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| Titel |
Explicit entrainment parameterization in the general circulation model ECHAM5-HAM |
| VerfasserIn |
Colombe Siegenthaler-Le Drian, Peter Spichtinger, Ulrike Lohmann |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036634
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| Zusammenfassung |
| The complex interactions affecting cloud lifetime and liquid water path (LWP) are not well
captured in state-of-the-art general circulation models (GCM). A recent climate model
intercomparison showed an overestimation of the positive correlation of LWP with aerosol
optical depth by a factor of two as compared to MODIS data for almost all participating
models (Quaas et al., 2009). As the authors suggest, a proper interaction of the boundary
layer dynamics, particularly the inclusion of cloud top entrainment may lead to an
improvement. ECHAM5 was one of the participating model. In this model, the
turbulent fluxes in the planetary boundary layer are simulated using a turbulent
kinetic energy – mixing length scheme. It has been showed that its performance
diminishes when the resolution decreases, the different fluxes being not represented
satisfactory with 31 vertical levels, particularly at the cloud top (Lenderink et al.,
2000). We thus replace the turbulent fluxes by the explicit entrainment closure by
Turton and Nicholls (1987) at the top of the stratocumulus capped boundary layers.
The turbulent fluxes are weighted with the cloud cover to apply the entrainment
closure only above clouds. In addition, we use an explicit term for the radiative
cooling contribution in the buoyancy production term. We use the new version of the
Hamburg general circulation model ECHAM5-HAM (Roeckner et al., 2003; Stier et al.,
2005). The cloud scheme that is used for this study includes the double-moment
cloud microphysics scheme for cloud droplets and ice crystals (Lohmann et al.,
2007).
The principal effect of the explicit entrainment is to dry and warm the planetary boundary
layer. The averaged profiles are more stable and the inversion is reduced. The stratocumulus
deck is reduced in all typical stratocumulus regions. In a single column version of the model,
the diurnal cycle simulated in cloud cover or equivalentely in cloud water is much more
representative of observed subtropical stratocumulus when applying new parameterization.
Moreover, even if the entrainment parameterization does not explicitly depend on the number
of cloud droplets, the steep increase of liquid water path with increasing cloud droplet
number concentration is reduced.
Furthermore, the turbulent kinetic energy (TKE) is crucially affected. First, its vertical
profile is smoothed compared to the huge values in the standard version. Moreover, due to the
explicit addition of radiative cooling in the buoyancy flux, the maximum of TKE occurs at
cloud top (as in reality) and not at cloud base (as in the standard model version). Finally, the
trade wind cumulus are better represented in terms of cloud cover. Indeed, the TKE source at
cloud top enhances the latent heat flux, triggering the convective routine in shallow cumulus
regions.
References
[Lenderink et al., 2000]   Lenderink, G., Van Meijgaard, E., and Holtslag, A. M.
(2000). Evaluation of the ECHAM4 cloud-turbulence scheme for stratocumulus.
Meteorol. Z., 9(1):0041–47.
[Lohmann et al., 2007]   Lohmann, U., Stier, P., Hoose, C. et al. (2007).
Cloud microphysics and aerosol indirect effects in the global climate model
ECHAM5-HAM. Atmos. Chem. Phys., 7:3425–3446.
[Quaas et al., 2009]   Quaas, J., Ming, Y., Menon, S. et al. (2009). Aerosol indirect
effects – general circulation model intercomparison and evaluation with satellite
data. Atmos. Chem. Phys. Discuss., 9:12731–12779.
[Roeckner et al., 2003]   Roeckner, E., Bäuml, G., Bonaventura, L. et al. (2003).
The atmospheric general circulation modell echam5, part I: Model description.
Technical Report 349, Max-Planck-Institute for Meteorology, Hamburg,Germany.
[Stier et al., 2005]   Stier, P., Feichter, J., Kinne, S. et al. (2005). The
aerosol-climate model ECHAM5-HAM. Atmos. Chem. Phys., 5:1125–1156.
[Turton and Nicholls, 1987]   Turton, J. D. and Nicholls, S. (1987). A study of the
diurnal variation of stratocumulus using a multiple mixed layer model. Quart. J.
Roy. Meteor. Soc., 113:969–1009. |
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