A phase plane based perspective of energetics of large scale tropical convection

Abstract

Rainfall and convection in the tropics has huge impacts on the lives and livelihood of almost 40 percent of the global population that resides there. However, we still lack a complete understanding of the full spectrum of convective variability that is observed in the tropics. One such component of this spectrum is the cyclic increase and decrease in rainfall at sub-seasonal scales in the tropics. Such cyclic patterns, termed as convective life-cycles, have been observed ubiquitously throughout different regions in the tropics across a range of different timescales. This suggests that such cyclic evolution is fundamental to tropical convection and how it interacts with its environment. Further, global climate models seem to find it difficult to replicate such variability realistically. This makes it even more important for us to better understand the underlying mechanisms and where the models are going wrong. In this dissertation, I aim to provide a phase plane based framework to understand the different aspects and mechanisms associated with such cyclical modes of variability. The phase plane based framework presented in this study is built upon the moist static energy (MSE) budget framework. It provides a new way to visualize and extend the application of MSE budget. We test and verify the usefulness of this framework as a process-oriented diagnostic tool by applying it to different observational and reanalysis data sets, with the hope that future work can adapt this to be applied to understand model biases and errors. Chapter 2 introduces novel phase angle diagnostics to understand convective evolution. The phase angle is physically related to the notion of gross moist stability (GMS). It allows us to track the daily evolution of GMS which was not possible earlier due to the highly variable nature of the metric at shorter timescales. Chapter 2 also highlights that convective life-cycles are strongly linked to the evolution of moisture and can be considered as moisture recharge-discharge cycles. Results show that these recharge-discharge cycles are a type of moisture mode driven primarily by advection of moisture by the convective circulation. Interestingly, the cycles over different regions in the tropics seem to have a similar timescale. Chapter 3 applies the GMS phase plane to analyze easterly waves in the Eastern Pacific. We observe that easterly waves also show a cyclic evolution on the phase plane driven by advection of MSE similar to recharge-discharge cycles. However, not all cyclic evolution on the phase plane is associated with easterly waves events. This helps extend the applicability of the phase plane based framework to other tropical wave modes and for understanding moisture modes in general. In chapter 4, we focus on a more lagrangian perspective of the ubiquitous convective evolution by introducing a new MSE variance phase space. The use of MSE variance allows us to understand convective evolution in terms of convective aggregation. The results highlight that large-scale convective aggregation in the real world is in the form of aggregation-disaggregation cycles. The results also highlights the important role of advection of MSE in maintaining and driving aggregation-disaggregation cycles which has been relatively less studied compared to radiative feedbacks.