Convective initiation over heated terrain
Dr Dan Kirshbaum
Mountains serve as the focal points for convective initiation by mechanically forcing impinging air to rise and acting as elevated heat sources that drive horizontal convergence. This project focuses on the latter mechanism, which gives rise to deep convection in the tropics and in mid-latitude summers. Of particular interest is the ability of mountain convergence zones to weaken convective inhibition and moisten the mid-troposphere through shallow-cumulus detrainment. Such preconditioning is necessary in conditionally unstable but strongly inhibited and very dry flows that often exist in areas of weak synoptic forcing. Our cloud-resolving simulations of airflow over a diurnally-heated mountain (cf Figure 1) have revealed some key mechanisms behind deep-convection initiation. For a given thermodynamic environment, we find that the ambient winds are a critically important determinant for deep convection. Firstly, they act to ventilate heat away from the mountain and weaken local buoyancy gradients, which in turn weakens the thermally-driven convergence. Although this trend is qualitatively consistent with linear theory, the sensitivity is much greater in fully non-linear flows. Ambient winds also control the impacts of mid-level entrainment into the cumulus cores. By carrying mid-level flow that has been moistened by shallow-cumulus detrainment away from the mountain region, these winds prevent fresh updrafts emerging from the mountain convergence zone from benefitting from this moistening. Instead they face severe suppression as they entrain dry mid-level air. Another key aspect of mountain convection is that the rapid succession of cloud pulses emerging from the mountain convergence zone allow fresh thermals to pass through preexisting clouds before they have a chance to detrain. As a result, they are effectively shielded from the dry mid-level flow and may glaciate more rapidly due to seeding from preexisting ice particles in decaying cumuli. Results of some of this work are in press and will be published in Journal of the Atmospheric Sciences in 2011 (see my publications page for a copy of the manuscript).
Figure 1: Image of a band of cumulus congestus over a heated mountain in a cloud-resolving quasi-idealized simulation of IOP8b from the Convective and Orographically-induced Precipitation Study (COPS). 0.1 g/kg isosurface of liquid water content is shown in gray, and surface precipitation intensity is shown by coloured controus along the ground.