoyancy generation and dominates the sum of the buoyancy and water-loading generation. This means that, in addition to the thermodynamic generation of kinetic energy, shear generation should be included in the closure condition for the parameterization of organized convection in large-scale models.

A simple mass-flux-based parameterization scheme is outlined for organized convection that consistently treats dynamical and thermodynamical fluxes.

Full access
Qin Xu and Mitchell W. Moncrieff

Abstract

This paper develops an idealized (inviscid two fluid), two-dimensional, steady-state model of a density current circulation and its front propagating into a uniformly sheared environmental flow. This fully nonlinear analytical model is used to examine the kinematic and dynamic factors that control the depth and propagation speed of the density current and the geometric shape of the density current front. The results show that in comparison with the environmental inflow shear, the strength of the internal circulation within the cold pool of a density current plays a secondary role in controlling the depth and propagation speed of the density current, at least one having constant vorticity. The direction of the cold pool circulation can be either clockwise or anticlockwise, not affecting the propagation speed, depth, and geometric shape of an inviscid conservative density current. Physical interpretation of the results is provided in regard to the way that the inflow shear controls the shape of the density current head and produces the “optimal state” for supporting long-lived squall lines.

Full access
Changhai Liu and Mitchell W. Moncrieff

Abstract

The intertropical convergence zone (ITCZ) is one of the most important components of the global circulation. In order to understand the dynamical processes that regulate its formation, latitudinal preference, and structure, explicit two-dimensional numerical modeling of convection on an equatorial beta plane was conducted with a nonhydrostatic cloud-system-resolving model. The model was forced by energy fluxes associated with constant sea surface temperature (SST) and by horizontally homogeneous radiative cooling.

Two distinct patterns were identified for the spatial distribution of convective activity in the Tropics. The first was characteristic of enhanced off-equator convection, namely, a double ITCZ-like morphology (one more salient than the other) straddling the equator during the early period of the integration. The second featured enhanced equatorial convection, namely, a single ITCZ-like morphology on the equator during the later quasi-equilibrium period. Diagnostic analysis and two additional experiments, one excluding surface friction and the other having time- and space-independent surface fluxes, revealed that the wind-induced surface flux variability played an essential role in the development and maintenance of the equatorial maximum convection. Surface friction was largely responsible for the early asymmetric convective distribution with respect to the equator in the control simulation and acted to flatten the convective peaks.

One important discrepancy from observations concerned the too-weak trade wind convergence around enhanced convective regions. This unrealistic feature suggested that, as well as the meridional dynamics, latitudinal SST gradients, large-scale forcing, and other physical processes regulate the observed ITCZs.

Full access
Xiaoqing Wu and Mitchell W. Moncrieff

Abstract

Most atmospheric general circulation models (GCMs) and coupled atmosphere–ocean GCMs are unable to get the tropical energy budgets at the top of the atmosphere and the surface to simultaneously agree with observations. This aspect is investigated using a cloud-resolving model (CRM) that treats cloud-scale dynamics explicitly, a single-column model (SCM) of the National Center for Atmospheric Research (NCAR) Community Climate Model that parameterizes convection and clouds, and observations made during Tropical Oceans and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The same large-scale forcing and radiation parameterizations were used in both modeling approaches. We showed that the time-averaged top-of-atmosphere and surface energy budgets agree simultaneously with observations in a 30-day (5 December 1992–3 January 1993) cloud-resolving simulation of tropical cloud systems. The 30-day time-averaged energy budgets obtained from the CRM are within the observational accuracy of 10 W m−2, while the corresponding quantities derived from the SCM have large biases. The physical explanation for this difference is that the CRM realization explicitly represents cumulus convection, including its mesoscale organization, and produces vertical and horizontal distributions of cloud condensate (ice and liquid water) that interact much more realistically with radiation than do parameterized clouds in the SCM.

The accuracy of the CRM-derived surface fluxes is also tested by using the fluxes to force a one-dimensional (1D) ocean model. The 1D model, together with the surface forcing from the CRM and the prescribed advection of temperature and salinity, simulates the long-term evolution and diurnal variation of the sea surface temperature. This suggests that the atmosphere–ocean coupling requires accurate representation of cloud-scale and mesoscale processes.

Full access
Changhai Liu and Mitchell W. Moncrieff

Abstract

Idealized two-dimensional cloud-resolving numerical modeling was conducted to investigate the diurnal variability of deep tropical oceanic convection. The model was initialized with a horizontally homogeneous atmosphere upon which a uniform and time-independent large-scale forcing was imposed. The underlying surface was assumed to be an open ocean with a constant sea surface temperature. Emphasis was on two distinct regimes:(a) highly organized squall-line-like convection in strong ambient shear and (b) less organized nonsquall cloud clusters without ambient shear.

A pronounced diurnal cycle was simulated for the highly organized case; convective activity and intensity attained a maximum around predawn and a minimum in the late afternoon. A similar diurnal variability was obtained for the less organized case and was characterized by more precipitation during the night and early morning and less precipitation in the afternoon and evening.

The modeled diurnal variation was primarily attributed to the direct interaction between radiation and convection, whereas the cloud–cloud-free differential heating mechanism played a secondary role.

When the radiative effect of clouds was excluded, a diurnal cycle was still present. Moreover, the cloud radiative forcing had a negative influence on precipitation/convective activity, in contrast with general circulation modeling results.

Full access
Xiaoqing Wu and Mitchell W. Moncrieff

Abstract

Two sets of single-column model (SCM) simulations are performed to determine whether the SCM solutions are more sensitive to model parameterization schemes than to initial perturbations in temperature and moisture profiles. The first set of simulations (S3) used the Zhang and McFarlane scheme for the deep convection and the Hack scheme for the shallow convection, while the second set (S2) used the Hack scheme for all types of convection. The same random perturbation used by Hack and Pedretti is applied in S2 and S3. The observed total (horizontal and vertical) advections of temperature and moisture during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are used to force all simulations. A major difference in temperature and moisture biases occurs between the ensemble means of the two sets of simulations, and is much larger than the standard deviation of each set. Differences are also evident in cloud and radiative properties. This demonstrates that SCM solutions can be more sensitive to the model physics than to the initial perturbations. In other words, the deterministic aspects of SCM solutions dominate the nondeterministic aspects, which is important for their continued use in developing parameterization schemes of convection and clouds in large-scale models. This point is also supported by the SCM simulations using several available longer observational datasets over different regions.

Full access
Changhai Liu and Mitchell W. Moncrieff

Abstract

The effects of three distinct stratifications on density current dynamics are investigated using a nonhydrostatic numerical model: (i) a stably stratified layer underneath a deep neutrally stratified flow, representing a nocturnal boundary layer over land; (ii) a neutrally stratified layer underlying a deep stably stratified flow, representing a daytime boundary layer; and (iii) a continuously stratified atmosphere.

In the first case, a weak or intermediate stratification decreases the height of density currents and increases the propagation speed. The same result holds in strongly stratified situations as long as the generated disturbances in the neighborhood of the head do not propagate away. Classical density currents occur in weak stratification, multiheaded density currents in intermediate stratification, and multiheaded density currents with solitary wave–like or borelike disturbances propagating ahead of the current in strong stratification.

In the second case, the upper-layer stratification consistently reduces the density-current height and its propagation speed. The simulated system resembles laboratory density currents and is not much affected by the overlying stratification.

Finally, in continuously stratified flow, the effect of stratification is similar to the second case. The density current becomes shallower and moves more slowly as the stratification is increased. The modeled system has the basic features of density currents if the stratification is weak or moderate, but it becomes progressively less elevated as stratification increases. In strong stratification the density current assumes a wedgelike structure.

The simulation results are compared with the authors’ previously obtained analytical results, and the physical mechanisms for the effect of stratification are discussed.

Full access