01/23/2019
What Factors Matter in Simulations of the Nocturnal Low-Level Jet?
Observations of the low-level jet over the ARM Oklahoma observatory provide cases for evaluation of numerical model simulations.
The Science
The nocturnal low-level jet is defined as a maximum in the vertical profile of wind speed occurring overnight in the lowest kilometer of the atmosphere. The nocturnal low-level jet is a common and important meteorological phenomenon in the US Great Plains region due its role in transport of Gulf of Mexico moisture northward, initiation of deep convection and severe weather, potential hazardous wind shear for aviation, wind energy production, and increased shear and turbulence that impact wind turbines. Regional and global-scale models cannot resolve all of the details of the planetary boundary layer, and so have to rely on equations (known as parameterizations) that attempt to represent sub-grid-scale features of turbulent exchange processes in the atmosphere. Model simulations are sensitive to both the assumptions made in different formulations of these parameterizations and the horizontal and vertical resolution used in the models. In this study, scientists used detailed observational data from a field experiment at the Atmospheric Radiation Measurement (ARM) user facility observatory in Oklahoma to evaluate model simulations of the nocturnal low-level jet.
The Impact
Counter to the seemingly natural assumption that reducing grid spacing should improve predictions of atmospheric flow variables, findings of this study suggested that using 4-km horizontal grid spacing is no worse than using finer horizontal spacing (i.e., 1 or 2 km) for representing the nocturnal low-level jet. However, using finer vertical grid spacing did provide improvements to the morphology of the jet, especially to the representation of the temporal evolution and vertical structure of the jet. One of the boundary layer parameterizations was found to be quite sensitive to the choice of model top height. The study presents the optimal grid configuration and boundary layer parameterization choice for modeling the nocturnal low-level jet over the Great Plains. This information will be useful for scientists interested in simulating convective cloud processes or wind energy production in this region.
Summary
Previous studies have shown that the Weather Research and Forecasting (WRF) Model often under predicts the strength of the Great Plains nocturnal low-level jet (NLLJ), which has implications for weather, climate, aviation, air quality, and wind energy in the region. During the Lower Atmospheric Boundary Layer Experiment (LABLE) conducted in 2012, NLLJs were frequently observed at high temporal resolution, allowing for detailed documentation of their development and evolution throughout the night. Ten LABLE cases with observed NLLJs were chosen to systematically evaluate the WRF Model’s ability to reproduce the observed NLLJs. Model runs were performed with 4-, 2-, and 1-km horizontal spacing and with the default stretched vertical grid and a non-stretched 40-m vertically spaced grid to investigate which grid configurations are optimal for NLLJ modeling. These tests were conducted using three common boundary layer parameterization schemes: Mellor- Yamada Nakanishi Niino, Yonsei University, and Quasi-Normal Scale Elimination. It was found that refining horizontal spacing does not necessarily improve the modeled NLLJ wind, however increasing the number of vertical levels on a non-stretched grid does provide more information about the temporal evolution and vertical structure of the NLLJ. Simulations of the NLLJ were found to be less sensitive to boundary layer parameterization than to grid configuration. The Quasi-Normal Scale Elimination scheme was chosen for future NLLJ simulation studies.
Principal Investigator(s)
Elizabeth N. Smith
University of Oklahoma
[email protected]
Funding
Data were obtained from the Atmospheric Radiation Measurement (ARM) user facility, a U.S. Department of Energy (DOE) Office of Science user facility managed by The Office of Biological and Environmental Research. This research was completed with support by the National Science Foundation under Grant AGS-1359698. The first author was also supported in part by the Lockheed Martin Corporation-American Meteorological Society Graduate Fellowship.
References
Smith, EN, JA Gibbs, E Fedorovich, and PM Klein. “WRF Model Study of the Great Plains Low-Level Jet: Effects of Grid Spacing and Boundary Layer Parameterization.” Journal of Applied Meteorology and Climatology 57(10), 2375-2397 (2018). [DOI:10.1175/jamc-d-17-0361.1].