Abstract
AbstractThe present study examines mechanisms by which deep moist convection may be initiated through deepening of a well‐mixed convective boundary layer under large‐scale low‐level convergence. The process is examined using a standard formulation for the well‐mixed boundary layer, in which the depth increases exponentially with time under a constant convergence with height. Consideration is also given to a case with a more realistic mean vertical velocity profile which has a maximum in the middle troposphere and attenuates to zero at the tropopause. In the latter case, the unstable well‐mixed layer under convergence grows to a troposphere‐deep mixed layer. Importantly, under these scenarios, the so‐called convective inhibition (CIN) does not inhibit deepening of a well‐mixed convective boundary layer, nor does deep moist convection need to overcome CIN to initiate.
Highlights
It is phenomenologically well established that low-level convergence tends to induce deep moist convection (e.g., Byers and Braham, 1949; Weaver, 1979; Wilson and Schreiber, 1986)
By introducing a more realistic large-scale vertical velocity profile, we can infer the subsequent evolution of an unstable boundary layer; it no longer keeps growing, but it asymptotically approaches a state of a troposphere-deep well-mixed layer
The present study suggests that deep moist convection is initiated through the deepening of a well-mixed convective boundary layer under a large-scale low-level convergence
Summary
It is phenomenologically well established that low-level convergence tends to induce deep moist convection (e.g., Byers and Braham, 1949; Weaver, 1979; Wilson and Schreiber, 1986). The article presents initiation of deep moist convection as a natural consequence of continuous deepening of a well-mixed boundary layer under a presence of large-scale convergence. The present article emphasizes a simple (even trivial) fact that, under the presence of large-scale convergence, the well-mixed boundary layer continues to deepen with time, so long as standard mixed-layer assumptions remain valid. In the latter case, a solution of exponentially growing mixed-layer depth naturally arises under a condition of low-level convergence. To address the question of transformation of the well-mixed boundary layer into cumulus convection, the horizontal dependence of a growing perturbation must be considered explicitly by removing this assumption As it turns out, deriving a solution becomes much more involved. For more careful analysis, the actual formulation becomes much more involved; Section 4.2 gives further remarks
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