Jet Streak Research Articles

An Extreme Predecessor Rain Event in Central China Amplified by Upper-Level Jet Streak

Abstract A comprehensive study is conducted on a predecessor rain event (PRE) closely associated with the upper-level jet (ULJ) in central China during July 2021, utilizing the Weather Research and Forecasting Model (WRF). The ULJ and precipitation patterns are reasonably reproduced in the control experiment. In the sensitivity experiment (NOULJ), the ULJ is largely suppressed by the piecewise potential vorticity inversion method. The comparison of the two simulations indicates that the ULJ promotes the development of secondary circulation, lower-level jet (LLJ), moisture transport, and atmospheric instability. Synergistic effects among these processes amplify this PRE. The strong updrafts in the precipitation region are closely associated with the inverse secondary circulation that develops on the south side of ULJ entrance. The upper-level mass divergence associated with the ULJ enhances the lower-level convergence, promoting the development of lower-level vorticity and LLJ. The LLJ facilitates moisture transport into the precipitation region and increasing atmospheric instability. In contrast, the precipitation in NOULJ is decreased by 34% due to the weakening of favorable conditions.

An extreme precipitation event over Dronning Maud Land, East Antarctica - A case study of an atmospheric river event using the Polar WRF Model

Extreme precipitation events (EPEs) are crucial in Antarctica, impacting the Antarctic ice sheet's surface mass balance and stability. Comprehensive case studies are essential for better understanding these events and the underlying processes driving them. Here, we investigate an extreme snowfall event in Dronning Maud Land (DML), East Antarctica on November 8 and 9, 2015. This event contributed approximately 22 % of the annual accumulation in less than two days and exhibited high spatial variability in precipitation distribution. We employed a high-resolution atmospheric model specifically optimized for the polar regions (Polar WRF) and ERA5 reanalysis data to analyze the event in detail. Our findings highlight the importance of a blocking high-pressure ridge of record strength that effectively blocked and diverted a strong extratropical cyclone into DML, ultimately leading to the heavy snowfall event. The sudden deepening of the cyclone was initiated by a ‘jet streak’ in the upper atmosphere that steered the system southeastwards towards the Antarctic coast. Notably, we observed an anomalously high poleward moisture transport in the form of a strong atmospheric river on November 7, 2015. This atmospheric river originated in the South Atlantic Ocean and tracked poleward from the 30°S-40°S latitude band. Vertical cross-sections of the model outputs indicate that most of the precipitation was concentrated in regions with steep orography along the path of the atmospheric river. This interaction between the atmospheric river and the steep terrain led to the uplift of maritime air, resulting in heavy snowfall. This study highlights the significance of extreme upper and lower atmospheric conditions in driving intense moisture transport towards coastal DML. The interaction between the atmospheric river and the steep orography contributed to heavy snowfall, underscoring the importance of considering orographic influences in understanding EPEs in Antarctica.

Impact of WindBorne Observation Assimilation on Prediction of a TPV Merger Case From THINICE

AbstractThe impact of assimilating in situ temperature, moisture and wind observations from a WindBorne Systems balloon, with long duration and adjustable ballast, is evaluated using a case study from the 2022 THINICE field campaign. A case is selected wherein the WindBorne balloon directly sampled a jet streak associated with a tropopause polar vortex (TPV). The observed TPV merged with another TPV at the same time as a downstream Arctic cyclone (AC) redeveloped eastward. The case is used to better understand the role of the observed TPV in the evolution of the downstream AC. The assimilation of the WindBorne observations improves the forecast track and amplitude of the TPV during the ∼1 day forecast period that the TPV can be tracked as a distinct feature. The root mean square error of temperature at 350 hPa is improved by the WindBorne assimilation throughout the 2.5 day forecast period. The surface cyclone track forecast is improved by the WindBorne assimilation during the period of eastward redevelopment of the surface cyclone, and the sea level pressure RMSE in the surrounding region is reduced during the first ∼1.5 days of forecast lead time. Results demonstrate the capability of the long‐duration controllable WindBorne balloons to improve the analysis of the TPV associated with the jet streak, leading to improved forecast of both tropopause‐level and surface pressure features. Additionally, the importance of observing the upstream TPV amplitude for improving forecasts of the process of TPV merging and its impact on the evolution and longevity of the mature AC is confirmed.

Resolution Dependence of Extreme Wind Speed Projections in the Great Lakes Region

Abstract The effect of anthropogenic climate change on extreme near-surface wind speeds is uncertain. Observed trends are weak and difficult to disentangle from internal variability, and model projections disagree on the sign and magnitude of trends. Standard coarse-resolution climate models represent the fine structures of relevant physical phenomena such as extratropical cyclones (ETCs), upper-level jet streaks, surface energy fluxes, and land surface variability less skillfully than their high-resolution counterparts. Here, we use simulations with the NCAR Community Earth System Model with both uniform (110 km) resolution and the variable-resolution configuration (VR-CESM-SONT, from 110 to 7 km) to study the effect of refined spatial resolution on projections of extreme strong and weak wind speeds in the Great Lakes region under end-of-century RCP8.5 forcing. The variable-resolution configuration projects strengthening of strong-wind events in the refined region with the opposite occurring in the uniform-resolution simulation. The two configurations provide consistent changes to synoptic-scale circulations associated with high-wind events. However, only the variable-resolution configuration projects weaker static stability, enhanced turbulent vertical mixing, and consequentially enhanced surface wind speeds because boundary layer dynamics are better captured in the refined region. Both models project increased frequency of extreme weak winds, though only VR-CESM-SONT resolves the cold-season inversions and summertime high temperatures associated with stagnant wind events. The identifiable mechanism of the changes to strong winds in VR-CESM-SONT provides confidence in its projections and demonstrates the value of enhanced spatial resolution for the study of extreme winds under climate change. Significance Statement In this study, we compare climate change projections of high and low extreme wind speeds in the Great Lakes region between a standard coarse-resolution simulation and a high-resolution simulation performed using the same climate model. The fine-resolution simulation projects strengthening high wind speeds, opposite to the coarse-resolution simulation. Both project increasing frequency of extreme weak winds, but the human-health-related impacts of stagnant winds are only captured at fine resolution. The changes in the coarse-resolution simulation are explained by changes to large-scale circulation, while the fine-resolution changes are linked to local processes the coarse model does not resolve. This helps explain the diverging projections of strong winds and gives greater credibility to the fine-resolution simulation.

A quasi-geostrophic analysis of summertime southern African linear-regime westerly waves

Linear-regime westerly waves that propagate across the South African domain are often linked to well-known rainfall producing systems such as tropical temperate troughs and synoptic scale tropical low-pressure systems, and ridging South Atlantic Ocean anticyclones at the surface. It is accepted that the baroclinic waves that propagate across the domain provide the lifting mechanism that causes the required vertical motion for rainfall to occur. This study shows that there exists a jet streak embedded in these waves that is located downstream of the trough axis, to the east of which vertically upward motion is expected to occur. The entrance of the jet streak passes just south of the country, as the waves propagate past the domain. The study further shows that for this class of waves, the vertical motion that causes rainfall to occur is induced by the thermally direct transverse ageostrophic circulation that is located at this jet entrance. This is instead of the conventional upper air divergence that is located at the inflection point east of the trough axis. Using a method of decomposing the Q-vector into its transverse (Qn\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${Q}_n$$\\end{document}) and shear (Qs\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${Q}_s$$\\end{document}) components, the divergence fields of which are used to decompose the vertical motion into the corresponding components, i.e ωn\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega _n$$\\end{document} and ωs\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega _s$$\\end{document}, respectively; it was shown that the vertical motion over South Africa is explained more by the former than the latter. Therefore, the uplift over the country and that located at the infection point east of the trough are dynamically distinct processes. Taking the limitations of the quasi-geostrophic framework into consideration, the study concludes that during the passage of linear-regime waves vertical motion that might lead to rainfall is caused by the circulation at the jet entrance and not the divergence in the baroclinic wave.

Role of Midlatitude Baroclinic Condition in Heavy Rainfall Events Directly Induced by Tropical Cyclones in South Korea

Abstract Recurving tropical cyclones (TCs) in the western North Pacific often cause heavy rainfall events (HREs) in East Asia. However, how their interactions with midlatitude flows alter the characteristics of HREs remains unclear. The present study examines the synoptic–dynamic characteristics of HREs directly resulting from TCs in South Korea with a focus on the role of midlatitude baroclinic condition. The HREs are categorized into two clusters based on midlatitude tropopause patterns: strongly (C1) and weakly (C2) baroclinic conditions. C1, which is common in late summer, is characterized by a well-defined trough–ridge couplet and jet streak at the tropopause. As TCs approach, the trough–ridge couplet amplifies, but is anchored by divergent TC outflow. This leads to phase locking of the upstream trough with TCs and thereby prompts substantial structural changes of TCs reminiscent of extratropical transition. The synergistic TC–midlatitude flow interactions allow for widely enhanced quasigeostrophic forcing for ascent to the north of the TC center. This allows HREs to occur even before TC landfall with more inland rainfall than C2 HREs. In contrast, C2, which is mainly observed in midsummer, does not accompany the undulating tropopause. In the absence of strong interactions with midlatitude flows, TCs rapidly dissipate after HREs while maintaining their tropical features. The upward motion is confined to the inherent TC convection, and thus HREs occur only when TCs are located in the vicinity of the country. These findings suggest that midlatitude baroclinic condition determines the spatial extent of TC rainfall and the timing of TC-induced HREs in South Korea. Significance Statement This study suggests that the midlatitude flows can substantially modulate heavy rainfall events directly caused by tropical cyclones. By analyzing the 42-yr tropical cyclone–induced heavy rainfall events in South Korea, it is found that tropical cyclones and midlatitude flows strongly interact with each other, especially when the midlatitude flows meander in conjunction with a strong jet stream. Their synergistic interactions result in a poleward expansion of the tropical cyclones’ precipitation shields, leading to heavy rainfall events even before they make landfall in the country. Consequently, it is advisable to carefully monitor the midlatitude conditions as well as tropical cyclones themselves as earlier heavy rainfall warnings may be necessary depending on the former.

Comparing Synoptic Conditions and Environmental Characteristics for Fronts with and without Prefrontal Convection Initiation and Heavy Rain over Coastal South China

Abstract To understand why convection initiation and heavy rain sometimes occur ahead of fronts over South China in the presummer rainy season but sometimes do not, a climatology of 137 fronts is constructed, in which 34% of the fronts exhibit no prefrontal convection initiation (NoPCI), 31% of the fronts exhibit prefrontal convection initiation (PCI), and 35% of the fronts exhibit prefrontal convection initiation and heavy rain (PCI+HR). An anticyclonically curved upper-level jet streak and midtropospheric QG forcing produce synoptic-scale descent for the prefrontal region in NoPCI events, whereas the right-entrance region of a straight upper-level jet streak and forcing for ascent dominate the prefrontal region in PCI and PCI+HR events. Whether prefrontal convection and heavy rain occur is also related to the character of low-level flows. NoPCI features anticyclonic southerly winds, with an environment having low dewpoint throughout the troposphere, unfavorable for convection initiation. However, synoptic circulation of PCI and PCI+HR events favors a broad prefrontal surface low, which determines the greater cyclonic character of airflows in PCI+HR events, in contrast with that of the PCI events. Convective available potential energy is useful in distinguishing NoPCI and PCI events, and the three events have statistically significant differences in precipitable water. Moreover, larger magnitudes of precipitable water and bulk wind shear in PCI+HR events are conducive for prefrontal convection to produce heavy rain compared to those of PCI events. These results indicate the importance of the upper-level forcing on the prefrontal convection initiation, and heavy rain is sensitive to the changes in prefrontal airflow and moisture. Significance Statement Convection and heavy rain sometimes occur a few hundred kilometers ahead of fronts in the warm air over South China in early summer. To understand atmospheric conditions favoring or inhibiting convection and heavy rain ahead of fronts, we examine 46 fronts without prefrontal convection, 43 fronts with prefrontal convection, and 48 fronts with prefrontal convection and heavy rain. These scenarios have similarities in environmental behaviors but different large-scale conditions that favor or inhibit ascent in the prefrontal area. Specifically, prefrontal heavy rain tends to occur in a very moist environment with a prefrontal surface low. These findings help researchers and operational forecasters better discriminate the subtle conditions that favor or inhibit prefrontal convection and heavy rain over South China.

The increased summer monsoon rainfall in Northwest India: Coupling with the Northwestern Arabian Sea warming and modulated by the Silk Road Pattern since 2000

This study investigates changes in rainfall patterns over Northwest India (NWI) and the warming of the Northwestern Arabian Sea (NWAS) since 2000. These changes are attributed to the Silk Road Pattern (SRP) phase change in the late 1990s. The SRP positive phase triggers geostrophic deformation over the Tibetan Plateau (TP) by inducing an anomalous circulation pattern along the westerly jet, at 200 hPa. This deformation along the westerly Jet leads to Jet streaks and thermally indirect ageostrophic circulation over TP. In addition, the intensified heating over the TP led to a westward shift of the South Asian High (SAH). Associated with this, the interaction of positive temperature advection and northward ageostrophic wind (Jet entrance) over south-central Asia generates a southward component of ageostrophic wind (Jet Exit), causing upper-level convergence at southeast TP, excites the sinking cold air thereby strengthen the surface pressure below, over TP. This amplifies the anomalous anticyclone effect over there resulting in strong easterly wind anomalies along the monsoon trough at 850 hPa, disrupting the monsoon circulation. These anomalies, combined with a poleward shifting moisture-laden Low-Level Jet (LLJ), contribute to the warming of the NWAS and increased rainfall over NWI. The NWAS has shown significant summer warming over the past two decades. The poleward shift of the LLJ and SRP-induced circulation changes also impact the ocean dynamics. Given the occurrence of severe floods in this region of the Indian subcontinent, these findings have broader implications for predicting and projecting monsoon variability and extremes.

Jet stream dynamics from a potential vorticity gradient perspective: The method and its application to a kilometre‐scale simulation

AbstractThe influence of adiabatic and diabatic processes on the midlatitude circulation is a formidable research question, especially considering their projected changes under global warming. This study presents the prospects, merits, and caveats of a potential vorticity (PV) gradient perspective as a means to disentangle the contributions of adiabatic and diabatic processes affecting the midlatitude circulation. Theoretical considerations reassess the link between the PV gradient and the jet stream. They reveal that the maximum isentropic PV gradient is consistently located on the stratospheric side of the jet, whereas the gradient of is shifted to the tropospheric side but, in general, is better aligned with the jet axis. The stratospheric shift of the PV gradient results from variations in stability across the tropopause, whereas the tropospheric shift of the gradient results from variations in vorticity. Regions of high PV gradient may serve as a proxy for the curvature of the wind field in the case of sufficiently small variations in stability. Otherwise, they depict variations in both wind and thermal stratification along tropopause‐intersecting isentropic surfaces. Lagrangian “PV gradient thinking” is demonstrated in two case studies of jet streak evolution in a simulation with 1.1 km grid spacing performed with the graphics‐processing‐unit‐enabled numerical weather prediction model Consortium for Small‐Scale Modelling featuring on‐line air parcel trajectories. Dry deformation drives the Lagrangian evolution of the PV gradient in the first case, whereas there is a pronounced influence of diabatic modification in the second case. The Lagrangian PV gradient perspective presented offers fresh insight into adiabatic and diabatic processes underlying the midlatitude circulation variability and change.

A Comparison of Stratospheric Gravity Waves in a High‐Resolution General Circulation Model With 3‐D Satellite Observations

AbstractAtmospheric gravity waves (GWs) play a key role in determining the thermodynamical structure of the Earth's middle atmosphere. Despite the small spatial and temporal scales of these waves, a few high‐top general circulation models (GCMs) that can resolve them explicitly have recently become available. This study compares global GW characteristics simulated in one such GCM, the Japanese Atmospheric GCM for Upper‐Atmosphere Research (JAGUAR), with those derived from three‐dimensional (3‐D) temperatures observed by the Atmospheric Infrared Sounder (AIRS) aboard NASA's Aqua satellite. The target period is from 15 December 2018 to 8 January 2019, including the onset of a major sudden stratospheric warming (SSW). The 3‐D Stockwell transform method is used for GW spectral analysis. The amplitudes and momentum fluxes of GWs in JAGUAR are generally in good quantitative agreement with those in the AIRS observations in both magnitude and distribution. As the SSW event progressed, the GW amplitudes and eastward momentum flux increased at low latitudes in the summer hemisphere in both the model and observation datasets. Case studies demonstrate that the model is able to reproduce comparable wave events to those in the AIRS observations with some differences, especially noticeable at low latitudes in the summer hemisphere. Through a comparison between the model results with and without the AIRS observational filter applied, it is suggested that the amplitudes of GWs near the entrance or exit of an eastward jet streak are underestimated in AIRS observations.

Synoptic‐scale and mesoscale controls for tornadogenesis on cold fronts: A tornadic cold front in amplifying northwesterly flow

AbstractThe structure and evolution of a cold front that produced a tornadic narrow cold‐frontal rainband (NCFR) over the UK on 20 November 2013 is explored using reanalysis data, high‐resolution model simulations and observations. Physical links are found to exist across a range of spatiotemporal scales, through which the evolving large‐scale flow field exerts an influence on the timing and location of miso‐scale vortex genesis, and therefore the potential for tornado genesis. The synoptic‐scale flow pattern exhibited amplification, consisting of upstream ridge building and downstream trough extension. A prominent jet streak and associated positive potential vorticity (PV) anomaly and tropopause fold moved rapidly southeastward on the rear flank of the extending upper‐level trough, the leading edge of these features eventually overspreading the surface cold front over the UK. Increasing 850 hPa frontogenesis occurred underneath the left exit of the jet streak and associated, intensifying, PV anomaly. A filament of dry, high‐PV air was extruded from the overlying tropopause fold within an intensifying front‐transverse circulation in this region. This dry filament eventually penetrated to low levels immediately behind the front, where it undercut the upper, rearward parts of the wide cold‐frontal precipitation band. Model fields and observations suggest that diabatic cooling, associated with sublimation of solid hydrometeors falling into the dry filament, led to the development of local downdraft and near‐surface divergence maxima and a prominent cold pool immediately behind the surface cold front over central England. Increases in horizontal convergence, updraft speed, and horizontal temperature gradients along the frontal boundary immediately ahead of the cold pool resulted in development of a locally well‐defined cold front and associated NCFR, where the surface front was formerly (and elsewhere continued to be) relatively weak. Tornadoes occurred in association with miso‐scale vortices that developed rapidly along an intensifying vertical vortex sheet at the NCFR.

Downstream development during ridging South Atlantic Ocean anticyclones

Ridging South Atlantic high pressure systems (ridging highs) are often accompanied by cut-off low (COL) pressure systems aloft, but may also occur without them, in which case a linear baroclinic wave would be observed propagating across the South African domain in the upper troposphere. Using 41 years of ERA-5 reanalysis data, this study documents differences between the characteristics of the prevailing dynamical processes and associated local eddy kinetic energy generation, its downstream transfer and dissipation during these two scenarios. The study shows that when COLs are present then baroclinic conversion is strong and it is confined east of the Greenwich Meridian, whereas it is located downstream of South Africa and it is much weaker, when ridging occurs without COLs. The differences in strength and locations of the baroclinic conversion are associated with the differing jet streak configurations between the two scenarios; which lead to Rossby wave breaking and the absence thereof when there are COLs and when ridging occurs without COLs, respectively. The presence of breaking during COLs leads to trans-ridge downstream development that facilitates energy transfer from the midlatitudes into the South African domain. When there are no COLs present, the trans-trough downstream development is stronger than it is across the upstream ridge. Barotropic conversion from eddy kinetic energy to mean kinetic energy occurs in the South African domain during COLs, but occurs much further downstream when there are no COLs during ridging highs. The difference in the characteristics identified in this study can be traced back to the differences in the potential vorticity anomaly structures, which are largely due to whether the waves break or do not during the evolution of ridging events in the South African domain.

Formation Mechanisms of the Mesoscale Environment Conducive to a Downslope Windstorm over the Cuyamaca Mountains Associated with Santa Ana Wind during the Cedar Fire (2003)

Abstract Numerical simulations were conducted to investigate the upstream environment’s impacts on the airflow over the lee slope of the Cuyamaca Mountains (CM) near San Diego, California, during the Cedar Fire that occurred from 25 to 29 October 2003. The upstream environment was largely controlled by a southwest–northeast-oriented upper-tropospheric jet streak that rotated around a positively tilted ridge within the polar jet stream. Three sequential dynamical processes were found to be responsible for modifying the mesoscale environment conducive to low-level momentum and dry air that sustained the Cedar Fire. First, the sinking motion associated with the indirect circulation of the jet streak’s exit region strengthened the midtropospheric flow over the southern Rockies and the lee slope of the Sawatch and San Juan Ranges, thus modestly affecting the airflow by enhancing the downslope wind over the CM. Second, consistent with the coupling process between the upper-level sinking motion, downward momentum transfer, and developing lower-layer mountain waves, a wave-induced critical level over the mountain produced wave breaking, which was characterized by a strong turbulent mixed region with a wind reversal on top of it. This critical level helped to produce severe downslope winds leading to the third stage: a hydraulic jump that subsequently enhanced the downstream extent of the strong winds conducive to the favorable lower-tropospheric environment for rapid fire spread. Consistent with these findings was the deep-layer resonance between the mountain surface and tropopause, which had a strong impact on strengthening the severe downslope winds over the lee slope of the CM accompanying the elevated strong easterly jet at low levels.

Why Do Some Post-Tropical Cyclones Impact Europe?

Abstract Post-tropical cyclones (PTCs) can bring high winds and extreme precipitation to Europe. Although the structure and intensity of observed Europe-impacting PTCs has been investigated in previous studies, a quantitative understanding of the factors important for PTCs to reach Europe has not been established. By tracking and identifying the full life cycle of tropical cyclones (TCs) in the ERA5 reanalysis, we investigate why some PTCs impact Europe and why others do not, using a composite analysis. We show that PTCs that impact Europe are typically ∼4–6 m s−1 stronger at their lifetime maximum intensity and throughout the extratropical transition process. They are also twice as likely to reintensify in the midlatitudes. During ET, the Europe-impacting PTCs interact more strongly with an upstream upper-level trough in a significantly more baroclinic environment. The Europe-impacting PTCs are steered on a more poleward trajectory across a midlatitude jet streak. It is during the crossing of the jet that these cyclones often undergo their reintensification. Using contingency table analysis, TC lifetime maximum intensity, and whether post-ET reintensification occurs are shown to be significantly associated with the odds that a PTC reaches Europe. This supports our composite analysis and further indicates that TC intensity and reintensification both modulate the likelihood that a PTC will impact Europe. Significance Statement Some post-tropical cyclones (PTCs) reach Europe, often associated with extreme precipitation and high winds. It is currently unclear what factors allow this to occur. In this study, we track cyclones in two reanalyses using a feature tracking scheme and identify the PTCs by matching (in space and time) reanalysis tracks with observed tracks. Using a composite analysis, we show that 1) tropical cyclones (TCs) that are more intense, and 2) TCs that reintensify after extratropical transition, are more likely to reach Europe. TCs that reintensify interact strongly with an upper-level upstream trough and cross a midlatitude jet streak. Reintensification occurs as the cyclones cross this jet streak.

Effects of Distant Organized Convection on Forecasts of Widespread Clear-Air Turbulence

Abstract Two cases of observed widespread moderate-or-greater (MOG) clear-air turbulence (CAT) in different synoptic patterns are investigated using a nested high-resolution NWP model. Both of these cases occurred in confluent entrance regions of upper-tropospheric–lower-stratospheric (UTLS) jet streaks, where large-scale anticyclonic outflow from distant organized moist convection strengthened the UTLS jet. Both the strength and vertical sharpness of the resulting jet influence the altitudes of MOG turbulence and the details of simulated turbulence onset mechanisms. In a strong and narrow UTLS jet downstream of a weak synoptic ridge, MOG turbulence arises from Kelvin–Helmholtz (KH) waves that overturn in opposite directions on the vertical flanks of the jet. In broader UTLS jets, MOG turbulence arising from KH waves was favored in strong vertical shear layers beneath the wind maximum, but was inhibited above it due to static stability increases near the tropopause. However, vertically propagating internal gravity waves initiated above KH wave breaking beneath the UTLS jet amplify within the lower stratosphere above the jet, constituting another possible source of turbulence. Turbulence onset mechanisms were often apparent in simulations with minimum horizontal grid spacings of Δx = 1 km. However, amplitudes of the associated grid-resolved vertical motions were unreliable when compared with simulations having minimum horizontal grid spacings of Δx = 1/3 km. In spite of this, turbulence forecasting systems driven by input from coarser-resolution operational NWP models are demonstrated to provide good diagnoses of this type of convectively influenced CAT when the NWP model accurately forecasts upstream convection. Significance Statement In this study we document the role of distant convection on observed widespread strong clear-air turbulence near upper-level jet streams in different weather patterns. Results from nested NWP model simulations, together with similar examples from previous case studies, suggest a strong association of widespread turbulence with jet stream enhancements related to outflow from upstream convection. We also demonstrate how model horizontal grid spacings of <1 km are required to adequately resolve common turbulence onset mechanisms (e.g., Kelvin–Helmholtz instability, gravity wave breaking). Nevertheless, examples are provided showing that turbulence forecasting systems driven by lower-resolution operational NWP models can provide potentially valuable guidance on these widespread turbulence events when those models accurately forecast the timing and locations of upstream convection.

Aircraft observations and reanalysis depictions of trends in the North Atlantic winter jet stream wind speeds and turbulence

AbstractMultiple studies have considered whether increased anthropogenic CO2 will affect the wind speeds and turbulence associated with the winter North Atlantic polar‐front jet stream in the upper atmosphere. Key questions are whether any effects can already be seen and, if so, can they be seen independent of computer models of the atmosphere. In this study we use two reanalyses, NCEP/NCAR and the ECMWF ERA5, and two large observational archives, AMDAR/ACARS and the Global Aircraft Data Set (GADS), to try to answer these questions for the period 2002–2020 when automated aircraft observations were plentiful over the North Atlantic. We focus on eastbound, New York to London, flights. No significant increase appears in reanalyses during the last roughly 40 years (1979–2020) which is our best estimate for the modern satellite era. In contrast, for the last roughly 20 years (2002–2020) both the ERA5 reanalysis (2.5% per year) and the GADS archive (1.2% to 1.4% per year) show a statistically significant rise in the wind speed in the North Atlantic jet streak exit region. These results must be considered in the context of atmospheric oscillations, changes to the North Atlantic Track System (NATS), and the effects of aircraft step climbs. We estimate that up to 0.5% of the rise may be due to improvements in the NATS operations and an unknown additional amount may be due to the substantial increase in automated aircraft observations starting in 1997. We also examine the impact of aircraft observations on one's confidence in drawing conclusions from secular changes in the reanalyses. For turbulence, the Light turbulence trends are not statistically significant. Our confidence in the turbulence results is more limited since these observations reflect medium‐term changes in tactical and strategic aircraft operational procedures as well as the underlying prevalence of turbulence.

Mach and Froude Numbers on Mars

Mars atmospheric global circulation models exhibit transonic jet streaks during northern winter, which motivates this study of the Mach number, Ma (the ratio of flow speed to the speed of sound), and Froude number, Fr (the ratio of flow speed to the speed of buoyancy waves), as a function of season and location. Two global reanalyses spanning Mars Years (MY) 24 to 32 are used as input, EMARS and OpenMARS. The study’s vertical coordinate is potential temperature, θ, ranging from θ = 400 to 1100 K (from z ≈ 32 to 66 km); the floor is set to the lowest level that avoids intersecting mountains. Area-weighted global means and standard deviations and 5 yr temporal means using the complete years MY 25, 26, 29, 30, and 31 are compared. EMARS and OpenMARS show general agreement below θ = 700 K (z ≈ 53 km), where the observational constraints are strongest, but can vary significantly at higher levels. Both reanalyses contain transonic jet streaks in every northern winter sampled. The Fr signal is roughly twice the Ma number signal, as expected where the temperature lapse rate is small compared to the dry adiabatic lapse rate. Mach numbers are similar in both reanalyses but show larger year-to-year variability in OpenMARS. Maps of standard deviations indicate a depression between the main peaks in Tharsis and higher variability in Mare Boreum than Mare Australe. The main conclusion is that the atmosphere of Mars routinely operates in the compressible regime (0.3 < Ma < 0.8), unlike Earth. Aspects of all flow-speed meteorology are discussed.

An Analysis of Northward-Moving Tornadoes within an Open Warm Sector Across Eastern Texas

Tornadoes in eastern Texas generally track to the east as predominant westerly upper flow acts on their parent storms. However, an examination of tornadoes from 2000 to 2018 finds that 22% of all tornadoes in the region move in much more northward directions. These tornadoes’ parent storms develop in the open warm sector prior to the arrival of a main linear forcing mechanism (e.g., front, dryline). In fact, some of the more notable tornado outbreaks in recent years across Texas have occurred from northward-moving thunderstorms. This bifurcation of storm/tornado motions is important to understand for forecasting, warning, and messaging of these events. The results show these tornadoes typically occur eastward of large, slow moving, mid to upper-level long-wave troughs and underneath the left quadrant exit-region of an upper-level jet streak. The composite pattern also shows that a low-level jet in eastern Texas, a surface low centered in west-central Texas, and a warm/stationary front extending northeast of the surface low were common for these events. The typical air mass was indicative of weak instability, low convective inhibition, and high shear. Radar analysis of the northerly moving, tornadic storms showed mesocyclonic circulations with smaller diameters and lower rotational shear when compared with tornadic storms that moved in an easterly direction.

The Severe Multi-Day October 2019 Snow Storm Over Southern Manitoba, Canada

ABSTRACT A devastating storm struck southern Manitoba, Canada on 10–13 October 2019, producing a large region of mainly sticky and wet snow. Accumulations reached 75 cm, wind gusts exceeded 100 km h−1, and surface temperature (T) remained near 0°C (−1°C ≤ T ≤ 1°C) for up to 88 h. It produced the largest October snowfall and was the earliest to produce at least 20 cm since 1872 in Winnipeg. These factors led to unparalleled damage and power restoration challenges for Manitoba Hydro and, with leaves still largely on vegetation, the most damaging storm to Winnipeg’s trees ever recorded. The storm’s track was uncommon, and produced elevated convection related to buoyancy-driven instability and conditional symmetric instability (CSI), with a moist absolutely unstable layer (MAUL) near 500 hPa. Instabilities were released via lift through lower-tropospheric warm advection and frontogenesis, differential cyclonic vorticity advection, and jet streak dynamics. Precipitation bands, elevated convection, and lake effect snow bands enhanced local snowfall. Snow adhering to structures was not always wet but, when present, it sometimes occurred because of incomplete freezing of particles partially melted aloft in a near-surface (<100 m deep) inversion. Although other storms over the historical record have produced a similar combination of severe precipitation, temperature and wind conditions, none have done this for such a long period.

Multiscale Aspects of the 26–27 April 2011 Tornado Outbreak. Part II: Environmental Modifications and Upscale Feedbacks Arising from Latent Processes

Abstract One of the most prolific tornado outbreaks ever documented occurred on 26–27 April 2011 and comprised three successive episodes of tornadic convection that culminated with the development of numerous long-track, violent tornadoes over the southeastern United States during the afternoon of 27 April. This notorious afternoon supercell outbreak was preceded by two quasi-linear convective systems (hereinafter QLCS1 and QLCS2), the first of which was an anomalously severe nocturnal system that rapidly grew upscale during the previous evening. Here in Part II, we use a series of RUC 1-h forecasts and output from convection-permitting WRF-ARW simulations configured both with and without latent heat release to investigate how environmental modifications and upscale feedbacks produced by the two QLCSs contributed to the evolution and exceptional severity of this multiepisode outbreak. QLCS1 was primarily responsible for amplifying the large-scale flow pattern, inducing two upper-level jet streaks, and promoting secondary surface cyclogenesis downstream from the primary baroclinic system. Upper-level divergence markedly increased after QLCS1 developed, which yielded strong isallobaric forcing that rapidly strengthened the low-level jet (LLJ) and vertical wind shear over the warm sector and contributed to the system’s upscale growth and notable severity. Moreover, QLCS2 modified the mesoscale environment prior to the supercell outbreak by promoting the downstream formation of a pronounced upper-level jet streak, altering the midlevel jet structure, and furthering the development of a highly ageostrophic LLJ over the Southeast. Collectively, the flow modifications produced by both QLCSs contributed to the notably favorable shear profiles present during the afternoon supercell outbreak. Significance Statement The tornado outbreak that impacted the United States on 26–27 April 2011 was part of an extended outbreak that produced 343 tornadoes and numerous fatalities. This paper is Part II of a study that describes the meteorological factors supporting such a prolific event. Herein we investigate the convectively forced environmental modifications that occurred during a 36-h period encompassing three successive convective episodes. The first two episodes collectively altered the upper-level flow pattern and markedly enhanced low-level winds throughout the warm sector. These modifications served as upscale feedbacks that contributed to the first episode’s exceptional severity and to the remarkable vertical shear profiles that supported numerous long-track and violent tornadoes during the final episode on the afternoon of 27 April.