Spontaneous flow reversal at the equator in two-dimensional moist turbulence

Abstract:

On any given day in the tropics, satellite observations reveal a rich variety of clouds, accompanied by a range of precipitation types and strengths – even tropical cyclones. Within this sporadic activity, a striking feature is the formation of localized deep convection centers, in which most of the rain occurs. Severe thunderstorms may occur on a daily basis. While individual convection centers are quite short lived and variable, observations spanning several days reveal an organized aggregation of moist convective activity. The cause & effect of this aggregation is still not understood in the scientific community. Spectral analyses of moist convection-related quantities like outgoing long wave radiation show significant power at discrete spatial scales up to ≈ 10 000 km embedded in a red-noise spectrum originating from a fully turbulent atmosphere. The observed maximum variability in brightness temperature corresponds to theoretical equatorial wave dispersion relations on the β-plane. The tropical convection can be seen to also aggregate in large scales driven by features found in dry dynamics, like tropical Rossby, gravity, Kelvin waves, and the MJO - one prominent 60-90 day time scale planetary wave, which has a strong influence on weather in the tropics and outside (influencing seasonal prediction skill).

The interaction and influence of moisture on these large scale patterns is still not understood at a basic level. In computer simulations, groups around the globe are beginning to include the role of moisture in simple shallow water flow to better understand the challenge it poses from a physical perspective, as it makes the physics irreversible. We present results from an ensemble of 1000 simulations showing multiple-equilibria in the tropical flow. The multiple equilibria occur in a dry, as well as a moist environment in stochastic forcing with statistically similar random numbers. In the ensemble of runs, we see that latent heat release in a moist environment encourages super-rotation. We examine the budget of angular momentum enabling this flow-reversal at the equator in a moist environment, to understand the role of moisture in this process (Zurita-Gotor 2019).