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Hugh A. Rand and Christopher S. Bretherton


Mesoscale variability in entrainment across the inversion capping the cloud-topped atmospheric boundary layer (CTBL) has been proposed as an explanation for mesoscale variability in cloud thickness. The relevance of this mechanism, called mesoscale entrainment instability, or MEI, to some typical atmospheric boundary layers is investigated. The results indicate that MEI is of relevance only if the potential temperature jump across the inversion is small, ∼1–2 K, and the stable layer virtual potential temperature above is very strongly stratified, ∼40 K km−1. Thus, MEI does not appear to be a viable explanation for mesoscale cellular convection in most CTBLs.

Two parameters are also investigated whose effect on the growth rate can be substantial. They are the rate of horizontal turbulent diffusion and the effect of variations in solar heating due to cloud thickness fluctuations. Decreases in the horizontal turbulent transport rate do not greatly affect the growth rate but can substantially decrease the wavelength of the instability. The solar heating effect can as much as double the growth rate but probably not enough to make the instability significant.

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Matthew C. Wyant, Christopher S. Bretherton, Hugh A. Rand, and David E. Stevens


A two-dimensional eddy-resolving model is used to study the transition from the stratocumulus topped boundary layer to the trade cumulus boundary layer. The 10-day simulations use an idealized Lagrangian trajectory representative of summertime climatological conditions in the subtropical northeastern Pacific. The sea surface temperature is increased steadily at 1.5 K day−1, reflecting the southwestward advection of the subtropical marine boundary layer by the trade winds, while the free tropospheric temperature remains unchanged. Results from simulations with both a fixed diurnally averaged shortwave radiative forcing and a diurnally varying shortwave forcing are presented.

A two-stage model for the boundary layer evolution consistent with these simulations is proposed. In the first stage, decoupling is induced by increased latent heat fluxes in the deepening boundary layer. After decoupling, cloud cover remains high, but the cloudiness regime changes from a single stratocumulus layer to sporadic cumulus that detrain into stratocumulus clouds. In the second stage, farther SST increase causes the cumuli to become deeper and more vigorous, penetrating farther into the inversion and entraining more and more dry above-inversion air. This evaporates liquid water in cumulus updrafts before they detrain, causing the eventual dissipation of the overlying stratocumulus. Diurnal variations of insolation lead to a large daytime reduction in stratocumulus cloud amount, but they have little impact on the systematic evolution of boundary layer structure and cloud. The simulated cloudiness changes are not consistent with existing criteria for cloud-top entrainment instability.

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