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Kristen L. Corbosiero and John Molinari

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The influence of vertical wind shear on the azimuthal distribution of cloud-to-ground lightning in tropical cyclones was examined using flash locations from the National Lightning Detection Network. The study covers 35 Atlantic basin tropical cyclones from 1985–99 while they were over land and within 400 km of the coast over water. A strong correlation was found between the azimuthal distribution of flashes and the direction of the vertical wind shear in the environment. When the magnitude of the vertical shear exceeded 5 m s−1, more than 90% of flashes occurred downshear in both the storm core (defined as the inner 100 km) and the outer band region (r = 100–300 km). A slight preference for downshear left occurred in the storm core, and a strong preference for downshear right in the outer rainbands. The results were valid both over land and water, and for depression, storm, and hurricane stages. It is argued that in convectively active tropical cyclones, deep divergent circulations oppose the vertical wind shear and act to minimize the tilt. This allows the convection maximum to remain downshear rather than rotating with time.

The downshear left preference in the core is stronger for hurricanes than for weaker tropical cyclones. This suggests that the helical nature of updrafts in the core, which is most likely for the small orbital periods of hurricanes, plays a role in shifting the maximum lightning counterclockwise from updraft initiation downshear. The downshear right maximum outside the core resembles the stationary band complex of Willoughby et al. and the rain shield of Senn and Hiser. The existence and azimuthal position of this feature appears to be controlled by the magnitude and direction of the vertical wind shear.

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Patrick Duran and John Molinari

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Upper-level static stability (N 2) variations can influence the evolution of the transverse circulation and potential vorticity in intensifying tropical cyclones (TCs). This paper examines these variations during the rapid intensification (RI) of a simulated TC. Over the eye, N 2 near the tropopause decreases and the cold-point tropopause rises by up to 4 km at the storm center. Outside of the eye, N 2 increases considerably just above the cold-point tropopause and the tropopause remains near its initial level. A budget analysis reveals that the advection terms, which include differential advection of potential temperature θ and direct advection of N 2, are important throughout the upper troposphere and lower stratosphere. These terms are particularly pronounced within the eye, where they destabilize the layer near and above the cold-point tropopause. Outside of the eye, a radial–vertical circulation develops during RI, with strong outflow below the tropopause and weak inflow above. Differential advection of θ near the outflow jet provides forcing for stabilization below the outflow maximum and destabilization above. Turbulence induced by vertical wind shear on the flanks of the outflow maximum also modifies the vertical stability profile. Meanwhile, radiative cooling tendencies at the top of the cirrus canopy generally act to destabilize the upper troposphere and stabilize the lower stratosphere. The results suggest that turbulence and radiation, alongside differential advection, play fundamental roles in the upper-level N 2 evolution of TCs. These N 2 tendencies could have implications for both the TC diurnal cycle and the tropopause-layer potential vorticity evolution in TCs.

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John F. Festa and Robert L. Molinari

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A volunteer observing ship (VOS)-expendable bathythermograph (XBT) network has been proposed for the Atlantic Ocean to satisfy World Ocean Circulation Experiment (WOCE) objectives in the upper water column. These objectives include measuring changes in upper-layer temperature. An evaluation of the proposed WOCE XBT network to resolve variability in sea surface temperature (SST), temperature distribution at 150 m (T150), and average temperature of the upper 400 m (T400L) between 25°S and 35°N is performed. A sampling design study based on an optimum interpolation (OI) of the historical XBT dataset is used to construct uncertainty distributions for various XBT networks. The OI technique requires statistical representations of the variability (in the form of structure functions) of the three variables that are derived from the historical database. The structure functions and various sampling grids are used to construct uncertainty maps. Two seasons are used in the analysis of SST. In both seasons, uncertainties in mapped SST values for the proposed WOCE grid range from 0.3° to 0.4°C in regions of adequate data coverage. Errors are larger along the boundary. Uncertainties in the T150 fields are larger (0.5°−0.7°C) because of the smaller scales of spatial variability at depth. Errors in T400L range from 0.3° to 0.4°C. The effect of alternative observing strategies on the error maps are shown. Finally, error maps derived from the XBT network as it exists today (i.e., incomplete) are given. The maps indicate that monthly resolution is not available from the incomplete network.

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John Molinari, David Vollaro, and Steven Skubis

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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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Deborah Hanley, John Molinari, and Daniel Keyser

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The objective of this study is to understand how interactions with upper-tropospheric troughs affect the intensity of tropical cyclones. The study includes all named Atlantic tropical cyclones between 1985 and 1996. To minimize other factors affecting intensity change, times when storms are over subcritical sea surface temperatures (≤26°C) or near landfall are removed from the sample. A trough interaction is defined to occur when the eddy momentum flux convergence calculated over a 300–600-km radial range is greater than 10 (m s−1) day−1.

The trough interaction cases are separated into four composites: (i) favorable superposition [tropical cyclone intensifies with an upper-tropospheric potential vorticity (PV) maximum within 400 km of the tropical cyclone center], (ii) unfavorable superposition, (iii) favorable distant interaction (upper PV maximum between 400 and 1000 km from the tropical cyclone center), and (iv) unfavorable distant interaction. For comparison, two additional composites are created: (v) favorable no trough, and (vi) unfavorable no trough.

Tropical cyclones over warm water and away from land are more likely to intensify than weaken after an interaction with an upper-level trough; 78% of superposition cases and 61% of distant interaction cases deepened. In the favorable superposition composite, intensification begins soon after a small-scale upper-tropospheric PV maximum approaches the storm center. As in previous studies, the PV maximum subsequently weakens, most likely due to diabatic heating, and never crosses the center and reverses the deepening.

In the favorable distant interaction composite, the upper PV maximum remains well to the west of the tropical cyclone center, and intensification is not due to superposition. Strong upper-level divergence occurs downshear of the center, and an upper-level jet is located poleward of the maximum divergence. The center of the intensifying tropical cyclone is located in the right entrance region of the jet, where upward motion is favored. It is argued that the tropical cyclone and upper-level jet develop in a coupled fashion.

In the unfavorable distant interaction composite, weakening is attributed to a slightly larger and stronger upper PV maximum than occurs in the favorable distant interaction composite, which induces about 5 m s−1 more vertical wind shear over the tropical cyclone center. The fairly subtle PV changes that bring about this increase in vertical shear may help account for the difficulty in forecasting tropical cyclone intensity change during distant trough interactions.

The no-trough composites have dramatically smaller azimuthal asymmetries than those involving trough interactions. The major distinguishing factor between deepening and filling storms in the no-trough composites is the magnitude of the vertical wind shear.

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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, and Kristen L. Corbosiero

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The development of Hurricane Danny (1997) from depression to hurricane was examined using cloud-to-ground lightning data, reconnaissance aircraft data, and satellite imagery. Vertical wind shear between 850 and 200 hPa of 5–11 m s−1 produced persistent downshear convective outbreaks that became progressively more intense and closer to the center during the development. Early in the period the storm intensified steadily in the presence of this downshear convection. During the last and most intense outbreak, a second vortex appeared to develop within the convection. Evidence is presented that the new downshear vortex became the dominant vortex and absorbed the original. Based on these events, it is hypothesized that the presence of moderate vertical wind shear accelerated the early development process.

Equivalent potential temperature fields within 500 m of the surface were examined. Only well after the period of vortex interaction did the characteristic mature tropical cyclone radial profile of equivalent potential temperature appear. This came about by the virtual elimination of both low θ e values in the core and high θ e values outside the core that had been present at previous hours.

The growth of Hurricane Danny is viewed in terms of the wind-induced surface heat exchange (WISHE) theory. During the tropical depression and early tropical storm (“pre-WISHE”) periods, few if any of the assumptions of WISHE were met: vertical wind shear exceeded 5 m s−1, considerable azimuthal asymmetry was present, transient highly buoyant convection occurred, and low values of θ e in the storm core suggested the presence of convective downdrafts. It is proposed that 1) vortex interactions and subsequent axisymmetrization produced a single dominant vortex at the surface, and 2) vertical mixing of moist entropy by strong convection moved the sounding toward moist neutrality. By this reasoning, the disturbance then met the key tenets of the known finite-amplitude WISHE instability, and the storm intensified to hurricane strength.

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Brian Crandall, John Molinari, and David Vollaro

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This case study examines the complex history of a tropical storm that formed southeast of a large subtropical gyre. In real time the tropical storm was incorrectly identified as being two separate storms, and at one time was mislocated by 465 km. The unique forecast problems associated with tropical cyclones within a subtropical gyre are described. The tropical storm propagated around the gyre and encountered a midlevel temperature gradient to the north. The interaction of the storm with this gradient produced a strong midtropospheric temperature dipole. Temperature advection within this feature produced a change in structure to a subtropical storm corotating with an upper low. The subtropical storm turned equatorward and nearly came to a halt as the upper low became aligned with the storm. As convection increased over warm water, the upper low shifted away from the center and the storm reversed direction and moved poleward. These sudden track changes have frequently been observed in the northwest Pacific, but the role of midtropospheric temperature gradients has not previously been addressed. Clear air at the gyre center coincided with a region of cold advection. A fishhook structure in the gyre cloudiness developed as a result of warm advection east and north of the gyre. The subtropical structure of the storm evolved within the fishhook. It is recommended that the Joint Typhoon Warning Center (JTWC) provide formal warnings on subtropical storms, because their baroclinic nature can produce dramatic track changes associated with the presence of upper lows near the center.

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John Molinari, Jaclyn Frank, and David Vollaro

Abstract

Tropical Storm Edouard (2002) experienced episodic outbreaks of convection downshear within the storm core in the presence of 11–15 m s−1 of ambient vertical wind shear. These outbreaks lasted 2–6 h and were followed by long periods with no deep convection. Flights from U.S. Air Force reconnaissance aircraft within the boundary layer were used to investigate the cause of one such oscillation. Low equivalent potential temperature θe air filled the boundary layer as convection ceased, creating a 4–6-K deficit in θe within the convective region. Soundings within 110 km of the center were supportive of convective downdrafts, with midlevel relative humidity below 15% and large downdraft CAPE. Deep convection ceased within 75 km of the center for more than 8 h. Tangential velocity reached hurricane force locally during the convective outbreak, then became nearly symmetric after convection stopped, arguably as a result of axisymmetrization, and the storm weakened. Nevertheless, the corresponding lack of convective downdrafts during this period allowed surface heat and moisture fluxes to produce substantial increases in boundary layer entropy. A new burst of convection followed. Consistent with recent papers it is argued that tropical cyclone intensification and decay can be understood as a competition between surface heat and moisture fluxes (“fuel”) and low-entropy downdrafts into the boundary layer (“antifuel”).

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John Molinari, Patrick Duran, and David Vollaro

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Dropsondes from the NOAA G-IV aircraft were used to examine the presence of low bulk Richardson numbers R B in tropical cyclones. At least one 400-m layer above z = 7.5 km exhibited R B < 1 in 96% of the sondes and R B ≤ 0.25 in 35% of the sondes. The latter represent almost certain turbulence. Sondes from major Hurricane Ivan (2004) were examined in detail. Turbulent layers fell into three broad groups. The first was found below cloud base near the edge of the central dense overcast (CDO) where relative humidity fell below 40%. Near-zero static stability existed within the turbulent layer with stability and shear maxima above it. This structure strongly resembled that seen previously from sublimation of precipitation beneath cloud base. The second type of turbulent layer was located within CDO clouds in the upper troposphere and was due almost entirely to near-zero static stability. This most likely arose as a result of cooling via longwave flux divergence below CDO top. The third type of turbulent layer existed well outside the CDO and was produced by large local vertical wind shear. The shear maxima associated with the beneath-cloud and outside-CDO turbulent layers produced a sharp transition from weak inflow below to strong outflow above. The results suggest that the CDO creates its own distinctive stability profile that strongly influences the distribution of turbulence and the transition to outflow in tropical cyclones.

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