Investigating the Assumptions in Current Methods of Observing and Modeling Tropical Vertical Motion Profiles

Abstract

This thesis investigates tropical vertical motion profiles using both observations and modelling work. Firstly, we investigate if we can use in-situ observations of heavy-water isotopologues in rainfall to understand vertical motion profiles in the tropics. Next, this thesis will also systematically investigate the different methodologies used to parameterize the large-scale tropical circulation in idealized small-domain simulations, and the differences between them, and whether we can assume that these different schemes are interchangeable. Lastly, I also discuss what these different parametrizations can tell us about convective organization in idealized large-domain simulations, and also within the tropics. Previous work has provided proof-of-concept in using in-situ observations of depletion of heavy-water isotopologues to understand vertical motion profiles in the tropical East Pacific. Using station observational data collected around the time of the OTREC field campaign and after, we show that top-heavy convection results in rainfall that is more depleted in heavy-water isotopologues such as HDO and H$_2^{18}$O. However, these trends of depletion are not spatially consistent, resulting in rainfall that may be more enriched in one location compared to other locations despite having similar rainfall rate and vertical-motion structure. We verify these results using realistic WRF simulations, and based on our results, we provide another metric to measure top-heaviness, one that is also partially dependent on vertical moisture advection. Next, we discuss how large-scale vertical motion profiles are parameterized in idealized small-domain models using the WTG framework. Ever since the canonical formulation of the WTG, adjustment schemes utilizing its explicit and implicit forms have become ubiquitous when modelling tropical convection in small-domain simulations. However, despite the prevalence of these schemes in modelling work for tropical climate, they often produce noticeably different vertical motion structures. For example, the original Weak Temperature Gradient formulation of Raymond and Zeng [2005] is known to produce vertical-motion structures that are more top-heavy than the Damped Gravity Wave formulation of Kuang [2008] and Blossey et al. [2009]. We show that the differences observed when using different WTG-adjustment schemes are often more pronounced under more idealized conditions, and can be traced back to how different WTG-adjustment schemes treat the different vertical modes. Lastly, most past research surrounding small-domain simulations in the WTG framework have focused on the analogues to the wet- and dry-regimes of self-aggregation in large-domain simulations. My work also shows that the WTG framework is potentially able to attain analogues to convectively-coupled waves (CCWs) in small domain simulations. Model runs in large-domain simulations are able to replicate these CCWs regardless of radiative scheme. We note that despite prevalence of convective organization in the form of CCWs across all large-domain simulations we have run, it is still a somewhat overlooked aspect of convective organization, as current work on this field predominantly focuses on self-aggregation of convection.Earth and Planetary Science