On the Role of Sloping Terrain in the Forcing of the Great Plains Low-Level Jet

2010 ◽  
Vol 67 (8) ◽  
pp. 2690-2699 ◽  
Author(s):  
Thomas R. Parish ◽  
Larry D. Oolman
Keyword(s):  
Great Plains ◽  
Mesoscale Model ◽  
Horizontal Resolution ◽  
Inertial Oscillation ◽  
Western Boundary ◽  
Boundary Current ◽  
Wind Maximum ◽  
Low Level ◽  
Low Level Jet ◽  
Sloping Terrain

Abstract The summertime Great Plains low-level jet (LLJ) has been the subject of numerous investigations during the past several decades. Characteristics of the LLJ include nighttime development of a pronounced wind maximum of typically 15–20 m s−1 at levels 300–800 m above the surface and a clockwise rotation of the wind maximum during the course of the night. Maximum frequency of occurrence of the LLJ is found in the southern Great Plains. Theories proposed to explain the diurnal wind maximum of the Great Plains LLJ include inertial oscillation of the ageostrophic wind, the diurnal oscillation of the horizontal pressure field associated with heating and cooling of the sloping terrain, and the western boundary current interpretations. A simple equation system and output from the 12-km horizontal resolution Weather Research and Forecasting Nonhydrostatic Mesoscale Model (NAM) for July 2008 are used to provide evidence as to the importance of the Great Plains topography in driving the LLJ. Summertime heating of the sloping terrain is critical in establishing the climatological position for the Great Plains LLJ. Heating enhances the background geostrophic flow associated with the Bermuda high, resulting in a maximum low-level mean summertime flow over the Great Plains region. Maximum geostrophic winds in the NAM are found during late afternoon, providing a large background wind on which frictional decoupling can act. The nighttime LLJ maximum is the result of an inertial oscillation of the unbalanced components that arise fundamentally from frictional decoupling. Diurnal heating of the sloping terrain forces a cycle in the geostrophic wind that is out of phase with the wind maximum.

scholarly journals A Comparative Study of the 3 June 2015 Great Plains Low-Level Jet

2016 ◽  
Vol 144 (8) ◽  
pp. 2963-2979 ◽  
Author(s):  
Thomas R. Parish
Keyword(s):  
Great Plains ◽  
Cross Sections ◽  
Geostrophic Wind ◽  
Gradient Force ◽  
Inertial Oscillation ◽  
Pressure Gradient Force ◽  
Wind Maximum ◽  
Airborne Measurements ◽  
Low Level ◽  
Low Level Jet

Abstract Detailed ground-based and airborne measurements were conducted of the summertime Great Plains low-level jet (LLJ) in central Kansas during the Plains Elevated Convection at Night (PECAN) campaign. Airborne measurements using the University of Wyoming King Air were made to document the vertical wind profile and the forcing of the jet during the nighttime hours on 3 June 2015. Two flights were conducted that document the evolution of the LLJ from sunset to dawn. Each flight included a series of vertical sawtooth and isobaric legs along a fixed track at 38.7°N between longitudes 98.9° and 100°W. Comparison of the 3 June 2015 LLJ was made with a composite LLJ case obtained from gridded output from the North American Mesoscale Forecast System for June and July of 2008 and 2009. Forcing of the LLJ was detected using cross sections of D values that allow measurement of the vertical profile of the horizontal pressure gradient force and the thermal wind. Combined with observations of the actual wind, ageostrophic components normal to the flight track can be detected. Observations show that the 3 June 2015 LLJ displayed classic features of the LLJ, including an inertial oscillation of the ageostrophic wind. Oscillations in the geostrophic wind as a result of diurnal heating and cooling of the sloping terrain are not responsible for the nocturnal wind maximum. Net daytime heating of the sloping Great Plains, however, is responsible for the development of a strong background geostrophic wind that is critical to formation of the LLJ.

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

2017 ◽  
Vol 74 (12) ◽  
pp. 3937-3953 ◽  
Author(s):  
Thomas R. Parish
Keyword(s):  
Pressure Gradient ◽  
Great Plains ◽  
Lower Atmosphere ◽  
Inertial Oscillation ◽  
Wind Maximum ◽  
Horizontal Pressure Gradient ◽  
Low Level ◽  
Thermal Wind ◽  
Horizontal Pressure ◽  
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 6–9-h 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 5-yr period (2008–12). 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 nonjet 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 because of 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.

scholarly journals A Simple Analytical Model of the Nocturnal Low-Level Jet over the Great Plains of the United States

2014 ◽  
Vol 71 (10) ◽  
pp. 3674-3683 ◽  
Author(s):  
Yu Du ◽  
Richard Rotunno
Keyword(s):  
Friction Coefficient ◽  
Analytical Model ◽  
Great Plains ◽  
Present Model ◽  
The United States ◽  
Simple Analytical Model ◽  
Thermal Forcing ◽  
Low Level ◽  
Low Level Jet ◽  
Sloping Terrain

Abstract A simple analytical model including both diurnal thermal forcing over sloping terrain (the “Holton” mechanism) and diurnally varying boundary layer friction (the “Blackadar” mechanism) is developed to account for the observed amplitude and phase of the low-level jet (LLJ) over the Great Plains and to understand better the role of each mechanism. The present model indicates that, for the pure Holton mechanism (time-independent friction coefficient), the maximum southerly wind speed occurs (depending on the assumed friction coefficient) between sunset and midnight local standard time, which is earlier than the observed after-midnight maximum. For the pure Blackadar mechanism (time-independent thermal forcing), the present model shows that generally occurs later (closer to sunrise) than observed and has a strong latitudinal dependence. For both mechanisms combined, the present model indicates that occurs near to the observed time, which lies between the time obtained in the pure Holton mechanism and the time obtained in the pure Blackadar mechanism; furthermore, is larger (and closer to that observed) than in each one considered individually. The amplitude and phase of the LLJ as a function of latitude can be obtained by the combined model by allowing for the observed latitude-dependent mean and diurnally varying thermal forcing.

scholarly journals A Unified Theory for the Great Plains Nocturnal Low-Level Jet

2016 ◽  
Vol 73 (8) ◽  
pp. 3037-3057 ◽  
Author(s):  
Alan Shapiro ◽  
Evgeni Fedorovich ◽  
Stefan Rahimi
Keyword(s):  
Great Plains ◽  
Slope Angle ◽  
Geostrophic Wind ◽  
Linear Functions ◽  
Free Atmosphere ◽  
Wind Maximum ◽  
Maximum Surface ◽  
Low Level ◽  
Low Level Jet ◽  
Surface Buoyancy

Abstract A theory is presented for the Great Plains low-level jet in which the jet emerges in the sloping atmospheric boundary layer as the nocturnal phase of an oscillation arising from diurnal variations in turbulent diffusivity (Blackadar mechanism) and surface buoyancy (Holton mechanism). The governing equations are the equations of motion, mass conservation, and thermal energy for a stably stratified fluid in the Boussinesq approximation. Attention is restricted to remote (far above slope) geostrophic winds that blow along the terrain isoheights (southerly for the Great Plains). Diurnally periodic solutions are obtained analytically with diffusivities that vary as piecewise constant functions of time and slope buoyancies that vary as piecewise linear functions of time. The solution is controlled by 11 parameters: slope angle, Coriolis parameter, free-atmosphere Brunt–Väisälä frequency, free-atmosphere geostrophic wind, radiative damping parameter, day and night diffusivities, maximum and minimum surface buoyancies, and times of maximum surface buoyancy and sunset. The Holton mechanism, by itself, results in relatively weak wind maxima but produces strong jets when paired with the Blackadar mechanism. Jets with both Blackadar and Holton mechanisms operating are shown to be broadly consistent with observations and climatological analyses. Jets strengthen with increasing geostrophic wind, maximum surface buoyancy, and day-to-night ratio of the diffusivities and weaken with increasing Brunt–Väisälä frequency and magnitude of minimum slope buoyancy (greater nighttime cooling). Peak winds are maximized for slope angles characteristic of the Great Plains.

scholarly journals The role of nocturnal Low-Level-Jet in nocturnal convection and rainfalls in the west Mediterranean coast: the episode of 14 December 2010 in northeast of Iberian Peninsula

2012 ◽  
Vol 8 (1) ◽  
pp. 27-31 ◽  
Author(s):  
J. Mazón ◽  
D. Pino
Keyword(s):  
Iberian Peninsula ◽  
Mesoscale Model ◽  
Return Flow ◽  
Inertial Oscillation ◽  
Air Mass ◽  
Cold Air ◽  
Low Level ◽  
Radar Images ◽  
Synoptic Wind ◽  
Low Level Jet

Abstract. The night of 14 December 2010 radar images of the Spanish Weather Agency recorded a large rain band that moved offshore at the Northeast coast of the Iberian Peninsula. MM5 mesoscale model is used to study the atmospheric dynamics during that day. A Nocturnal Low Level Jet (NLLJ) generated by an inertial oscillation that brings cold air to the coast from inland has been simulated in the area. This cold air interacts with a warmer air mass some kilometers offshore. According to the MM5 mesoscale model simulation, the cold air enhances upward movements of the warm air producing condensation. Additionally, there is a return flow to the coastline at 600–900 m high. This warm air mass interacts again with the cold air moving downslope, also producing condensation inland. The simulation for the night before this episode shows large drainage winds with a NLLJ profile, but no condensation areas. The night after the 14th the simulation also shows drainage winds but without a NLLJ profile. However, an offshore convergence area was produced with a returned flow, but no condensation inland occurred. This fact is in agreement with radar observations which reported no precipitation for these two days. Consequently, NLLJ in combination with a synoptic wind over the sea could enhance condensation and eventually precipitation rates in the Mediterranean Iberian coast.

scholarly journals Clarifying the Baroclinic Contribution to the Great Plains Low-Level Jet Frequency Maximum

2019 ◽  
Vol 147 (9) ◽  
pp. 3481-3493 ◽  
Author(s):  
Joshua G. Gebauer ◽  
Alan Shapiro
Keyword(s):  
Great Plains ◽  
Geostrophic Wind ◽  
Inertial Oscillation ◽  
Low Level ◽  
Differential Heating ◽  
Oklahoma Mesonet ◽  
Nocturnal Jet ◽  
Low Level Jet ◽  
Relative Prevalence ◽  
East West

Abstract The frequency and intensity of the Great Plains nocturnal low-level jet (LLJ) are enhanced by baroclinicity over the sloped terrain of the region. A classical description of baroclinic-induced diurnal wind oscillations over the Great Plains considers differential heating of the slope with respect to air at the same elevation far removed from the slope, but with buoyancy constant along the slope (Holton mechanism). Baroclinicity can also occur due to differential heating of the slope itself, which creates a gradient in buoyancy along the slope. The relative prevalence of the two types of baroclinicity in this region has received scant attention in the literature. The present study uses 19 years of data from the Oklahoma Mesonet to evaluate the characteristics of along-slope buoyancy gradients over the region. A mean negative afternoon along-slope buoyancy gradient (east–west gradient) is found over Oklahoma. The sign of this afternoon buoyancy gradient is favorable for LLJ formation, as it results in the strongest southerly geostrophic wind near the ground around sunset, which is conducive to nocturnal jet formation via the inertial oscillation mechanism. The negative afternoon buoyancy gradient is at least partially created by an east–west gradient in diurnal heating and is stronger and more consistent in the summer months, which is when LLJs are most frequent. The contribution of the along-slope buoyancy gradient to the low-level geostrophic wind was found to be as important as the contribution of the Holton mechanism. Overall, these results indicate that along-slope buoyancy gradients should be accounted for in studies of LLJ dynamics over the Great Plains.

scholarly journals Regional Model Simulations of the Bodélé Low-Level Jet of Northern Chad during the Bodélé Dust Experiment (BoDEx 2005)

2008 ◽  
Vol 21 (5) ◽  
pp. 995-1012 ◽  
Author(s):  
Martin C. Todd ◽  
Richard Washington ◽  
Srivatsan Raghavan ◽  
Gil Lizcano ◽  
Peter Knippertz
Keyword(s):  
Diurnal Cycle ◽  
Climate Models ◽  
Mesoscale Model ◽  
Regional Climate ◽  
Dust Emission ◽  
Horizontal Resolution ◽  
Emission Model ◽  
Low Level ◽  
Low Level Jet ◽  
Key Features

Abstract The low-level jet (LLJ) over the Bodélé depression in northern Chad is a newly identified feature. Strong LLJ events are responsible for the emission of large quantities of mineral dust from the depression, the world’s largest single dust source, and its subsequent transport to West Africa, the tropical Atlantic, and beyond. Accurate simulation of this key dust-generating atmospheric feature is, therefore, an important requirement for dust models. The objectives of the present study are (i) to evaluate the ability of regional climate models (RCMs) and global analyses/reanalyses to represent this feature, and (ii) to determine the driving mechanisms of the LLJ and its strong diurnal cycle. Observational data obtained during the Bodélé Dust Experiment (BoDEx 2005) are utilized for comparison. When suitably configured, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) RCM can represent very accurately many of the key features of the jet including the structure, diurnal cycle, and day-to-day variability. Surface winds are also well reproduced, including the peak winds, which activate dust emission. Model fidelity is, however, strongly dependent on the boundary layer parameterization scheme, surface roughness, and vertical resolution in the lowest layers. A model horizontal resolution of a few tens of kilometers is sufficient to resolve most of the key features of the LLJ, while in global analyses/reanalyses many features of the LLJ are not adequately represented. Idealized RCM simulations indicate that under strong synoptic forcing the surrounding orography of the Tibesti and Ennedi Mountains acts to focus the LLJ onto the Bodélé and to accelerate the jet by ∼40%. From the RCM experiments it is diagnosed that the pronounced diurnal cycle of the Bodélé LLJ is largely a result of varying eddy viscosity, with elevated heating/cooling over the Tibesti Mountains to the north as a second-order contribution.

scholarly journals Observing and Simulating the Summertime Low-Level Jet in Central Iowa

2015 ◽  
Vol 143 (6) ◽  
pp. 2319-2336 ◽  
Author(s):  
Brian J. Vanderwende ◽  
Julie K. Lundquist ◽  
Michael E. Rhodes ◽  
Eugene S. Takle ◽  
Samantha L. Irvin
Keyword(s):  
Great Plains ◽  
Vertical Wind Shear ◽  
Ground Level ◽  
Turbine Rotor ◽  
Inertial Oscillation ◽  
Energy Forecasting ◽  
Site Assessment ◽  
Low Level ◽  
Low Level Jet ◽  
Wind Profiles

Abstract In the U.S. state of Iowa, the increase in wind power production has motivated interest into the impacts of low-level jets on turbine performance. In this study, two commercial lidar systems were used to sample wind profiles in August 2013. Jets were systematically detected and assigned an intensity rating from 0 (weak) to 3 (strong). Many similarities were found between observed jets and the well-studied Great Plains low-level jet in summer, including average jet heights between 300 and 500 m above ground level, a preference for southerly wind directions, and a nighttime bias for stronger jets. Strong vertical wind shear and veer were observed, as well as veering over time associated with the LLJs. Speed, shear, and veer increases extended into the turbine-rotor layer during intense jets. Ramp events, in which winds rapidly increase or decrease in the rotor layer, were also commonly observed during jet formation periods. The lidar data were also used to evaluate various configurations of the Weather Research and Forecasting Model. Jet occurrence exhibited a stronger dependence on the choice of initial and boundary condition data, while reproduction of the strongest jets was influenced more strongly by the choice of planetary boundary layer scheme. A decomposition of mean model winds suggested that the main forcing mechanism for observed jets was the inertial oscillation. These results have implications for wind energy forecasting and site assessment in the Midwest.

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