Environmental controls of vertical motion over the maritime tropics

Abstract

Understanding what controls vertical motion profile shape is fundamental to understanding tropical precipitation patterns. There is not a comprehensive understanding of what controls the top-heaviness of vertical motion in the tropics and through this dissertation I will present novel research towards that task. In the first research chapter, I investigate the two controls which have been proposed in the literature: the thermodynamic profiles of the environment and the dynamics imposed by sea surface temperature (SST) patterns. To fit these two perspectives together, we focus on two regions with distinctly top and bottom-heavy vertical motion: The Western Pacific and the Central Eastern Pacific. The top-heaviness angle is introduced to describe this difference. To study thermodynamic controls on vertical motion profile shape, we use weak temperature gradient (WTG) simulations. We are able to simulate the shape differences between our two regions from the thermodynamics and show that the temperature and SST are the most important for the vertical motion shape differences between our two regions. We show that the qualitative shape differences can be explained using a simple entraining plume model. We hypothesize that the SST gradients lead to a cooler equilibrium lower tropospheric temperature compared with no gradient, this leads to a more conducive thermodynamic environment to bottom-heaviness and the dynamics mechanism controls top-heaviness through the thermodynamics. In the second research chapter, We present an analysis of observations from the Organization of Tropical East Pacific Convection (OTREC) field campaign. We find that moisture and temperature anomalies are related to the top-heaviness angle. To characterize the moisture profile variability, we introduce a new metric called the moisture dipole coefficient (MDC). We use it in conjunction with the saturation fraction (SF) to show they describe the amount of vertical motion and top-heaviness of vertical motion. We also demonstrate a correlation between the MDC and a measure of dry static stability, which indicates an evolution of moisture and vertical motion profiles such that the atmosphere is driven towards a critical moisture profile that is set by the temperature profile. A simple model of an entraining plume is used to demonstrate how thermodynamic profiles lead to top-heaviness variations and vice versa leading to a balance, which we call vertically resolved moisture quasi-equilibrium (VR-MQE). Finally, the authors show that the process of VR-MQE is not well represented in the forecast model used during the field program. In the third research chapter, Weak Temperature Gradient (WTG) modeling using a small cloud resolving model (CRM) admits multiple equilibria depending upon the initial model conditions. A new kind of equilibrium is presented here which undergoes a periodic and steady state oscillation which is emergent from static boundary conditions. The periodic oscillation is shown to be a vertical motion moisture mode generated by the interaction between vertical motion and radiation. A simple two-mode model based on decomposing the vertical motion profile is able to describe the oscillation.