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The sensitivity of a GCM's marine stratocumulus to cloud‐top entrainment
Author(s) -
Lock A. P.
Publication year - 2004
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1256/qj.03.114
Subject(s) - entrainment (biomusicology) , environmental science , atmospheric sciences , marine stratocumulus , parametrization (atmospheric modeling) , climatology , advection , meteorology , radiative transfer , geology , physics , thermodynamics , aerosol , quantum mechanics , rhythm , acoustics
It appears that the Met Office general‐circulation model (GCM) represents stratocumulus over the northeast subtropical Pacific Ocean quite accurately—the mean cloud cover is realistically high, the observed mean and diurnal cycle of liquid‐water path is well represented and the capping inversion is at a reasonable height. This suggests that the boundary‐layer thermodynamic budget must also be well modelled. However, one of the dominant terms in this budget is cloud‐top entrainment, which is parametrized explicitly in this model, and is still the subject of much debate in the literature. In order to quantify the sensitivity of the GCM to this parametrization, the results of simulations applying three perturbations of entrainment formulation are presented. One change, to the enhancement of entrainment in the buoyancy‐reversal regime, is found to have little impact. The other two led to a significant reduction of the cloudiness in the stratocumulus area, giving worse agreement with observations. The first of these was to increase the parametrized entrainment rate by a factor of two. The second was to remove the coupling of the entrainment fluxes to the radiative and resolved advective fluxes, an aspect of the implementation previously found necessary to prevent serious numerical errors which generated spurious additional entrainment. It is further hypothesized that the numerical errors can be associated with a signal in the surface heat fluxes in the north‐east Pacific stratocumulus area. This signal can be seen in many other GCMs, suggesting that numerical problems associated with mixing across sharp inversions may be prevalent. © Crown copyright, 2004.