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Horizontal convective rolls

November 30, 2023

"Cloud street" redirects here. For the book by Tim Winton, see Cloudstreet. For the television adaptation of the book, see Cloudstreet (miniseries).

Horizontal convective rolls, also known as horizontal roll vortices or cloud streets, are long rolls of counter-rotating air that are oriented approximately parallel to the ground in the planetary boundary layer. 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.[1] Cloud streets are usually more or less straight; rarely, cloud streets assume paisley patterns when the wind driving the clouds encounters an obstacle. Those cloud formations are known as von Kármán vortex streets.

Horizontal convective rolls
Horizontal convective rolls producing cloud streets (lower left portion of the image) over the Bering Sea.
Simple schematic of the production of cloud streets by horizontal convective rolls.
Lines of clouds streets stretch from north-west to south-east in this natural-colour satellite view of New England.

Characteristics edit

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[2] and/or dynamic inflection point instabilities in the mean wind profile.[3] Early theory[3][4][5][6][7] on the features 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.[5][7][8]

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.[6][7][9] Experimental studies have shown that the aspect ratio (a ratio of roll wavelength to boundary layer depth) 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.[4][10][6][7]

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 produced by cloud streets enabling them to fly straight for long distances, hence the name “cloud streets”.

Development and required environmental conditions edit

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.[6][7][11][12][13][1]

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 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.[3] 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.[7][1] 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.[11][12][1]

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 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.

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 environments generally produce cellular convection.[7][1][8]

Although this solution has been verified with numerous observations, it is complicated, involving chaos theory mathematics, and has not been widely used.[3][6][7][11][12] 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 significant contribution to the theory of chaos for organization of turbulence.

See also edit

References edit

  1. ^ a b c d e Etling, D.; R.A. Brown (1993). "Roll Vortices in the Planetary Boundary Layer: A Review". Boundary-Layer Meteorology. 65 (3): 215–248. Bibcode:1993BoLMe..65..215E. doi:10.1007/BF00705527. S2CID 119535446.
  2. ^ Kuo, H. (1963). "Perturbations of Plane Couette Flow in Stratified Fluid and Origin of Cloud Sheets". Physics of Fluids. 6 (2): 195–211. Bibcode:1963PhFl....6..195K. doi:10.1063/1.1706719.
  3. ^ a b c d Brown, R.A. (1970). "A Secondary Flow Model for the Planetary Boundary Layer". Journal of the Atmospheric Sciences. 27 (5): 742–757. Bibcode:1970JAtS...27..742B. doi:10.1175/1520-0469(1970)027<0742:ASFMFT>2.0.CO;2. ISSN 1520-0469.
  4. ^ a b Brown, R.A. (1972). "On the Inflection Point Instability of a Stratified Ekman Boundary Layer". Journal of the Atmospheric Sciences. 29 (5): 851–859. Bibcode:1972JAtS...29..850B. doi:10.1175/1520-0469(1972)029<0850:OTIPIO>2.0.CO;2.
  5. ^ a b LeMone, M. (1973). "The Structure and Dynamics of Horizontal Vorticities in the Planetary Boundary Layer". Journal of the Atmospheric Sciences. 30 (6): 1077–1091. Bibcode:1973JAtS...30.1077L. doi:10.1175/1520-0469(1973)030<1077:TSADOH>2.0.CO;2.
  6. ^ a b c d e 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, ISBN 0470111607.
  7. ^ a b c d e f g h Brown, R.A. (1980). "Longitudinal Instabilities and Secondary Flows in the Planetary Boundary Layer: A Review". Reviews of Geophysics and Space Physics. 18 (3): 683–697. Bibcode:1980RvGSP..18..683B. doi:10.1029/RG018i003p00683.
  8. ^ a b Weckworth, T.M.; J.W. Wilson; R.M. Wakimoto; N.A. Crook (1997). "Determining the Environmental Conditions Supporting their Existence and Characteristics". Monthly Weather Review. 125 (4): 505–526. Bibcode:1997MWRv..125..505W. doi:10.1175/1520-0493(1997)125<0505:HCRDTE>2.0.CO;2. S2CID 124381300.
  9. ^ Stull, Roland (1988). An Introduction to Boundary Layer Meteorology (2nd ed.). Kluwer Academic Publishers. ISBN 9027727694.
  10. ^ Kelly, R. (1982). "A Single Doppler Radar Study of Horizontal-Roll Convection in a Lake-Effect Snow Storm". Journal of the Atmospheric Sciences. 39 (7): 1521–1531. Bibcode:1982JAtS...39.1521K. doi:10.1175/1520-0469(1982)039<1521:asdrso>2.0.co;2.
  11. ^ a b c Brown, R.A. (1981). "On the Use of Exchange Coefficients and Organized Large Scale Eddies in Modeling Turbulent Flows". Boundary-Layer Meteorology. 20 (1): 111–116. Bibcode:1981BoLMe..20..111B. doi:10.1007/BF00119927. S2CID 120165198.
  12. ^ a b c Brown, R.A. and T. Liu (1982). "An Operational Large-scale Marine Planetary Boundary Layer Model". Journal of Applied Meteorology. 21 (3): 261–269. Bibcode:1982JApMe..21..261B. doi:10.1175/1520-0450(1982)021<0261:AOLSMP>2.0.CO;2. ISSN 1520-0450.
  13. ^ Brown, R.A. (1991). “Fluid Mechanics of the Atmosphere”, International Geophysics Series, 47, Academic Press, San Diego, ISBN 0-12-137040-2

Further reading edit

  • Dunlop, Storm (2002) The Weather Identification Handbook Guilford, Connecticut: The Lyons Press. ISBN 1-58574-857-9
  • Scorer, Verkaik (1989) Spacious Skies David & Charles ISBN 0-7153-9139-9
  • . NASA Earth Observatory. Archived from the original on 2006-10-01. Retrieved 2006-05-01.
  • "Cloud Streets Photographed over Gulf of Mexico: Gallery of Cloud Streets Images". Meteorology News. Retrieved 2009-10-29.

November 30, 2023
horizontal, convective, rolls, cloud, street, redirects, here, book, winton, cloudstreet, television, adaptation, book, cloudstreet, miniseries, also, known, horizontal, roll, vortices, cloud, streets, long, rolls, counter, rotating, that, oriented, approximat. Cloud street redirects here For the book by Tim Winton see Cloudstreet For the television adaptation of the book see Cloudstreet miniseries Horizontal convective rolls also known as horizontal roll vortices or cloud streets are long rolls of counter rotating air that are oriented approximately parallel to the ground in the planetary boundary layer 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 1 Cloud streets are usually more or less straight rarely cloud streets assume paisley patterns when the wind driving the clouds encounters an obstacle Those cloud formations are known as von Karman vortex streets Horizontal convective rollsHorizontal convective rolls producing cloud streets lower left portion of the image over the Bering Sea Simple schematic of the production of cloud streets by horizontal convective rolls Lines of clouds streets stretch from north west to south east in this natural colour satellite view of New England Contents 1 Characteristics 2 Development and required environmental conditions 3 See also 4 References 5 Further readingCharacteristics editHorizontal 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 2 and or dynamic inflection point instabilities in the mean wind profile 3 Early theory 3 4 5 6 7 on the features 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 5 7 8 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 6 7 9 Experimental studies have shown that the aspect ratio a ratio of roll wavelength to boundary layer depth 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 4 10 6 7 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 produced by cloud streets enabling them to fly straight for long distances hence the name cloud streets Development and required environmental conditions editThe 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 6 7 11 12 13 1 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 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 3 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 7 1 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 11 12 1 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 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 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 environments generally produce cellular convection 7 1 8 Although this solution has been verified with numerous observations it is complicated involving chaos theory mathematics and has not been widely used 3 6 7 11 12 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 significant contribution to the theory of chaos for organization of turbulence See also editAtmospheric convection Wave cloudReferences edit a b c d e Etling D R A Brown 1993 Roll Vortices in the Planetary Boundary Layer A Review Boundary Layer Meteorology 65 3 215 248 Bibcode 1993BoLMe 65 215E doi 10 1007 BF00705527 S2CID 119535446 Kuo H 1963 Perturbations of Plane Couette Flow in Stratified Fluid and Origin of Cloud Sheets Physics of Fluids 6 2 195 211 Bibcode 1963PhFl 6 195K doi 10 1063 1 1706719 a b c d Brown R A 1970 A Secondary Flow Model for the Planetary Boundary Layer Journal of the Atmospheric Sciences 27 5 742 757 Bibcode 1970JAtS 27 742B doi 10 1175 1520 0469 1970 027 lt 0742 ASFMFT gt 2 0 CO 2 ISSN 1520 0469 a b Brown R A 1972 On the Inflection Point Instability of a Stratified Ekman Boundary Layer Journal of the Atmospheric Sciences 29 5 851 859 Bibcode 1972JAtS 29 850B doi 10 1175 1520 0469 1972 029 lt 0850 OTIPIO gt 2 0 CO 2 a b LeMone M 1973 The Structure and Dynamics of Horizontal Vorticities in the Planetary Boundary Layer Journal of the Atmospheric Sciences 30 6 1077 1091 Bibcode 1973JAtS 30 1077L doi 10 1175 1520 0469 1973 030 lt 1077 TSADOH gt 2 0 CO 2 a b c d e 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 ISBN 0470111607 a b c d e f g h Brown R A 1980 Longitudinal Instabilities and Secondary Flows in the Planetary Boundary Layer A Review Reviews of Geophysics and Space Physics 18 3 683 697 Bibcode 1980RvGSP 18 683B doi 10 1029 RG018i003p00683 a b Weckworth T M J W Wilson R M Wakimoto N A Crook 1997 Determining the Environmental Conditions Supporting their Existence and Characteristics Monthly Weather Review 125 4 505 526 Bibcode 1997MWRv 125 505W doi 10 1175 1520 0493 1997 125 lt 0505 HCRDTE gt 2 0 CO 2 S2CID 124381300 Stull Roland 1988 An Introduction to Boundary Layer Meteorology 2nd ed Kluwer Academic Publishers ISBN 9027727694 Kelly R 1982 A Single Doppler Radar Study of Horizontal Roll Convection in a Lake Effect Snow Storm Journal of the Atmospheric Sciences 39 7 1521 1531 Bibcode 1982JAtS 39 1521K doi 10 1175 1520 0469 1982 039 lt 1521 asdrso gt 2 0 co 2 a b c Brown R A 1981 On the Use of Exchange Coefficients and Organized Large Scale Eddies in Modeling Turbulent Flows Boundary Layer Meteorology 20 1 111 116 Bibcode 1981BoLMe 20 111B doi 10 1007 BF00119927 S2CID 120165198 a b c Brown R A and T Liu 1982 An Operational Large scale Marine Planetary Boundary Layer Model Journal of Applied Meteorology 21 3 261 269 Bibcode 1982JApMe 21 261B doi 10 1175 1520 0450 1982 021 lt 0261 AOLSMP gt 2 0 CO 2 ISSN 1520 0450 Brown R A 1991 Fluid Mechanics of the Atmosphere International Geophysics Series 47 Academic Press San Diego ISBN 0 12 137040 2Further reading editDunlop Storm 2002 The Weather Identification Handbook Guilford Connecticut The Lyons Press ISBN 1 58574 857 9 Scorer Verkaik 1989 Spacious Skies David amp Charles ISBN 0 7153 9139 9 Cloud Streets Pave Hudson Bay NASA Earth Observatory Archived from the original on 2006 10 01 Retrieved 2006 05 01 Cloud Streets Photographed over Gulf of Mexico Gallery of Cloud Streets Images Meteorology News Retrieved 2009 10 29 Retrieved from https en wikipedia org w index php title Horizontal convective rolls amp oldid 1181768542, wikipedia, wiki, book, books, library,

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