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Abstract Over the Texas-Louisiana Shelf in the Northern Gulf of Mexico, the eutrophic, fresh Mississippi/Atchafalaya river plume isolates saltier waters below, supporting the formation of bottom hypoxia in summer. The plume also generates strong density fronts, features of the circulation that are known pathways for the exchange of water between the ocean surface and the deep. Using high-resolution ocean observations and numerical simulations, we demonstrate how the summer land-sea breeze generates rapid vertical exchange at the plume fronts. We show that the interaction between the land-sea breeze and the fronts leads to convergence/divergence in the surface mixed layer, which further facilitates a slantwise circulation that subducts surface water along isopycnals into the interior and upwells bottom waters to the surface. This process causes significant vertical displacements of water parcels and creates a ventilation pathway for the bottom water in the northern Gulf. The ventilation of bottom water can bypass the stratification barrier associated with the Mississippi/Atchafalaya river plume and might impact the dynamics of the region’s dead zone.
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Environmental Characteristics Supporting Warm-Season Coastal Convection Initiation near Houston, Texas
Abstract Convection initiation (CI) remains a formidable forecasting challenge, particularly along the coast. Examining Houston, Texas, radar observations of isolated cells’ CI spatiotemporal patterns for Junes from 2017 to 2022 revealed three patterns: cells forming exclusively over land (LAND), over coastal waters (GULF), or domainwide, initiating first over land (DW-L) or the water (DW-G). CI and dissipation times varied by regime. LAND events tended to typify the diurnal cycle, whereas GULF events tended to initiate overnight; both had durations < 10 h. In contrast, DW events began overnight and lasted until evening, with durations often exceeding 10–15 h. Synoptic-scale composites for each regime revealed only minimal forcing for ascent, suggesting the local environment’s importance for CI. Composite vertical profiles for CI locations revealed surface-based CAPE > 1500 J kg−1and CIN > −40 J kg−1for each regime. LAND had the hottest and driest lowest 1 km AGL, but was moistest between 1 and 2 km, suggesting LAND parcels originating below 1 km may be susceptible to entrainment and require moister midlevels for successful CI. We also found conditional instability below 1 km AGL for all regimes but a stable layer for GULF and neutral layers for LAND and DW-G between 1 and 2 km. This indicates saturation of air parcels within this layer is insufficient for CI, and mechanical lifting (e.g., sea breeze) would be necessary for CI. Indeed, all regimes featured potential instability throughout the lowest 4 km. However, only the LAND regime had a coastal density gradient conducive to sea-breeze formation; this indicates other lifting mechanisms may be important in the other regimes. Significance StatementForecasting the timing and location of storm formation is a major challenge, particularly in coastal areas. We endeavor to understand storm formation patterns in the Houston, Texas, area, with the main goal of better understanding how precursor atmospheric conditions may favor or disfavor such storm formation. We find four spatial patterns of storm formation: only over the land, only over coastal gulf waters, or over both land and gulf (but starting over land or the gulf). Average large-scale and local conditions were similar for each regime, with only subtle differences in their low-level temperature and humidity profiles. Results suggest that small-scale features like sea breezes thus are required for initiation, but only the LAND regime has sea-breeze-favorable conditions.
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- Award ID(s):
- 2020000
- PAR ID:
- 10678662
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Weather and Forecasting
- Volume:
- 40
- Issue:
- 6
- ISSN:
- 0882-8156
- Page Range / eLocation ID:
- 937 to 958
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Data obtained during the NSF-funded Experiment of Sea Breeze Convection, Aerosols, Precipitation, and Environment (ESCAPE) field campaign and conventional observations are used to evaluate the physical mechanisms responsible for convection initiation (CI) over coastal Texas during the 2 June 2022 intensive observational period. Failed CI attempts first occurred about 1300 UTC (8 am local time, LT) along a decaying land breeze located parallel to and offshore of the Texas coastline. Subsequent successful CI, which occurred around 1430 UTC (9:30 am LT), required the assistance of gravity waves that formed as an upstream MCS cold pool perturbed the stable lower troposphere. Upon the interaction between the leading radar-detected gravity wave and the offshore remnants of the land breeze, convective cells developed along the length of the offshore boundary. Outflow from this line of cells moved inland and initiated new convective cells resulting in widespread convection over the coastal zone that continued past 2000 UTC (3:00 pm LT). Results from this work illustrate the complex chain of mechanisms that can lead to CI over coastal regions, and emphasizes the challenges encountered by forecasters in predicting coastal CI.more » « less
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McTaggart-Cowan, R (Ed.)Abstract This study creates a composite sounding for nocturnal convection initiation (CI) events under weakly forced conditions and utilizes an idealized numerical simulation to assess the impact of atmospheric bores on these environments. Thirteen soundings were used to create this composite sounding. Common conditions associated with these weakly forced environments include a nocturnal low-level jet and a Brunt–Väisälä frequency of 0.011 s−1above 900 hPa. The median lift needed for parcels to realize any convective instability is 490 m, the median convective available potential energy of these convectively unstable parcels is 992 J kg−1, and the median initial pressure of these parcels is 800 hPa. An idealized numerical simulation was utilized to examine the potential influence of bores on CI in an environment based on composite sounding. The characteristics of the simulated bore were representative of observed bores. The vertical velocities associated with this simulated bore were between 1 and 2 m s−1, and the net upward displacement of parcels was between 400 and 650 m. The vertical displacement of air parcels has two notable phases: lift by the bore itself and smaller-scale lift that occurs 100–150 km ahead of the bore passage. The prebore lift is between 50 and 200 m and appears to be related to low-frequency waves ahead of the bores. The lift with these waves was maximized in the low to midtroposphere between 1 and 4 km AGL, and this lift may play a role in assisting CI in these otherwise weakly forced environments.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1175/WAF-D-24-0104.1
