Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling

WRF-Simulated Low-Level Jets over Iowa: Characterizationand Sensitivity Studies

TitleWRF-Simulated Low-Level Jets over Iowa: Characterizationand Sensitivity Studies
Publication TypeJournal Article
Year of Publication2020
AuthorsAird, Jeanie A., Barthelmie Rebecca J., Shepherd Tristan J., and Pryor S. C.
JournalWind Energy Science
Volume6
Pages1015-1030
Abstract / Summary

Output from high-resolution simulations with the Weather Research and Forecasting (WRF) model is analyzed to characterize local low-level jets (LLJ) over Iowa. Analyses using a detection algorithm wherein the wind speed above and below the jet maximum must be below 80% of the jet wind speed within a vertical window of approximately 20 m – 530 m a.g.l. indicate the presence of an LLJ in at least one of the 14700 4 km by 4 km grid cells over Iowa on 98% of nights. Nocturnal LLJ are most frequently associated with stable stratification and low TKE and hence are more frequent during the winter months. The spatiotemporal mean LLJ maximum (jet core) wind speed is 9.55 ms-1 and the mean height is 182 m. Locations of high LLJ frequency and duration across the state are seasonally varying with a mean duration of 3.5 hours. LLJ are most frequent in the topographically complex northwest of the state in winter, and in the flatter northeast of the state in spring. Sensitivity of LLJ characteristics to the: i) LLJ definition and ii) vertical resolution at which the WRF output is sampled are examined. LLJ definitions commonly used in LLJ literature are considered in the first sensitivity analysis. These sensitivity analyses indicate that LLJ characteristics are highly variable with LLJ definition. Further, when the model output is down-sampled to lower vertical resolution, the maximum LLJ wind speed and mean height 20 decrease, but spatial distributions of regions of high frequency and duration are conserved.

URLhttp://dx.doi.org/10.5194/wes-2020-113
DOI10.5194/wes-2020-113
Journal: Wind Energy Science
Year of Publication: 2020
Volume: 6
Pages: 1015-1030
Publication Date: 11/2020

Output from high-resolution simulations with the Weather Research and Forecasting (WRF) model is analyzed to characterize local low-level jets (LLJ) over Iowa. Analyses using a detection algorithm wherein the wind speed above and below the jet maximum must be below 80% of the jet wind speed within a vertical window of approximately 20 m – 530 m a.g.l. indicate the presence of an LLJ in at least one of the 14700 4 km by 4 km grid cells over Iowa on 98% of nights. Nocturnal LLJ are most frequently associated with stable stratification and low TKE and hence are more frequent during the winter months. The spatiotemporal mean LLJ maximum (jet core) wind speed is 9.55 ms-1 and the mean height is 182 m. Locations of high LLJ frequency and duration across the state are seasonally varying with a mean duration of 3.5 hours. LLJ are most frequent in the topographically complex northwest of the state in winter, and in the flatter northeast of the state in spring. Sensitivity of LLJ characteristics to the: i) LLJ definition and ii) vertical resolution at which the WRF output is sampled are examined. LLJ definitions commonly used in LLJ literature are considered in the first sensitivity analysis. These sensitivity analyses indicate that LLJ characteristics are highly variable with LLJ definition. Further, when the model output is down-sampled to lower vertical resolution, the maximum LLJ wind speed and mean height 20 decrease, but spatial distributions of regions of high frequency and duration are conserved.

DOI: 10.5194/wes-2020-113
Citation:
Aird, J, R Barthelmie, T Shepherd, and SC Pryor.  2020.  "WRF-Simulated Low-Level Jets over Iowa: Characterizationand Sensitivity Studies."  Wind Energy Science 6: 1015-1030.  https://doi.org/10.5194/wes-2020-113.