On the Forcing of the Summertime Great Plains Low-Level Jet

On the Forcing of the Summertime Great Plains Low-Level Jet AbstractThe low-level jet (LLJ) is a ubiquitous feature of the lower atmosphere over the Great Plains during summer. The LLJ is a nocturnal phenomenon, developing during the six- to nine-hour period after sunset. Forcing of the LLJ has been debated for over 60 years, the focus being on two processes: decoupling of the residual layer from the surface owing to nighttime cooling and diurnal heating and cooling of the sloping Great Plains topography.To examine characteristics and forcing mechanisms for the LLJ, composite grids were compiled from the North American Mesoscale Forecast System for the summertime months of June and July over a five-year period 2008-2012. One composite set was assembled from well-developed LLJ episodes during which the maximum nocturnal jet magnitude at 0900 UTC over northwestern Oklahoma exceeded 20 m s−1. A second set consists of non-jet conditions for which the maximum nighttime wind magnitude in the lowest 3 km did not exceed 10 m s−1.The intensity of the horizontal pressure gradient and hence background geostrophic flow at jet level was the dominant difference between composite cases. The horizontal pressure gradient forms in response to the thermal wind above jet level that results primarily from seasonal heating of the sloping Great Plains. Thermal wind forcing is thus the key link between the Great Plains and the high frequency of LLJ occurrence. The nocturnal wind maximum develops primarily due to the inertial oscillation of the ageostrophic wind occurring after decoupling of the lower atmosphere from the surface owing to radiational cooling in the early evening. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Atmospheric Sciences American Meteorological Society

On the Forcing of the Summertime Great Plains Low-Level Jet

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Publisher
American Meteorological Society
Copyright
Copyright © American Meteorological Society
ISSN
1520-0469
D.O.I.
10.1175/JAS-D-17-0059.1
Publisher site
See Article on Publisher Site

Abstract

AbstractThe low-level jet (LLJ) is a ubiquitous feature of the lower atmosphere over the Great Plains during summer. The LLJ is a nocturnal phenomenon, developing during the six- to nine-hour period after sunset. Forcing of the LLJ has been debated for over 60 years, the focus being on two processes: decoupling of the residual layer from the surface owing to nighttime cooling and diurnal heating and cooling of the sloping Great Plains topography.To examine characteristics and forcing mechanisms for the LLJ, composite grids were compiled from the North American Mesoscale Forecast System for the summertime months of June and July over a five-year period 2008-2012. One composite set was assembled from well-developed LLJ episodes during which the maximum nocturnal jet magnitude at 0900 UTC over northwestern Oklahoma exceeded 20 m s−1. A second set consists of non-jet conditions for which the maximum nighttime wind magnitude in the lowest 3 km did not exceed 10 m s−1.The intensity of the horizontal pressure gradient and hence background geostrophic flow at jet level was the dominant difference between composite cases. The horizontal pressure gradient forms in response to the thermal wind above jet level that results primarily from seasonal heating of the sloping Great Plains. Thermal wind forcing is thus the key link between the Great Plains and the high frequency of LLJ occurrence. The nocturnal wind maximum develops primarily due to the inertial oscillation of the ageostrophic wind occurring after decoupling of the lower atmosphere from the surface owing to radiational cooling in the early evening.

Journal

Journal of the Atmospheric SciencesAmerican Meteorological Society

Published: Sep 6, 2017

References

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