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

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

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.

Abstract

A numerical model investigation is conducted of the effects of ambient flow and shear upon the propagation and morphology of density currents. The model is initialized with a horizontally homogeneous wind profile superimposed on a cold-air source that initiates and maintains the density currents. The base state is neutrally stratified and free-slip lower and upper boundary conditions are used.

A headwind (i.e., relative flow in the direction opposing the system movement) raises the density current head compared to calm surroundings, while a tailwind has the opposite effect. A weak or moderate shear elevates the head for the downshear-traveling system and a shallow multihead structure appears in strong shear. In contrast, the upshear-moving system is largely insensitive to the shear. In uniform flow, the propagation speed is linearly proportional to the ambient wind speed, reduced or enhanced by about three-quarters depending on the airflow direction. In uniform shear, a linear relationship approximates the relationship between the advance rate of density current and the value of the shear, particularly for the upshear-moving system.

An idealized dynamical model is developed for the moderate shear case in terms of a Froude number ℱ. The model has three branches, namely, a borelike region, an overturning updraft, and a stagnant region that moves bodily with the system. The Froude number calculated from the numerical model data is ℱ ≈ 0.7, which lies within die range of analytic solutions obtained.

With regard to the initiation of convection over an island or peninsula in an unsheared or weakly sheared ambient flow, a sea-breeze circulation will preferentially cause convection on the leeward side and a land breeze on the windward side. The opposite occurs when the ambient flow has moderate to strong low-level shear—that is, the sea breeze will cause convection on the windward side and a land breeze on the leeward side. The mean-flow momentum and mean-flow shear thus affect convection initiation in opposing ways. There is a dearth of observational data on density currents in shear flow with which to evaluate our dynamical model—in particular, the role of the overturning updraft, which is a new concept as regards density current dynamics.

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. Sensitivity is strongest in weaker forcing. Cloud-interactive radiation has little direct effect on a 6-day timescale. In particular, the effects of shear on convective momentum transport and cloud fraction are large enough to be potentially significant when included in parameterizations for climate models.

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.

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.

Abstract

Numerical simulations are performed to investigate organized convection observed in the Asian summer monsoon and documented as a category of mesoscale convective systems (MCSs) over the U.S. continent during the warm season. In an idealized low-inhibition and unidirectional shear environment of the mei-yu moisture front, the structure of the simulated organized convection is distinct from that occurring in the classical quasi-two-dimensional, shear-perpendicular, and trailing stratiform (TS) MCS. Consisting of four airflow branches, a three-dimensional, eastward-propagating, downshear-tilted, shear-parallel MCS builds upshear by initiating new convection at its upstream end. The weak cold pool in the low-inhibition environment negligibly affects convection initiation, whereas convectively generated gravity waves are vital. Upstream-propagating gravity waves form a saturated or near-saturated moist tongue, and downstream-propagating waves control the initiation and growth of convection within a preexisting cloud layer. A sensitivity experiment wherein the weak cold pool is removed entirely intensifies the MCS and its interaction with the environment. The horizontal scale, rainfall rate, convective momentum transport, and transverse circulation are about double the respective value in the control simulation. The positive sign of the convective momentum transport contrasts with the negative sign for an eastward-propagating TS MCS. The structure of the simulated convective systems resembles shear-parallel organization in the intertropical convergence zone (ITCZ).