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structures), indicate that the diffusivity approximation is not adequate. Ekman's solution has an intrinsic inflectional wind profile that was found to be unstable to long waves corresponding to the organized large vortices scale. The nonlinear theory showed that the growth of these finite perturbation waves modifies the mean flow, eliminating the dynamic inflectional instability energy so that equilibrium is obtained. The modified mean flow corresponds well with observations. This solution for the layer containing the PBL-scale roll wavelength requires a modification of the flux transports to accommodate modeling of the advective motion of the large vortices.
59:
85:. Although horizontal convective rolls, also known as cloud streets, have been clearly seen in satellite photographs for the last 30 years, their development is poorly understood, due to a lack of observational data. From the ground, they appear as rows of cumulus or cumulus-type clouds aligned parallel to the low-level wind. Research has shown these eddies to be significant to the vertical transport of momentum, heat, moisture, and air pollutants within the boundary layer. Cloud streets are usually more or less straight; rarely, cloud streets assume
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predict that the vortices may be aligned up to 30° to the left for stably stratified environments, 18° to the left for neutral environments, and nearly parallel to the mean wind for unstably stratified (convective) environments. This theory has been supported by aircraft observations from several field experiments.
101:
Horizontal rolls are counter-rotating vortex rolls that are nearly aligned with the mean wind of the
Planetary Boundary Layer (PBL). They can be caused by convection in the presence of a moderate wind and/or dynamic inflection point instabilities in the mean wind profile. Early theory on the features
109:
If the environmental air is near saturation, condensation may occur in updrafts produced from the vortex rotation. The sinking motion produced between alternating pairs of rolls will evaporate clouds. This, combined with the updrafts, will produce rows of clouds. Glider pilots often use the updrafts
105:
The depth of a vortex is usually the depth of the boundary layer, which is generally on the order of 1–2 km. A vortex pair usually has a lateral to vertical dimension ratio of around 3:1. Experimental studies have shown that the aspect ratio (a ratio of roll wavelength to boundary layer depth)
126:
The most favorable conditions for the formation of the rolls occur when the lowermost layer of air is unstable, but is capped by an inversion-by a stable layer of air. There must be a moderate wind. This often occurs when upper air is subsiding, such as under anticyclonic conditions, and is also
130:
Turbulent energy derived from dynamic instabilities is produced from wind shear energy. Higher wind favors this roll development while convective energy modifies it. Convection in the presence of low speed produces rolls as instability growth in shear is suppressed. Convection in very low wind
138:
mathematics, and has not been widely used. However, when incorporated into the NCEP forecast models using satellite surface wind data, it significantly improved the forecasts. The nonlinear solution, with explicit description of the finite perturbation coherent structure rolls constitutes a
122:
The linear approximation, the eddy diffusivity equation with an eddy diffusion coefficient K, allowed Ekman to obtain a simple logarithmic spiral solution. However the frequent presence of the horizontal roll vortices in the PBL, which represent an organization of the turbulence (coherent
118:
The exact process that leads to the formation of horizontal rolls is complicated. The basic stress mechanism in the PBL is turbulent flux of momentum, and this term must be approximated in the fluid dynamic equations of motion in order to model the Ekman layer flow and fluxes.
411:
Brown, R.A. (1974). “Analytic
Methods in Planetary Boundary Layer Modeling”, Adam Analytic Methods in Planetary Boundary Layer Modeling, Adam Hilger LTD., London, and Halstead Press, John Wiley and Sons, New York,
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frequently found when radiation fog has formed overnight. Convection occurs below the inversion, with air rising in thermals below the clouds and sinking in the air between the streets.
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has been found to vary between 2:1 and 6:1, however, in some situations, the aspect ratio may be as large as 10:1. The lifetime of a convective roll can last from hours to days.
751:
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703:
Brown, R.A. (1991). “Fluid
Mechanics of the Atmosphere”, International Geophysics Series, 47, Academic Press, San Diego,
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Brown, R.A. (1981). "On the Use of
Exchange Coefficients and Organized Large Scale Eddies in Modeling Turbulent Flows".
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Brown, R.A. (1980). "Longitudinal
Instabilities and Secondary Flows in the Planetary Boundary Layer: A Review".
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produced by cloud streets enabling them to fly straight for long distances, hence the name “cloud streets”.
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81:, are long rolls of counter-rotating air that are oriented approximately parallel to the ground in the
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Lines of clouds streets stretch from north-west to south-east in this natural-colour satellite view of
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Kuo, H. (1963). "Perturbations of Plane
Couette Flow in Stratified Fluid and Origin of Cloud Sheets".
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patterns when the wind driving the clouds encounters an obstacle. Those cloud formations are known as
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Although this solution has been verified with numerous observations, it is complicated, involving
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Horizontal convective rolls producing cloud streets (lower left portion of the image) over the
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Etling, D.; R.A. Brown (1993). "Roll
Vortices in the Planetary Boundary Layer: A Review".
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485:"Determining the Environmental Conditions Supporting their Existence and Characteristics"
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563:"A Single Doppler Radar Study of Horizontal-Roll Convection in a Lake-Effect Snow Storm"
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364:"The Structure and Dynamics of Horizontal Vorticities in the Planetary Boundary Layer"
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Simple schematic of the production of cloud streets by horizontal convective rolls.
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775:"Cloud Streets Photographed over Gulf of Mexico: Gallery of Cloud Streets Images"
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significant contribution to the theory of chaos for organization of turbulence.
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320:"On the Inflection Point Instability of a Stratified Ekman Boundary Layer"
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657:"An Operational Large-scale Marine Planetary Boundary Layer Model"
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Weckwerth, T.M.; J.W. Wilson; R.M. Wakimoto; N.A. Crook (1997).
19:"Cloud street" redirects here. For the book by Tim Winton, see
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10.1175/1520-0450(1982)021<0261:AOLSMP>2.0.CO;2
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10.1175/1520-0469(1982)039<1521:asdrso>2.0.co;2
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10.1175/1520-0493(1997)125<0505:HCRDTE>2.0.CO;2
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10.1175/1520-0469(1973)030<1077:TSADOH>2.0.CO;2
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10.1175/1520-0469(1972)029<0850:OTIPIO>2.0.CO;2
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10.1175/1520-0469(1970)027<0742:ASFMFT>2.0.CO;2
268:"A Secondary Flow Model for the Planetary Boundary Layer"
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environments generally produce cellular convection.
114:Development and required environmental conditions
23:. For the television adaptation of the book, see
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534:An Introduction to Boundary Layer Meteorology
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726:Guilford, Connecticut: The Lyons Press.
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724:The Weather Identification Handbook
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567:Journal of the Atmospheric Sciences
368:Journal of the Atmospheric Sciences
324:Journal of the Atmospheric Sciences
272:Journal of the Atmospheric Sciences
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16:Long rolls of counter-rotating air
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752:"Cloud Streets Pave Hudson Bay"
655:Brown, R.A. and T. Liu (1982).
661:Journal of Applied Meteorology
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71:Horizontal convective rolls
34:Horizontal convective rolls
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802:Atmospheric thermodynamics
609:Boundary-Layer Meteorology
538:Kluwer Academic Publishers
183:Boundary-Layer Meteorology
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91:von Kármán vortex streets
83:planetary boundary layer
75:horizontal roll vortices
25:Cloudstreet (miniseries)
736:Scorer, Verkaik (1989)
467:10.1029/RG018i003p00683
756:NASA Earth Observatory
722:Dunlop, Storm (2002)
532:Stull, Roland (1988).
489:Monthly Weather Review
149:Atmospheric convection
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740:David & Charles
318:Brown, R.A. (1972).
266:Brown, R.A. (1970).
673:1982JApMe..21..261B
621:1981BoLMe..20..111B
579:1982JAtS...39.1521K
501:1997MWRv..125..505W
459:1980RvGSP..18..683B
380:1973JAtS...30.1077L
362:LeMone, M. (1973).
336:1972JAtS...29..850B
284:1970JAtS...27..742B
238:1963PhFl....6..195K
195:1993BoLMe..65..215E
807:Gliding technology
629:10.1007/BF00119927
561:Kelly, R. (1982).
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64:New England
21:Cloudstreet
796:Categories
784:2009-10-29
766:2006-05-01
547:9027727694
418:0470111607
160:References
154:Wave cloud
44:Bering Sea
691:1520-0450
637:120165198
519:124381300
302:1520-0469
211:119535446
143:See also
669:Bibcode
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677:doi
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