Low-Level Jet Over Southern Great Plains
Global climate models have difficulty reproducing the correct location and timing of precipitation over the central United States. One possible reason for this difficulty involves the Southern Great Plains “low-level jet (LLJ)”, a phenomenon of enhanced wind speeds at heights below 3 km that plays an important role in transporting moisture from the Gulf of Mexico to the Great Plains. A team of Department of Energy (DOE) researchers used data from DOE’s Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site to identify LLJ characteristics and evaluate how well six commonly used reanalysis products, which combine numerical weather models with data assimilation models, were able to reproduce the characteristics. The study focused on data from the Mid-latitude Continental Convective Clouds Experiment (MC3E) collected over the ARM SGP site in April and May 2011, as well as a 10-year period from 2001 to 2010 that provides a comparison with the MC3E study. The team compared all six reanalysis products to MC3E data and only three of them to the 10-year data. They found that reanalyses are able to represent most aspects of the composite LLJ profile, but there are large discrepancies in the number of observed jets and those derived from reanalyses. Underestimating the frequency of strong LLJs leads to an underestimation of the moisture transport. When the 10-year period is considered, all three reanalyses underestimate the moisture transport associated with strong LLJs by factors ranging between 1.4 and 2.7, impacting the models’ ability to produce accurate timing and location of precipitation in the Great Plains. There are indications that increased horizontal and vertical resolution improves the ability of the reanalyses to produce strong LLJs, but other factors not addressed in this study might also be important.
Berg, L. K., L. D. Riihimaki, Y. Qian, H. Yan, and M. Huang. 2015. “The Low-Level Jet over the Southern Great Plains Determined from Observations and Re-analysis and Its Impact on Moisture Transport,” Journal of Climate 28(17), 6682–706. DOI:10.1175/JCLI-D-14-00719.1.