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Travis Griggs
,
James Flynn
,
Yuxuan Wang
,
Sergio Alvarez
,
Michael Comas
, and
Paul Walter

Abstract

Photochemical modeling outputs showing high ozone concentrations over the Gulf of Mexico and Galveston Bay during ozone episodes in the Houston–Galveston–Brazoria (HGB) region have not been previously verified using in situ observations. Such data were collected systematically, for the first time, from July to October 2021 from three boats deployed for the Galveston Offshore Ozone Observations (GO3) and Tracking Aerosol Convection Interactions Experiment—Air Quality (TRACER-AQ) field campaigns. A pontoon boat and a commercial vessel operated in Galveston Bay, while another commercial vessel operated in the Gulf of Mexico offshore of Galveston. All three boats had continuously operating sampling systems that included ozone analyzers and weather stations, and the two boats operating in Galveston Bay had a ceilometer. The sampling systems operated autonomously on the two commercial boats as they traveled their daily routes. Thirty-seven ozonesondes were launched over water on forecast high ozone days in Galveston Bay and the Gulf of Mexico. During the campaigns, multiple periods of ozone exceeding 100 ppbv were observed over water in Galveston Bay and the Gulf of Mexico. These events included previously identified conditions for high ozone events in the HGB region, such as the bay/sea-breeze recirculation and postfrontal environments, as well as a localized coastal high ozone event after the passing of a tropical system (Hurricane Nicholas) that was not well forecast.

Open access
Morgan E O’Neill
and
Daniel R. Chavas
Open access
Teryn J. Mueller
,
Christina M. Patricola
, and
Emily Bercos-Hickey

Abstract

The El Niño–Southern Oscillation (ENSO) influences seasonal Atlantic tropical cyclone (TC) activity by impacting environmental conditions important for TC genesis. However, the influence of future climate change on the teleconnection between ENSO and Atlantic TCs is uncertain, as climate change is expected to impact both ENSO and the mean climate state. We used the Weather Research and Forecasting model on a tropical channel domain to simulate 5-member ensembles of Atlantic TC seasons in historical and future climates under different ENSO conditions. Experiments were forced with idealized sea-surface temperature configurations based on the Community Earth System Model (CESM) Large Ensemble representing: a monthly-varying climatology, Eastern Pacific El Niño, Central Pacific El Niño, and La Niña. The historical simulations produced fewer Atlantic TCs during Eastern Pacific El Niño compared to Central Pacific El Niño, consistent with observations and other modeling studies. For each ENSO state, the future simulations produced a similar teleconnection with Atlantic TCs as in the historical simulations. Specifically, La Niña continues to enhance Atlantic TC activity, and El Niño continues to suppress Atlantic TCs, with greater suppression during Eastern Pacific El Niño compared to Central Pacific El Niño. In addition, we found a decrease in Atlantic TC frequency in the future relative to historical regardless of ENSO state, which was associated with a future increase in northern tropical Atlantic vertical wind shear and a future decrease in the zonal tropical Pacific SST gradient, corresponding to a more El Niño-like mean climate state. Our results indicate that ENSO will remain useful for seasonal Atlantic TC prediction in the future.

Restricted access
Jingjie Yu
,
Bolan Gan
,
Haiyuan Yang
,
Zhaohui Chen
,
Lixiao Xu
, and
Lixin Wu

Abstract

Subtropical mode water (STMW) is a thick layer of water mass characterized by homogeneous properties within the main pycnocline, important for oceanic oxygen utilization, carbon sequestration, and climate regulation. North Pacific STMW is formed in the Kuroshio Extension region, where vigorous mesoscale eddies strongly interact with the atmosphere. However, it remains unknown how such mesoscale ocean-atmosphere (MOA) coupling affects the STMW formation. By conducting twin simulations with an eddy-resolving global climate model, we find that approximately 25% more STMW is formed with the MOA coupling than without it. This is attributable to a significant increase in ocean latent heat release primarily driven by higher wind speed over the STMW formation region, which is associated with the southward deflection of storm tracks in response to oceanic mesoscale imprints. Such enhanced surface latent heat loss overwhelms the stronger upper-ocean restratification induced by vertical eddy and turbulent heat transport, leading to the formation of colder and denser STMW in the presence of MOA coupling. Further investigation of a multi-model and multi-resolution ensemble of global coupled models reveals that the agreement between the STMW simulation in eddy-present/rich coupled models and observations is superior to that of eddy-free ones, likely due to more realistic representation of MOA coupling. However, the ocean-alone model simulations show significant limitations in improving STMW production, even with refined model resolution. This indicates the importance of incorporating the MOA coupling into Earth system models to alleviate biases in STMW and associated climatic and biogeochemical impacts.

Restricted access
Jannick Fischer
,
Johannes M. L. Dahl
,
Brice E. Coffer
,
Jana Lesak Houser
,
Paul M. Markowski
,
Matthew D. Parker
,
Christopher C. Weiss
, and
Alex Schueth

Abstract

Over the last decade, supercell simulations and observations with ever increasing resolution have provided new insights into the vortex-scale processes of tornado formation. This article incorporates these and other recent findings into the existing three-step model by adding an additional fourth stage. The goal is to provide an updated and clear picture of the physical processes occurring during tornadogenesis. Specifically, we emphasize the importance of the low-level wind shear and mesocyclone for tornado potential, the organization and interaction of relatively small-scale pre-tornadic vertical vorticity maxima, and the transition to a tornado-characteristic flow. Based on these insights, guiding research questions are formulated for the decade ahead.

Open access
Christopher J. Schultz
,
Phillip M. Bitzer
,
Michael Antia
,
Jonathan L. Case
, and
Christopher R. Hain

Abstract

Twenty-six years of lightning data were paired with over 68 000 lightning-initiated wildfire (LIW) reports to understand lightning flash characteristics responsible for ignition in between 1995 and 2020. Results indicate that 92% of LIW were started by negative cloud-to-ground (CG) lightning flashes and 57% were single stroke flashes. Moreover, 62% of LIW reports did not have a positive CG within 10 km of the start location, contrary to the science literature’s suggestion that positive CG flashes are a dominant fire-starting mechanism. Nearly ⅓ of wildfire events were holdovers, meaning 1 or more days elapsed between lightning occurrence and fire report. However, fires that were reported less than a day after lightning occurrence statistically burned more acreage. Peak current was not found to be a statistically significant delineator between fire starters and non–fire starters for negative CGs but was for positive CGs. Results highlighted the need for reassessing the role of positive CG lightning and subsequently long-continuing current in wildfire ignition started by lightning. One potential outcome of this study’s results is the development of real-time tools to identify ignition potential during lightning events to aid in fire mitigation efforts.

Restricted access
Pieter B. Smit
,
Galen Egan
, and
Isabel A Houghton

Abstract

Peak periods estimated from finite resolution frequency spectra are necessarily discrete. For wind generated surface gravity waves, conflicting considerations of robust (quasi)-stationary statistics, and high spectral resolution, combined with the inverse relation between frequency and period, this typically implies that swell periods (above 10 s) are resolved at best at 𝒪(1) s intervals. Here we consider a method to improve peak period estimates for finite resolution spectra. Specifically, we propose to define the peak period based on continuous spectra derived from a spline-based interpolation of the discretely sampled monotone cumulative distribution function. The method may directly be applied to existing discrete spectra—the original time-domain data (which may not be available) are not required. We compare reconstructed spectra and derived peak periods to parametric shapes and field data. Peak estimates are markedly improved, allowing for better tracking of e.g., swells. The proposed method also marginally improves spectral levels and shape for a given discretely sampled estimate.

Restricted access
Eun-Pa Lim
,
Harry H. Hendon
,
Amy H. Butler
,
David W. J. Thompson
,
Zachary D. Lawrence
,
Adam A. Scaife
,
Theodore G. Shepherd
,
Inna Polichtchouk
,
Hisashi Nakamura
,
Chiaki Kobayashi
,
Ruth Comer
,
Lawrence Coy
,
Andrew Dowdy
,
Rene D. Garreaud
,
Paul A. Newman
, and
Guomin Wang
Open access
Matthew Patterson
,
Christopher O’Reilly
,
Jon Robson
, and
Tim Woollings

Abstract

The coupled nature of the ocean-atmosphere system frequently makes understanding the direction of causality difficult in ocean-atmosphere interactions. This study presents a method to decompose turbulent surface heat fluxes into a component which is directly forced by atmospheric circulation, and a residual which is assumed to be primarily ‘ocean-forced’. This method is applied to the North Atlantic in a 500-year pre-industrial control run using the Met Office’s HadGEM3-GC3.1-MM model. The method shows that atmospheric circulation dominates interannual to decadal heat flux variability in the Labrador Sea, in contrast to the Gulf Stream where the Ocean primarily drives the variability. An empirical orthogonal function analysis identifies several residual heat flux modes associated with variations in ocean circulation. The first of these modes is characterised by the ocean warming the atmosphere along the Gulf Stream and North Atlantic Current and the second by a dipole of cooling in the western subtropical North Atlantic and warming in the sub-polar North Atlantic. Lead-lag regression analysis suggests that atmospheric circulation anomalies in prior years partly drive the ocean heat flux modes, however there is no significant atmospheric circulation response in years following the peaks of the modes. Overall, the heat flux dynamical decomposition method provides a useful way to separate the effects of the ocean and atmosphere on heat flux and could be applied to other ocean basins and to either models or reanalysis datasets.

Open access
Ning Yang
,
Debin Su
,
Luyao Sun
, and
Tao Wang

Abstract

Atmospheric ducting is a highly refractive propagation condition that frequently occurs at sea and significantly impacts radar and communication equipment. This paper analyzes the spatiotemporal distribution of lower atmospheric ducts (LAD) in the South China Sea (SCS) and the variation of their occurrence rate with the monsoon by using reanalysis data from the ECMWF from 1980 to 2022. Additionally, the study discusses the relationship between ducting occurrences and atmospheric and oceanic conditions. The results indicate that wind dynamics in the SCS significantly impact ducting incidents. During the high-incidence period of LAD, humidity-gradient-constructed ducts are the primary mechanism. Before the onset of the monsoon, the mountains in the western part of Luzon Island obstruct the easterly wind, resulting in high temperatures and strong evaporation along the western coast of the mountains. Meanwhile, low temperatures and humidity prevail in the eastern part of the mountains, and they lead to a stratified atmosphere characterized by dry and cold upper layers and warm and humid lower layers in the western part of Luzon Island, which causes a distinct decrease in humidity with height. After the onset of the monsoon, the air from the Indochina Peninsula to the ocean is dry and cold, but the high-altitude area blocks it. This weakens the horizontal mobility of the low-level humid atmosphere over the sea, resulting in atmospheric stratification in the eastern coastal area of the Indochina Peninsula. This stratification leads to dry and cold upper layers and warm and humid lower layers.

Significance Statement

Atmospheric ducting is a superrefractive propagation condition that frequently occurs at sea and has a significant impact on radar and communication equipment and is related to large-scale or medium- and small-scale atmospheric stratification. The distribution of land and sea around the South China Sea (SCS) and the monsoon are important factors affecting the existence of atmospheric ducts in this region. Many scholars have studied the mechanism of atmospheric ducts in local areas based on observation data (or reanalysis data). The literature on the atmospheric ducts in the SCS mainly focuses on the spatial and temporal statistical distribution of seasons, months, and days, and gives the spatial and temporal distribution characteristics of the region within the statistical time, emphasizing the important influence of the monsoon on the duct, but there is no relevant research on the reasons for the existence of the specific relationship and its temporal and spatial distribution characteristics. The manuscript analyzes the temporal and spatial distribution of lower atmospheric ducts in the SCS and the variation of their occurrence rate with the monsoon, quantifies the contributions of temperature, humidity, and air pressure to the ducting occurrence, meanwhile discussing the ducting occurrence relation with atmospheric and oceanic conditions. In the end, we demonstrate that the development of high-incidence areas for SCS ducts prior to and following the onset of monsoon season is connected to factors such as wind patterns, seawater evaporation, and topography. Furthermore, unstable vertical transport of water vapor in both the atmosphere and oceanic conditions plays a crucial role in facilitating the creation of humidity-type ducts.

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