On the Production of Severe Convective Storm Environments in North and South America

Funing Li, Purdue University

Abstract

This work centers on a fundamental question related to the geography of severe weather on Earth – why the United States has the most tornadoes in the world? We mainly use global climate model (GCM) experiments, combined with observations and theories, to test several hypotheses toward understanding this question. Ultimately, our results emphasize the role of large-scale surface roughness in modulating continental severe thunderstorm and tornadic environments and low-level circulation, which adds a critical missing ingredient in the conventional conceptual understanding of the geographic controls of severe weather hotspots on Earth.We first provide a comprehensive climatological analysis and evaluation of severe thunderstorm environments over North America as well as the associated synoptic-scale features that frequently generate them, in a new high-resolution global reanalysis dataset (ERA5) and a GCM (CESM CAM6) historical simulation. Overall, they both reasonably reproduce severe thunderstorm environments and the relevant synoptic-scale features.We then combine theory and the MERRA-2 reanalysis data to provide foundations to a scaling CAPE (convective available potential energy) framework. We demonstrate that the scaling CAPE formula, which uses environmental data alone without lifting a hypothetical air parcel, can easily estimate true CAPE given several assumptions. Using the scaling CAPE framework, we further evaluate the performance of 13 CMIP6 models in simulating the historical climatology of severe thunderstorm environments in the United States. Overall, model errors in severe thunderstorm environments arise primarily due to errors in the average temperature and moisture of near-surface air, though a few models (MPI and CNRM) quantitatively well reproduce the magnitude and spatial pattern of severe thunderstorm environments.The frequent and intense severe thunderstorm environments over the eastern half of North America have been ascribed to the existence of elevated terrain to the west and the Gulf of Mexico to the south. Yet, to what extent each of them is necessary for producing severe thunderstorm environments in North America has not been examined. We conduct GCM experiments using CAM6, with North American topography removed and the Gulf of Mexico converted to land, respectively, to show that these severe thunderstorm environments depend strongly on upstream elevated terrain but more weakly on the Gulf of Mexico, though the Gulf of Mexico affects their spatial footprint.A similar geographic setup is also found in South America, where the Andes Mountains stretch from north to south similar to the Rockies, and the Amazon basin is as warm and moist as tropical oceans. However, quite fewer tornadoes occur in South America than North America though both have frequent severe thunderstorm. We further show, by conducting GCM and idealized GCM experiments, that the upstream surface roughness drives the contrast in tornado potential between North and South America. A smoother upstream surface permits stronger easterly trade winds that feed the poleward low-level winds flowing downstream into the continental interiorLastly, we show a tug-of-way between elevated terrain and large-scale surface roughness in modulating continental low-level atmospheric circulation. With GCM experiments conducted in North America, we demonstrate that the uplift of elevated terrain intensifies low-level winds along the foothills by strengthening synoptic-scale processes including lee cyclogenesis, whereas the strong large-scale surface roughness slows down low-level winds by increasing surface friction that deepens boundary layer. These results provide valuable insights into understanding the climatology of the poleward low-level winds including lowlevel jet in central North and South America, whose variance strongly modulates moisture transport and hence the continental-scale weather and climate.

Degree

Ph.D.

Advisors

Chavas, Purdue University.

Subject Area

Energy|Atmospheric sciences|Meteorology

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