Duane E. Stevens

Potential Vorticity Modeling of the ITCZ and the Hadley Circulation

Abstract A simple zonally symmetric balanced model of the Hadley circulation is presented. The model is based on potential vorticity arguments and consists of a predictive equation for the potential pseudodensity and an invertibility principle to diagnose the associated balanced wind and mass fields. When the theory is formulated in the potential latitude coordinate, the meridional advection is implicit in the coordinate transformation, which makes the prediction equation for potential pseudodensity analytically solvable. For convective heating patterns that simulate the ITCZ, the model produces upper and lower tropospheric potential vorticity anomalies of opposite sign. The associated winds are easterly at low levels and westerly aloft, except between the equator and the ITCZ, where there are low-level westerlies and upper-level easterlies. Since the potential vorticity anomalies develop within a background state that has potential vorticity increasing to the north, reversed poleward gradients of potenti...

The Role of Baroclinic Processes in Tropical Cyclone Motion: The influence of Vertical Tilt

Abstract The numerical study presented here focuses on baroclinic processes that contribute to tropical cyclone (TC) propagation. A three-dimensional, semispectral, primitive equation model of baroclinic vortex was developed to study TC motion. In a tilted vortex, interaction between upper- and lower-level vorticity anomalies leads to vortex propagation relative to the steering flow. On a, β plane, with no environmental flow, the vortex is tilted toward the south and the interaction between the layers reduces the westward movement of the vortex. The vortex tilting can also occur due to the vertical shear in the environmental wind. On an f plane, the interaction between the layers causes the northward movement of the vortex in westerly linear shear, and southward movement in easterly linear shear, with a meridional velocity of about 1 m s−1. Ibis velocity increases with increasing vortex intensity and vertical motion.

On symmetric stability and instability of zonal mean flows near the equator

Abstract Obserations of longitudinally-averaged zonal flows in the atmosphere and ocean tend to display north–south symmetry about the equator, with a characteristic wind maximum or minimum, and therefore little horizontal wind shear locally near the equator. It is shown that this configuration is required for balanced flow on a sphere to be inertially stable. If dissipation can be neglected, any horizontal wind shear at the equator will cause inertial instability to develop. effectively eliminating the horizontal shear. It follows that the potential vorticity (q) must vanish at the equator for the symmetric circulation to be stable; and that it must increase to the north and decrease to the south. Balanced cross-equatorial flow can occur only if there is a north–south gradient in the torque or the diabatic heating at the equator. These conclusions are obtained under the assumption of a balanced zonal flow; i.e., acceleration and dissipation are explicitly neglected in the meridional momentum equation. Th...

Developing versus Nondeveloping Disturbances for Tropical Cyclone Formation. Part II: Western North Pacific*

GlobaldailyreanalysisfieldsfromtheNavyOperationalGlobalAtmosphericPredictionSystem(NOGAPS) are used to analyze Northern Hemisphere summertime (June‐September) developing and nondeveloping disturbances for tropical cyclone (TC) formation from 2003 to 2008. This is Part II of the study focusing on the westernNorthPacific(WNP),followingPartIfortheNorthAtlantic(NATL)basin.Tropicalcyclonegenesisin the WNP shows different characteristics from that in the NATL in both large-scale environmental conditions and prestorm disturbances. A box difference index (BDI) is used to identify parameters in differentiating between the developing and nondeveloping disturbances. In order of importance, they are 1) 800-hPa maximum relative vorticity, 2) rain rate, 3) vertically averaged horizontal shear, 4) vertically averaged divergence, 5) 925‐400-hPa water vapor content, 6) SST, and 7) translational speed. The study indicates that dynamic variables are more important in TC genesis in the WNP, while in Part I of the study the thermodynamic variables are identified as more important in the NATL. The characteristic differences between the WNP and the NATL are compared.

Response of the Hadley Circulation to Convective Forcing in the ITCZ

Abstract Through the use of a zonal balance model we investigate the response of the mean meridional circulation to a specified diabatic forcing for both resting and nonresting zonal flows. The use of a potential latitude coordinate and transformed meridional circulation components results in a simplified meridional circulation equation in which the variable coefficients are the normalized potential vorticity and inertial stability. Solutions of this equation illustrate how latent heat release away from the equator forces a winter hemisphere Hadley cell that is more intense than the summer hemisphere cell. This asymmetric response is due primarily to the anisotropy associated with the spatial variation of the inertial stability field. Despite the sensitivity of the meridional circulation to the location and breadth of the forcing, the low latitude thermodynamic response is for the most part insensitive as long as the total latent heat release remains the same. Numerical solutions of the zonal balance mode...

Developing versus Nondeveloping Disturbances for Tropical Cyclone Formation. Part I: North Atlantic*

AbstractThis study investigates the characteristic differences of tropical disturbances that eventually develop into tropical cyclones (TCs) versus those that did not, using global daily analysis fields of the Navy Operational Global Atmospheric Prediction System (NOGAPS) from the years 2003 to 2008. Time filtering is applied to the data to extract tropical waves with different frequencies. Waves with a 3–8-day period represent the synoptic-scale disturbances that are representatives as precursors of TCs, and waves with periods greater than 20 days represent the large-scale background environmental flow. Composites are made for the developing and nondeveloping synoptic-scale disturbances in a Lagrangian frame following the disturbances. Similarities and differences between them are analyzed to understand the dynamics and thermodynamics of TC genesis. Part I of this study focuses on events in the North Atlantic, while Part II focuses on the western North Pacific.A box difference index (BDI), accounting for...

A Study of Planetary Waves in the Southern Winter Troposphere and Stratosphere. Part II: Life Cycles

Abstract Large-amplitude planetary waves in the southern winter stratosphere are observed to occur episodically, the result of episodic tropospheric forcing. This work is an observational study of the dynamics of the planetary waves, focusing on the evolution through a typical life cycle. Time lag correlations of wave amplitude with the Eliassen-Palm flux vector reveal the characteristic heat and momentum flux patterns associated with wave evolution. Energetic studies clearly show that the stratospheric waves can be understood in terms of a life cycle of vertical propagation from the troposphere, followed by decay from barotropic interactions with the zonal mean flow. Although usually of secondary importance baroclinic decay of stratospheric wave energy is also observed, resulting from equatorward heat flux in the lower stratosphere. Good agreement in the energy balances discounts in situ instability in the stratosphere as a source of wave activity. An average or composite over several clearly propagating...

Tropical Wave-CISK with a Moisture Budget and Cumulus Friction

Abstract The wave-CISK problem is investigated in a linearized context with two significant departures from previous models: 1) the cumulus heating parameterization is made thermodynamically consistent by requiring that cumulus heating exactly equal the latent heat of moisture converged in the wave; and 2) cumulus friction is included in the model. The parameterization of vertical transport of horizontal momentum by cumulus clouds follows the formulation of Schneider and Lindzen (1976); with a constant mean zonal wind, cumulus friction is proportional to the mean cloud mass flux. Results from the model of Stevens et at. (1977) are presented for various scales of motion. For magnitudes of cloud mass flux typical of the ITCZ, no wave-CISK modes appear on the synoptic scale of several thousand kilometers. As the horizontal scales of these long-period waves are decreased, the disturbances become frictionally controlled, neutral wave-CISK modes. When the mean cloud mass flux is reduced by a factor of 2, instab...

Temporal Variations of the Tropical 40-50 Day Oscillation

Abstract In recent years there has been a great deal of interest in a quasi-periodic tropical oscillation of zonal winds, which was first reported by Madden and Julian. An attempt to determine the temporal variation of the oscillation parameters is presented here. Using a 4-year duration global time series and a 25-year station time series, we find that although the nonseasonal variations are large, any seasonal cycle in the oscillation amplitude and frequency must be very small. The small seasonal signal in the oscillation frequency seems to argue against explanations for the time scale based on Doppler-shifted traveling waves.

Inertial Instability of Horizontally Sheared Flow away from the Equator

Abstract We investigate the temporal and spatial characteristics of unstable normal modes in a horizontally sheared flow on a sphere using the shallow water equations. Both inertial and barotropic instabilities are identified in cases where the appropriate necessary conditions are satisfied. A primary focus is determining what conditions favor asymmetric modes of inertial instability rather than symmetric modes. With the Bickley jet profile, the region of instability [f(f + &xi) ≤ 0] is confined to the anticyclonic side of the jet in a limited region. We find that symmetric instability is preferred only for modes of very small vertical wide, for which the pressure gradient force is secondary. Relatively small dissipation is needed to stabilize these modes. With deeper vertical scales, asymmetric instabilities are preferred in which the zonal scale of the instability is comparable to the width of the unstable region. This study extends previous results for linear shear on an equatorial beta plane to the mi...