The sensitivity of deep convective initiation (the inception of precipitation producing convective clouds ­ DCI) to the lapse rate (vertical gradient of temperature) of the atmosphere~Rs cloud-bearing layer is undertaken. Experiments are conducted using an idealized, 2D, cloud-resolving numerical model and are initialized with environments that possess identical surface-based values of convective available potential energy (CAPE), convective inhibition (CIN), and levels of free convection (LFC). An attempt is made to trigger deep convection in each experiment with a preexisting airmass boundary.

Despite the similarities in surface/near-surface conditions and initiation mechanisms, DCI failed to occur in initial environments whose cloud-bearing lapse rates were below a threshold value. Trajectory analysis reveals that embryonic deep convective clouds are composed of air lifted from ~1.3 km above the surface (although air originating near the surface is found to contribute to the convective clouds later in cloud evolution). It is concluded that the likelihood of DCI is proportional to lapse rate because the LFC of these ~Selevated~T air sources are lower in the larger lapse rate environments. Thus, for the same forced (non-buoyant) upward displacement of air, the larger lapse rate environments are more likely to support the release of thermal instability.

In the single experiment for which thermal instability was released but DCI failed to occur, the smaller size of embryonic convective clouds, compared to the experiments that supported DCI, is found to be the limiting factor. These smaller clouds are shown to be more vulnerable to dilution that, in combination with the reduced buoyancy stemming from a smaller cloud-bearing lapse rate, prevents DCI. Ultimately, cloud size is also found to be a function of lapse rate.

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