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www.nonlin-processes-geophys.net/17/187/2010/
doi:10.5194/npg-17-187-2010
© Author(s) 2010. This work is distributed
under the Creative Commons Attribution 3.0 License.
Estimating the diffusive heat flux across a stable interface forced by convective motions
1NCAS-Weather, Centre for Atmospheric & Instrumentation Research, University of Hertfordshire, Hatfield, UK
2Laboratoire des Ecoulements Géophysiques et Industriels, CNRS/UJF/INPG, Grenoble, France
Abstract. Entrainment at the top of the convectively-driven boundary layer (CBL) is revisited using data from a high-resolution large-eddy simulation (LES). In the range of values of the bulk Richardson number RiB studied here (about 15–25), the entrainment process is mainly driven by the scouring of the interfacial layer (IL) by convective cells. We estimate the length and time scales associated with these convective cells by computing one-dimensional wavenumber and frequency kinetic energy spectra. Using a Taylor assumption, based upon transport by the convective cells, we show that the frequency and wavenumber spectra follow the Kolmogorov law in the inertial range, with the multiplicative constant being in good agreement with previous measurements in the atmosphere. We next focus on the heat flux at the top of the CBL,
, which is parameterized in classical
closure models for the entrainment rate we at the interface. We show
that can be computed exactly using the method proposed by
Winters et al. (1995), from which the values of a turbulent diffusivity
across the IL can be inferred. These values are recovered
by tracking particles within the IL using a Lagrangian stochastic model
coupled with the LES. The relative difference between the Eulerian and
Lagrangian values of is found to be lower than 10%. A simple
expression of we as a function of is also proposed.
Our results are finally used to assess the validity of the classical
"first-order'' model for we. We find that, when RiB is
varied, the values for we derived from the "first-order'' model
with the exact computation of agree to better than 10% with
those computed directly from the LES (using its definition). The simple
expression we propose appears to provide a reliable estimate of we
for the largest values of RiB only.Full Article (PDF, 902 KB)
Citation: Chemel, C., Staquet, C., and Chollet, J.-P.: Estimating the diffusive heat flux across a stable interface forced by convective motions, Nonlin. Processes Geophys., 17, 187-200, doi:10.5194/npg-17-187-2010, 2010. Bibtex EndNote Reference Manager XML
