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  • Author or Editor: William J. Gutowski Jr x
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William J. Gutowski Jr.

Abstract

A model designed for studying the interaction between vertical eddy beat fluxes and the vertical temperature structure in midlatitudes is presented. A temperature profile is obtained for the model by computing an equilibrium among heating rates from simplified representations of the large-scale vertical eddy heat flux, moist convection and radiation. In particular, the eddy flux profile is obtained from the quasi-geostrophic, linear baroclinic instability of a single wave, and the eddy amplitude is either specified or else obtained from a closure assumption. Tests using a variety of input conditions indicate that the most appropriate single wave to use is the most unstable mode of the instability problem.

The model's temperature and Brunt-Väisälä frequency profiles am compared with observed profiles. The most unstable mode computations, in particular, reproduce well the observed profiles for both winter and summer conditions. Model runs with various eddy amplitudes show that particular aspects of the observed profiles, such as the strong decrease of Brunt- Väis¨lä frequency with height in the lower troposphere, can be understood in terms of the vertical eddy flux's influence on temperature structure. Also, the eddy flux tends to alter the lapse rate more than the tropopause height as its strength varies. This particular influence is part of a negative feedback between temperature structure and the eddy flux strength: an increase in flux strength causes the lapse rate to decrease, which in turn causes the flux to weaken. The lapse rate alteration occurs primarily in the lower troposphere, indicating that the eddy flux places a significant constraint on how the vertical temperature structure in midlatitudes change when the climate changes.

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William J. Gutowski Jr.

Abstract

An assessment is made of how baroclinically unstable waves might stabilize a zonal mean Bow by adjusting vertical temperature profiles. To this end, temperature profiles for a continuous atmosphere an derived which are baroclinically stable. Features of these profiles of relevance to the atmosphere are discussed, and a requirement is derived for the minimal amount of profile adjustment necessary to stabilize the zonal mean flow. Neither the atmosphere nor realistic models can perform the profile adjustment necessary to attain a rigorously stable state, but a model used here which includes vertical heat fluxes from baroclinically unstable waves can attain an effectively stable state that is very similar to the rigorously stable state. Observed temperature profiles, as well as model profiles that are not effectively stable, also display features of the adjusted profiles, indicating that much of the temperature variation with height in the midlatitude troposphere can be understood in terms of the adjustment. The model results also show that vertical eddy heat fluxes arising from baroclinic instability try to produce the adjustment to a stable state but other processes, most notably those controlling temperature near the surface, prevent the fluxes from being successful. Finally, the model results are used to show similarities between the continuous atmosphere adjustment presented here and the two-layer atmosphere adjustment studied by others.

The work presented here suggests that vertical heat fluxes from baroclinically unstable waves can produce significant variations of midlatitude lapse rates in both height and time; midlatitude models that omit these variations may be missing an important aspect of wave-mean flow interaction.

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William J. Gutowski Jr. and Weidong Jiang

Abstract

The authors examine the role of convection in the dynamics of eddy life cycles through numerical experiments using initial states that are baroclinically and conditionally unstable in midlatitudes. The location of wave-induced convection and its influence on the growing wave depends on how strongly the wave is coupled to the lower boundary through surface fluxes. For all three convective schemes used here (Emanuel, modified Grell, modified Kuo), convective destabilization is favored in the wave’s warm sector when there are no surface fluxes included in the simulation and in the cold sector when there are. Convection is also shallower when it occurs in the cold sector, though still precipitating. For simulations using Emanuel convection, the relatively shallow convection plays a central role in a water cycle wherein 1) evaporation gives moisture to the cold sector’s boundary layer, 2) convection pumps some of the moisture into the lower troposphere above the boundary layer, 3) the large-scale circulation transports the moisture eastward and upward into the wave’s warm sector, and 4) stable precipitation condenses the moisture into precipitation. The additional condensation catalyzes a more energetic life cycle by inducing stronger vertical motion and, hence, a greater conversion of available potential energy to kinetic energy. This enhancement, however, is parameterization dependent, with the key factor being how much lower-tropospheric moistening a convection scheme produces.

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Weidong Jiang and William J. Gutowski Jr.

Abstract

The influence of convective heating on baroclinic instability in the presence of surface sensible heat and moisture fluxes is investigated. Following previous numerical work, a two-dimensional continuous model on an f plane incorporates diabatic heating effects due to cumulus convection and surface sensible heat flux using parameterizations based on a wave-induced unstable boundary layer and associated moist convective destabilization. The temperature-damping effect of surface sensible heat flux is assumed to decrease exponentially with height, and the vertical distribution of convective heating uses a prescribed profile. The atmosphere is assumed to overlie an oceanic surface. In this configuration, convective heating occurs in the wave’s cold sector.

General forms of the dispersion relation and eigenfunction are derived analytically. Results show that the most unstable wave is modified by the effect of convective latent heating. With weak convection, the wave’s structure does not change much, while the wave’s energy generation is hampered by the negative contribution of convection. In the presence of moderate convective heating, although the wave’s energy generation is decreased by convection, the wave adjusts its structure to minimize the negative effect of convection and retain growth. In the region with strong convective heating, convective heating significantly changes the wave’s temperature structure. Above and below the strong heating region, the wave structure still retains some features of the Eady mode. The results have bearing on how the structure of oceanic storms may be altered by convection.

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William J. Gutowski Jr., Lee E. Branscome, and Douglas A. Stewart

Abstract

We use a global, primitive equation model to study the evolution of waves growing in a zonal mean state that is initially baroclinically unstable. The waves produce changes in the zonal mean state that we compare with changes predicted by baroclinic adjustment theories We examine mean state adjustment by representative zonal wavenumbers 3, 7 or 12.

In the absence of surface processes, as the wave grows to its maximum amplitude, it reduces the zonal mean state's potential vorticity gradient through the lower troposphere, in accord with adjustment theories. Over the latitudes with largest wave amplitude, changes in the static stability and the zonal wind's vertical shear contribute about equally to the potential vorticity gradient adjustment. However, during the last day of a wave's growth, momentum fluxes strengthen the barotropic component of the zonal wind and the potential vorticity gradient in the middle troposphere, changes that are not anticipated by adjustment theory. The static stability adjustment occurs across the latitudinal band occupied by the growing wave. Further experiments show that the static stability adjustment alone is very effective in reducing the instability of the flow and restricting the maximum amplitude attained by growing waves. Adjustment of the zonal wind's vertical shear is confined to a narrower range of latitude and is partially reversed as the wave decays. Additional experiments indicate that the barotropic governor mechanism of James does not contribute strongly to the mean flow's stabilization in the cases we examine, though it way inhibit secondary growth at latitudes adjacent to the initial disturbance.

When the model includes surface friction and heat flux, the waves adjust the zonal mean state less effectively, especially near the surface. Surface heat flux inhibits static stability adjustment, and surface friction inhibits adjustment of the zonal wind's vertical shear. In the absence of surface processes, the adjusted state produced by the wave is quite different from observed mean structures. However, with both surface processes included, the vertical profiles of the adjusted static stability, wind shear and potential vorticity gradient are similar to observed profiles. The model' interaction between the waves and the mean flow corroborates results from previous studies of baroclinic adjustment that used simpler representations of atmospheric dynamics.

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Lee E. Branscome, William J. Gutowski Jr., and Douglas A. Stewart

Abstract

The nonlinear development of baroclinically unstable waves in the presence of surface friction and heat flux is studied, using a global primitive equation model. The experiments use zonal wavenumber 3.7 or 12 and a variety of initial conditions, mostly representative of observed initial states. Other initial states consist of solidbody rotation with vertical shear of the zonal wind. In addition to comparisons of inviscid and dissipative experiments, the effect of linear and nonlinear drag formulations is compared. Starting from a small-amplitude perturbation in the temperature field, a modal structure emerges and grows exponentially for a few days. Unstable waves assume a structure that reduces frictional energy IOU when surface drag is present, but they still retain a normal mode character during a period of rapid growth. As the wave grows in amplitude, the ratio of upper-level to low-level eddy kinetic energy increases substantially in the presence of nonlinear surface drag. In the absence of surface drag or in the presence of linear drag the waves experience less structural change. Surface processes reduce the maximum amplitude achieved by the wave and damp the slowly growing wavenumber-3 and shallow wavenumber-12 disturbances more effectively than the rapidly growing, deep wavenumber 7.In the mature wave, surface momentum drag and heat flux suppress eddy velocity and temperature fields near the surface, causing the meridional heat flux to peak at about 800 mb rather than near the surface as itdoes when surface fluxes are excluded. When surface fluxes are present, the structures of mature waves resemble observations more closely than when the fluxes are absent. When initial conditions are similar to those used by Simmons and Hoskins, the Eliassen-Palm flux produced by the mature wave tends to converge in the upper troposphere, primarily as the result of the vertical gradient in poleward heat flux. However, the convergence is sensitive to initial conditions and is spread more broadly through the troposphere for other configurations of the initial state.

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William J. Gutowski Jr., Lee E. Branscome, and Douglas A. Stewart

Abstract

The interaction between moisture and baroclinic eddies was examined through eddy life-cycle experiments using a global, primitive equation model. How condensation affects the structural evolution of eddies, their fluxes of heat, moisture, and momentum, and their subsequent interaction with the zonal average state was examined. Initial states corresponded to climatological winter and summer zonal average states. For most experiments the perturbation had a fundamental zonal wavenumber 7, representing an appropriate scale for transient eddies that reach substantial amplitudes in the atmosphere. Additional experiments used fundamental wavenumber 4, 10, or 14.

The wave's vertical motion produced midtropospheric supersaturation whose heating further amplified the vertical motion. Consequently, the largest effects of condensation were associated with vertical transports. Compared to corresponding dry experiments, intensified vertical motions increased the maximum kinetic energy attained by the wave, but they also depleted the eddy available potential energy more rapidly, thus inducing a faster evolution of the life cycle. Even greater condensation occurred near the surface as warm, moist air moving poleward became supersaturated by heat loss into a cooler surface. However, the latent heat thus released was balanced by the heat loss into the surface and so produced no dynamical effect. The hydrological cycle induced by the wave was largely confined to the lower troposphere, but the strongest effects of condensation on eddy dynamics occurred in the upper troposphere, so the condensational heating altered only weakly the intensity of the wave-induced moisture cycle.

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