Abstract
Abstract This paper proposes a new formulation of the deep convection triggering for general circulation model convective parameterizations. This triggering is driven by evolving properties of the strongest boundary layer thermals. To investigate this, a statistical analysis of large-eddy simulation cloud fields in a case of transition from shallow to deep convection over a semiarid land is carried out at different stages of the transition from shallow to deep convection. Based on the dynamical and geometrical properties at cloud base, a new computation of the triggering is first proposed. The analysis of the distribution law of the maximum size of the thermals suggests that, in addition to this necessary condition, another triggering condition is required, that is, that this maximum horizontal size should exceed a certain threshold. This is explicitly represented stochastically. Therefore, the new formulation integrates the whole transition process from the first cloud to the first deep convective cell and can be decomposed into three steps: (i) the appearance of clouds, (ii) crossing of the inhibition layer, and (iii) deep convection triggering.
Highlights
Many features of tropical deep convection are accounted for by the quasi-equilibrium (QE) hypothesis
Since our concern is the triggering of deep convection, we focus on high velocities, and we need to make sure that the hypothesis of independent Gaussian drafts is, at least, robust for the distribution tail
This threshold governs the transition from a regime in which cumulus clouds cannot reach their level of free convection (LFC) to a transient regime where at least some cumulus overshoot the convective inhibition (CIN) but do not reach the high troposphere in significant numbers
Summary
Subcloud lifting processes and convective inhibition (CIN) are known to exert a strong control on deep convection onset and intensity, modulating the entropy flux. In observations and in high-resolution simulations of moist convection, the triggering (or onset) of deep convection is the time when cumulus clouds reach the highest levels of the troposphere (i.e., congestus and cumulonimbus) Prior to this sharp transition, the convective boundary layer enters a transient regime (transition stage), during which cumulus clouds become gradually wider and deeper, but still remain in the low troposphere (Chaboureau et al 2004; Guichard et al 2004; Grabowski et al 2006; Khairoutdinov and Randall 2006). Chaboureau et al (2004) show that, during the transition phase, the updraft vertical velocities at cloud base are large enough to overcome the convective inhibition but that entrainment of exceedingly dry air limits the cloud vertical development It is only when the lower free troposphere is moist enough that the sharp transition to deep convection occurs.
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