Doppler lidar measurements of wind variability and LLJ Properties in Central Oklahoma during the August 2017 Land-Atmosphere Feedback Experiment.

Abstract

Low-level jets (LLJs) are an important nocturnal source of wind energy in the U.S. Great Plains. An August 2017 lidar-based field-measurement campaign (LAFE) studied LLJs over the Central SGP site in Oklahoma, and found nearly equal occurrences of the usual southerly jets, and postfrontal northeasterly jets—typically rare during this season—for an opportunity to compare the two types of LLJs during this month. Southerly winds were stronger than the north-easterlies by more than 4 ms−1 on average, reflecting a significantly higher frequency of winds stronger than 12 ms−1. The analysis of this dataset has been expanded to other SGP Doppler-lidar sites to quantify the variability of winds and LLJ properties between sites of different land use. Geographic variations of winds over the study area were noted: on southerly-wind nights, the winds blew stronger at the highest, westernmost sites by 2 ms−1, whereas on the northeasterlyflow nights, the easternmost sites had the strongest wind speeds. Lidar measurements at 5 sites during August 2017, contrasted to the 2016-2021 summertime data, revealed unusual wind and LLJ conditions. Temporal hodographs using hourly-averaged winds at multiple heights revealed unorganized behavior in the turbulent stable boundary layer (SBL) below the jet nose. Above the nose, some nights showed veering qualitatively similar to inertial-oscillation (IO) behavior, but at amplitudes much smaller than expected for an IO, whereas other nights showed little veering. Vertical hodographs had a linear shape in the SBL, indicating little directional shear there, and veering above, resulting in a hook-shaped hodograph with height.