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Scott A. Braun

Abstract

A numerical simulation of Hurricane Bob (1991) is conducted using the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) with a horizontal grid spacing of 1.3 km on the finest nested mesh. The model produces a realistic hurricane that intensifies slowly during the period of finescale simulation. The time-averaged structure is characterized by a wavenumber-1 asymmetry with maximum low-level vertical motions and near-surface inflow in the left-front quadrant relative to the nearly aligned storm motion and mean wind shear vectors and strong outflow just above the boundary layer collocated with the updrafts. Instantaneous distributions of radial flow, vertical motion, and precipitation are strongly modified by a wavenumber-2 asymmetry that rotates cyclonically around the center at about half the speed of the mean tangential winds, consistent with the theory for vortex Rossby waves.

The time-mean asymmetric vertical motion is comprised of small-scale convective updrafts that at any given time cover only a small portion of the eyewall area, but account for a majority of the updraft mass flux, consistent with the concept of hot towers. Calculations of buoyancy indicate that eyewall updrafts are positively buoyant with respect to an environment that includes the vortex-scale warm core structure. Air parcels entering the eyewall in the boundary layer are initially accelerated upward by vertical pressure gradient forces, but once above the boundary layer, they accelerate upward because of buoyancy forces. The buoyancy is typically achieved along outward-sloping paths in a layer characterized by conditional symmetric instability. However, the strong low-level outflow above the boundary layer in the eyewall displaces rising parcels out from under the warm core so that they become unstable to vertical displacements. Consequently, the eyewall updrafts are generally the result of convective rather than symmetric instability. A key source for the buoyancy is the energy gained from surface fluxes of moisture and heat by select parcels that originate from outside of the eyewall in the lowest part of the boundary layer, penetrate furthest into the eye, and then accelerate outward sharply while rising out of the boundary layer. Occasionally, air within the eye is drawn into the eyewall updrafts, suggesting episodic rather than continuous venting of the eye air into the eyewall.

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Scott A. Braun

Abstract

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate Hurricane Bonnie at high resolution (2-km spacing) in order to examine budgets of water vapor, cloud condensate, and precipitation. Virtually all budget terms are derived directly from the model (except for the effects of storm motion). The water vapor budget reveals that a majority of the condensation in the eyewall occurs in convective hot towers, while outside of the eyewall most of the condensation occurs in weaker updrafts, indicative of a larger role of stratiform precipitation processes. The ocean source of water vapor in the eyewall region is only a very small fraction of that transported inward in the boundary layer inflow or that condensed in the updrafts. In contrast, in the outer regions, the ocean vapor source is larger owing to the larger area, counters the drying effect of low-level subsidence, and enhances the moisture transported in toward the eyewall. In this mature storm, cloud condensate is consumed as rapidly as it is produced. Cloud water peaks at the top of the boundary layer and within the melting layer, where cooling from melting enhances condensation. Unlike in squall lines, in the hurricane, very little condensate produced in the eyewall convection is transported outward into the surrounding precipitation area. Most of the mass ejected outward is likely in the form of small snow particles that seed the outer regions and enhance in situ stratiform precipitation development through additional growth by vapor deposition and aggregation.

This study also examines artificial source terms for cloud and precipitation mass associated with setting to zero negative mixing ratios that arise from numerical advection errors. Although small at any given point and time, the cumulative effect of these terms contributes an amount of mass equivalent to 13% of the total condensation and 15%–20% of the precipitation. Thus, these terms must be accounted for to balance the model budgets, and the results suggest the need for improved model numerics.

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Scott A. Braun

Abstract

The existence of the Saharan air layer (SAL), a layer of warm, dry, dusty air frequently present over the tropical Atlantic Ocean, has long been appreciated. The nature of its impacts on hurricanes remains unclear, with some researchers arguing that the SAL amplifies hurricane development and with others arguing that it inhibits it. The potential negative impacts of the SAL include 1) vertical wind shear associated with the African easterly jet; 2) warm air aloft, which increases thermodynamic stability at the base of the SAL; and 3) dry air, which produces cold downdrafts. Multiple NASA satellite datasets and NCEP global analyses are used to characterize the SAL’s properties and evolution in relation to tropical cyclones and to evaluate these potential negative influences. The SAL is shown to occur in a large-scale environment that is already characteristically dry as a result of large-scale subsidence. Strong surface heating and deep dry convective mixing enhance the dryness at low levels (primarily below ∼700 hPa), but moisten the air at midlevels. Therefore, mid- to-upper-level dryness is not generally a defining characteristic of the SAL, but is instead often a signature of subsidence. The results further show that storms generally form on the southern side of the jet, where the background cyclonic vorticity is high. Based upon its depiction in NCEP Global Forecast System meteorological analyses, the jet often helps to form the northern side of the storms and is present to equal extents for both strengthening and weakening storms, suggesting that jet-induced vertical wind shear may not be a frequent negative influence. Warm SAL air is confined to regions north of the jet and generally does not impact the tropical cyclone precipitation south of the jet.

Composite analyses of the early stages of tropical cyclones occurring in association with the SAL support the inferences from the individual cases noted above. Furthermore, separate composites for strongly strengthening and for weakening storms show few substantial differences in the SAL characteristics between these two groups, suggesting that the SAL is not a determinant of whether a storm will intensify or weaken in the days after formation. Key differences between these cases are found mainly at upper levels where the flow over strengthening storms allows for an expansive outflow and produces little vertical shear, while for weakening storms, the shear is stronger and the outflow is significantly constrained.

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Liguang Wu and Scott A. Braun

Abstract

The influence of uniform large-scale flow, the beta effect, and vertical shear of the environmental flow on hurricane intensity is investigated in the context of the induced convective or potential vorticity asymmetries in the core region with a hydrostatic primitive equation hurricane model. In agreement with previous studies, imposition of one of these environmental effects weakens the simulated tropical cyclones. In response to the environmental influence, significant wavenumber-1 asymmetries develop. Asymmetric and symmetric tendencies of the mean radial and azimuthal winds and temperature associated with the environment-induced convective asymmetries are evaluated. The inhibiting effects of environmental influences are closely associated with the resulting eddy momentum fluxes, which tend to decelerate tangential and radial winds in the inflow and outflow layers. The corresponding changes in the symmetric circulation tend to counteract the deceleration effect. The net effect is a moderate weakening of the mean tangential and radial winds. The reduced radial wind can be viewed as an anomalous secondary radial circulation with inflow in the upper troposphere and outflow in the lower troposphere, weakening the mean secondary radial circulation.

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Scott A. Braun and Liguang Wu

Abstract

A high-resolution numerical simulation of Hurricane Erin (2001) is used to examine the organization of vertical motion in the eyewall and how that organization responds to a large and rapid increase in the environmental vertical wind shear and subsequent decrease in shear. During the early intensification period, prior to the onset of significant shear, the upward motion in the eyewall was concentrated in small-scale convective updrafts that formed in association with regions of concentrated vorticity (herein termed mesovortices) with no preferred formation region around the eyewall. Asymmetric flow within the eye was weak. As the shear increased, an azimuthal wavenumber-1 asymmetry in storm structure developed with updrafts tending to occur on the downshear to downshear-left side of the eyewall. Continued intensification of the shear led to increasing wavenumber-1 asymmetry, large vortex tilt, and a change in eyewall structure and vertical motion organization. During this time, the eyewall structure was dominated by a vortex couplet with a cyclonic (anticyclonic) vortex on the downtilt-left (downtilt-right) side of the eyewall and strong asymmetric flow across the eye that led to strong mixing of eyewall vorticity into the eye. Upward motion was concentrated over an azimuthally broader region on the downtilt side of the eyewall, upstream of the cyclonic vortex, where low-level environmental inflow converged with the asymmetric outflow from the eye. As the shear diminished, the vortex tilt and wavenumber-1 asymmetry decreased, while the organization of updrafts trended back toward that seen during the weak shear period. Based upon the results for the Erin case, as well as that for a similar simulation of Hurricane Bonnie (1998), a conceptual model is developed for the organization of vertical motion in the eyewall as a function of the strength of the vertical wind shear. In weak to moderate shear, higher wavenumber asymmetries associated with eyewall mesovortices dominate the wavenumber-1 asymmetry associated with the shear so that convective-scale updrafts form when the mesovortices move into the downtilt side of the eyewall and dissipate on the uptilt side. Under strong shear conditions, the wavenumber-1 asymmetry, characterized by a prominent vortex couplet in the eyewall, dominates the vertical motion organization so that mesoscale ascent (with embedded convection) occurs over an azimuthally broader region on the downtilt side of the eyewall. Further research is needed to determine if these results apply more generally.

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Scott A. Braun and Robert A. Houze Jr.

Abstract

Thermodynamic and microphysical retrieval techniques are applied to dual-Doppler synthesized air motion fields for a midlatitude squall line, which passed through the Oklahoma-Kansas Preliminary Regional Experiment for the Stormscale Operational and Research Meteorology Program (PRE-STORM) observational array in Kansas and Oklahoma on 10–11 June 1985. The retrieved pressure and potential temperature fields are consistent with surface network and sounding data, while the retrieved microphysical fields show the characteristic secondary maximum of radar reflectivity in the stratiform region and the band of low reflectivity, or transition zone, lying between the leading convective line and the secondary maximum.

The retrieved fields indicate the processes producing the secondary maximum and transition zone minimum of radar reflectivity more quantitatively than has been possible in previous studies. The primary processes accounting for these features of the radar reflectivity pattern were 1) the substantial increase in precipitation mass concentrations by vapor deposition within the region of mesoscale ascent in the stratiform region and the increase in particle size resulting from the strong aggregation of ice particles above the bright band in the region of the secondary band, 2) the suppression of growth in the middle to upper level descent just behind the convective region, which enhanced the minimum of radar reflectivity in that zone, and 3) the trajectories of ice particles detrained from the convective line, which qualitatively accounted for the general location of the secondary band. Additional insights into these processes are discussed.

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Scott A. Braun and Robert A. Houze Jr.

Abstract

Mesoscale analysis of surface observations and mesoscale modeling results show that the 10–11 June squall line, contrary to prior studies, did not form entirely ahead of a cold front. The primary environmental features leading to the initiation and organization of the squall line were a low-level trough in the lee of the Rocky Mountains and a midlevel short-wave trough. Three additional mechanisms were active: a southeastward-moving cold front formed the northern part of the line, convection along the edge of cold air from prior convection over Oklahoma and Kansas formed the central part of the line, and convection forced by convective outflow near the lee trough axis formed the southern portion of the line.

Mesoscale model results show that the large-scale environment significantly influenced the mesoscale circulations associated with the squall line. The qualitative distribution of along-line velocities within the squall line is attributed to the larger-scale circulations associated with the lee trough and midlevel baroclinic wave. Ambient rear-to-front (RTF) flow to the rear of the squall line, produced by the squall line’s nearly perpendicular orientation to strong westerly flow at upper levels, contributed to the exceptional strength of the rear inflow in this storm. The mesoscale model results suggest that the effects of the line ends and the generation of horizontal buoyancy gradients at the back edge of the system combined with this ambient RTF flow to concentrate the strongest convection and back-edge sublimative cooling along the central portion of the line, which then produced a core of maximum rear inflow with a horizontal scale of approximately 100–200 km. The formation of the rear-inflow core followed the onset of strong sublimative cooling at the back edge of the storm and suggests that the rear inflow maximum was significantly influenced by microphysical processes. In a sensitivity test, in which sublimative cooling was turned off midway through the simulation, the core of strong rear inflow failed to form and the squall line rapidly weakened.

The evolution of the low-level mesoscale to synoptic-scale pressure field contributed to the dissipation of the squall line. Cyclogenesis occurred over Missouri, ahead of the squall line, and caused the presquall flow to veer from southeasterly to southwesterly, which decreased the low-level inflow and line-normal vertical wind shear. The reduction in low-level wind shear decreased the effectiveness of the cold pool in sustaining deep convection along the gust front.

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Scott A. Braun and Robert A. Houze Jr.

Abstract

The water and heat budgets for a midlatitude squall line are estimated from single- and dual-Doppler-radar data and thermodynamic data from rawinsonde and thermodynamic retrieval (from dual-Doppler winds). These data, along with models to retrieve the freezing, melting, and radiative heating rates, yield vertical profiles of the heating within the convective, stratiform, and overhanging anvil areas of the squall line and differentiate the processes contributing to the total heating. The use of radar-derived vertical velocity information provides a more accurate delineation of the convective and stratiform components of the heating than can be obtained from rawinsonde data.

The effects of diabatic heating on the potential vorticity (PV) field are calculated using a simple two-dimensional model and the vertical profiles of heating from the heat budget. The diabatic heating produced a deep column of high PV air coincident with the convective region and produced several regions of negative PV. Similar regions of negative PV were observed in the squall line. Upper-level negative PV within and to the rear of the stratiform precipitation region suggests that symmetric or inertial instability might favor intensification of the upper-level line-normal outflow there. Anticyclonic inertial turning of this outflow contributes to the formation of a strong upper-level jet in the line-parallel flow to the rear of the squall line.

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Scott A. Braun and Wei-Kuo Tao

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The fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model is used to simulate Hurricane Bob (1991) using grids nested to high resolution (4 km). Tests are conducted to determine the sensitivity of the simulation to the available planetary boundary layer parameterizations, including the bulk aerodynamic, Blackadar, Medium-Range Forecast (MRF) model, and Burk–Thompson boundary layer schemes. Significant sensitivity is seen, with minimum central pressures varying by up to 16 mb and maximum winds by 15 m s−1. The Burk–Thompson and bulk aerodynamic boundary layer schemes produced the strongest storms while the MRF scheme produced the weakest storm. Simulated horizontal precipitation structures varied substantially between the different PBL schemes, suggesting that accurate forecasts of precipitation in hurricanes can be just as sensitive to the formulation of the PBL as they are to the cloud microphysical parameterizations.

Each PBL scheme is different in its formulation of the vertical mixing within the PBL and the surface fluxes, with the exception of the MRF and Blackadar schemes, which share essentially the same surface flux parameterization. Detailed analyses of the PBL schemes describe the key differences in the surface fluxes and how they impact storm intensity. In order to isolate the effects of vertical mixing and surfaces fluxes, simulations were conducted in which each of the surface flux schemes was used in conjunction with the same vertical mixing scheme, and vice versa. These experiments indicate that simulated intensity is largely determined by the surface fluxes rather than by the vertical mixing, with the exception of the MRF PBL case, in which excessively deep vertical mixing acts to dry the lower PBL and reduce hurricane intensity. Simulations that vary only the surface fluxes suggest that the intensity of the simulated hurricane increases with increasing values of the ratio of the exchange coefficients for enthalpy and momentum, C k/C D. However, even for identical values of C k/C D, the simulated intensity varies depending on the wind speed dependence of the surface roughness parameter z 0.

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