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Changhai Liu
and
Mitchell W. Moncrieff

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.

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Changhai Liu
and
Mitchell W. Moncrieff

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.

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Mitchell W. Moncrieff
and
Changhai Liu

Abstract

Steady-state analytic models establish two key points concerning the impact of vertical shear on density currents and the implication for convection initiation. First, shear decreases the horizontal convergence, and therefore the mean ascent, associated with downshear propagating currents. Second, shear has a basic effect on the dynamical organization. If the downshear current travels at the speed of the ambient flow at a critical (steering) level, an overturning circulation provides deep lifting. Although mean ascent is increased by shear in the case of upshear propagating currents, the lifting is comparatively shallow because jumplike ascent occurs rather than deep overturning. The convection initiation mechanism involving the downshear current is therefore very different from the upshear case.

These basic principles are borne out in two-dimensional numerical simulations. Density currents generated by a stationary cold source imposed on an initially horizontally homogeneous, sheared, and neutrally stratified ambient flow are explored. Results show that (i) if the surface flow and low-level shear vectors are in the same direction, as in a low-level jet, the effects of shear and surface flow on the density current head height counteract one another; and (ii) if they oppose one another, as in a surface jet, both conspire to lower the density current head on the downwind side but raise it on the upwind side.

As regards convection initiation by sea breezes, point (i) above shows an approximately equal but weak preference for convection exists on the leeward and windward coasts. Point (ii) shows that initiation is strongly suppressed on the windward coast, but strongly enhanced on the leeward one. The hypothesis that sea breezes are more intense in offshore flow therefore holds only if shear and surface flow have opposite sign or if the flow is unsheared.

Concerning convection initiation by thunderstorm outflows, downshear propagating outflows provide the deepest lifting if they move at the speed of the ambient flow at a critical level, despite the fact that low-level convergence is decreased by shear. While shear strengthens the mean ascent in upshear propagating outflows there is no steering level to anchor the incipient convection to the organized ascent.

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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.

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Changhai Liu
and
Mitchell W. Moncrieff

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.

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Changhai Liu
and
Mitchell W. Moncrieff

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.

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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.

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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.

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Changhai Liu
and
Mitchell W. Moncrieff

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.

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Changhai Liu
and
Mitchell W. Moncrieff

Abstract

The effects of latent heating, gravity waves, and planetary rotation on numerically simulated convective cloud systems are investigated. First, the nonlinear response of an initially motionless, uniformly stratified, dry atmosphere to steady heating that interacts with the environment through inertial–gravity waves is examined. Planetary rotation confines the subsidence-induced adiabatic warming to the neighborhood of the heated region on a time scale comparable to the lifetime of mesoscale convective systems. In a moist atmosphere, rotation-induced localized descent stabilizes and dries the near environment and decreases the convective available potential energy. The Tropics is therefore a preferred region for convective clustering.

This hypothesis is tested in two sets of multiday convection-resolving simulations on f planes representative of the Tropics, subtropics, and midlatitudes. Convection is maintained by radiative cooling and surface fluxes of heat and moisture. In a motionless mean state, convective clustering is most prominent in the Tropics. In constant easterly flow, tropical convection organizes on three scales. Eastward-propagating convectively coupled gravity waves generate large-scale envelopes of cloudiness. Embedded within these envelopes are westward-traveling mesoscale convective systems that, in turn, contain westward-traveling deep convective cores.

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