Gary M. Barnes

Mesoscale and Convective Structure of a Hurricane Rainband

Abstract The mesoscale thermodynamic, kinematic, and radar structure of a Hurricane Floyd rainband observed on 7 September 1981 is presented. Data are from 26 aircraft passes through the rainband from 150 to 6400 m. A composite technique which presents rainband structure as a function of distance from the storm circulation center reveals inflow from the outer edge of the band and a partial barrier to this flow below 3 km. In the direction parallel to rainband orientation, radar reveals cellular reflectivity structure on the upwind and central portions of the rainband; the frequency of cellular precipitation decreases in favor of stratiform precipitation further downwind as the band spirals gradually towards the eyewall. In the radial direction, a decrease of 12 K in θe, is observed across the rainband in the subcloud layer. Convective scale up- and downdrafts that are associated with cellular reflectivity structure are hypothesized to be responsible for the thermodynamic modification of the cloud and subc...

Momentum flux by lines of cumulonimbus over the tropical oceans

Abstract Examination of aircraft and rawinsonde data gathered in nine tropical mesoscale convective line cases indicates that all but two lines systematically increased front-to-rear momentum at heights greater than about 4 km, and rear-to-front momentum at lower levels, where “front” is defined as the direction toward which the line is moving. The convective lines were characterized by a leading 10–20 km wide band of convective clouds, and a trailing region of stratiform cloudiness. Most wore “propagating” lines, moving into the wind at all levels. Consistent with mixing-length theory, the vertical transport of the horizontal wind component parallel to the lines was down the vertical gradient of the component, resulting in a decrease of its vertical shear. Smaller, more random cloud groups and the upper portions of a convective line with isolated towers transported both components of horizontal momentum downgradient. Normalization of the vertical flux of horizontal momentum normal to the line (u′¯w′¯) su...

A Convective Cell in a Hurricane Rainband

Abstract On 10 October 1983 the two NOAA WP-3D aircraft completed a mission designed to provide airborne Doppler radar data for a convective cell embedded in a weak rainband on the trailing side of Hurricane Raymond. Comparisons of the wind field produced from the pseudo-dual-Doppler radar technique with in situ wind measurements suggest that the larger convective-scale feature may be resolved if the sampling time is kept to a minimum. The convective cell was found to move downband faster than any environmental winds but slightly slower than the winds found in the reflectivity core that delineates the cell. In the core of the cell the tangential wind is increased and the radial inflow turns to outflow with respect to the circulation center. The flow field demonstrates that the downband stratiform portion of a rainband is not from cells currently active since the updraft detrains upwind relative to the cell but rather it is due to the fallout from ice particles placed into the upper troposphere by clouds t...

Subcloud Layer Energetics of Precipitating Convection

Abstract The thermodynamic modification of the subcloud layer in the GATE area is shown to be a function of precipitating convection. A critical rate of 2 mm h−1, based on the Z – R relationship, in conjunction with 4 km × 4 km scale 15 min mean radar maps, distinguishes between evaporation of precipitation in the subcloud layer (no change in moist static energy h) and vertical mass transport associated with penetrative downdrafts (decreases in h) into this layer from near and above cloud base. The spatial extent of the outflow of the active downdrafts is limited to a convective-mesoscale area directly under and as much as 15 km downwind of the precipitation causing the change. A more extensive wake region occurs on the upwind side of the precipitating region. The initial thermodynamic environment directly affects energy transport per unit mass by moist convection. Precipitating cells which operate upon an initially undisturbed atmosphere cause a net transfer of 60% more energy per unit mass than those co...

Low-Level Thermodynamic, Kinematic, and Reflectivity Fields of Hurricane Guillermo (1997) during Rapid Intensification

Abstract From 0600 UTC 2 August to 1200 UTC 3 August Hurricane Guillermo (1997) deepened by 54 hPa over the eastern North Pacific Ocean, easily exceeding the thresholds that define rapid intensification (RI). The NOAA WP-3Ds observed a portion of this RI with similar two-aircraft missions on consecutive days. The aircraft jettisoned 70 successful global positioning system (GPS) dropwindsondes (or GPS sondes), which reveal how conditions in the lower troposphere on the octant to quadrant scale evolved within 250 km of the eye. Reflectivity fields demonstrate that the deepening is correlated with a spiraling in of the northern eyewall that reduces the eye diameter by 10 km. This behavior contrasts the more uniform contraction witnessed during eyewall replacement cycles. Mixing between the lower eye and eyewall, as detailed by other investigators, appears to have triggered the reduction in the eye diameter. After RI the eyewall remains asymmetrical with the tallest echo tops and heaviest rain rates located o...

Evolution of the Inflow Boundary Layer of Hurricane Gilbert (1988)

Abstract On 12 September 1988 the two NOAA WP-3D aircraft conducted an experiment in and around an intense, outer rainband located 175 km southeast of the center of Hurricane Gilbert. Radial-height cross sections along a constant azimuth reveal a rapid and an exceptionally large increase of the equivalent potential temperature θe of the inflow but in a region radially outward from the rainband. Kinematic analyses that incorporate both in situ and pseudo-dual-Doppler data illustrate that the inflow is only 2 km deep and strongly divergent prior to reaching the convective core of the band. The Doppler-derived wind fields, which compare favorably with the in situ wind fields, demonstrate that there is a radially outward or return flow directly above the inflow. Soundings show that this return flow is unusually moist despite being dominated by mesoscale descent, which contrasts the dry conditions found under the anvil of virtually all tropical mesoscale convective systems. A one-dimensional general structure ...

Inflow Layer Energetics of Hurricane Bonnie (1998) near Landfall

Abstract On 26 August 1998, a NOAA WP-3D aircraft executed a curved track that mimics an inflow trajectory to the eyewall of Hurricane Bonnie. Global positioning system (GPS) sondes and airborne expendable bathythermographs jettisoned along the trajectory provide the observations to conduct an energy budget for the 1600-m-deep inflow to the eyewall. Surface fluxes are estimated via the bulk aerodynamic equations and the flux at the top of the inflow is solved as a residual. From 170- to 125-km radial distance from the circulation center the mean θe of the inflow remains constant despite combined sensible and latent surface fluxes in excess of 500 W m−2. Convective cells remove energy from the inflow boundary layer at a rate similar to the inputs from the sea. From 125 to 100 km, in the annulus adjacent to the eyewall, mean θe increases 4.5 K in response to higher surface fluxes and little loss through the inflow top. Energy balance may be achieved by either entrainment of higher θe through the top of the ...

Atypical Thermodynamic Profiles in Hurricanes

The global positioning system dropwindsondes deployed in Hurricane Bonnie on 26 August 1998 with supporting deployments in Hurricanes Mitch (1998) and Humberto (2001) are used to identify three unusual thermodynamic structures in the lower-cloud and subcloud layers. Two of these structures impact the energy content of the inflow and therefore the intensity of the hurricane. First, positive lapse rates of equivalent potential temperature are found near the top of the inflow. These layers insulate the inflow from the negative impacts of entrainment mixing and promote rapid energy increases, especially near the eyewall. The second structure is a rapid decrease of equivalent potential temperature adjacent to the sea surface. This is essentially a prominent surface layer that owes its existence to both higher moisture content and a superadiabatic lapse rate. The steep lapse rate most often occurs under and near the eyewall where wind speeds at the surface exceed hurricane force. The author speculates that water loading from spray increases the residence time of air parcels in the surface layer, contributing to the creation of this structure. The third feature is a moist absolutely unstable layer previously identified by Bryan and Fritsch for the midlatitudes. These layers are found adjacent to the eyewall, in rainbands, and in the hub cloud within the eye and are evidence of mesoscale or vortex-scale convergence and the very modest instabilities often found in the core of a hurricane.

Perturbation Pressure Fields Measured by Aircraft around the Cloud-Base Updraft of Deep Convective Clouds

Abstract Perturbation pressure fields are measured by aircraft around the cloud base updrafts of seven clouds ranging in size from weak cumulus congestus to intense cumulonimbus during CCOPE (1981). The fields are characterized by a high-low pressure couplet of similar size to the updraft, but a quarter-wavelength out of Phase, with the minimum pressure downshear of the updraft maximum. An estimate of the terms in the Poisson equation for pressure show that the pressure perturbation results chiefly from the interaction of the updraft with the vertical shear of the environmental horizontal wind. The behavior of the pressure oscillation is well predicted by inserting sinusoidal functions in the corresponding terms in the Poisson equation. The amplitude of the pressure oscillation is proportional to the wavelengths of the pressure and vertical-velocity fields, the amplitude of the vertical-velocity oscillation, and the vertical shear of the horizontal environmental wind through cloud base, measured in the di...

Inner Core Strength of Atlantic Tropical Cyclones

Abstract The evolution of the wind field beyond the radius of maximum winds is studied for 18 Atlantic tropical cyclones (TCs) with 989 research and reconnaissance flight legs. Inner core strength, defined as the storm relative mean tangential wind from 65 to 140 km from the circulation center for a given flight leg, is shown to be linearly correlated with tropical cyclone intensity. Inner core strengthening coincides with deepening, but as a hurricane decays, the inner core may exhibit a wider range of behavior. During an eyewall replacement cycle inner core strength and intensity become out of phase. Inner core strength tends to be axisymmetric as no quadrant maintains a higher inner core strength than the other quadrants for more than a day. Increases of inner core strength occur throughout the entire 65–140-km radial distance and, thus, are not due to the higher winds found in rainbands alone. The authors speculate that inner core strength, being relatively close to the circulation center, responds ef...