Downshear-left Quadrant Research Articles

Examining Polarimetric Radar Observations of Bulk Microphysical Structures and Their Relation to Vortex Kinematics in Hurricane Arthur (2014)

Dual-polarization radar observations were taken of Hurricane Arthur prior to and during landfall, providing needed insight into the microphysics of tropical cyclone precipitation. A total of 30 h of data were composited and analyzed by annuli capturing storm features (eyewall, inner rainbands, and outer rainbands) and by azimuth relative to the deep-layer environmental wind shear vector. Polarimetric radar variables displayed distinct signatures indicating a transition from convective to stratiform precipitation in the downshear-right to downshear-left quadrants, which is an organization consistent with the expected kinematic asymmetry of a sheared tropical cyclone. In the downshear-right quadrant, vertical profiles of differential reflectivity ZDRand copolar correlation coefficient ρHVwere more vertically stretched within and above the melting layer at all annuli, which is attributed to convective processes. An analysis of specific differential phase KDPindicated that nonspherical ice particles had an increased presence in two layers: just above the melting level and near 8-km altitude. Here, convective updrafts generated ice particles in the lower layer, which were likely columnar crystals, and increased the available moisture in the upper layer, leading to increased planar crystal growth. A sharp transition in hydrometeor population occurred downwind in the downshear-left quadrant where ZDRand ρHVprofiles were more peaked within the melting layer. Above the melting layer, these signatures indicated reduced ice column counts and shape diversity owing to aggregation in a predominantly stratiform regime. The rainband quadrants exhibited a sharper transition compared to the eyewall quadrants owing to weaker winds and longer distances that decreased azimuthal mixing of ice hydrometeors.

Thermodynamic and Kinematic Influences on Precipitation Symmetry in Sheared Tropical Cyclones: Bertha and Cristobal (2014)

Prior studies have shown an association between symmetrically distributed precipitation and tropical cyclone (TC) intensification. Although environmental vertical wind shear typically forces an asymmetric precipitation distribution in TCs, the magnitude of this asymmetry can exhibit considerable variability, even among TCs that experience similar shear magnitudes. This observational study examines the thermodynamic and kinematic influences on precipitation symmetry in two such cases: Bertha and Cristobal (2014). Consistent with the impact of the shear, both TCs exhibited a tilted vortex as well as a pronounced azimuthal asymmetry, with the maximum precipitation occurring in the downshear-left quadrant. However, Bertha was characterized by more symmetrically distributed precipitation and relatively modest vertical motions, while Cristobal was characterized by more azimuthally confined precipitation and much more vigorous vertical motions. Observations showed three potential hindrances to precipitation symmetry that were more prevalent in Cristobal than in Bertha: (i) convective downdrafts that transported low entropy air downward into the boundary layer, cooling and stabilizing the lower troposphere downstream in the left-of-shear and upshear quadrants; (ii) subsidence in the upshear quadrants, which acted to increase the temperature and decrease the relative humidity of the midtroposphere, resulting in capping of the boundary layer; and (iii) lateral advection of midtropospheric dry air from the environment, which dried the TC’s upshear quadrants.

Electrification and Lightning in Idealized Simulations of a Hurricane-Like Vortex Subject to Wind Shear and Sea Surface Temperature Cooling

Abstract Relationships between intensity fluctuations, cloud microphysics, lightning variations, and electrical structures within idealized tropical cyclones are investigated with the cloud-resolving Collaborative Model for Multiscale Atmospheric Simulation (COMMAS). An initial strong tropical cyclone is subjected to either steady-state control conditions (CTRL), increased wind shear (SHEAR), or a reduction in sea surface temperature (SST). In CTRL, nearly all the lightning (>95%) occurred in the outer region (100 < r ≤ 300 km) and was overall very episodic in the inner core (r ≤ 100 km), consistent with observations. The inner-core updrafts were weaker and experienced greater depletion of cloud water by warm rain processes, which, in contrast to the deeper updrafts in the rainband convection, reduced the mixed-phase cloud depth and confined the bulk of the charging and lightning initiations to lower levels. Notably, larger flash rates were produced in the asymmetric inner core of the SHEAR case, with the majority of the flashes located in the downshear left quadrant, consistent with prior observational works. In contrast to CTRL, the more vigorous inner-core convection in SHEAR resulted in the formation of a prominent negative charge region and enhanced production of negative ground flashes. With a nearly identical filling rate as SHEAR, the introduction of cooler sea surface temperature in the SST case caused lightning activity to fade rapidly in both the inner core and rainbands.

On the asymmetric distribution of shear-relative typhoon rainfall

The Tropical Rainfall Measuring Mission (TRMM) 3B42 precipitation, the National Centers for Environmental Prediction (NCEP) Final analysis and the Regional Specialized Meteorological Center (RSMC) Tokyo best-track data during 2000–2015 are used to compare spatial rainfall distribution associated with Northwest Pacific tropical cyclones (TCs) with different vertical wind shear directions and investigate possible mechanisms. Results show that the maximum TC rainfall are all located in the downshear left quadrant regardless of shear direction, and TCs with easterly shear have greater magnitudes of rainfall than those with westerly shear, consistent with previous studies. Rainfall amount of a TC is related to its relative position and proximity from the western Pacific subtropical high (WPSH) and the intensity of water vapor transport, and low-level jet is favorable for water vapor transport. The maximum of vertically integrated moisture flux convergence (MFC) are located on the downshear side regardless of shear direction, and the contribution of wind convergence to the total MFC is far larger than that of moisture advection. The cyclonic displacement of the maximum rainfall relative to the maximum MFC is possibly due to advection of hydrometeors by low- and middle-level cyclonic circulation of TCs. The relationship between TC rainfall and the WPSH through water vapor transport and vertical wind shear implies that TC rainfall may be highly predictable given the high predictability of the WPSH.

Idealized Tropical Cyclone Responses to the Height and Depth of Environmental Vertical Wind Shear

Abstract Three sets of idealized, cloud-resolving simulations are performed to investigate the sensitivity of tropical cyclone (TC) structure and intensity to the height and depth of environmental vertical wind shear. In the first two sets of simulations, shear height and depth are varied independently; in the third set, orthogonal polynomial expansions are used to facilitate a joint sensitivity analysis. Despite all simulations having the same westerly deep-layer (200–850 hPa) shear of 10 m s−1, different intensity and structural evolutions are observed, suggesting the deep-layer shear alone may not be sufficient for understanding or predicting the impact of vertical wind shear on TCs. In general, vertical wind shear that is shallower and lower in the troposphere is more destructive to model TCs because it tilts the TC vortex farther into the downshear-left quadrant. The vortices that tilt the most are unable to precess upshear and realign, resulting in their failure to intensify. Shear height appears to modulate this tilt response by modifying the thermodynamic environment above the developing vortex early in the simulations, while shear depth modulates the tilt response by controlling the vertical extent of the convective vortex. It is also found that TC intensity predictability is reduced in a narrow range of shear heights and depths. This result underscores the importance of accurately observing the large-scale environmental flow for improving TC intensity forecasts, and for anticipating when such forecasts are likely to have large errors.

A Study on the Asymmetric Rapid Intensification of Hurricane Earl (2010) Using the HWRF System

Abstract In this study, the results of a forecast from the operational Hurricane Weather Research and Forecast (HWRF) system for Hurricane Earl (2010) are verified against observations and analyzed to understand the asymmetric rapid intensification of a storm in a sheared environment. The forecast verification shows that HWRF captured well Earl’s observed evolution of intensity, convection asymmetry, wind field asymmetry, and vortex tilt in terms of magnitude and direction in the pre rapid and rapid intensification (RI) stages. Examination of the high-resolution forecast data reveals that the tilt was large at the RI onset and decreased quickly once RI commenced, suggesting that vertical alignment is the result instead of the trigger for RI. The RI onset is associated with the development of upper-level warming in the eye, which results from upper-level storm-relative flow advecting the warm air caused by subsidence warming in the upshear-left region toward the low-level storm center. This scenario does not occur until persistent convective bursts (CB) are concentrated in the downshear-left quadrant. The temperature budget calculation indicates that horizontal advection plays an important role in the development of upper-level warming in the early RI stage. The upper-level warming associated with the asymmetric intensification process occurs by means of the cooperative interaction of the convective-scale subsidence, resulting from CBs in favored regions and the shear-induced mesoscale subsidence. When CBs are concentrated in the downshear-left and upshear-left quadrants, the subsidence warming is maximized upshear and then advected toward the low-level storm center by the storm-relative flow at the upper level. Subsequently, the surface pressure falls and RI occurs.

Observations of the Eyewall Structure of Typhoon Sinlaku (2008) during the Transformation Stage of Extratropical Transition

AbstractA unique dataset observing the life cycle of Typhoon Sinlaku was collected during The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) in 2008. In this study observations of the transformation stage of the extratropical transition of Sinlaku are analyzed. Research flights with the Naval Research Laboratory P-3 and the U.S. Air Force WC-130 aircraft were conducted in the core region of Sinlaku. Data from the Electra Doppler Radar (ELDORA), dropsondes, aircraft flight level, and satellite atmospheric motion vectors were analyzed with the recently developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) software with a 1-km horizontal- and 0.5-km vertical-node spacing. The SAMURAI analysis shows marked asymmetries in the structure of the core region in the radar reflectivity and three-dimensional wind field. The highest radar reflectivities were found in the left of shear semicircle, and maximum ascent was found in the downshear left quadrant. Initial radar echos were found slightly upstream of the downshear direction and downdrafts were primarily located in the upshear semicircle, suggesting that individual cells in Sinlaku’s eyewall formed in the downshear region, matured as they traveled downstream, and decayed in the upshear region. The observed structure is consistent with previous studies of tropical cyclones in vertical wind shear, suggesting that the eyewall convection is primarily shaped by increased vertical wind shear during step 2 of the transformation stage, as was hypothesized by Klein et al. A transition from active convection upwind to stratiform precipitation downwind is similar to that found in the principal rainband of more intense tropical cyclones.

Asymmetric Hurricane Boundary Layer Structure from Dropsonde Composites in Relation to the Environmental Vertical Wind Shear

Abstract This study investigates the asymmetric structure of the hurricane boundary layer in relation to the environmental vertical wind shear in the inner core region. Data from 1878 GPS dropsondes deployed by research aircraft in 19 hurricanes are analyzed in a composite framework. Kinematic structure analyses based on Doppler radar data from 75 flights are compared with the dropsonde composites. Shear-relative quadrant-mean composite analyses show that both the kinematic and thermodynamic boundary layer height scales tend to decrease with decreasing radius, consistent with previous axisymmetric analyses. There is still a clear separation between the kinematic and thermodynamic boundary layer heights. Both the thermodynamic mixed layer and height of maximum tangential wind speed are within the inflow layer. The inflow layer depth is found to be deeper in quadrants downshear, with the downshear right (DR) quadrant being the deepest. The mixed layer depth and height of maximum tangential wind speed are alike at the eyewall, but are deeper outside in quadrants left of the shear. The results also suggest that air parcels acquire equivalent potential temperature θe from surface fluxes as they rotate through the upshear right (UR) quadrant from the upshear left (UL) quadrant. Convection is triggered in the DR quadrant in the presence of asymmetric mesoscale lifting coincident with a maximum in θe. Energy is then released by latent heating in the downshear left (DL) quadrant. Convective downdrafts bring down cool and dry air to the surface and lower θe again in the DL and UL quadrants. This cycling process may be directly tied to shear-induced asymmetry of convection in hurricanes.

Rapidly Intensifying Hurricane Guillermo (1997). Part II: Resilience in Shear

This paper examines the structure and evolution of a mature tropical cyclone in vertical wind shear (VWS) using airborne Doppler radar observations of Hurricane Guillermo (1997). In Part I, the modulation of eyewall convection via the rotation of vorticity asymmetries through the downshear-left quadrant was documented during rapid intensification. Here, the focus is on the relationship between VWS, vortex tilt, and associated asymmetry within the tropical cyclone core region during two separate observation periods. A method for estimating local VWS and vortex tilt from radar datasets is further developed, and the resulting vertical structure and its evolution are subjected to statistical confidence tests. Guillermo was a highly resilient vortex, evidenced by its small tilt magnitude relative to the horizontal scale of the vortex core. The deep-layer tilt was statistically significant, oriented on average ~60° left of shear. Large-scale vorticity and thermal asymmetries oriented along the tilt direction support a response of Guillermo to shear forcing that is consistent with balanced dynamics. The time-averaged vertical motion asymmetry within the eyewall exhibited maximum ascent values ~40° left of the deep-layer shear, or in this case, right of the deep-layer tilt. The observation-based analysis of Guillermo’s interaction with VWS confirms findings of recent theoretical and numerical studies, and serves as the basis for a more comprehensive investigation of VWS and tropical cyclone intensity change using a recently constructed multistorm database of Doppler radar analyses.

Mesoscale vortex generation and merging process: A case study associated with a post-landfall tropical depression

An observational analysis of satellite blackbody temperature (TBB) data and radar images suggests that the mesoscale vortex generation and merging process appeared to be essential for a tropical-depression-related heavy rain event in Shanghai, China. A numerical simulation reproduced the observed mesoscale vortex generation and merging process and the corresponding rain pattern, and then the model outputs were used to study the related dynamics through diagnosing the potential vorticity (PV) equation. The tropical depression (TD) was found to weaken first at lower levels and then at upper levels due to negative horizontal PV advection and diabatic heating effects. The meso-vortices developed gradually, also from the lower to the upper levels, as a result of positive horizontal PV advection and diabatic heating effects in the downshear left quadrant of the TD. One of these newly-generated vortices, V1, replaced the TD ultimately, while the other two, V2 and V3, merged due to the horizontal PV advection process. Together with the redevelopment of V1, the merging of V2 and V3 triggered the very heavy rain in Shanghai.

Rapidly Intensifying Hurricane Guillermo (1997). Part I: Low-Wavenumber Structure and Evolution

Abstract The structure and evolution of rapidly intensifying Hurricane Guillermo (1997) is examined using airborne Doppler radar observations. In this first part, the low-azimuthal-wavenumber component of the vortex is presented. Guillermo’s intensification occurred in an environmental flow with 7–8 m s−1 of deep-layer vertical shear. As a consequence of the persistent vertical shear forcing of the vortex, convection was observed primarily in the downshear left quadrant of the storm. The greatest intensification during the ∼6-h Doppler observation period coincided with the formation and cyclonic rotation of several particularly strong convective bursts through the left-of-shear semicircle of the eyewall. Some of the strongest convective bursts were triggered by azimuthally propagating low-wavenumber vorticity asymmetries. Mesoscale budget analyses of axisymmetric angular momentum and relative vorticity within the eyewall are presented to elucidate the mechanisms contributing to Guillermo’s structural evolution during this period. The observations support a developing conceptual model of the rapidly intensifying, vertically sheared hurricane in which shear-forced mesoscale ascent in the downshear eyewall is modulated by internally generated vorticity asymmetries yielding episodes of anomalous intensification.

Some Aspects of Azimuthal Wavenumber-One Structure of Typhoons Represented in the JMA Operational Mesoscale Analyses

Azimuthal wavenumber-one asymmetric structures in the inner-core region of typhoon are examined primarily using the operational mesoscale analyses from the Japan Meteorological Agency (JMA) for the four typhoon seasons from 2004 to 2007. The results show that both the cold and high specific humidity anomalies at low-tomiddle-tropospheric levels tend to be located in the downshear-left quadrant (looking down at the direction of environmental vertical wind shear), consistent with some theoretical predictions. In addition, the vertical tilt of typhoon vortex is directed predominantly downshear to downshear-left with a magnitude being roughly proportional to shear strength, in agreement with previous numerical case studies.In the present study, the wavenumber-one structures of near-surface winds are also investigated using both theoretical and statistical approaches. In the theoretical approach, a set of analytical formulae that relate the asymmetries of radial and tangential winds to convective asymmetry in the eyewall region are derived by applying a scaling argument to the result from a numerical simulation of Typhoon Chaba (2004). The formulae predict that the maximum in storm-relative tangential wind occurs 90° azimuthally downwind of the enhanced updraft region. The statistical approach using the mesoscale analysis data reveals that the azimuthal location of tangential wind maximum relative to storm direction depends strongly on the directional difference between shear and storm motion, under relatively strong shear conditions, and the wind maximum, contrary to the majority of cases, tends to occur to the left of motion in cases where the directional difference is very small. Considering the strong dependence of convective asymmetry in the inner-core region on the shear, the results are in line with expectations from the analytic theory.

Extreme Helicity and Intense Convective Towers in Hurricane Bonnie

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.

Effects of Ambient Vertical Wind Shear on the Inner-Core Asymmetries and Vertical Tilt of a Simulated Tropical Cyclone

The distributions of precipitation, radar reectivity, and vertical motion associated with tropical cy clones frequently exhibit a signi cant degree of wavenumber-one asymmetry within the intense inner core. Recent observational and numerical studies attributed the convective asymmetry to the ambient vertical wind shear in which the storms were embedded. However, the precise mechanisms by which the environmental shear controls the convective asymmetry are not yet fully established. Although a previous numerical study by the author suggested that a thermal-wind adjustment to the shear-induced vortex tilt may be a primary mechanism responsible for the asymmetry, the reason why the rainfall maxima were observed predominantly in the downshear-left quadrant (looking downshear) in the Northern Hemisphere rather than right downshear remains to be answered.In the present study, an analytical formula for the amplitude of shear-induced vertical motion asymme try developed in the previous study based on the proposed mechanism was applied to the numerical simu lation data obtained for Typhoon Chaba in 2004. As a result, high correlation coefcients were calculated between simulated and formula-predicted vertical motion asymmetries, especially at low levels where the diabatic enhancement of upward motion was not so signicant, conrming that the proposed mechanism may be a primary triggering mechanism for the vertical motion asymmetry.In the study, to discover the processes that govern the directional preference for the rainfall maxima, a Lagrangian trajectory approach was applied to the simulation data. The results suggested that asym metric water vapor distribution caused by shear-induced vertical motion was a primary factor to locate the rainfall maxima on the downshear-left side rather than right downshear of the storm center. In addition, it was found that the near-collocation in azimuth of upward motion maxima and vortex tilts, which was com monly observed in this and other simulations, was inextricably linked to the formation mechanism for the asymmetric water vapor distribution.

The formation of hurricaneHumberto (2001): The importance of extra-tropical precursors

The authors examine the formation of hurricane Humberto (2001) in the Atlantic basin. Precursor disturbances originating in the extratropics appear critical for inducing tropical cyclogenesis. One precursor near the tropopause arose during the development of hurricane Erin on 7 September and could be tracked continuously for 12 days until 1200 UTC 19 September. A fully explicit numerical simulation with the PSU/NCAR mesoscale model produces a generally realistic evolution of the developing storm during a 4-day period beginning 0000 UTC 19 September. Diagnostics of the simulations reveal that moderate vertical shear was crucial for organizing convection, and this shear was enhanced on the periphery of an upper-level vortex, within a lower- to mid-tropospheric baroclinic zone. Downdraughts and evaporative cooling produced a rearward propagating bore that organized new convection in a notably moistened environment with small CAPE. Vorticity generation maximized near 1 km above mean sea level and the vortex built upward with time. This evolution contrasts markedly with ‘downward development’ conceptual models of tropical cyclogenesis and points to the crucial need for observations in the lower troposphere before depression formation. Convective-scale dynamics were governed by intense convective cores which produced most of the diabatic heating and lower-tropospheric vertical vorticity. They were constrained by vertical shear to populate the downshear-left quadrant of the vortex, and therefore their collective behaviour was organized despite being quasi-random within the favoured region. Ultimately, the shear that helped organize the convection in the developing depression hampered Humberto's subsequent intensification. Copyright © 2006 Royal Meteorological Society

Structure of Highly Sheared Tropical Storm Chantal during CAMEX-4

Abstract Tropical Storm Chantal during August 2001 was a storm that failed to intensify over the few days prior to making landfall on the Yucatan Peninsula. An observational study of Tropical Storm Chantal is presented using a diverse dataset including remote and in situ measurements from the NASA ER-2 and DC-8 and the NOAA WP-3D N42RF aircraft and satellite. The authors discuss the storm structure from the larger-scale environment down to the convective scale. Large vertical shear (850–200-hPa shear magnitude range 8–15 m s−1) plays a very important role in preventing Chantal from intensifying. The storm had a poorly defined vortex that only extended up to 5–6-km altitude, and an adjacent intense convective region that comprised a mesoscale convective system (MCS). The entire low-level circulation center was in the rain-free western side of the storm, about 80 km to the west-southwest of the MCS. The MCS appears to have been primarily the result of intense convergence between large-scale, low-level easterly flow with embedded downdrafts, and the cyclonic vortex flow. The individual cells in the MCS such as cell 2 during the period of the observations were extremely intense, with reflectivity core diameters of 10 km and peak updrafts exceeding 20 m s−1. Associated with this MCS were two broad subsidence (warm) regions, both of which had portions over the vortex. The first layer near 700 hPa was directly above the vortex and covered most of it. The second layer near 500 hPa was along the forward and right flanks of cell 2 and undercut the anvil divergence region above. There was not much resemblance of these subsidence layers to typical upper-level warm cores in hurricanes that are necessary to support strong surface winds and a low central pressure. The observations are compared to previous studies of weakly sheared storms and modeling studies of shear effects and intensification. The configuration of the convective updrafts, low-level circulation, and lack of vertical coherence between the upper- and lower-level warming regions likely inhibited intensification of Chantal. This configuration is consistent with modeled vortices in sheared environments, which suggest the strongest convection and rain in the downshear left quadrant of the storm, and subsidence in the upshear right quadrant. The vertical shear profile is, however, different from what was assumed in previous modeling in that the winds are strongest in the lowest levels and the deep tropospheric vertical shear is on the order of 10–12 m s−1.