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.

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.

Abstract

The dynamical theory of mass and momentum transport by organized convection, produced by Moncrieff, is extended using a hydrodynamical model, a two-dimensional buoyant model, and a quasi-three-dimensional buoyant model. Each is characterized by three relative flow branches that idealize the structure of squall-line cloud systems.

Despite the physical and structural diversity, a clear similarity in mass and momentum transports holds for the entire hierarchy, such as the negative-dominant momentum flux by systems propagating in the positive x-direction. Shear and buoyancy are shown to alter the details but not the overall nature of the dynamical transports. In particular. both mass and momentum fluxes are insensitive to the Froude numbers in the hydrodynamical model. The two-dimensional buoyant model enhances the momentum flux amplitude but has a much less noticeable impact on mass fluxes. In contrast, the three-dimensional buoyant model has a larger mass flux and raises the heights of the mass and momentum flux extrema. The low-level inflow shear has a similar effect in these models by increasing both mass and momentum fluxes. Buoyancy affects transports largely through modifying the flow field while the inflow shear influences transports by strengthening the low-level convergence.

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.

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.

Abstract

The mesoscale organization of precipitating convection is highly relevant to next-generation global numerical weather prediction models, which will have an intermediate horizontal resolution (grid spacing about 10 km). A primary issue is how to represent dynamical mechanisms that are conspicuously absent from contemporary convective parameterizations. A hybrid parameterization of mesoscale convection is developed, consisting of convective parameterization and explicit convectively driven circulations.

This kind of problem is addressed for warm-season convection over the continental United States, although it is argued to have more general application. A hierarchical strategy is adopted: cloud-system-resolving model simulations represent the mesoscale dynamics of convective organization explicitly and intermediate resolution simulations involve the hybrid approach. Numerically simulated systems are physically interpreted by a mechanistic dynamical model of organized propagating convection. This model is a formal basis for approximating mesoscale convective organization (stratiform heating and mesoscale downdraft) by a first-baroclinic heating couplet.

The hybrid strategy is implemented using a predictor–corrector strategy. Explicit dynamics is the predictor and the first-baroclinic heating couplet the corrector. The corrector strengthens the systematically weak mesoscale downdrafts that occur at intermediate resolution. When introduced to the Betts–Miller–Janjic convective parameterization, this new hybrid approach represents the propagation and dynamical structure of organized precipitating systems. Therefore, the predictor–corrector hybrid approach is an elementary practical framework for representing organized convection in models of intermediate resolution.

Abstract

A nonlinear analytic model is used to study the bulk characteristics of energy conserving density currents in stratified and sheared environments. The idealized representation of latent heating in a stratified flow is a unique feature that interactively couples the dynamic and thermodynamic fields.

A stable stratification decreases the height of density currents but increases the corresponding propagation speed. In contrast, the density current is deeper and moves more slowly once latent heating is included. As for the effect of shear, the depth and the translation speed of density currents increase as the ambient shear varies from negative to positive (in the direction of propagation), with the exception of a strongly stable environment. A key addition to density current dynamics is the upper-level overturning circulation ahead of the system. This feature is very different from the blocked or choked upper-level structure found in the companion paper of Liu and Moncrieff. The distinction is attributed to the effect of different shear profiles on density current dynamics.

These analytic results quantifying the role of shear and latent heating in density-current-like phenomena in the atmosphere should now be evaluated against high-resolution numerical simulations and observations.

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.

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.

Abstract

This paper investigates the effects of cloud microphysics parameterizations on simulations of warm-season precipitation at convection-permitting grid spacing. The objective is to assess the sensitivity of summertime convection predictions to the bulk microphysics parameterizations (BMPs) at fine-grid spacings applicable to the next generation of operational numerical weather prediction models. Four microphysical parameterization schemes are compared: simple ice (Dudhia), four-class mixed phase (Reisner et al.), Goddard five-class mixed phase (Tao and Simpson), and five-class mixed phase with graupel (Reisner et al.). The experimentation involves a 7-day episode (3–9 July 2003) of U.S. midsummer convection under moderate large-scale forcing. Overall, the precipitation coherency manifested as eastward-moving organized convection in the lee of the Rockies is insensitive to the choice of the microphysics schemes, and the latent heating profiles are also largely comparable among the BMPs. The upper-level condensate and cloudiness, upper-level radiative cooling/heating, and rainfall spectrum are the most sensitive, whereas the domain-mean rainfall rate and areal coverage display moderate sensitivity. Overall, the three mixed-phase schemes outperform the simple ice scheme, but a general conclusion about the degree of sophistication in the microphysics treatment and the performance is not achievable.