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Shushi Zhang, David B. Parsons, and Yuan Wang

Abstract

This study investigates a nocturnal mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) field campaign. A series of wavelike features were observed ahead of this MCS with extensive convective initiation (CI) taking place in the wake of one of these disturbances. Simulations with the WRF-ARW Model were utilized to understand the dynamics of these disturbances and their impact on the MCS. In these simulations, an “elevated bore” formed within an inversion layer aloft in response to the layer being lifted by air flowing up and over the cold pool. As the bore propagated ahead of the MCS, the lifting created an environment more conducive to deep convection allowing the MCS to discretely propagate due to CI in the bore’s wake. The Scorer parameter was somewhat favorable for trapping of this wave energy, although aspects of the environment evolved to be consistent with the expectations for an n = 2 mode deep tropospheric gravity wave. A bore within an inversion layer aloft is reminiscent of disturbances predicted by two-layer hydraulic theory, contrasting with recent studies that suggest bores are frequently initiated by the interaction between the flow within stable nocturnal boundary layer and convectively generated cold pools. Idealized simulations that expand upon this two-layer approach with orography and a well-mixed layer below the inversion suggest that elevated bores provide a possible mechanism for daytime squall lines to remove the capping inversion often found over the Great Plains, particularly in synoptically disturbed environments where vertical shear could create a favorable trapping of wave energy.

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David B. Parsons, Kevin R. Haghi, Kelton T. Halbert, Blake Elmer, and Junhong Wang

Abstract

This investigation explores the relationship among bores, gravity waves, and convection within the nocturnal environment through the utilization of measurements taken during the International H2O Project (IHOP_2002) over the Southern Great Plains. The most favorable conditions for deep convection were found to occur within the boundary layer during the late afternoon and early evening hours in association with the diurnal cycle of solar insolation. At night, the layers most favorable for deep convection occur at and above the height of the nocturnal southerly low-level jet in association with distinct maxima in both the southerly and westerly components of the wind. Observations taken during the passage of 13 nocturnal wave disturbances over a comprehensive profiling site show the average maximum and net upward displacements with these waves were estimated to be ~900 and ~660 m, respectively. The lifting was not limited to the stable boundary layer, but reached into the conditionally unstable layers aloft. Since the net upward displacements persisted for many hours as the disturbances propagated away from the convection, areas well in excess of 10 000 km2 are likely impacted by this ascent. This lifting can directly maintain existing convection and aid in the initiation of new convection by reducing the convective inhibition in the vicinity of the active convection. In agreement with past studies, strong ascent in the lowest ~1.5 km was generally consistent with the passage of a bore. However, separate wave responses also occurred well above the bores, and low-frequency gravity waves may explain such disturbances.

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Alan Shapiro, Evgeni Fedorovich, and Joshua G. Gebauer

Abstract

A theory for gentle but persistent mesoscale ascent in the lower troposphere is developed in which the vertical motion arises as an inertia–gravity wave response to the sudden decrease of turbulent mixing in a horizontally heterogeneous convective boundary layer (CBL). The zone of ascent is centered on the local maximum of a laterally varying buoyancy field (warm tongue in the CBL). The shutdown also triggers a Blackadar-type inertial oscillation and associated low-level jet (LLJ). These nocturnal motions are studied analytically using the linearized two-dimensional Boussinesq equations of motion, thermal energy, and mass conservation for an inviscid stably stratified fluid, with the initial state described by a zero-order jump model of a CBL. The vertical velocity revealed by the analytical solution increases with the amplitude of the buoyancy variation, CBL depth, and wavenumber of the buoyancy variation (larger vertical velocity for smaller-scale variations). Stable stratification in the free atmosphere has a lid effect, with a larger buoyancy frequency associated with a smaller vertical velocity. For the parameter values typical of the southern Great Plains warm season, the peak vertical velocity is ~3–10 cm s−1, with parcels rising ~0.3–1 km over the ~6–8-h duration of the ascent phase. Data from the 2015 Plains Elevated Convection at Night (PECAN) field project were used as a qualitative check on the hypothesis that the same mechanism that triggers nocturnal LLJs from CBLs can induce gentle but persistent ascent in the presence of a warm tongue.

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Thomas R. Parish

Abstract

The 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.

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Evgeni Fedorovich, Jeremy A. Gibbs, and Alan Shapiro

Abstract

Nocturnal low-level jets (LLJs) over gently sloping terrain typical of the U.S. Great Plains are investigated by means of direct numerical simulation. Such LLJs develop in a tilted atmospheric boundary layer as a result of inertia–gravity oscillations initiated by a change of the surface thermal forcing during the evening transition. External parameters are the free-atmospheric geostrophic wind, ambient atmospheric stratification, surface buoyancy forcing, and slope angle. The governing momentum and buoyancy balance equations are written in slope-following coordinates, and solved numerically in the Boussinesq approximation. The surface forcing is prescribed in a form of surface buoyancy or buoyancy flux, both of which are slope-uniform but change in time. LLJs over slopes are contrasted with LLJs over flat terrain.

Slope-induced effects essentially modify the entire structure of nocturnal LLJs. The shape of the LLJ wind profile over a slope is characterized by a sharper and larger-magnitude maximum. The presence of the slope causes the along-slope advection of environmental potential temperature during the night. This advection can reignite static instability in the LLJ flow developing after the evening transition. The resulting turbulence leads to a complete or partial remix of the boundary layer flow and drastically changes the appearance of the LLJ in terms of its shape and vertical position. A pronounced nighttime jet can also develop from the daytime convective boundary layer in the absence of any free-atmospheric geostrophic forcing. The daytime flow preconditioning, an important precursor of the nocturnal LLJ development, plays an especially important role in LLJs over a slope.

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