On the Formation of Organized Convective Systems Observed over the Eastern Atlantic

Ogura, Yusuke; Chen, Y. L.; Russell, James O. H.; Soong, Su-Tzai

doi:10.1175/1520-0493(1979)107<0426:otfooc>2.0.co;2

Cited by 26 publications

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“…10 and 11, we observe that the severe storms formed in an area where the horizontal gradient of temperature in the boundary layer was large, and the strong differential development of the mixed layer had occurred in the prestorm period. This situation was somewhat similar to that observed earlier in the formation of a squall line over western Oklahoma on 8 June 1966 by Ogura and Chen (1977). They noted that low-level convergence was present in the prestorm situation, and speculated that the prestorm convergence was associated with boundary layer processes.…”

Section: Analysis Of Data From the Storm-scale Networksupporting

confidence: 86%

“…We attempted to compute horizontal divergence at selected pres- sure levels from the wind data over the storm-scale network, by applying a simple objective analysis scheme that we used earlier in analyzing data from the spring programs of the National Severe Storms Laboratory (Ogura and Chen, 1977) and the GARP Atlantic Tropical Experiment . The result of our objective analysis was disappointing in that the computed divergence fields were not coherent in time or in the vertical, in most cases.…”

Section: Analysis Of Data From the Storm-scale Networkmentioning

confidence: 99%

“…Ogura (1971) numerically investigated wavenumber selection in finite-amplitude perturbations in a simple situation corresponding to 2-dimensional Benard-Rayleigh convection. He found that, when the Rayleigh number exceeds the critical Rayleigh number, the wavenumber in the steady-state solution does not necessarily coincide with that of the maximum growth rate predicted from the linear stability analysis, but depends upon the wavenumber of the imposed initial perturbation .…”

Section: Symmetric Instabilitymentioning

confidence: 99%

“…The premise of our investigation is that the formation of mesoscale convective systems is triggered by a low-level mesoscale convergence in a convectively unstable atmosphere (e.g., Ogura and Chen, 1977;. The question we addressed in this analysis was what physical processes were responsible for generating the low-level mesoscale convergence in the prestorm situation and, more specifically, why the convective activity…”

Section: Introductionmentioning

confidence: 99%

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Possible Triggering Mechanisms for Severe Storms in SESAME-AVE IV (9–10 May 1979)

Ogura

Juang

Zhang

et al. 1982

Bulletin of the American Meteorological Society

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The SESAME-AVE IV (9-10 May 1979) rawinsonde data were analyzed to uncover possible triggering mechanisms for severe storms that developed over western Oklahoma and Texas. The high frequency of observations (at 3 h intervals) and high vertical resolution of reported data (at 25 mb intervals) at all stations permitted investigation of the diurnal variation of the planetary boundary layer on the synoptic scale. Thunderstorms developed first just ahead of a stationary front over the Texas panhandle on the afternoon of 9 May. This area was characterized by the absence of a strong inversion (or "lid") that represented an interface between very warm and dry air aloft, and warm moist tropical air below. Apparently, mesoscale low-level ascending motion associated with frontal lifting and/or the inland sea breeze effect led to the removal of the lid. Another noteworthy feature in this storm event was the strong vertical wind shear at low and middle levels over the storm area. When combined with the development of a deep boundary layer with weak stratification during the daytime, the Richardson number became less than one in the boundary layer in the prestorm situation. The results of our numerical linear stability analysis indicate that the observed basic states were indeed symmetrically unstable. This may suggest that the triggering processes were argumented by symmetric instability. Although a well defined dry line was present, it does not seem to have contributed directly to initiation of storms in this case. It also was observed that, as the thermal low began to weaken in the early evening, the cold air behind a stationary front started advancing eastward and helped to extend the line of thunderstorms deep into central Texas. This may be another process whereby some storms prefer to develop in the late afternoon or early evening.

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Section: Analysis Of Data From the Storm-scale Networksupporting

confidence: 86%

Section: Analysis Of Data From the Storm-scale Networkmentioning

confidence: 99%

Section: Symmetric Instabilitymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Possible Triggering Mechanisms for Severe Storms in SESAME-AVE IV (9–10 May 1979)

Ogura

Juang

Zhang

et al. 1982

Bulletin of the American Meteorological Society

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“…For examples, Cb cloud systems developed in tropical oceanic areas have been studied by Ruprecht and Gray (1976a and b) and Ogura et al (1979).…”

Section: Introductionmentioning

confidence: 99%

A Medium-Scale Cloud Cluster in a Baiu Front

Akiyama¹

1984

Journal of the Meteorological Society of Japan

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Observation features and evolution process of a long-lived medium-scale cloud cluster have been analyzed in Part I of the present paper. Thermal and kinematic fields and heat budget of this cloud cluster are investigated in Part II. The special attention is given in its formation and developing stages over the Continental China region.The following results were obtained. 1). A cloud cluster developed in the uniform unstable tropical air over the Continent. Thermal fields in and around the cloud cluster exhibited local variations due to convections and 3-dimensional moisture advection.2). Kinematic fields in the cloud cluster over the Continent also showed particular localized features.That is, medium-scale cyclonic circulation in the lower troposphere, anticyclonic circulation in the upper layer and upward motion are found.They indicated time variations throughout the formation and developing stages.3). An apparent moisture sink in the lower tropospere and an apparent heat source in the upper troposphere existed in the cloud cluster.The both quantities reached the maxima at the time when observed rainfall reached the maximum (*80mm/6 hour). 4). Calculated rainfall (vertically integrated apparent moisture sink) showed similar time variation to that in the observed rainfall:The calculated rainfall amount showed a variation of about one day period, that was consistent with the observed variation analyzed in Part I of the present paper. 5). Thermal and kinematic fields in and around the cloud cluster investigated in this study, which developed in the uniform unstable air over the Continent, showed similar features to those of medium-scale cloud clusters frequently developed in the Baiu front (subtropical baroclinic zone) over the Japan Islands and the Pacific Ocean, in spite of different environmental situations. The present study indicates that the cloud cluster transformed into a frontal depression under the influence of a short wave trough in the subtropical baroclinic zone. It is conjectured that in the atmosphere without baroclinicity the cloud cluster remained as a cloud cluster without transforming into a depression, even though a large amount of heat was released and redistributed through convections.

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A Numerical Simulation of Quasi‐Stationary Tropical Convective Bands

Dudhia

Moncrieff

1987

Quart J Royal Meteoro Soc

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S~' M M A K YA method of imposing large-scale ascent on H three-dimensional numerical model is introduced and this is shown to be crucial in maintaining the quasl-stationary convective bands observed in GATE. The effects of ascent on the overall convective development are to enhance updraughts and particularly to maintain the development of new convection cells. 'The magnitutlc o f the forcing is varied to show the sensitivity of growth on convergence.A quasi-stationary convective band, aligned approximately parallel to the low-level wind shear, is simulated in an initial environment defined from soundings obtained from the GATE ships Reseurcher and Oceanographer on 3 September 1974: the organization is therefore quite distinct from that of fast-moving squall lines aligned at right angles to the shear. Thc simulated convective system has marked dynamical similarity to the observed bands: these consisted of a number of cumulonimhus cells with new cells developing on the eastern end of the hand with deeper cells to the west. The cells generally travel along the line.'The large shear due to the strong low-lcvcl jet is crucial in influencing the band development and oricntation. The dynamical structure of the convective band is investigated by a trajectory analysis. which indicates an organized three-dimensional structure.The transports of heat, moisture and momentum arc calculated, and a quasi-balance between large-scale ascent and cloud-induced descent on a time scale of a few hours is shown to exist. Apparent heat sources and moisture sinks are calculated and these have a hi-modal vertical structure, in general agreement with those in observed tropical cloud clusters.

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On the Formation of Organized Convective Systems Observed over the Eastern Atlantic

Cited by 26 publications

References 0 publications

Possible Triggering Mechanisms for Severe Storms in SESAME-AVE IV (9–10 May 1979)

Possible Triggering Mechanisms for Severe Storms in SESAME-AVE IV (9–10 May 1979)

A Medium-Scale Cloud Cluster in a Baiu Front

A Numerical Simulation of Quasi‐Stationary Tropical Convective Bands

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