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John Molinari and David Vollaro

Abstract

Helicity was calculated in Hurricane Bonnie (1998) using tropospheric-deep dropsonde soundings from the NASA Convection and Moisture Experiment. Large helicity existed downshear of the storm center with respect to the ambient vertical wind shear. It was associated with veering, semicircular hodographs created by strong, vortex-scale, radial-vertical flow induced by the shear. The most extreme values of helicity, among the largest ever reported in the literature, occurred in the vicinity of deep convective cells in the downshear-left quadrant. These cells reached as high as 17.5 km and displayed the temporal and spatial scales of supercells.

Convective available potential energy (CAPE) averaged 861 J kg−1 downshear, but only about one-third as large upshear. The soundings nearest the deep cells were evaluated using two empirical supercell parameters that make use of CAPE, helicity, and/or shear. These parameters supported the possible existence of supercells as a consequence of the exceptional helicity combined with moderate but sufficient CAPE. Ambient vertical wind shear exceeded 12 m s−1 for 30 h, yet the hurricane maintained 50 m s−1 maximum winds. It is hypothesized that the long-lived convective cells enabled the storm to resist the negative impact of the shear.

Supercells in large-helicity, curved-hodograph environments appear to provide a useful conceptual model for intense convection in the hurricane core. Helicity calculations might also give some insight into the behavior of vortical hot towers, which share some characteristics with supercells.

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Patrick Duran and John Molinari

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Dropsondes with horizontal spacing as small as 4 km were released from the stratosphere in rapidly intensifying Hurricane Patricia (2015) during the Office of Naval Research Tropical Cyclone Intensity experiment. These observations provide cross sections of unprecedented resolution through the inner core of a hurricane. On 21 October, Patricia exhibited a strong tropopause inversion layer (TIL) across its entire circulation, with a maximum magnitude of 5.1 K (100 m)−1. This inversion weakened between 21 and 22 October as potential temperature θ increased by up to 16 K just below the tropopause and decreased by up to 14 K in the lower stratosphere. Between 22 and 23 October, the TIL over the eye weakened further, allowing the tropopause to rise by 1 km. Meanwhile over Patricia’s secondary eyewall, the TIL restrengthened and bulged upward by about 700 m into what was previously the lower stratosphere. These observations support many aspects of recent modeling studies, including eyewall penetration into the stratosphere during rapid intensification (RI), the existence of a narrow inflow layer near the tropopause, and the role of subsidence from the stratosphere in developing an upper-level warm core. Three mechanisms of inner-core tropopause variability are hypothesized: destabilization of the TIL through turbulent mixing, weakening of the TIL over the eye through upper-tropospheric subsidence warming, and increasing tropopause height forced by overshooting updrafts in the eyewall. None of these processes are seen as the direct cause of RI, but rather part of the RI process that includes strong increases in boundary layer moist entropy.

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John Molinari and David Vollaro

Abstract

The previous study of helicity, CAPE, and shear in Hurricane Bonnie (1998) was extended to all eight tropical cyclones sampled by NASA during the Convection and Moisture Experiments (CAMEX). Storms were categorized as having large or small ambient vertical wind shear, with 10 m s−1 as the dividing line. In strongly sheared storms, the downshear mean helicity exceeded the upshear mean by a factor of 4. As in the previous study, the helicity differences resulted directly from the tropical cyclone response to ambient shear, with enhanced in-up-out flow and veering of the wind with height present downshear. CAPE in strongly sheared storms was 60% larger downshear. Mean inflow near the surface and the depth of the inflow layer each were 4 times larger downshear. At more than 30% of observation points outside the 100-km radius in the downshear right quadrant, midlatitude empirical parameters indicated a strong likelihood of supercells. No such points existed upshear in highly sheared storms. Much smaller upshear–downshear differences and little likelihood of severe cells occurred in storms with ambient wind shear below 10 m s−1. In addition to these azimuthal asymmetries, highly sheared storms produced 30% larger area-averaged CAPE and double the area-averaged helicity versus relatively unsheared storms. The vortex-scale increase in these quantities lessens the negative impact of large vertical wind shear.

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Patrick Duran and John Molinari

Abstract

High-vertical-resolution rawinsondes were used to document the existence of low–bulk Richardson number (R b) layers in tropical cyclones. The largest frequency of low R b existed in the inner 200 km at the 13.5-km level. This peak extended more than 1000 km from the storm center and sloped downward with radius. The presence of an extensive upper-tropospheric low-R b layer supports the assumption of Richardson number criticality in tropical cyclone outflow by Emanuel and Rotunno.

The low-R b layers were found to be more common in hurricanes than in tropical depressions and tropical storms. This sensitivity to intensity was attributed to a reduction of upper-tropospheric static stability as tropical cyclones intensify. The causes of this destabilization include upper-level cooling that is related to an elevation of the tropopause in hurricanes and greater longwave radiative warming in the well-developed hurricane cirrus canopy. Decreased mean static stability makes the production of low R b by gravity waves and other perturbations easier to attain.

The mean static stability and vertical wind shear do not exhibit diurnal variability. There is some indication, however, that low Richardson numbers are more common in the early morning than in the early evening, especially near the 200–300-km radius. The location and timing of this diurnal variability is consistent with previous studies that found a diurnal cycle of infrared brightness temperature and rainfall in tropical cyclones.

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John Molinari and David Vollaro

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A weak tropical storm (Gabrielle in 2001) experienced a 22-hPa pressure fall in less than 3 h in the presence of 13 m s−1 ambient vertical wind shear. A convective cell developed downshear left of the center and moved cyclonically and inward to the 17-km radius during the period of rapid intensification. This cell had one of the most intense 85-GHz scattering signatures ever observed by the Tropical Rainfall Measuring Mission (TRMM). The cell developed at the downwind end of a band in the storm core. Maximum vorticity in the cell exceeded 2.5 × 10−2 s−1. The cell structure broadly resembled that of a vortical hot tower rather than a supercell.

At the time of minimum central pressure, the storm consisted of a strong vortex adjacent to the cell with a radius of maximum winds of about 10 km that exhibited almost no tilt in the vertical. This was surrounded by a broader vortex that tilted approximately left of the ambient shear vector, in a similar direction as the broad precipitation shield. This structure is consistent with the recent results of Riemer et al.

The rapid deepening of the storm is attributed to the cell growth within a region of high efficiency of latent heating following the theories of Nolan and Vigh and Schubert. This view is supported by a rapid growth of wind speed and vorticity in the storm core during the 1-h lifetime of the cell, and by the creation of a narrow 7°C spike in 700-hPa temperature adjacent to the cell and coincident with the lowest pressure. The cell is not seen as the cause of rapid intensification. Rather, it is part of a multiscale process: (i) development of a new circulation center within the downshear precipitation shield, (ii) continued ambient shear creating a favored region for cell formation just downshear of the new center, and (iii) the development of the intense cell that enhanced diabatic heating close to the center in a region of high efficiency of kinetic energy production. This sheared, asymmetric rapid intensification of Tropical Storm Gabrielle is contrasted with the nearly symmetric composite given by Kaplan and DeMaria.

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Anantha R. Aiyyer and John Molinari

Abstract

A linear shallow water model is used to simulate the evolution of mixed Rossby–gravity (MRG) waves in background states representative of the convective phase of the Madden–Julian oscillation (MJO). Initial MRG wave structures are obtained analytically. The MJO basic state is defined by the steady response of the tropical atmosphere to localized heating. Results from the simulations reveal that variations in the background flow play a significant role in the evolution of the MRG waves. When the basic state is symmetric about the equator, the MRG wave amplifies within the convergent region of the background flow and the ensuing development remains symmetric. When the heating is asymmetric, both the basic state and the MRG wave evolution exhibit significant asymmetries. Prominent features of this simulation are the development and growth of a series of small-scale, off-equatorial eddies that resemble tropical-depression-type disturbances.

The results suggest that a persistent large-scale heating that is asymmetric with respect to the equator may lead to the growth of off-equatorial disturbances from an equatorial mode. These disturbances, approximately 1000–2000 km in scale, are considerably smaller than the initial wavelength of the MRG wave. It is suggested that the cyclonic elements among them could serve as seedlings for tropical cyclones. This process may be particularly relevant to cyclogenesis in the tropical western Pacific, a region where the MJO and MRG waves are frequently observed.

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Kay L. Shelton and John Molinari

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Hurricane Claudette developed from a weak vortex in 6 h as deep convection shifted from downshear into the vortex center, despite ambient vertical wind shear exceeding 10 m s−1. Six hours later it weakened to a tropical storm, and 12 h after the hurricane stage a circulation center could not be found at 850 hPa by aircraft reconnaissance. At hurricane strength the vortex contained classic structure seen in intensifying hurricanes, with the exception of 7°–12°C dewpoint depressions in the lower troposphere upshear of the center. These extended from the 100-km radius to immediately adjacent to the eyewall, where equivalent potential temperature gradients reached 6 K km−1. The dry air was not present prior to intensification, suggesting that it was associated with vertical shear–induced subsidence upshear of the developing storm. It is argued that weakening of the vortex was driven by cooling associated with the mixing of dry air into the core, and subsequent evaporation and cold downdrafts. Evidence suggests that this mixing might have been enhanced by eyewall instabilities after the period of rapid deepening. The existence of a fragile, small, but genuinely hurricane-strength vortex at the surface for 6 h presents difficult problems for forecasters. Such a “temporary hurricane” in strongly sheared flow might require a different warning protocol than longer-lasting hurricane vortices in weaker shear.

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Leon T. Nguyen and John Molinari

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Hurricane Irene (1999) rapidly intensified from 65 to 95 kt (~33.4 to 48.9 m s−1) in 18 h. During the rapid intensification (RI) period, the northeastward storm motion increased from 10 to 18 m s−1, the ambient southwesterly vertical wind shear increased from 6–7 to 10–13 m s −1, and the downshear tilt of the inner core vortex increased. The azimuthal wavenumber-1 asymmetric convection that developed was consistent with a superposition of shear-induced and storm motion–induced forcing for vertical motion downshear and ahead of the center. Although the diabatic heating remained strongly asymmetric, it was of sufficient intensity to dramatically increase the azimuthally averaged heating. This heating occurred almost entirely inside the radius of maximum winds, a region known to favor rapid warm core development and spinup of the vortex. It is hypothesized that asymmetric forcing from the large vertical wind shear and rapid storm motion were responsible for RI. An unanswered question is what determines whether the heating will develop within the radius of maximum winds. Extraordinarily deep cells developed in the inner core toward the end of the RI period. Rather than causing RI, these cells appeared to be an outcome of the same processes noted above.

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Kristen L. Corbosiero and John Molinari

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The influence of the direction of storm motion on the azimuthal distribution of electrified convection in 35 Atlantic basin tropical cyclones from 1985 to 1999 was examined using data from the National Lightning Detection Network. In the inner 100 km, flashes most often occurred in the front half of storms, with a preference for the right-front quadrant. In the outer rainbands (r = 100–300 km), flashes occurred predominantly to the right of motion, although the maximum remained in the right-front quadrant. The results are shown to be consistent with previous studies of asymmetries in rainfall, radar reflectivity, and vertical motion with respect to tropical cyclone motion. The motion effect has been attributed to the influence of asymmetric friction in the tropical cyclone boundary layer.

The authors previously found a strong signature in the azimuthal distribution of lightning with respect to vertical wind shear. Because both effects show clearly, vertical wind shear and storm motion must themselves be systematically related. It was found that more than three-quarters of 12-hourly periods contained a storm motion vector that was left of (i.e., counterclockwise from) the shear vector. These results support the importance of a downshear shift in the upper anticyclone, which produces motion left of shear for all directions of shear. The results are further broken down by direction of shear, and it is shown that the beta effect also plays a significant role in the relationship between motion and vertical wind shear. These results also suggest that substantial downshear tilt of the cyclonic part of the tropical cyclone vortex is uncommon, because that alone produces motion right of shear.

The relative importance of asymmetric friction and vertical wind shear on the azimuthal asymmetry of convection was determined by examining circumstances in which the two effects would place maximum lightning in different quadrants. Without exception, the influence of vertical wind shear dominated the distribution. Although asymmetric friction creates vertical motion asymmetries at the top of the boundary layer, these apparently do not produce deep convection if vertical wind shear–induced circulations oppose them.

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Leon T. Nguyen and John Molinari

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The downshear reformation of Tropical Storm Gabrielle (2001) was simulated at 1-km horizontal resolution using the Weather Research and Forecasting (WRF) Model. The environmental shear tilted the initial parent vortex downshear left and forced azimuthal wavenumber-1 kinematic, thermodynamic, and convective asymmetries. The combination of surface enthalpy fluxes and a lack of penetrative downdrafts right of shear allowed boundary layer moist entropy to increase to a maximum downshear right. This contributed to convective instability that fueled the downshear convection. Within this convection, an intense mesovortex rapidly developed, with maximum boundary layer relative vorticity reaching 2.2 × 10−2 s−1. Extreme vortex stretching played a key role in the boundary layer spinup of the mesovortex. Cyclonic vorticity remained maximized in the boundary layer and intensified upward with the growth of the convective plume.

The circulation associated with the mesovortex and adjacent localized cyclonic vorticity anomalies comprised a developing “inner vortex” on the downshear-left (downtilt) periphery of the parent cyclonic circulation. The inner vortex was nearly upright within a parent vortex that was tilted significantly with height. This inner vortex became the dominant vortex of the system, advecting and absorbing the broad, tilted parent vortex. The reduction of tropical cyclone (TC) vortex tilt from 65 to 20 km in 3 h reflected the emerging dominance of this upright inner vortex. The authors hypothesize that downshear reformation, resulting from diabatic heating associated with asymmetric convection, can aid the TC’s resistance to shear by reducing vortex tilt and by enabling more diabatic heating to occur near the center, a region known to favor TC intensification.

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