Mesoscale Ascent in Nocturnal Low-Level Jets

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.

Nocturnal Convection Initiation during PECAN 2015

“…6, a veering, horizontally heterogeneous LLJ results in an elevated convergence region along its eastern edge. The veering of the LLJ also results in differential moisture advection across its axis, which, especially when combined with various mechanisms of mesoscale ascent (e.g., Trier et al 2017;Shapiro et al 2018), can lead to significant increases in relative humidity at the top eastern edge of the LLJ core (Gebauer et al 2018). The effect of persistent mesoscale ascent manifests as weak upward displacements of lower-tropospheric isentropes near the location of elevated NCI (Fig.…”

Linking Small-Scale Flight Manoeuvers and Density Profiles to the Vertical Movement of Insects in the Nocturnal Stable Boundary Layer

“…A,B). These weak but persistent nocturnal ascents might be caused by the same circumstances that result in the frequent low-level jet over Oklahoma 19 . Application of our methodology 10 to the test cases is straightforward, but because of the weak ascent, results in a complicated set of governing equations which appear to be analytically intractable.…”

“…18 for a comprehensive review on wave-turbulence interactions relevant to the NBL). Other vertical motions in the NBL may result from the combination of the shutdown of turbulent mixing at sunset occurring over a laterally-varying buoyancy field, which can produce weak but persistent ascent of magnitude 3-10 cm s −1 alongside a strong nocturnal Blackadar-type low-level jet horizontal wind speed profile 19 .…”

An Overview of Low‐Level Jet Winds and Corresponding Mixed Layer Depths During PECAN

“…While in‐depth case studies of PECAN LLJs influencing CI have been published (Gebauer et al, ; Reif & Bluestein, ; Shapiro et al, ; Trier et al, ), we present here jet coincidence with PNCI for the entire PECAN campaign. This is not meant to be an exhaustive study, but rather a brief opportunistic presentation of a known important jet impact, made available by the preceding work of categorizing the jets.…”

“…Fedorovich et al () used numerical modeling and sounding profiles to discuss LLJ impacts on turbulence and advection of environmental potential temperature and how these feed back to affect LLJ structure, also showing that weak LLJs can form on the sloped terrain of the Great Plains in the absence of the geostrophic wind that is fundamental to IO theory. PECAN data have also been used to investigate LLJ influence on nocturnal CI, both related to surface fronts (Trier et al, ) and more pristine environments (Gebauer et al, ; Shapiro et al, ). Hitchcock et al () performed an in‐depth study of all PECAN mesoscale convective system (MCS) environments, highlighting the LLJ's contribution to MCS development.…”