Multicloud parametrization of mesoscale convective systems for the ITCZ

Friday, 19 December 2014
Boualem Khouider, University of Victoria, Victoria, BC, Canada and Mitchell W Moncrieff, National Center for Atmospheric Research, Boulder, CO, United States
Mesoscale convective systems (MCS), aligned approximately parallel to the background low-level wind shear, are ubiquitous in the Eastern Pacific intertropical convergence zone (ITCZ). They are believed to control the local Hadley circulation and have a nontrivial momentum feedback on the ambient shear. They also play a central role in the two-way interactions between convection and the synoptic and planetary scale waves. They do so by serving as both the building block for organized convection, which involves congestus cloud decks that moisten and precondition the environment for deep convection which in turn is lagged by stratiform anvils, and as a conveyer belt for convective momentum transport (CMT). Here, we propose an extension of the multicloud model of Khouider and Majda (2006) to make the stratiform anvils more sensitive to the background wind shear profile. We do so by invoking two layers of moisture in the free troposphere instead of one, in addition to the boundary layer. Linear stability, in a wind shear background consisting of both mid-level and low-level easterly jets, representing, simultaneously, the Tropical Easterly and African Easterly jets, features the usual synoptic scale instability of the multicloud model plus two new instability bands at the meso-alpha and meso-beta scales, respectively. The meso-alpha and meso-beta modes constitute a paradigm for the dynamics of shear parallel convective systems with the meso-alpha waves being the quasi-stationary systems. In this talk we will present limited domain 3D simulations, without rotation, of realistic shear parallel lines of convection with parallel stratifrom anvils moving eastward, with a steering level in the upper troposphere, as a mesoscale envelope of the individual convective cells moving inwards, with a steering level in the lower troposphere. This provides, among other things, an excellent example of nontrivial CMT effect on the background low-level wind. It results in a narrow channel whose width is controlled solely by the size of the convective cells, along which the parallel lines form and propagate.