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Gary M. Barnes

Abstract

A Queen Air, instrumented to make 1-Hz measurements of the kinematic, dynamic, and thermodynamic fields, and radar, mesonet, and soundings from the Cooperative Convective Precipitation Experiment 1981 is used to monitor the evolution of the updraft at cloud base of a large cumulus congestus over the High Plains. The environment is characterized by modest instability and strong horizontal wind shear. Twelve passes completed by the Queen Air just below cloud base from the late growth to the dissipation stage reveal that the main updraft splits into two with the south updraft rotating cyclonically and moving to the right of the mean winds. This cell is associated with a pressure perturbation in excess of 1 mb that is most likely caused by the interaction of the updraft with the shear of the horizontal wind. Saturation-point analyses of the updraft and the subcloud layer demonstrate that in the early stages air from near the surface ascended into the cloud, but as the cloud ages, air from the upper subcloud and transition layers contributes to the updraft. This air has little or no buoyancy, which loads to cloud collapse. Mass flux and saturation-point analyses predict the cloud's demise adequately, in contrast to the trends of vertical velocity, virtual potential temperature, or moisture at cloud base. A pressure perturbation caused by updraft–shear interaction is an important mechanism for cloud intensification, but it must act in concert with another forcing mechanism, typically a gust front, to tap the most unstable air found in the lower subcloud layer in the High Plains.

The observations support the numerical simulations of cumulonimbi in the presence of strong shear, albeit for a much smaller cloud. Congestus clouds merit attention as they are suitable targets for a variety of platforms and will lead to a more complete understanding of the convective cloud spectrum.

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Gary M. Barnes

Abstract

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.

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Carl E. Barnes and Gary M. Barnes

Abstract

Eye and eyewall traits were ascertained for 209 images from 37 tropical cyclones (TCs) using the lower-fuselage 5.6-cm radar, aboard the two National Oceanic and Atmospheric Administration WP-3Ds. These TCs were almost entirely from the Atlantic basin and were sampled from 1997 to 2012. For the eye these traits included area, maximum diameter, and roundness; for the eyewall the traits included area, completeness, maximum width, maximum reflectivity value and location, number of local reflectivity maxima, and mean rain rate. These variables were compared to TC intensity and motion characteristics from the best-track dataset, and environmental characteristics from the Statistical Hurricane Intensity Prediction Scheme.

Interrelationships between eyewall features revealed that eyewall reflectivity features became more homogeneous as eye and eyewall areas shrank, and maximum reflectivity and rain rate increased as the eyewall became wider and more complete. As the TC intensified, the eye area decreased, while the eyewall area increased due to increasing completeness and width. Rain rate was also found to be higher for faster-moving TCs. Stronger vertical shear of the horizontal wind was found to be associated with more asymmetric eyewall reflectivity. The maximum reflectivity value occurred most often on the downshear side of the eyewall, and to the right of the storm motion, verifying prior research. There were no relationships found between the reflectivity and sea surface temperature or environmental relative humidity. A schematic incorporating typical eye and eyewall traits is presented.

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Rebecca Schneider and Gary M. Barnes

Abstract

During 11 h on 26 August 1998, two NOAA WP-3D aircraft deployed 85 Global Positioning System (GPS) dropwindsondes within 2° of latitude of the circulation center of Hurricane Bonnie as it made landfall in North Carolina. About 75% of the sondes successfully collected data, which were used to create a series of storm-relative horizontal maps of kinematic and thermodynamic variables from 10 m to 2 km. Reflectivity fields were analyzed from the Weather Surveillance Radar-1988 Dopplers (WSR-88Ds) located at Wilmington and Morehead City, North Carolina, and the tail and lower fuselage radars aboard the WP-3Ds.

GPS sonde performance and deployment spacing is adequate to identify several aspects of the vortex. These include 1) warm, dry, stable air in the offshore flow that results in reduced equivalent potential temperatures entering the southern portion of the eyewall, 2) cooler air collocated with the upwelled water in the right-rear quadrant and under the eyewall, and 3) an atypical radial wind pattern with strong inflow southwest of the circulation center and outflow northeast of the center. The strongly asymmetric structure found at 10 m becomes much more homogeneous by 2-km altitude.

No intense rainbands developed over land in the onshore flow nor did the bands in the onshore flow undergo any significant changes once they made landfall. Beyond the eyewall the offshore flow contained much less precipitation than the onshore portion of the storm.

Characteristics beyond the eyewall appear to have been modulated by the proximity to land but hurricane intensity did not vary. The authors infer that the lower energy content of the inflow was offset by the contraction of the eyewall.

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Klaus Dolling and Gary M. Barnes

Abstract

At 0600 UTC 22 September 2001, Humberto was a tropical depression with a minimum central pressure of 1010 hPa. Twelve hours later, when the first global positioning system dropwindsondes (GPS sondes) were jettisoned, Humberto’s minimum central pressure was 1000 hPa and it had attained tropical storm strength. Thirty GPS sondes, radar from the WP-3D, and in situ aircraft measurements are utilized to observe thermodynamic structures in Humberto and their relationship to stratiform and convective elements during the early stage of the formation of an eye.

The analysis of Tropical Storm Humberto offers a new view of the pre-wind-induced surface heat exchange (pre-WISHE) stage of tropical cyclone evolution. Humberto contained a mesoscale convective vortex (MCV) similar to observations of other developing tropical systems. The MCV advects the exhaust from deep convection in the form of an anvil cyclonically over the low-level circulation center. On the trailing edge of the anvil an area of mesoscale descent induces dry adiabatic warming in the lower troposphere. The nascent warm core at low levels causes the initial drop in pressure at the surface and acts to cap the boundary layer (BL). As BL air flows into the nascent eye, the energy content increases until the energy is released from under the cap on the down shear side of the warm core in the form of vigorous cumulonimbi, which become the nascent eyewall. This series of events show one possible path in which a mesoscale convective system may evolve into a warm-cored structure and intensify into a hurricane.

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Gary M. Barnes and Michael Garstang

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 convective clouds which operate upon regions previously modified by precipitation and downdrafts. Results suggest that large, linearly shaped, moving cloud lines are the centers of the most efficient energy transfer per unit mass.

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Gary M. Barnes and Paul Fuentes

Abstract

Over 4.5 days, NOAA and U.S. Air Force personnel in reconnaissance aircraft deployed 44 global positioning system dropwindsondes (GPS sondes) in the eye of Hurricane Lili (2002). The vertical profiles derived from these GPS sondes were used to determine the evolution of the height of the inversion, presence, and height of the hub cloud, the height of the lifted condensation layer, and the depth of the mixed layer. As Lili deepened, underwent rapid intensification (RI), and eventually rapid decay, the lower portion of the eye moistened and the lapse rate became moist adiabatic. The inversion layer rose as Lili intensified and then quickly fell over 1500 m at the beginning of RI. Comparison of the equivalent potential temperature θe of the eye with that in the eyewall revealed that like many other hurricanes, the eye was a reservoir for the warmest θe. The authors define a variable called eye excess energy that is a function of the difference in θe between the eye and the eyewall and the depth over which this difference occurs and present evidence that this quantity became small during RI. The authors hypothesize that the warm θe in the eye served as a boost for convection in the eyewall that may, in turn, initiate RI. However, the small volume of eye excess energy available and the rapidity at which it was transferred to the eyewall demonstrate that eye excess energy cannot sustain RI, which typically continues for many hours. The results are discussed in light of eye–eyewall mixing arguments.

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Mark Croxford and Gary M. Barnes

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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 efficiently to heat and momentum sources in the eyewall. The behavior differences between inner core strength in these hurricanes and outer core strength in typhoons discussed in earlier works are chiefly a function of distance from the eyewall and appear consistent in light of currently accepted theoretical arguments.

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Matthew Sitkowski and Gary M. Barnes

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 on the east or trailing side of the hurricane and to the left of the deep-layer shear vector. Net latent heat release within 60 km of the circulation center increases 21% from 2 to 3 August and is matched by a 30% increase in the inflow below 2 km at the 100-km radius. The GPS sondes, combined with aircraft in situ data for the eyewall region, reveal that the tropical cyclone (TC) establishes an annulus adjacent to and under the eyewall where the tangential wind component and equivalent potential temperature increase substantially. The radial extent of this annulus is constrained by the rainbands that remain robust throughout RI. The results support the argument that RI is controlled by processes within 100 km of the circulation center, and in particular within the eyewall.

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Klaus Dolling and Gary M. Barnes

Abstract

In 2001, the National Oceanic and Atmospheric Administration and the National Aeronautical and Space Administration marshaled their resources to sample Hurricane Humberto for 3 successive days during the fourth Convection and Moisture Experiment (CAMEX-4). Humberto developed from a tropical storm into a category-2 hurricane despite the deep-layer vertical shear of the environmental horizontal wind (VWS) increasing markedly on the second and third days of sampling. As exhibited in earlier studies, the eyewall convection developed an azimuthal wavenumber-1 (n = 1) asymmetry as the VWS increased. Horizontal divergence and vertical stability within 100 km of the eye exhibited persistent relationships to the VWS vector.

The warm core evolved in an unexpected way. The warm anomaly was initially located in the lower troposphere and built upward as the storm intensified. The maximum temperature anomaly remained in the lower troposphere on all 3 days while the development of the upper-tropospheric warm anomaly appeared to be inhibited by the increasing VWS and the entrainment of dry environmental air into the core at midlevels.

The warm core of this higher-latitude (33°N) storm displayed large differences when compared to most numerical simulations, wind-induced surface heat exchange theory, and observations of tropical cyclones in the deep tropics acquired nearly 50 years ago. The results were similar to some recent numerical simulations.

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