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Rodger A. Brown and Rebecca J. Meitín

Abstract

During the late afternoon and early evening of 27 June 1989. Three splitting thunderstorms formed over Standing Rock Indian Reservation in the southern portion of the North Dakota Thunderstorm Project area. The first two storms are the subject of this study. The entire life cycles of both storms were documented using a single ground-based Doppler radar. Radar reflectivity signatures of updraft summits and Doppler velocity signatures of divergence near storm top were used to deduce updraft evolution within the storms. Dual-Doppler radar observations from a ground-based radar and an airborne Doppler radar provided fragmentary documentation of the storms’ life cycles.

The splitting storms on that day were unusual in two distinct ways: (a) the left members of the splitting storms were the dominant and longer-lasting ones, and (b) none of the deduced updrafts were collocated with centers of vorticity signatures that would have indicated updraft rotation. Both of the left-moving storms had 10 sequential primary updrafts, whereas their right-hand counterparts had 3 or 4 primary updrafts. Initial formation of the right-flank updrafts lagged behind the initial formation of the left-flank updrafts by 40–70 min. All the individual updraft summits moved in the general direction of the mean wind. Sequential updraft development on the left and right flanks of the storms suggested that expanding gust fronts provided the propagational component of storm motion.

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Margaret A. LeMone and Rebecca J. Meitin

Abstract

Evidence indicates that fair-weather to towering cumulus clouds over the East Atlantic Ocean during GATE were frequently organized into mesoscale structures. Three examples of such structures are examined, using gust-probe aircraft data collected in parallel straight-and-level flight tracks at 150 m, and covering an area greater than 30×30 km. The aircraft (two cases) or rawinsonde (one case) data provide vertical profiles of mean wind, temperature and mixing ratio. Cloud patterns are revealed from an upward-looking infrared sensor on the aircraft and radar and satellite pictures.

The data show that the cumulus were organized into bands with horizontal wavelengths of 15–25 km. The circulations appear to extend through the subcloud layer, with all the fields at 150 m well related to the cloudiness overhead. Since the circulations are aligned with the subcloud-layer shear and travel in a direction parallel to the subcloud-layer wind (in the two cases for which band movement is documented), it is believed that they are primarily subcloud-layer phenomena. The subcloud-layer depth is about 600 m, giving aspect ratios of the bands from 25 to 50, in the range of mesoscale cellular convection observed in midlatitudes.

Several physical mechanisms which might explain the bands are discussed.

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Edward J. Zipser, Rebecca J. Meitín, and Margaret A. LeMone

Abstract

The structure of the convective band of 14 September in the dense GATE observing array is determined using wind and thermodynamic data primarily from multiple aircraft penetrations, which are well distributed in the vertical and in time.

The well-defined mesoscale features in the line, which are 10–40 km in scale, quasi-two-dimensional, and persist for several hours, determine the distribution of the convective-scale features, which are 5 km or less in size, three-dimensional, not generally detectable for more than one flight leg. At the leading edge, a 30 km zone of strong ascent is computed from two-dimensional continuity. Here, lifting of the ambient air creates a favorable environment—not found elsewhere—for deep cumulonimbus clouds to develop. Their updrafts are weak, 2–4 m s−1 on the average. Behind the updraft zone, below 3–4 km, is a broad descent zone. It corresponds to the stratiform rain area, and has little convection, and some drying at lower levels. On the average, the mass flux by the mesoscale and convective-scale drafts of the updraft zone is about twice as much as that of the descent zone. The rainfall rate in the updraft zone is generally in excess of 8 mm h−1, while that in the downdraft region is less. The horizontal winds normal to the line are strongly modified by pressure forces, while those parallel to the line are changed mainly through mixing. Strong vertical vorticity is created in the line by tilting of the mean shear of the parallel component.

As the system matures, the downdraft mass flux increases relative to the updraft mass flux, so that the net mass flux becomes negative during the decay phase. The fraction of the total rain falling in the stratiform zone increases with time. However, considerable rain still falls from intense convective cells as well as the stratiform “anvil” even when the net mass flux goes to zero in the lowest kilometer.

The structure and evolution of the line is similar to that of tropical squall lines, but it is less spectacular. Winds are weaker, there is less mass flow through the system, movement is slower, and there is less drying in the rain area. The line is aligned with the wind and shear, rather than across it, as is the case for many squall lines.

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Kenneth Sassen, Arlen W. Huggins, Alexis B. Long, Jack B. Snider, and Rebecca J. Meitín

Abstract

A comprehensive analysis of a deep winter storm system during its passage over the Tushar Mountains of southwestern Utah is reported. The case study, drawn from the 1985 Utah/NOAA cooperative weather modification experiment, is divided into descriptions of the synoptic and kinematic properties in Part I, and storm structure and composition here in Part II. In future parts of this series, the turbulence structure and indicated cloud seeding potential will be evaluated. The analysis presented here in Part II focuses on multiple remote sensor and surface microphysical observations collected from a midbarrier (2.57 km MSL) field site. The collocated remote sensors were a dual-channel microwave radiometer, a polarization lidar, and a Ka-band Doppler radar. These data are supplemented by upwind, valley-based C-band Doppler radar observations, which provided a considerably larger-scale view of the storm.

In general, storm properties above the barrier were either dominated by barrier-level orographic clouds or propagating mesoscale cloud systems. The orographic cloud component consisted of weakly (−3° to −10°C) supercooled liquid water (SLW) clouds in the form of an extended barrier-wide cap cloud that contained localized SLW concentrations. The spatial SLW distribution was linked to topographical features surrounding the midbarrier site, such as abrupt terrain rises and nearby ridges. This orographic cloud contributed to precipitation primarily through the riming of particles sedimenting from aloft, and also to some extent through an ice multiplication process involving graupel growth. In contrast, mesoscale precipitation bands associated with a slowly moving cold front generated much more significant amounts of snowfall. These precipitation bands periodically disrupted the shallow orographic SLW clouds. Mesoscale vertical circulations appear to have been particularly important in SLW and precipitation production along the leading edges of the bands. Since the SLW clouds during the latter part of the storm were based at the frontal boundary, SLW and precipitation gradually diminished as the barrier became submerged under the cold front.

Based on a winter storm conceptual model, we conclude that low-level orographic SLW clouds, when decoupled from the overlying ice cloud layers of the storm, are generally inefficient producers of precipitation due to the typically warm temperatures at these altitudes in our region.

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Bruce A. Boe, Jeffrey L. Stith, Paul L. Smith, John H. Hirsch, John H. Helsdon Jr., Andrew G. Detwiler, Harold D. Orville, Brooks E. Mariner, Roger F. Reinking, Rebecca J. Meitín, and Rodger A. Brown

The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification.

Of primary interest were tracer experiments that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF6), carbon monoxide, ozone, radar chaff, and silver iodide.

Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunderstorms has resulted.

The objectives of the various project experiments and the specific facilities employed are described. Project highlights and some preliminary results are also presented.

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