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John F. Gamache

Abstract

Two-dimensional images of ice particles observed by a NOAA WP-3D research aircraft during the Summer Monsoon Experiment (SMONEX) are examined. These images were obtained in the temperature interval from −25° to 0°C. The particle structures and size distributions found in convective and stratiform clouds are compared.

Branched crystals were located predominantly in stratiform clouds while column-shaped crystals were located commonly in both stratiform and convective clouds. Stratiform clouds, particularly those observed at temperature warmer than −7°C, had a much greater percentage concentration of large ice particles (>0.8 mm in diameter), and many of these ice particles were aggregates or branched crystals. The importance of aggregation and deposition above the melting level in the stratiform clouds is strongly suggested by these findings.

Ice particle number concentrations measured with the cloud probe were often very high in convective clouds, with a maximum value of approximately 800 L−1. The average convective-cloud concentration was approximately 230 L−1, while the average concentration in the stratiform clouds was approximately 20 L−1. Liquid water was almost completely absent in the convective updrafts, at temperatures between −10° and −22°C. This suggests that the convective updrafts may have been nearly completely glaciated, and the microphysics were dominated by deposition.

The high particle concentrations in the convective updrafts suggest that the updrafts may provide most of the ice particles found in the stratiform cloud. Significant modification in particle structures and size distributions have occurred, however, by the time these suspended particles fall out of the stratiform clouds. These modifications appear to arise from aggregation and deposition.

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John F. Gamache and Robert A. Houze Jr.

Abstract

Composites of radar and wind observations in a coordinate system attached to a moving tropical squall line confirm that such a squall system is composed of two separate circulation features: a convective squall-line region and a stratiform anvil region. The squall-line region is characterized by mesoscale boundary-layer convergence, which feeds deep convective updrafts, and mid-to-upper-level divergence associated with outflow from the cells. The anvil region is characterized by mid-level convergence, which feeds both a mesoscale downdraft below the anvil and a mesoscale updraft within the anvil cloud. Before this study, the mesoscale updraft in the anvil cloud of the tropical squall system had been somewhat speculative, and both the anvil updraft and downdraft had been inferred only qualitatively. The occurrence of the anvil updraft is now proven and quantitative profiles of the mesoscale anvil updraft and downdraft have been obtained.

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John F. Gamache and Robert A. Houze Jr.

Abstract

A squall-line cloud cluster observed in the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) is studied as an example of a mesoscale convective system in the tropics. The system is divided into convective and stratiform regions. Composite wind, vertical motion, humidity, radar and satellite data fields have been derived for the system and are used to calculate the components of the water budgets of each region. Particular attention is devoted to understanding the sources of condensate for the stratiform region. The mesoscale updraft in the stratiform cloud accounts for 25–40% of the condensate making up the stratiform cloud, while the remaining 60–75% is supplied by horizontal transfer to the stratiform region of condensate generated in the cumulonimbus towers of the convective region.

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Sylvie Lorsolo, John Gamache, and Altug Aksoy

Abstract

The Hurricane Research Division Doppler radar analysis software provides three-dimensional analyses of the three wind components in tropical cyclones. Although this software has been used for over a decade, there has never been a complete and in-depth evaluation of the resulting analyses. The goal here is to provide an evaluation that will permit the best use of the analyses, but also to improve the software. To evaluate the software, analyses are produced from simulated radar data acquired from an output of a Hurricane Weather Research and Forecasting (HWRF) model nature run and are compared against the model “truth” wind fields. Comparisons of the three components of the wind show that the software provides analyses of good quality. The tangential wind is best retrieved, exhibiting an overall small mean error of 0.5 m s−1 at most levels and a root-mean-square error less than 2 m s−1. The retrieval of the radial wind is also quite accurate, exhibiting comparable errors, although the accuracy of the tangential wind is generally better. Some degradation of the retrieval quality is observed at higher altitude, mainly due to sparser distribution of data in the model. The vertical component of the wind appears to be the most challenging to retrieve, but the software still provides acceptable results. The tropical cyclone mean azimuthal structure and wavenumber structure are found to be very well captured. Sources of errors inherent to airborne Doppler measurements and the effects of some of the simplifications used in the simulation methodology are also discussed.

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John F. Gamache and Robert A. Houze Jr.

Abstract

An objective analysis technique is applied to the time-composite wind and thermodynamic fields of the 12 September GATE tropical squall line. Previous subjective analyses described by Gamache and Houze are confirmed and several new results are obtained.

In the previous analyses, mesoscale upward motion was found in the upper troposphere of the stratiform precipitation region immediately trailing the squall line. Mesoscale downward motion was found in the lower troposphere of the stratiform region. The convective clouds were found to be the source of condensate for more than half of the stratiform precipitation, but mesoscale-updraft condensation was also found to be substantial. In these previous studies, thermodynamic structure was not analyzed, the wind analyses were limited by the number of levels included and vorticity was not analyzed. By employing an objective analysis method in the present study, we have refined and extended the previous work by including more levels, computing vorticity and analyzing the thermodynamic fields.

In the stratiform region, the level of zero vertical motion separating the mesoscale updraft in the upper troposphere from the mesoscale downdraft below is found to be at the 520 mb level (a higher altitude than was indicated by the previous subjective analyses). Maximum convergence in the stratiform region occurred near this level (at 500 mb), but maximum positive vorticity is found to have been at a somewhat lower altitude (650 mb).

The thermodynamic structure of the mesoscale updraft in the stratiform region is indicated by the objective analysis to have been more complex than previously estimated. In its central layer the mesoscale updraft contained a warm anomaly with a humidity that was saturated with respect to ice. Cool anomalies are indicated to have existed near the top of the stratiform cloud deck and (possibly) at the base of the mesoscale updraft.

The structure of the squall system was apparently strongly affected by interaction with the wake of an earlier squall line and with a convective line existing immediately ahead of the squall and intersecting it at nearly right angles. The portion of the squall line feeding on the stabilized wake air associated with these two convective lines was characterized by systematically lower cell tops, as determined by radar, than the remainder of the line. The portion of the stratiform region trailing this part of the line exhibited a distinctly different thermodynamic stratification than was observed to the rear of the deeper-cell section of the squall line. This difference is attributed to the lower altitudes at which condensate and water vapor were determined from this portion of the line are inferred to have advected into the stratiform region.

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John F. Gamache, Frank D. Marks Jr., and Frank Roux

Abstract

Three different airborne Doppler radar sampling strategies were tested in Hurricane Gustav (1990) on 29 August 1990. The two new strategies were the fore-aft scanning technique (FAST) and airborne dual-platform Doppler sampling. FAST employs radar mans in cones pointing alternately fore and aft of the vertical plane that is perpendicular to the flight track. The airborne dual-platform sampling uses two Doppler radars, each aboard a separate aircraft. The Doppler radars scan strictly in the vertical plant normal to the flight track. The aircraft fly simultaneously along different, preferably perpendicular, tracks. The third strategy tested in Hurricane Gustav was single-platform sampling, which uses one Doppler radar on one aircraft that flies two consecutive, usually orthogonal, flight tracks. The antenna scans in the plane normal to the flight track. The third technique had been used previously in hurricanes and other disturbed weather.

The rms differences between the aircraft in situ winds and the Doppler winds derived near the aircraft by single-platform sampling, dual-platform sampling, and FAST are found to be 7.8, 5.1, and 2.5 m s−1, respectively. These results suggest that in hurricanes dual-platform flat-plane sampling and FAST both enable substantial improvements in the accuracy and temporal resolution of airborne Doppler wind fields over those obtained from single-platform, fiat-plane scanning. The FAST results should be applicable to dual-beam sampling, which began in 1991. The actual rms errors of Doppler winds far from the flight tracks, at levels well above flight level, and in highly sheared environments may be significantly higher than the above differences.

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Sylvie Lorsolo, Jun A. Zhang, Frank Marks Jr., and John Gamache

Abstract

Hurricane turbulent kinetic energy (TKE) was computed using airborne Doppler measurements from the NOAA WP-3D tail radars, and TKE data were retrieved for a variety of storms at different stages of their life cycle. The geometry of the radar analysis coupled with the relatively small beam resolution at ranges <8 km allowed for the estimation of subkilometer turbulent processes. Two-dimensional profiles of TKE were constructed and revealed that the strongest turbulence was generally located in convective regions, such as the eyewall, with magnitudes often exceeding 15 m2 s−2 and in the boundary layer with values of 5–10 m2 s−2 in the lowest kilometer. A correlation analysis showed that the strong turbulence was generally associated with strong horizontal shear of vertical and radial wind components in the eyewall and strong vertical shear of horizontal wind in the boundary layer. Mean vertical profiles of TKE decrease sharply above the hurricane boundary layer and level off at low magnitude for all regions outside the radius of maximum wind.

The quality of the retrieval method was evaluated and showed very good agreement with TKE values directly calculated from the three-dimensional wind components of in situ measurements. The method presented here provides a unique opportunity to assess hurricane turbulence throughout the storm, especially in high-wind regions, and can be applied on extensive datasets of past and future airborne hurricane penetrations.

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Paul D. Reasor, Matthew D. Eastin, and John F. Gamache

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.

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Robert Rogers, Sylvie Lorsolo, Paul Reasor, John Gamache, and Frank Marks

Abstract

The multiscale inner-core structure of mature tropical cyclones is presented via the use of composites of airborne Doppler radar analyses. The structure of the axisymmetric vortex and the convective and turbulent-scale properties within this axisymmetric framework are shown to be consistent with many previous studies focusing on individual cases or using different airborne data sources. On the vortex scale, these structures include the primary and secondary circulations, eyewall slope, decay of the tangential wind with height, low-level inflow layer and region of enhanced outflow, radial variation of convective and stratiform reflectivity, eyewall vorticity and divergence fields, and rainband signatures in the radial wind, vertical velocity, vorticity, and divergence composite mean and variance fields. Statistics of convective-scale fields and how they vary as a function of proximity to the radius of maximum wind show that the inner eyewall edge is associated with stronger updrafts and higher reflectivity and vorticity in the mean and have broader distributions for these fields compared with the outer radii. In addition, the reflectivity shows a clear characteristic of stratiform precipitation in the outer radii and the vorticity distribution is much more positively skewed along the inner eyewall than it is in the outer radii. Composites of turbulent kinetic energy (TKE) show large values along the inner eyewall, in the hurricane boundary layer, and in a secondary region located at about 2–3 times the radius of maximum wind. This secondary peak in TKE is also consistent with a peak in divergence and in the variability of vorticity, and they suggest the presence of rainbands at this radial band.

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Michael S. Fischer, Paul D. Reasor, Robert F. Rogers, and John F. Gamache

Abstract

This analysis introduces a novel airborne Doppler radar database, referred to as the Tropical Cyclone Radar Archive of Doppler Analyses with Recentering (TC-RADAR). TC-RADAR is comprised of over 900 analyses from 273 flights into TCs in the North Atlantic, eastern North Pacific, and central North Pacific basins between 1997–2020. This database contains abundant sampling across a wide range of TC intensities, which facilitated a comprehensive observational analysis on how the three-dimensional, kinematic TC inner-core structure is related to TC intensity. To examine the storm-relative TC structure, we implemented a novel TC center-finding algorithm. Here, we show that TCs below hurricane intensity tend to have monopolar radial profiles of vorticity and a wide range of vortex tilt magnitudes. As TC intensity increases, vorticity becomes maximized within an annulus inward of the peak wind, the vortex decays more slowly with height, and the vortex tends to be more aligned in the vertical. The TC secondary circulation is also strongly linked to TC intensity, as more intense storms have shallower and stronger lower-tropospheric inflow as well as larger azimuthally-averaged ascent. The distribution of vertical velocity is found to vary with TC intensity, height, and radial domain. These results—and the capabilities of TC-RADAR—motivate multiple avenues for future work, which are discussed.

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