Stability of a Plane Parallel Flow with Variable Vertical Shear and Unstable Stratification

Asai, Tomio

doi:10.2151/jmsj1965.48.2_129

Cited by 120 publications

(86 citation statements)

References 28 publications

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“…Asai (1972) investigated the nature of instability in unstably stratified shear flows turning the direction with height, which is a similar situation in this case. He showed that two di¤erent types of instability are obtained in the case of directional shear flows with points of inflection as in the unidirectional shear flow case shown in Asai (1970b). One is the shear-perpendicular mode driven primary by the vertical shear and the other is the shear-parallel mode driven by the buoyancy.…”

Section: Discussionmentioning

confidence: 98%

“…Yagi (1985) and Yagi et al (1986) analyzed GMS images and upper-air sounding data and indicated that the axes of the transversal cloud bands were parallel to the vertical wind shear vector of horizontal winds. In addition, by applying the linear theory of the thermal instability in the vertical shear flow (Asai 1970a(Asai , 1970b(Asai and 1972, Yagi (1985) proposed that the transversal cloud bands were produced from longitudinal-mode (shear-parallel mode) roll convections. Recently, Shimizu and Tsuboki (2005) analyzed the transversal cloud band near the coast of the Hokuriku district on the basis of dual-Doppler radar observations.…”

Section: Introductionmentioning

confidence: 99%

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The Structure and Formation Mechanism of Transversal Cloud Bands Associated with the Japan-Sea Polar-Airmass Convergence Zone

Eito

Murakami

Muroi

et al. 2010

Journal of the Meteorological Society of Japan

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During a cold-air outbreak, a broad cloud band is occasionally observed over the Japan-Sea Polar-Airmass Convergence Zone (JPCZ) that forms over the Sea of Japan from the base of the Korean Peninsula to the Japanese Islands. On 14 January 2001, a broad cloud band associated with the JPCZ (JPCZ cloud band) extended in a southeastward direction from the base of the Korean Peninsula to Wakasa Bay, and it stagnated for half a day. The JPCZ cloud band consisted of two cloud regions: one was a long cloud band extending along its southwestern edge (a developed convective cloud band), and the other was the region consisting of cloud bands normal to a wind direction of winter monsoon (transversal cloud bands). The structure and formation mechanism of the transversal cloud bands were examined on the basis of observations (e.g., satellite images, in situ measurement and cloud-profiling radar data from an instrumented aircraft and upper-air soundings from observation vessels) and simulation results of a cloud-resolving model with a horizontal resolution of 1 km.The transversal cloud bands had the following characteristic structures; they extended along a northeastsouthwest direction, which was parallel to the direction pointed by the vertical shear vector of horizontal wind Corresponding author and present a‰liation: Hisaki Eito, Numerical Prediction Division, Forecast Department, Japan Meteorological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan. E-mail: heito@naps.kishou.go.jp 1 Present a‰liation: Numerical Prediction Division, Forecast Department, Japan Meteorological Agency. 6 2010, Meteorological Society of Japan in the mixed layer, they mainly consisted of convective clouds, which slanted with height toward the downshear side, and they widened and deepened toward southwest, as the depth of the mixed layer increased. An examination of simulation results presented that the transversal cloud bands were accompanied by roll circulations. The axes of rolls were oriented nearly parallel to the direction of the vertical shear vector in the mixed layer. An analysis of the eddy kinetic energy budget indicated that the roll circulations derived most of its energy from the mean vertical shear and the buoyancy.

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Section: Discussionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

The Structure and Formation Mechanism of Transversal Cloud Bands Associated with the Japan-Sea Polar-Airmass Convergence Zone

Eito

Murakami

Muroi

et al. 2010

Journal of the Meteorological Society of Japan

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“…These induced surface heterogeneities are the link between radiative transfer and buoyancy-driven convection (Lohou and Patton, 2014;Horn et al, 2015;Gronemeier et al, 2016). Our focus is therefore more on buoyancy-driven roll vortices in a linear shear environment (Asai, 1970) and less so on inflectionpoint instabilities. To that end, we omit cross-wind shear by neglecting Coriolis force and correspondingly neglect the horizontal turning of the wind as it would be the case in an Ekman boundary layer.…”

Section: Introductionmentioning

confidence: 99%

The role of 1-D and 3-D radiative heating in the organization of shallow cumulus convection and the formation of cloud streets

Jakub

Mayer

2017

Atmos. Chem. Phys.

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Abstract. The formation of shallow cumulus cloud streets was historically attributed primarily to dynamics. Here, we focus on the interaction between radiatively induced surface heterogeneities and the resulting patterns in the flow. Our results suggest that solar radiative heating has the potential to organize clouds perpendicular to the sun's incidence angle.To quantify the extent of organization, we performed a high-resolution large-eddy simulation (LES) parameter study. We varied the horizontal wind speed, the surface heat capacity, the solar zenith and azimuth angles, and radiative transfer parameterizations (1-D and 3-D). As a quantitative measure we introduce a simple algorithm that provides a scalar quantity for the degree of organization and the alignment. We find that, even in the absence of a horizontal wind, 3-D radiative transfer produces cloud streets perpendicular to the sun's incident direction, whereas the 1-D approximation or constant surface fluxes produce randomly positioned circular clouds. Our reasoning for the enhancement or reduction of organization is the geometric position of the cloud's shadow and its corresponding surface fluxes. Furthermore, when increasing horizontal wind speeds to 5 or 10 m s −1 , we observe the development of dynamically induced cloud streets. If, in addition, solar radiation illuminates the surface beneath the cloud, i.e., when the sun is positioned orthogonally to the mean wind field and the solar zenith angle is larger than 20 • , the cloud-radiative feedback has the potential to significantly enhance the tendency to organize in cloud streets. In contrast, in the case of the 1-D approximation (or overhead sun), the tendency to organize is weaker or even prohibited because the shadow is cast directly beneath the cloud. In a land-surface-type situation, we find the organization of convection happening on a timescale of half an hour. The radiative feedback, which creates surface heterogeneities, is generally diminished for large surface heat capacities. We therefore expect radiative feedbacks to be strongest over land surfaces and weaker over the ocean. Given the results of this study we expect that simulations including shallow cumulus convection will have difficulties producing cloud streets if they employ 1-D radiative transfer solvers or may need unrealistically high wind speeds to excite cloud street organization.

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“…In such situations, the energy for the circulations comes from the buoyancy and, if of sufficient magnitude, the vertical wind shear organizes the resulting circulations to minimize shear in the cross-band direction, forming horizontal convective rolls (e.g. Kuo, 1963;Asai, 1970;Miura, 1986;Shirer, 1986). Instead of vertical wind shear, many authors use near-surface wind velocity to determine the band orientation.…”

Section: Introductionmentioning

confidence: 99%

Snowbands over the English Channel and Irish Sea during cold‐air outbreaks

Norris

Vaughan

Schultz

2012

Quart J Royal Meteoro Soc

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Persistent northerly-to-easterly cold-air outbreaks affected the UK during the winters of 2009-10 and 2010-11, with the resulting convection frequently organizing into snowbands over the English Channel and Irish Sea. Sounding data and composite radar reflectivity images from the Met Office Nimrod precipitation radar network reveal that these bands formed along the major axis of each body of water (or sea) when the boundary-layer flow was roughly parallel to each of those axes (along-channel). For both seas, a band was present the majority of times that the 850 hPa flow was along-channel. Of these times of along-channel flow, the 850 hPa wind speed and surface-to-850 hPa temperature difference were significantly greater when bands were present than when they were not. For the English Channel only, the land-sea temperature difference was also significantly greater when bands were present than when they were not. In a real-data Weather Research and Forecasting model (WRF) control simulation of a typical band over the English Channel, a trough develops over the water and offshore air streams from either side converge along it. In the absence of surface fluxes, the trough, convergence and organized precipitation fail to develop altogether. Orography and roughness-length variations are less important in band development, affecting only the location and morphology.

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Stability of a Plane Parallel Flow with Variable Vertical Shear and Unstable Stratification

Abstract: A study is made of stability properties of perturbations superimposed on an unstably stratified plane parallel flow with variable vertical shear.

Cited by 120 publications

References 28 publications

The Structure and Formation Mechanism of Transversal Cloud Bands Associated with the Japan-Sea Polar-Airmass Convergence Zone

The Structure and Formation Mechanism of Transversal Cloud Bands Associated with the Japan-Sea Polar-Airmass Convergence Zone

The role of 1-D and 3-D radiative heating in the organization of shallow cumulus convection and the formation of cloud streets

Snowbands over the English Channel and Irish Sea during cold‐air outbreaks

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