Multiscale tropical convection: understanding the relationship between convective momentum transport and gravity wave momentum flux

Tropical convection is inherently multi-scalar, with spatial scales ranging from individual convective clouds up to the scales of large convective clusters. In addition to forming a crucial component of the water cycle and heat budget, tropical convection induces vertical fluxes of horizontal momentum. There are two main contributions to this momentum transport. The first contribution resides entirely in the troposphere and is due to vertical ascent, descent, mixing, and organized circulations within convective systems. Organized “convective momentum transport” is strongly linked to tilted convective systems and varies significantly throughout the system lifetime. The second component of this momentum transport occurs in the troposphere, stratosphere, and further aloft and is caused by vertically propagating gravity waves. This “gravity wave momentum flux” has important influences on the momentum budget of the middle atmosphere. Furthermore, whilst in the troposphere, gravity waves can also contribute to the initiation and organization of convection, providing a feedback mechanism between the cloud population and the waves it generates. Both the convective momentum transport and the gravity wave momentum flux are processes that must be parameterized in general circulation models. In existing schemes these processes are treated independently, whereas in reality, these two momentum fluxes are inherently linked. Therefore, to ensure internal model consistency, parameterization of these processes should be coupled in some way. This study examines the relationship between the convective momentum flux and the gravity wave momentum transport.

This study utilizes a series of idealized model simulations that produce multiscale tropical convection in a variety of wind shear conditions. The relatively long cloud-system resolving simulations allow clouds to evolve, decay, form organized clusters, propagating lines, and generate gravity waves that interact with their environment in a more realistic way than has been considered in the past. The set of simulations produce convective systems with a variety of modes of convective organization, different convective momentum transports, and different gravity wave spectra. Also, in some simulations there is clear evidence of resonant feedbacks between the gravity waves and the cloud population. These results elucidate the important connections between the convective momentum transport and the gravity wave momentum flux.