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1. Wave Disturbances and Their Role in the Maintenance, Structure, and Evolution of a Mesoscale Convection System

   https://doi.org/10.1175/JAS-D-18-0348.1  

   Zhang, Shushi; Parsons, David B.; Wang, Yuan (January 2020, Journal of the Atmospheric Sciences)

   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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2. The Complexity and Importance of Representing Atmospheric Bores in Numerical Weather Prediction Systems

   Parsons, David B; Zhang, Shushi (June 2025, American Meteorological Society)

   Geerts, Bart (Ed.)

   Atmospheric bores are a type of gravity waves generated when a density current moves into a stable layer. Early research on atmospheric bores often focused on the "morning glory" over Australia formed from the collision of two sea breezes. Subsequent studies of bores over North America, including seminal work by Steven Koch, found that bores often formed when cold pools generated by mesoscale convective systems moved into the stable nocturnal boundary layer containing a nocturnal low-level jet. Research has shown that these bores often have upward displacements of ~1 km in the lower troposphere creating favorable conditions for nocturnal convection over areas of 10,000 km^2. This lifting plays a role on generating and/or maintaining nocturnal convection. Recent work on bore over China has further revealed the complexity and variation in bore structure and the interaction with convective system. This talk will focus on recent research that has shown: i) Bores structure is complex as bores occur not only in the nocturnal boundary layer but also at within stable layers aloft in the middle troposphere; ii) ii) When stable boundary layers exist over the oceans, bores can persist over the sea into the daylight hours and help to sustain mesoscale convective systems over the sea. When the MCS associated convectIve outflows move onshore, these features play a role generating severe weather over land;. iii) Variations in the mesoscale environment can generate significant variations occur in the mechanisms that lead to the initiation of bores (e.g., cold pools, sea breezes, downdrafts), in bore structure and in the ability of bores to impact convection. iv) The presence of bores likely plays a significant role in the current challenge of accurately predicting nocturnal convection 

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3. The Development of Atmospheric Bores in Non-Uniform Baroclinic Environments and Their Roles in the Maintenance, Structure, and Evolution of an MCS

   Zhang, S; Parsons, D; Xu, X; Huang, H; Xu, F; Wu, T; Chen, G; Abulikemu, A; Zhao, Y; Zhang, S; et al (December 2023, Journal of geophysical research Atmospheres)

   Cheng, Y (Ed.)

   Abstract This study used radar observations and a high-resolution numerical simulation to explore the interactions between an mesoscale convective system (MCS), cold pool outflows, and atmospheric bores in a non-uniform baroclinic environment. The bores were generated by a nocturnal MCS that occurred on 2–3 June 2017 over the southern North China Plain. The goal of this investigation is to determine how the structure of bores varied within this non-uniform environment and whether and how the bores would maintain the MCS and alter its structure. To the southwest of the MCS, where there was large CAPE and a well-mixed boundary layer, discrete convection initiation occurred behind a single radar fine line (RFL) maintaining the propagation of the MCS. To the southeast of the MCS, multiple RFLs were found suggesting the generation of an undular bore in an environment containing an intense nocturnal stable boundary layer with dry upper layers and little CAPE. Hydraulic and nonlinear theory were applied to the simulation of the MCS revealing that the differences in the bore evolution depended on both the characteristics of the cold pool and the variations in the ambient environment. Thus, the characteristics of the ambient environment and the associated differences in bore structure impacted the maintenance and organization of the MCS. This study implies the importance of an accurate representation of the low-level ambient environment and the microphysics and kinematics within the MCS to accurately simulate and forecast cold pools, the generation and evolution of bores, and their impact on nocturnal MCSs. 

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4. The Heat-Flux Imbalance: The Role of Advection and Dispersive Fluxes on Heat Transport Over Thermally Heterogeneous Terrain

   https://doi.org/10.1007/s10546-021-00687-1  

   Morrison, Travis; Pardyjak, Eric R.; Mauder, Matthias; Calaf, Marc (February 2022, Boundary-Layer Meteorology)

   Data from the Idealized Planar-Array experiment for Quantifying Spatial heterogeneity are used to perform a control volume analysis (400 × 400 × × 2 m^3) on the total derivative of the temperature tendency equation. Analysis of the heat-flux imbalance, which is defined as the ratio of the sum of advective, dispersive, and turbulence-flux terms to the turbulence-flux term, are presented. Results are divided amongst free-convective and forced-convective days, as well as high-wind-speed and quiescent nocturnal periods. Findings show that the median flux imbalance is greater on forced-convective days (a 168% turbulence-flux overestimation, or relative importance of the advection to dispersive flux to the turbulence flux) when compared to free-convective periods (79% turbulence-flux overestimation). During nocturnal periods, a median turbulence-flux underestimation of 146% exists for quiescent nights and a 43% underestimation of the flux for high-wind-speed nights. These results support the existing literature, suggesting that mean air-temperature heterogeneities lead to strong bulk advection and dispersive fluxes. A discussion of the impact of the flux imbalance on the surface energy balance and numerical-weather-prediction modelling is presented. 

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5. Environmental controls on simulated deep moist convection initiation occurring during RELAMPAGO-CACTI

   https://doi.org/10.1175/JAS-D-21-0226.1  

   Nelson, T. Connor; Marquis, James; Peters, John M.; Friedrich, Katja (March 2022, Journal of the Atmospheric Sciences)

   Abstract This study synthesizes the results of 13 high-resolution simulations of deep convective updrafts forming over idealized terrain using environments observed during the RELAMPAGO and CACTI field projects. Using composite soundings from multiple observed cases, and variations upon them, we explore the sensitivity of updraft properties (e.g., size, buoyancy, and vertical pressure gradient forces) to influences of environmental relative humidity, wind shear, and mesoscale orographic forcing that support or suppress deep convection initiation (CI). Emphasis is placed on differentiating physical processes affecting the development of updrafts (e.g., entrainment-driven dilution of updrafts) in environments typifying observed successful and null (i.e., no CI despite affirmative operational forecasts) CI events. Thermally-induced mesoscale orographic lift favors the production of deep updrafts originating from ∼1–2-km-wide boundary layer thermals. Simulations without terrain forcing required much larger ( ∼5-km-wide) thermals to yield precipitating convection. CI outcome was quite sensitive to environmental relative humidity; updrafts with increased buoyancy, depth, and intensity thrived in otherwise inhospitable environments by simply increasing the free tropospheric relative humidity. This implicates the entrainment of free-tropospheric air into updrafts as a prominent governor of CI, consistent with previous studies. Sensitivity of CI to the environmental wind is manifested by: 1) low-level flow affecting the strength and depth of mesoscale convergence along the terrain, and 2) clouds encountering updraft-suppressing pressure gradient forces while interacting with vertical wind shear in the free-troposphere. Among the ensemble of thermals occurring in each simulation, the widest deep updrafts in each simulation were the most sensitive to environmental influences. 

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