The Role of Eddy Diffusivity Profiles on Stratocumulus Liquid Water Path Biases

Stephan R. de Roode Royal Netherlands Meteorological Institute (KNMI), De Bilt, Netherlands

Search for other papers by Stephan R. de Roode in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

Results from simulations of the stratocumulus-topped boundary layer with one-dimensional versions of general simulation models typically exhibit a wide range of spread in the modeled liquid water path (LWP). These discrepancies are often attributed to differences in the modeled entrainment rate. Results from a large eddy simulation of the First International Satellite Cloud Climatology Project Regional Experiment I stratocumulus case are analyzed. The diagnosed eddy diffusivities for heat and moisture are found to differ by about a factor of 3. Moreover, both have a much larger magnitude than the ones typically applied in boundary layer parameterization schemes. Motivated by these results mean state solutions are analyzed for the specific case in which the vertical fluxes of heat and moisture are prescribed, whereas eddy diffusivity profiles are systematically varied by multiplication with a constant factor. The solutions demonstrate that any value, ranging from zero to a maximum adiabatic value, can be obtained for the LWP. In the subtropical parts over the ocean where horizontally extended stratocumulus fields persist, the surface sensible heat flux is typically small, whereas surface evaporation and entrainment of relatively dry air from above the surface can result in significant moisture fluxes. If the eddy diffusivity values are small, then the mean specific humidity will tend to decrease quite rapidly with height in order to support the humidity flux. This results in erroneous low humidity values in the upper part of the boundary layers causing low LWP values.

Corresponding author address: Stephan R. de Roode, Royal Netherlands Meteorological Institute (KNMI), P. O. Box 201, 3730 AE De Bilt, Netherlands. Email: roode@knmi.nl

Abstract

Results from simulations of the stratocumulus-topped boundary layer with one-dimensional versions of general simulation models typically exhibit a wide range of spread in the modeled liquid water path (LWP). These discrepancies are often attributed to differences in the modeled entrainment rate. Results from a large eddy simulation of the First International Satellite Cloud Climatology Project Regional Experiment I stratocumulus case are analyzed. The diagnosed eddy diffusivities for heat and moisture are found to differ by about a factor of 3. Moreover, both have a much larger magnitude than the ones typically applied in boundary layer parameterization schemes. Motivated by these results mean state solutions are analyzed for the specific case in which the vertical fluxes of heat and moisture are prescribed, whereas eddy diffusivity profiles are systematically varied by multiplication with a constant factor. The solutions demonstrate that any value, ranging from zero to a maximum adiabatic value, can be obtained for the LWP. In the subtropical parts over the ocean where horizontally extended stratocumulus fields persist, the surface sensible heat flux is typically small, whereas surface evaporation and entrainment of relatively dry air from above the surface can result in significant moisture fluxes. If the eddy diffusivity values are small, then the mean specific humidity will tend to decrease quite rapidly with height in order to support the humidity flux. This results in erroneous low humidity values in the upper part of the boundary layers causing low LWP values.

Corresponding author address: Stephan R. de Roode, Royal Netherlands Meteorological Institute (KNMI), P. O. Box 201, 3730 AE De Bilt, Netherlands. Email: roode@knmi.nl

Save
  • Albrecht, B. A., C. W. Fairall, D. W. Thomson, A. B. White, J. B. Snider, and W. H. Schubert, 1990: Surface-based remote sensing of the observed and the adiabatic liquid water content of stratocumulus clouds. Geophys. Res. Lett., 17 , 8992.

    • Search Google Scholar
    • Export Citation
  • Bougeault, P., and P. Lacarrère, 1989: Parameterization of orography-induced turbulence in a mesobeta-scale model. Mon. Wea. Rev., 117 , 18721890.

    • Search Google Scholar
    • Export Citation
  • Bretherton, C. S., and Coauthors, 2004: The EPIC 2001 stratocumulus study. Bull. Amer. Meteor. Soc., 85 , 967977.

  • De Roode, S. R., and P. G. Duynkerke, 1997: Observed Lagrangian transition of stratocumulus into cumulus during ASTEX: Mean state and turbulence structure. J. Atmos. Sci., 54 , 21572173.

    • Search Google Scholar
    • Export Citation
  • De Roode, S. R., P. G. Duynkerke, and A. P. Siebesma, 2000: Analogies between mass-flux and Reynolds-averaged equations. J. Atmos. Sci., 57 , 15851598.

    • Search Google Scholar
    • Export Citation
  • De Roode, S. R., H. J. J. Jonker, P. G. Duynkerke, and B. Stevens, 2004: Counter-gradient fluxes of conserved variables in the clear convective and stratocumulus-topped boundary layer: The role of the entrainment flux. Bound.-Layer Meteor., 112 , 179196.

    • Search Google Scholar
    • Export Citation
  • Duynkerke, P. G., and J. Teixeira, 2001: Comparison of the ECMWF reanalysis with FIRE I observations: Diurnal variation of marine stratocumulus. J. Climate, 14 , 14661478.

    • Search Google Scholar
    • Export Citation
  • Duynkerke, P. G., and Coauthors, 2004: Observations and numerical simulations of the diurnal cycle of the EUROCS stratocumulus case. Quart. J. Roy. Meteor. Soc., 130 , 32693296.

    • Search Google Scholar
    • Export Citation
  • Faloona, I., and Coauthors, 2005: Observations of entrainment in eastern Pacific marine stratocumulus using three conserved scalars. J. Atmos. Sci., 62 , 32683285.

    • Search Google Scholar
    • Export Citation
  • Holtslag, A. A. M., and B. A. Boville, 1993: Local versus nonlocal boundary layer diffusion in a global climate model. J. Climate, 6 , 18251842.

    • Search Google Scholar
    • Export Citation
  • Johnson, B. T., 2005: The semidirect aerosol effect: Comparison of a single-column model with large eddy simulation for marine stratocumulus. J. Climate, 18 , 119130.

    • Search Google Scholar
    • Export Citation
  • Jonker, H. J. J., P. G. Duynkerke, and J. W. M. Cuijpers, 1999: Mesoscale fluctuations in scalars generated by boundary layer convection. J. Atmos. Sci., 56 , 801808.

    • Search Google Scholar
    • Export Citation
  • Lappen, C. L., and D. A. Randall, 2001: Toward a unified parameterization of the boundary layer and moist convection. Part I: A new type of mass-flux model. J. Atmos. Sci., 58 , 20212036.

    • Search Google Scholar
    • Export Citation
  • Lenderink, G., and A. A. M. Holtslag, 2004: An updated length scale formulation for turbulent mixing in clear and cloudy boundary layers. Quart. J. Roy. Meteor. Soc., 130 , 34053428.

    • Search Google Scholar
    • Export Citation
  • Lenderink, G., and Coauthors, 2004: The diurnal cycle of shallow cumulus clouds over land: A single column model intercomparison study. Quart. J. Roy. Meteor. Soc., 130 , 33393364.

    • Search Google Scholar
    • Export Citation
  • Lock, A. P., A. R. Brown, M. R. Bush, G. M. Martin, and R. N. B. Smith, 2000: A new boundary layer mixing scheme. Part I: Scheme description and single-column model tests. Mon. Wea. Rev., 128 , 31873199.

    • Search Google Scholar
    • Export Citation
  • Louis, J. F., 1979: A parametric model of vertical fluxes in the atmosphere. Bound.-Layer Meteor., 17 , 187202.

  • Nicholls, S., and J. D. Turton, 1986: An observational study of the structure of stratiform cloud sheets: Part II. Entrainment. Quart. J. Roy. Meteor. Soc., 112 , 461480.

    • Search Google Scholar
    • Export Citation
  • Sánchez, E., and J. Cuxart, 2004: A buoyancy based mixing length proposal for cloudy boundary layers. Quart. J. Roy. Meteor. Soc., 130 , 33853404.

    • Search Google Scholar
    • Export Citation
  • Soares, P. M. M., P. M. A. Miranda, A. P. Siebesma, and J. Teixeira, 2004: An eddy-diffusivity/mass-flux approach for dry and shallow cumulus convection. Quart. J. Roy. Meteor. Soc., 130 , 33653384.

    • Search Google Scholar
    • Export Citation
  • Stevens, B., 2000: Quasi-steady analysis of a PBL model with an eddy-diffusivity profile and nonlocal fluxes. Mon. Wea. Rev., 128 , 824836.

    • Search Google Scholar
    • Export Citation
  • Stevens, B., 2002: Entrainment in stratocumulus-topped mixed layers. Quart. J. Roy. Meteor. Soc., 128 , 26632690.

  • Stevens, B., and Coauthors, 2005: Evaluation of large-eddy simulations via observations of nocturnal marine stratocumulus. Mon. Wea. Rev., 133 , 14431462.

    • Search Google Scholar
    • Export Citation
  • Stull, R. B., 1988: An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers, 666 pp.

  • Troen, I., and L. Mahrt, 1986: A simple model of the atmospheric boundary layer; sensitivity to surface evaporation. Bound.-Layer Meteor., 37 , 129148.

    • Search Google Scholar
    • Export Citation
  • Wyngaard, J. C., and R. A. Brost, 1984: Top-down and bottom-up diffusion of a scalar in the convective boundary layer. J. Atmos. Sci., 41 , 102112.

    • Search Google Scholar
    • Export Citation
  • Zhu, P., and Coauthors, 2005: Intercomparison and interpretation of single-column model simulations of a nocturnal stratocumulus-topped marine boundary layer. Mon. Wea. Rev., 133 , 27412758.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 89 23 1
PDF Downloads 44 15 1