Boundary-Layer Meteorology

, Volume 65, Issue 1–2, pp 29–53 | Cite as

Comparison of a computer-simulated stratus-topped boundary layer with aircraft observations

  • Shaohua Shen
  • Chin-Hoh Moeng
Article
  • 49 Downloads

Abstract

To assess the realism of large-eddy simulation (LES) of the stratus-topped boundary layer and its predicted turbulent structure, we performed detailed data analyses on a LES (which has a 12.5 m grid size in all three directions), in a manner similar to those used by Nicholls (1989) on aircraft measurements. The first analysis retrieves the primary convective elements, i.e., the negatively buoyant downdrafts, which are driven mainly by cloud-top radiative cooling, through a conditional sampling technique. Comparison shows that the LES of this resolution reflects most of the observed downdraft features; most of the discrepancies that exist between the obervations and the LES can be explained by decoupling of the cloud layer from the underlying flow that exists in the former but not in the latter. The second analysis shows the vertical velocity spectrum and its agreement with the measurements. In the third analysis, showing the turbulent kinetic energy budgets, the discrepancy in the turbulent transport term (i.e., the divergence of the third-moment quantity\(\overline {wE} \), the turbulent-kinetic-energy flux) between the LES and measurements exists even with such a fine resolution LES. This discrepancy is related mainly to the different behavior in\(\overline {w^3 } \) between the LES and observations, which may again be associated with decoupling.

An advantage of LES over aircraft observations is that the former can provide three-dimensional flow structure at any instant. In this paper, we examined the instantaneous flow structure and observed closed cellular patterns near the cloud top in which updrafts occupy the broad centers and relatively strong downdrafts occur in the narrow edges. In the intersections of these cell boundaries, there exist weak downdrafts, consisting of relatively cold and dry air, that are the most likely origins of the strong downdrafts extending throughout the mixed layer.

Keywords

Turbulent Kinetic Energy Aircraft Measurement Turbulent Kinetic Energy Budget Kinetic Energy Budget Aircraft Observation 

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

References

  1. Brost, R. A., Lenschow, D. H., and Wyngaard, J. C.: 1982a, ‘Marine Stratocumulus Layer. Part I: Mean Conditions’,J. Atmos. Sci. 3, 800–817.Google Scholar
  2. Brost, R. A., Wyngaard, J. C., and Lenschow, D. H.: 1982b, ‘Marine Stratocumulus Layer. Part II: Turbulence Budgets’,J. Atmos. Sci. 3, 818–836.Google Scholar
  3. Caughey, S. J., Crease, B. A., and Roach, W. T.: 1982, ‘A Field Study of Nocturnal Stratocumulus. II: Turbulence Structure and Entrainment’,Quart. J. Roy. Meteorol. Soc. 108, 125–144.Google Scholar
  4. Clark, T. L.: 1979, ‘Numerical Simulations with a Three-Dimensional Cloud Model: Lateral Boundary Condition Experimenta and Multicellular Severe Storm Stimulations’,J. Atmos. Sci. 36, 2191–2215.Google Scholar
  5. Deardorff, J. W.: 1974, ‘Three-Dimensional Numerical Study of Turbulence in an Entraining Mixed Layer’,Boundary-Layer Meteorol. 7, 199–226.Google Scholar
  6. Deardorff, J. W.: 1980a, ‘Stratocumulus-Capped Mixed Layers Derived from a Three-Dimensional Model’,Boundary-Layer Meteorol. 18, 495–527.Google Scholar
  7. Deardorff, J. W.: 1980b, ‘Clotid-Top Entrainment Instability’,J. Atmos. Sci. 37, 131–147.Google Scholar
  8. Kaimal, J. C., Wyngaard, J. C., Haugen, D. A., Coté, O. R., Izumi, Y., Caughey, S. J., and Readings, C. J.: 1976, ‘Turbulence Structure in the Convective Boundary Layer’,J. Atmos. Sci. 33, 2152–2169.Google Scholar
  9. Khalsa, S. J. S. and Greenhut, G. K.: 1985, ‘Conditional Sampling of Updrafts and Downdrafts in the Marine Atmospheric Boundary Layer’,J. Atmos. Sci. 42, 2550–2562.Google Scholar
  10. Lenschow, D. H. and Stephens, P.: 1980, ‘The Role of Thermals in the Convective Boundary Layer’,Boundary-Layer Meteorol. 19, 509–531.Google Scholar
  11. Mason, P. J.: 1989, ‘Large-Eddy Simulation of the Convective Atmospheric Boundary Layer’,J. Atmos. Sci. 4, 1192–1516.Google Scholar
  12. Moeng, C.-H.: 1984, ‘A Large-Eddy-Simulation Model for the Study of Planetary Boundary-Layer Turbulence’,J. Atmos. Sci. 41, 2052–2062.Google Scholar
  13. Moeng, C.-H.: 1986, ‘Large-Eddy Simulation of a Stratus-Topped Boundary Layer. Part I: Structure and Budgets’,J. Atmos. Sci. 43, 2886–2900Google Scholar
  14. Moeng, C.-H. and Schumann, U.: 1991, ‘Composite Structure of Plumes in Stratus-Topped Boundary Layers’,J. Atmos. Sci. 48, 2280–2291.Google Scholar
  15. Moeng, C.-H. and Rotunno, R.: 1990, ‘Vertical-Velocity Skewness in the Buoyancy Driven Boundary Layer’,J. Atmos. Sci. 47, 1149–1162.Google Scholar
  16. Moeng, C.-H. and Wyngaard, J. C.: 1984, ‘Statistics of Conservative Scalars in the Convective Boundary Layer’,J. Atmos. Sci. 41, 3161–3169.Google Scholar
  17. Moeng, C.-H. and Wyngaard, J. C.: 1986, ‘An Analysis of Closures for Pressure-Scalar Covariances in the Convective Boundary Layer’.J. Atmos. Sci. 43, 2499–2513.Google Scholar
  18. Moeng, C. and Wyngaard, J. C.: 1989, ‘Evaluation of Turbulent Transport and Dissipation Closures in Second-Order Modeling’.J. Atmos. Sci. 46, 2311–2330.Google Scholar
  19. Nicholls, S.: 1984, ‘The Dynamics of Stratocumulus: Aircraft Observations and Comparisons with a Mixed Layer Model’,Quart. J. Roy. Meteorol. Soc. 110, 783–820.Google Scholar
  20. Nicholls, S.: 1989, ‘The Structure of Radiatively Driven Convection in Stratocumulus’,Quart. J. Roy. Meteorol. Soc. 115, 487–511.Google Scholar
  21. Nicholls, S. and Leighton, J. R.: 1986, ‘An Observational Study of the Structure of Stratiform Cloud Sheets. Part 1. Mean Structure.Quart. J. Roy. Meteorol. Soc. 112, 431–460.Google Scholar
  22. Nicholls, S. and Readings, C. J.: 1981, ‘Spectral Characteristics of Surface Layer Turbulence Over the Sea’,Quart. J. Roy. Meteorol. Soc. 107, 591–614.Google Scholar
  23. Nicholls, S. and Turton, J. D.: 1986, ‘An Observational Study of the Structure of Stratiform Cloud Sheets. Part: Entrainment’,Quart. J. Roy. Meteorol. Soc. 112, 461–480.Google Scholar
  24. Nucciarone, J. J. and Young, G. S.: 1991, ‘Aircraft Measurements of Turbulence Spectra in the Marine Stratocumulus-Topped Boundary Layer’,J. Atmos. Sci. 48, 2382–2392.Google Scholar
  25. Randall, D. A.: 1980, ‘Conditional Instability of the First Kind Upside-Down’,J. Atmos. Sci. 37, 125–130.Google Scholar
  26. Schmidt, H. and Schumann, U.: 1989, ‘Coherent Structure of the Convective Boundary Layer Derived from Large-Eddy Simulations,J. Fluid Mech. 200, 511–562.Google Scholar
  27. Schumann, U. and Moeng, C.-H.: 1991a, ‘Plume Fluxes in Clear and Cloudy Convective Boundary Layers’,J. Atmos. Sci. 48, 1746–1757.Google Scholar
  28. Schumann, U. and Moeng, C.-H.: 1991b, ‘Plume Budgets in Clear and Cloudy Convective Boundary Layers’,J. Atmos. Sci. 48, 1758–1770.Google Scholar
  29. Slingo, A., Brown, R., and Wrench, C. L.: 1982, ‘A Field Study of Nocturnal Stratocumulus III. High Resolution Radiative and Microphysical Observations’,Quart. J. Roy. Meteorol. Soc. 108, 145–165.Google Scholar
  30. Young, G. S.: 1988a, ‘Turbulence Structure of the Convective Boundary Layer. II: Phoenix 78 Aircraft Observations of Thermals and their Environment’,J. Atmos. Sci. 45, 727–735.Google Scholar
  31. Young, G. S.: 1988b, ‘Turbulence Structure of the Convective Boundary Layer. III: The Vertical Velocity Budgets of Thermals and their Environment’,J. Atmos. Sci. 45, 2039–2049.Google Scholar

Copyright information

© Kluwer Academic Publishers 1993

Authors and Affiliations

  • Shaohua Shen
    • 1
  • Chin-Hoh Moeng
    • 1
  1. 1.National Center for Atmospheric ResearchBoulderUSA

Personalised recommendations