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G. M. Barnes
and
G. J. Stossmeister

Abstract

The mesoscale thermodynamic, kinematic and reflectivity structure of a rainband observed in Hurricane Irene on 27 September 1981 is presented. Data are from 16 aircraft passes, from 150 to 4700 m MSL through the rainband which was located SSW of the circulation center of the northward moving hurricane. Cross sections of the rainband as a function of height and radial distance from the storm center reveal the following:

1) little or no decrease of θ e in the subcloud layer associated with the weak convective cells in the rainband;

2) large differences in static stability between the north and south sides of the band;

3) a change of the radial wind component in the subcloud layer, from flow toward the circulation center to flow away from it as the rainband weakens.

The decay of the rainband is believed to be related to mesoscale subsidence and the location of the rainband relative to the motion of the hurricane. Melting and evaporation of hydrometeors falling between the eyewall and the rainband are the most plausible causes of the subsidence. The collapse of the rainband is considered to be an example of the negative feedback possible among mesoscale features in the hurricane.

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G. M. Barnes
and
K. Sieckman

Abstract

Rawinsonde data from GATE are composited to examine the environmental similarities and differences between fast-moving (line speed VL > 7 m s−1) and slow-moving VL < 3 m s−1) mesoscale convective cloud lines. Thermodynamic structures for the two types of line are similar with respect to the mixed-layer values of θ e and convective available potential energy, but the fast-moving lines have a more pronounced minimum in θ e at ∼700 mb. Kinematic structure shows that the vertical shows of the horizontal wind is normal to the leading edge of convention in the fast-moving convective lines but parallel to the leading edge of the slow-moving cloud lines. The results suggest that the type of line (fast versus slow) which forms may be affected by the initial environmental conditions.

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J. C. Fankhauser
,
G. M. Barnes
, and
M. A. LeMone

Abstract

Data from five Doppler radars, the surface mesonet, aircraft, and rawinsondes from the Cooperative Convective Precipitation Experiment (CCOPE) are used to document the structure and evolution of a squall line with unusually persistent cells and an anvil that spreads downwind in strong upper-level westerlies. The environmental sounding showed linear shear of ∼4 m s−1 km−1 through the troposphere, a convective available potential energy of 600 m2 s−2, and a convective Richardson number of 10, based on the wind in the lowest 6 km.

The orientation of the squall line, comprised of high-reflectivity centers spaced 20–40 km apart, changed with time. Initially, the squall-line axis was normal to the environmental shear, but with time it became parallel to the shear vector, as the northeastern portion of the subcloud cold dome merged with cold air generated by individual storms that had formed ahead of the line. The intensity of the cells within the squads line diminished as its axis became more parallel to the shear.

Trajectory analyses based on the Doppler-derived wind field show that three-dimensional airflow is crucial to the maintenance of the squall line. Boundary-layer air directly ahead of the strongest reflectivity centers fed the associated updrafts while air on their flanks rose slightly, was cooled by evaporation of rain, and then descended to become the primary source of air in the subcloud cold dome. In contrast to typical midlatitude squall lines, there was no evidence of organized rear-to-front system-relative airflow in the subcloud air. This is explained in terms of the initial front-to-rear momentum of the cold-dome source air, with frictional effects also playing a role for air near the surface. Since the ground is traveling rearward relative to the storm, frictional effects oppose the pressure gradient ahead of the cold-dome pressure maximum and keep the near-surface air moving rearward throughout the cold dome. Only a small fraction of the subcloud air originated at midcloud levels, probably because evaporation above cloud base was inhibited by high relative humidities in the environment and because comparatively weak updrafts produced only modest amounts of condensate for water loading.

The persistence of squall-line elements is discussed in light of (a) their resemblance to supercells as represented in numerical simulations, and (b) recent theories involving the balance of vorticity between vertical shear in the low-level environment and the cold dome in the subcloud layer. The squall line is representative of that part of the spectrum of mesoscale convective systems that does not have a rear inflow jet, does not produce a trailing stratiform precipitation region, and does not rely upon penetrative downdrafts to sustain the air mass within the subcloud cold dome.

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G. M. Barnes
,
J. F. Gamache
,
M. A. LeMone
, and
G. J. Stossmeister

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 that have since dissipated. The mass flux of this cell is estimated to be 5%–10% of the mass flux accomplished by an eyewall of a moderate tropical cyclone. This finding supports the concept that large, convectively active rainbands have a major effect on the subcloud layer air flowing toward the eyewall.

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B. F. Ryan
,
G. M. Barnes
, and
E. J. Zipser

Abstract

On 18 January 1987 the three aircraft from the Equatorial Mesoscale Experiment (EMEX) completed a mission designed to identify the mesoscale reflectivity, kinematic, and thermodynamic structure of a convectively active rainband on the leading edge of developing Tropical Cyclone Irma. The reflectivity structure, determined from land-based and aircraft radars, reveals that the band consisted of an unusually wide (80 km) region of convective clouds that did not maintain a linear organization. High-θ e air entered from both sides in the subcloud and lower cloud layer. Thermodynamically, the rainband was benign in the sense that it did not produce cold downdrafts or large regions of low-θ e air that would limit the energy of the low-level inflow to the developing eyewall region. Kinematically, the band was a preferred location for the enhancement of the wind component parallel to the band. The low-level flow was along rather than across the major axis of the band. Above 4.5 km flow was from the inner to the outer side of the band, but this air was also converging, which contributed to the widespread heavy rain encountered by the aircraft. The chaotic reflectivity, minor thermodynamic modification of the low-level inflow, and the tendency of the flow to be along the band are characteristics that contrast sharply with those of convectively active bands observed on the leading edge of mature tropical cyclones.

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G. M. Barnes
,
E. J. Zipser
,
D. Jorgensen
, and
F. Marks Jr.

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 θ e , 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 subcloud layers.

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C. A. Knight
,
M. B. Baker
,
G. M. Barnes
,
G. B. Foote
,
M. A. LeMone
, and
G. Vali
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M. A. LeMone
,
G. M. Barnes
,
E. J. Szoke
, and
E. J. Zipser

Abstract

The tilt with height of the leading edge of seven mesoscale convective lines in GATE is determined by two independent methods. When averaged over time and along the line axis, the tilt is found to he surprisingly shallow, between 20 and 35 degrees from the horizontal. This is distinct from the slopes of the individual towers, which can be much steeper. The line leading-edge slope corresponds to the ratio of the vertical to horizontal velocity, relative to the line motion, of “representative” embedded convective cores.

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J. C. Fankhauser
,
G. M. Barnes
,
L. J. Miller
, and
P. M. Roskowski

Photographs of some variform cloud features observed in the inflow sector of an intense thunderstorm that occurred in southeastern Montana on 11 July 1981 are described. Associated meteorological conditions are interpreted within the context of mesonetwork, aircraft, and radar data collected by the Cooperative Convective Precipitation Experiment (CCOPE). Three transient cloud forms identified as collar, pedestal, and scud (or arcus) clouds occurred within a 30 min period beneath the uniformly flat base of the shelf cloud and near the center of a mesocyclone resolved by multiple-Doppler radial velocity measurements. In contrast to these lowered cloud base anomalies, a cloud-free vault that penetrated upward into the base of the surrounding shelf cloud also is documented. A remarkable bluish coloration in its vertical walls is attributed to backscattering from the celestial dome. All of the departures in cloud base height from the horizontally uniform base of the shelf cloud can be explained in terms of a moisture excess or deficit in various branches of the 3-dimensional circulation around the mesocyclone.

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B. Guenther
,
W. Barnes
,
E. Knight
,
J. Barker
,
J. Harnden
,
R. Weber
,
M. Roberto
,
G. Godden
,
H. Montgomery
, and
P. Abel

Abstract

The Moderate-Resolution Imaging Spectroradiometer (MODIS) is a key instrument on the NASA Earth Observing System. It is a multispectral sensor that will be used to track long-term global change in the land, atmosphere, and ocean components of the earth. Major advances are being made with MODIS over previous sensors in the form of improved on-orbit sensor characterization and calibration using a system of onboard calibrators. This article describes those calibrators and provides an early estimate of the expected accuracy for the MODIS calibrated datasets resulting from the use of these calibrators. The focus of the paper is the calibration approach that is being implemented at-launch for the top-of-the-atmosphere data products.

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