A multicloud model for meso-scale convective systems

Organized convection in the tropics involves a wide spectrum of scales ranging from the convective cell of a few kilometers to synoptic and planetary scale disturbances such as the Madden-Julian Oscillation (MJO) and convectively coupled Kelvin waves. Mesoscale convective systems (MCS) and tropical squall lines (TSL), aligned approximately perpendicular to the background low-level wind shear, are believed to play a central roles 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 is then followed by stratiform anvils, and as a conveyer belt for convective momentum transport. This conceptual picture is used by Khouider and Majda (2006 and subsequent papers) to build a prototype cumulus parametrization—the multicloud model—which captures the full spectrum of convectively coupled waves with scale-selective instabilities at synoptic scales and simulates reasonably well the MJO in an aquaplanet GCM. Also, arguably, the success of super-parametrization simulations of the MJO can be rooted to their ability to represent MCS's and TSL's of some sort via the 2d cloud resolving model embedded in each GCM grid cell. On the other hand, Dudhia and Moncrieff (1987) found that large-scale ascent induced by the background wind shear is crucial for maintaining MCS's in the form of quasi-stationary convective bands aligned with the background low-level wind shear in the tropical eastern Atlantic during GATE. They argued that both the background wind shear and convective available potential energy (CAPE) are important for the simulations of MCS's and TSL's. Parker and Johnson (2004) classify MCS's into various categories according to their morphology and shape, including Front Fed Trailing Stratiform, Front Fed Leading Stratiform, and Rear Fed Leading Stratiform as well as Parallel Stratiform systems. It is demonstrated that the vertical profile of the wind shear alone is responsible for these differences. Here, we propose an extension of the multicloud model of Khouider and Majda (2006) to make the stratifrom 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, and by allowing stratiform heating to respond directly to moisture disturbances in the upper troposphere. Preliminary linear stability results show that in the absence of a background shear the scale-selective instability feature of the original multicloud model is faithfully preserved. When a wind shear consisting of both mid-level and low-level easterly jets, representing, simultaneously, the Tropical Easterly and African Easterly jets, the instability diagram dramatically changes. While the synoptic scale instability is still preserved, two new instability bands appear at the meso-alpha and meso-beta scales. The wave modes associated with these three instability bands have the features of convectively coupled Kelvin waves, MSC's and TSL's, 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 carrying large stratiform anvils in which embedded meso-beta modes propagate mainly as deep convective towers, consistent with the GATE observations and with the simulations of Dudhia and Moncrieff (1987).

References:

Dudhia, J. and M.W.Moncrieff, 1987: A numerical simulation of quasi-stationary tropical convective bands. Quart. J. Roy. Meteor. Soc., 113, 929–967

Khouider and Majda, 2006: A Simple Multicloud Parameterization for Convectively Coupled Tropical Waves. Part I: Linear Analysis. J. Atmos. Sci., 63, 1308–1323.

Parker, M. D., and R. H. Johnson, 2004: Structures and dynamics of quasi-2D mesoscale convective systems. J. Atmos. Sci., 61, 545-567