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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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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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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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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 Re-centering (TC-RADAR). TC-RADAR comprises over 900 analyses from 273 flights into TCs in the North Atlantic, eastern North Pacific, and central North Pacific basins between 1997 and 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.

Significance Statement

Acquiring observations of the inner core of tropical cyclones (TCs) is a challenge due to the hazardous conditions inherent to the storm. A proven method of sampling the TC core region is the use of airborne radar. This study presents a novel database comprising over 900 airborne radar analyses collected in storms between 1997 and 2020, which is freely available to the research community. Here we demonstrate the utility of the database by examining how the three-dimensional structure of the TC core region changes depending upon the intensity of the storm. By identifying how the baseline TC vortex structure varies with TC intensity, this work provides the foundation for multiple future research avenues and model evaluation efforts.

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John F. Gamache
,
Robert A. Houze Jr.
, and
Frank D. Marks Jr.

Abstract

The hydrometeor water budget of Hurricane Norbert on 24 September 1984 is computed using two micro- physical retrieval techniques. Three-dimensional distributions of condensation, evaporation, precipitation, and advection of cloud and precipitation are computed, and a bulk water budget is computed as the volume integral of these distributions.

The role of the microphysical retrievals is to provide the three-dimensional distribution of cloud water content, since it cannot be determined with the equipment available. Both retrieval methods use the steady-state continuity equation for water. The first method determines precipitation formation mechanisms from the radar-reflectivity and Doppler wind fields. The cloud water content is determined, through microphysical modeling, to be the amount necessary to explain the rate of precipitation formation. The second method (that of Hauser el al.) solves the water continuity equations as a boundary value problem, while also employing microphysical modeling. This method is applied in three dimensions for the first time.

Asymmetries in the water budget of Hurricane Norbert were important, apparently accounting for nearly half the net condensation. The most condensation and heaviest precipitation was to the left of the storm track, while the strongest evaporation was to the rear of the storm. Many of the downdrafts were unsaturated because they were downwind of the precipitation maximum where little water was available for evaporation. Since the evaporation in the downdrafts was significantly less than the condensation in their counterpart updrafts, net condensation (bulk condensation-bulk evaporation) was significantly greater than would be implied by the net upward mass flux. Much of the vapor required to account for the greater bulk condensation appears to have come from enhanced sea surface evaporation under the dry downdraft air to the right of the storm track.

The net outflow of condensate from the storm inner core was quite small, although there were appreciable outward and inward horizontal fluxes at certain locations. A maximum of ice outflow to the left of the storm track in the front of the storm corresponded well to the ice particle trajectories that Houze et al. suggested were feeding the stratiform precipitation found farther outward from the storm center.

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Robert A. Houze Jr.
,
Chee-Pong Cheng
,
Collen A. Leary
, and
John F. Gamache

Abstract

A set of equations for diagnosing the properties of precipitating clouds over a tropical ocean is developed by postulating a population of model clouds in which the vertical motions consist of convective up-drafts and downdrafts in cumulus-scale cells and mesocscale updrafts and downdrafts associated with anvil clouds. The properties of a population of precipitating clouds can be diagnosed with these equations by constraining the model clouds to explain either an observed large-scale heat budget (the synoptic approach) or an observed spectrum of precipitation (the radar approach). The results of either approach are dependent on certain parameters of the model clouds, which must be assumed. These parameters are identified, and, in this paper, they are held constant in a controlled experiment comparing the results of the radar and synoptic approaches obtained for the same cloud population (the average population in Phase 111 of GATE). This experiment shows that similar results can be obtained by either approach, giving confidence in both sets of data, the methods used to analyze them and the diagnostic equations themselves. In this experiment, however, the model parameters were adjusted to suppress the diagnosis of the mesoscale motions associated with precipitating anvil clouds. In other papers, the model parameters will be varied to test the model dependency of the diagnostic calculations, especially with regard to the inclusion of mesoscale motions.

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Paul D. Reasor
,
Michael T. Montgomery
,
Frank D. Marks Jr.
, and
John F. Gamache

Abstract

The asymmetric dynamics of the hurricane inner-core region is examined through a novel analysis of high temporal resolution, three-dimensional wind fields derived from airborne dual-Doppler radar. Seven consecutive composites of Hurricane Olivia’s (1994) wind field with 30-min time resolution depict a weakening storm undergoing substantial structural changes. The symmetric and asymmetric mechanisms involved in this transformation are considered separately.

To zeroth order the weakening of the primary circulation is consistent with the axisymmetric vortex spindown theory of Eliassen and Lystad for a neutrally stratified atmosphere. Vertical shear, however, increased dramatically during the observation period, leading to a strong projection of the convection onto an azimuthal wavenumber 1 pattern oriented along the maximum vertical shear vector. Recent theoretical ideas elucidating the dynamics of vortices in vertical shear are used to help explain this asymmetry.

The role of asymmetric vorticity dynamics in explaining some of the physics of hurricane intensity change motivates a special focus on Olivia’s vorticity structure. It is found that an azimuthal wavenumber 2 feature dominates the asymmetry in relative vorticity below 3-km height. The characteristics of this asymmetry deduced from reflectivity and wind composites during a portion of the observation period show some consistency with a wavenumber 2 discrete vortex Rossby edge wave. Barotropic instability is suggested as a source for the wavenumber 2 asymmetry through a series of barotropic numerical simulations.

Trailing bands of vorticity with radial wavelengths of 5–10 km are observed in the inner core approximately 20 km from the storm center, and may be symmetrizing vortex Rossby waves. Elevated reflectivity bands with radial scales comparable to those of the vorticity bands, also near 20–25-km radius, may be associated with these vorticity features.

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