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Observed relationship between the Turkana low-level jet and boreal summer convection

  • Edward K. VizyEmail author
  • Kerry H. Cook
Article

Abstract

The Turkana low-level jet is a circulation feature over northern Kenya that is not spatially well-resolved in many observational datasets. Here, high-resolution TRMM, IMERG, and CMORPH precipitation estimates, along with the ERA5 and MERRA2 reanalyses are analyzed to better understand the controls on jet strength and the relationship between the jet and boreal summer rainfall variability in the jet exit region over South Sudan. The height of the South Sudan wet season, August, is analyzed for 2000–2017. Climatologically, rainfall peaks over the jet exit region during the early morning hours, coinciding with the jet maximum. The strong jet enhances low-level wind convergence and briefly shifts the low-level zonal moisture gradient westward. Over the western South Sudan, rainfall peaks in the late afternoon/evening in association with daytime heating. The correlation between the daily Turkana Jet strength and precipitation indicates a significant negative relationship over eastern South Sudan, and over the Ethiopian and East African Highlands. Composite analysis of strong and weak jet periods reveal that the atmospheric conditions associated with a strong jet include enhanced low-level ridging along the East African coast south of the equator resulting in a stronger height gradient in the Turkana Channel, as well drier conditions over the channel and adjacent highlands. These conditions support a stronger jet that is further enhanced by katabatic flow due to the stronger nighttime cooling over the highlands, as well as enhanced dry air advection over the jet exit region. Thus, while low-level convergence may increase, convection weakens over eastern South Sudan due to the reduced atmospheric moisture content and weaker instability. The opposite occurs for the weak jet case.

Keywords

Turkana low-level jet East African rainfall Kenya South Sudan Ethiopia Diurnal cycle of rainfall Rainfall variability Katabatic winds ERA5 IMERG 

Notes

Acknowledgements

This work was funded by NSF Award #1701520. The authors acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing HPC and database resources that have contributed to the research results reported within this paper. URL: http://www.tacc.utexas.edu. The Grid Analysis and Display System software (GrADS) developed at COLA/IGES was used for generating the figures. The authors would like to thank Declan Finney and the anonymous reviewer for their constructive comments and suggestions.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Geological Sciences, Jackson School of GeosciencesThe University of Texas at AustinAustinUSA

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