Evaluation of the southerly low‐level jet climatology for the central United States as simulated by NARCCAP regional climate models

Abstract

An ensemble of simulations from four regional climate models (RCMs) driven by a global reanalysis was obtained from the North American Regional Climate Change Assessment Program (NARCCAP) and used to evaluate the ability of the RCMs to simulate the long-term (1979–2000) climatology of southerly low-level jets (S-LLJs) in the central United States. The RCM-derived S-LLJ climatologies were evaluated against rawinsonde observations for the same period. The use of a small ensemble of RCM simulations helped to identify model differences and assisted with interpretation. The RCMs generally reproduced the broad spatial patterns and temporal variations of jet frequency and average jet height and speed. No model consistently outperformed the others in all aspects of the evaluation, although differences existed between models in the placement, migration and relative strength of ‘hotspots’ of more frequent jet activity. In particular, three of the four models placed the centre of greatest nocturnal S-LLJ activity during the warm season in northern and central Texas, whereas for the other model the greatest jet activity was located in the south-central plains (Kansas/Oklahoma). The magnitude of a S-LLJ frequency maximum over south Texas also varied between models, with simulated frequencies exceeding observed frequencies for some models but substantially underestimating for others. The evaluation presented here highlights the potential applications of RCMs in S-LLJ research for future climate and other assessment studies that require three-dimensional data with relatively high spatial and temporal resolutions. The overall performance of the models in reproducing the long-term S-LLJ climatology supports the use of NARCCAP RCM simulations in climate assessments for the central United States where S-LLJs are an important contributor to the regional climatology.