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Kerry A. Emanuel

Abstract

Attempts were made during two field experiments to fly instrumented aircraft along absolute momentum (M) surfaces as a means of accurately determining slantwise convective stability. The application of this technique appears to have been quite successful. We present the results of four such efforts conducted in the ascent regions of midlatitude cyclones observed during the New England Winter Storms and Genesis of Atlantic Lows Experiments. In three of the four cases the atmosphere was almost exactly neutral to slantwise ascent while being quite stable to vertical displacements. In the fourth case, the atmosphere departed from neutrality but was also substantially drier, evidently due to subsidence. We find excellent agreement between assessments of stability based on the M surface flights and on cross sections constructed from rawinsonde observations. On the basis of these results I hypothesize that slantwise convective neutrality is characteristic of the ascent regions of baroclinic cyclones and discuss the implications of this finding for the dynamics of baroclinic systems.

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Kerry A. Emanuel

Abstract

We have constructed a simple, balanced, axisymmetric model as a means of understanding the existence of the threshold amplitude for tropical cyclogenesis discovered by Rotunno and Emanuel. The model is similar to Ooyama's but is phrased in Schubert and Hack's potential radius coordinates.

The essential difference between this and other balanced models lies in the representation of convective clouds. In the present model the cumulus updraft mass flux depends simply and directly on the buoyancy (on angular momentum surfaces) of lifted subcloud-layer air and is not explicitly constrained by moisture convergence. The downdraft mass flux is equal to the updraft flux multiplied by (1−ε), where ε is the precipitation efficiency. The complete spectrum of convective clouds in nature is here represented by two extremes: deep clouds with a precipitation efficiency of one, and shallow, nonprecipitating clouds. The former stabilize the atmosphere both by heating the free atmosphere and drying out the subcloud layer, whereas the shallow clouds stabilize only through drying of the subcloud layer. The two cloud types may coexist. In the crude vertical structure of the model, shallow clouds have the same thermodynamic effect as precipitation-induced downdrafts. Model runs without shallow clouds but with precipitation-induced downdrafts produce the same qualitative features as the runs with shallow clouds.

The existence of low-precipitation-efficiency clouds is crucial to the model hurricane development. When a weak vortex is placed in contact with the sea surface, the enhanced surface fluxes together with adiabatic cooling induced by Ekman pumping destabilize the atmosphere. The initial convective clouds that form have relatively low precipitation efficiency and thus only partially compensate for the adiabatic cooling associated with the Ekman pumping. They do, however, import low θe air into the subcloud layer. The vortex core therefore cools and the vortex decays. Only when the anomalous surface fluxes are strong enough, and /or the middle troposphere humid enough does the subcloud layer θe increase, and with it the temperature of the core and the amplitude of the cyclone.

The low-precipitation-efficiency clouds play a dual role, however. Once amplification begins, these clouds continue to dominate the convection outside the eyewall, keeping the boundary layer θe relatively low. Without low-precipitation-efficiency clouds, large heating occurs in the outer region and the vortex expands and weakens.

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Kerry A. Emanuel

Abstract

The energy cycle of the mature hurricane resides in the secondary circulation that passes through the storm’s eyewall. By equating the generation of energy in this cycle to boundary layer dissipation, an upper bound on wind speed is derived. This bound depends on the degree of thermodynamic disequilibrium between the tropical ocean and atmosphere, on the difference between sea surface and outflow absolute temperatures, and also on the ratio between the enthalpy exchange and surface drag coefficients. Such a bound proves to be an excellent predictor of maximum wind speeds in two different axisymmetric numerical models and does not appear to depend on the existence of the hurricane eye. But further consideration of the detailed dynamics of the eye and eyewall show that the intensification of hurricanes is accelerated by feedbacks associated with a component of eye subsidence forced by radial turbulent diffusion of momentum. This radial momentum diffusion is an inevitable by-product of the strong frontogenesis that the author here shows to be a fundamental characteristic of flow in the eyewall. Thus, while the upper bound on hurricane wind speed is independent of the eye dynamics, the intensification of hurricanes is indirectly accelerated by turbulent stresses that occur in the eye and eyewall.

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Kerry A. Emanuel

Abstract

A simple model describing the slantwise ascent of a two-dimensional horizontal air tube Subject to moist symmetric instability is developed under the assumptions that the Froude number is small and that mixing is absent. It is shown that, in general, the horizontal velocities attained by the tube are comparable to those of the mean flow and that vertical velocities of up to a few meters per second are possible. The tube ascends slantwise in such a way that its buoyancy remains nearly zero, unless the environment is my very nearly moist adiabatic, in which case ascent at an angle of 45° to the vertical is preferred. Results of the analysis support the contentions of Bennetts and Hoskins and Emanual that moist symmetric instability is the cause of some mesoscale rainbands. In a companion paper, it is demonstrated that the stability of the moist baroclinic atmosphere to two-dimensional slantwise displacements of arbitrary magnitude can be approximately assessed by reversibly lifting parcel along surfaces of constant angular momentum and comparing their density with that of their environment.

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Kerry A. Emanuel

Abstract

The standard parcel method of assessing the susceptibility of the atmosphere to moist convection using tephigrams is extended to account for the centrifugal as well as the gravitational potential energy of the displaced air parcel. This leads to a measure of the stability of the moist baroclinic atmosphere to finite slantwise reversible displacements of a two-dimensional air parcel; such a measure differs from previously derived measures of conditional symmetric instability which have considered only infinitesimal displacements in saturated atmospheres. It is demonstrated that the combined gravitational and centrifugal potential of a two-dimensional air parcel or “tube” can be assessed by displacing the tube slantwise along a surface of constant angular momentum, and that this combined potential energy can be estimated using a single atmospheric sounding. Several examples of the application of this technique are presented in the context of a case study of apparent slantwise convection. The results suggest that moist convection in a conditionally unstable baroclinic atmosphere proceeds in such a way as to render the atmosphere neutral to reversible slantwise displacements; such an atmosphere is characterized by moist adiabatic lapse rates along surfaces of constant angular momentum and may be considerably stable to purely vertical displacements.

The dynamics of slantwise moist convection are examined in a companion paper.

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Kerry A. Emanuel

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Kerry A. Emanuel

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Kerry A. Emanuel

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Kerry A. Emanuel

Abstract

A simple linear model is developed with the idea of demonstrating the basic physical processes that serve to distinguish the dynamics of precipitating convection from those of the nonprecipitating variety. In particular, it is shown that the hypothesis advanced by Seitter and Kuo to explain the slope and propagation of squall lines in the context of a fully nonlinear numerical model operates also within a linear model. With a hierarchy of linear models, it is demonstrated that 1) precipitating convection in a basic state consisting of a resting, uniform, unstable cloud can propagate and exhibit sloping up- and down-drafts; 2) subcloud evaporation of falling precipitation leads to modifications of the aforementioned instabilities and the formation of a new mode that travels rapidly and has peak amplitude in the subcloud layer; and 3) the introduction of a shear layer at the cloud base serves to couple the subcloud layer mode mentioned here with the cloud layer and yields a deep, rapidly growing, down-shear propagating mode which, while it has no critical level, nevertheless extracts kinetic energy from the mean shear. These models predict that small vertical shear favors slow-moving shear-parallel squall lines, somewhat larger shear leads to fast-moving shear-perpendicular lines, and very large shear favors three-dimensional convection.

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Kerry A. Emanuel

Abstract

Observations of strong convective lines in middle latitudes indicate a close association of the lines with the presence of vertical shear of the large-scale horizontal wind. Under the premise that this shear is necessary to the maintenance of mesoscale circulations accompanying the lines, it is found that the susceptibility of the large-scale momentum, temperature and moisture fields to such circulations is related to the inertial stability of the flow. Part I contains a description of a variational solution of the linear equations governing two-dimensional perturbations in a bounded, fully viscous, adiabatic and Boussinesq rotating fluid with constant vertical and horizontal shears. The principal finding of this analysis is that the horizontal length scale of the most unstable normal mode is determined primarily by the depth of the unstable domain and the slope of isentropic surfaces rather than by the diffusive properties of the fluid. The effects of moisture and the conditions under which inertial circulations are likely to develop in the atmosphere are examined in Part II and compared with observations of mesoscale convective systems.

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