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John Molinari and Steven Skubis

Abstract

The surface wind field in a developing tropical cyclone (Agnes, 1972) was analyzed over a 1660 km radius for four days using conventional surface data, as the storm evolved from a disorganized depression to a hurricane. The transition to hurricane intensity was characterized by a wavelike disturbance propagating inward at 15 m s−1 from the outermost radii to the storm core over a 36-hour period. This propagating disturbance was clearly visible in the radial and vertical motion fields as a surge of inflow and upward motion. Rapid intensification of the storm began within hours after the leading edge of the surge reached the storm center. The analysis of consecutive 12-hour periods without compositing of data from nonsynoptic times was essential for identification of this feature.

The surge had the same asymmetry as the upper-level outflow channel, indicating the possible involvement of the outflow layer in its initiation. No clear evidence of an external forcing mechanism for the surge, such as the passage of an easterly wave across the circulation, could be found. No instability theory could account for propagation of this feature across regions with such strongly varying dynamical properties. As a result, it remains uncertain whether the inflow surge represented an environmental trigger to hurricane formation or a manifestation of an internal instability.

The boundary layer momentum budget was dominated by Coriolis torque and frictional dissipation. The sum of these two terms acted as a momentum source primarily during the passage of the inflow surge across each radial region. Inward lateral flux of momentum contributed significantly only within 440 km of the center.

A distinct diurnal oscillation in pressure tendency occurred until hurricane strength was reached, with maximum deepening at 1200 local time, and minimum deepening at 0000 local time. Diurnal oscillations in other variables were more subtle and often at variance with those described in other tropical cyclones.

Because the inflow surge developed at outer radii 36 hours prior to rapid deepening and had a clear signature in the time change of radial mass flux, it provides a potential tool for forecasting tropical cyclogenesis 24 hours or more in advance which requires only the use of conventional data. More study is needed to determine whether such an early warning signal frequently occurs in intensifying tropical cyclones.

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John Molinari and Steven Skubis

Abstract

A simple framework is presented for adjusting the normal wind components in a polygon of data points which produces a vanishing vertical integral of horizontal divergence, allows correct calculation of flux and advective terms, and permits virtually any choice of vertical profile of divergence adjustment. The procedure was used to estimate precipitation as a residual from vertically integrated heat and moisture budgets for SESAME data, in order to evaluate the uncertainty introduced by commonly used approximations in diagnostic studies. Although the method cannot be applied on a grid in its current form, the results remain valid for gridded calculations.

Line integrals around the polygon were carried out analytically, allowing an exact calculation of eddy fluxes within the assumption of linearity along the edges. Finite difference approximations for nonlinear terms were shown to introduce significant errors, even under ordinary circumstances.

It is common practice to neglect the horizontal advecting velocity adjustment brought about by the adjusted divergence. Such an assumption produced negligible median errors in the integrated heat and moisture budgets. The median differences in calculated precipitation caused by differing choices of the divergence adjustment profile reached 1.34 andO.35 cm day−1 in the heat and moisture budgets, respectively. Because the true divergence adjustment profile is unknown, these values represent median lower bounds on the errors in budget estimates of precipitation in middle latitude convection.

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John Molinari, David Vollaro, and Steven Skubis

Abstract

The Eliassen balanced vortex model assumes gradient balance of the azimuthal mean flow. This assumption was tested by calculating mean and eddy terms in the radial momentum equation in the synoptic-scale environments of two tropical cyclones. The azimuthally averaged gradient balance was accurate to within 15%–25% in the free atmosphere outside the core, even in the asymmetric outflow layer. Balanced secondary circulations correlated well with circulations that included gradient thermal wind imbalance terms. Although the balanced model lacks Galilean invariance, balanced circulations were largely insensitive to use of a fixed coordinate or a coordinate moving with the storm. This occurred because changes in eddy heat and angular momentum fluxes largely offset one another. The two-dimensional balanced solutions provide a reasonably robust measure of circulations induced by azimuthal eddy processes in the tropical cyclone environment.

Nevertheless, individual forcing functions, such as the commonly examined lateral eddy flux convergence of angular momentum, often varied enormously between fixed and moving coordinates. Logic and available evidence suggest that such terms are meaningful only in a coordinate system moving with the storm.

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John Molinari, Steven Skubis, and David Vollaro

Abstract

The interaction of Hurricane Elena (1985) with a baroclinic wave was reexamined using both potential vorticity (PV) and a formulation for Eliassen-Palm fluxes in cylindrical coordinates. The hurricane began to deepen rapidly as a narrow upper-level positive PV anomaly became nearly superposed over the low-level center. The intensification appeared to represent an evaporation-wind feedback activated by constructive interference of the PV anomalies. Enhanced convection associated with this process eroded the upper PV anomaly and prevented it from crossing the hurricane and reversing the intensification.

The Eliassen–Palm flux divergence showed that maximum eddy activity remained in the upper troposphere prior to the reintensification. This activity was produced in part because the action of the outflow anticyclone of the hurricane contributed to synoptic-scale wave breaking. The upper-level PV anomaly that approached the center was much narrower in extent than the original synoptic-scale trough. The deep layer of vertical wind shear that would have prevented intensification of the hurricane was avoided.

It is concluded that the interaction of a tropical cyclone and a synoptic-scale trough cannot be viewed simply as the bringing together of positive PV anomalies. Rather, the outflow anticyclone, constantly reinforced by the large source of low PV air from the storm core, interacts with and resists shearing by the trough. Whether the hurricane intensifies during such interactions depends to a large extent upon the relative strengths of these positive and negative PV anomalies. Such outflow-layer interactions represent a fruitful area for further research into tropical cyclone intensity change.

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John Molinari, David Vollaro, Steven Skubis, and Michael Dickinson

Abstract

The genesis of Hurricane Hernan (1996) in the eastern Pacific was investigated using gridded analyses from the European Centre for Medium-Range Weather Forecasts and gridded outgoing longwave radiation. Hernan developed in association with a wave in the easterlies that could be tracked back to Africa in longitude–time plots of the filtered υ component of the wind (2–6-day period) at 700 mb. The wave crossed Central America near Lake Nicaragua with little change in its southwest–northeast tilt, but the most intense convection shifted from near the wave axis in the Caribbean to west of the wave axis in the Pacific. The wave intensified as it moved through a barotropically unstable background state (defined by a low-pass filter with a 20-day cutoff) in the western Caribbean and eastern Pacific. A surge in the southwesterly monsoons and enhanced convection along 10°N occurred to the west of the 700-mb wave in the Pacific and traveled with the wave. This had the effect of enhancing low-level vorticity over a wide region ahead of the 700-mb wave. Available evidence suggests that additional low-level vorticity was produced by enhanced flow from the north through the Isthmus of Tehuantepec as the 700-mb wave approached. Depression formation did not occur until 6–12 h after the 700-mb wave reached this region of large low-level vorticity in the Gulf of Tehuantepec.

Eastern Pacific SST and vertical wind shear magnitude are typically favorable for tropical cyclone development in Northern Hemisphere summer and early fall. Because the favorable mountain interaction and the surge in the low-level monsoons appear to relate directly to the wave in the easterlies, it is argued that the strength of such waves reaching Central America from the east is the single most important factor in whether subsequent eastern Pacific cyclogenesis occurs. Possible parallels with western Pacific cyclogenesis are discussed.

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John Molinari, David Knight, Michael Dickinson, David Vollaro, and Steven Skubis

Abstract

A significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310-K isentropic surface (near 700 mb) over the Caribbean Sea. The Charney–Stern necessary condition for instability of the mean flow is met in this region. It is speculated that the sign reversal permits either invigoration of African waves or actual generation of easterly waves in the Caribbean.

During the same season, a correlation existed between the strength of the negative potential vorticity gradient in the Caribbean and subsequent cyclogenesis in the eastern Pacific. The meridional PV gradient, convective heating measured by outgoing longwave radiation data, and eastern Pacific cyclogenesis all varied on the timescale of the Madden–Julian oscillation (MJO). It is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the MJO (or any other convective forcing) and the associated enhanced downstream tropical cyclogenesis.

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Ronald B. Smith, Steven Skubis, James D. Doyle, Adrian S. Broad, Christoph Kiemle, and Hans Volkert

Abstract

A stationary mountain wave, embedded in southwesterly flow over Mont Blanc in the Alps, was observed simultaneously by three research aircraft and three types of remote sensing: GPS dropsondes, airborne light detecting and ranging (lidar), and rapid-scan satellite imagery. These observations provide a basis for testing linear and nonlinear theories of how mountain waves over complex terrain are controlled by the ambient wind profile, especially the effects of a low-level stagnant layer and the jet stream aloft. The layer of blocked flow near the ground reduced the amplitude of the wave generation. The strong wind and weak stability in the upper troposphere forced the wave into a decaying “evanescent” state. In spite of this evanescent condition, no lee waves were observed. The authors resolve this paradox by demonstrating that the stagnant layer below 3 km played an additional role. It was able to absorb downward reflected waves, preventing the formation of a resonant cavity. Linear theory, including this low-level absorption, predicts the observed wave structure quite well and captures the wave absorption process found in the fully nonlinear Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. In spite of wave decay through the upper troposphere, there is evidence from satellite images and model simulation that the waves reached the uppermost troposphere.

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John Molinari, Steven Skubis, David Vollaro, Frank Alsheimer, and Hugh E. Willoughby

Abstract

The interaction of marginal Tropical Storm Danny (1985) with an upper-tropospheric positive potential vorticity anomaly was examined. The intensification mechanism proposed earlier for mature Hurricane Elena appears to be valid for Danny as well, despite significant differences in the synoptic-scale environment and in the stage of the tropical cyclone prior to the interaction. Both storms experienced rapid pressure falls as a relatively small-scale positive upper potential vorticity anomaly began to superpose with the low-level tropical cyclone center.

The interaction is described in terms of a complex interplay between vertical wind shear, diabatic heating, and mutual advection among vortices at and below the level of the outflow anticyclone. Despite this complexity, the superposition principle appears to be conceptually useful to describe the intensification of tropical cyclones during such interactions.

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