Mesoscale weather influenced by auroral gravity waves contributing to conditional symmetric instability release?

Forecasting weather has significantly improved but continues to present challenges, such as prediction of flash floods, tornado outbreaks, and rapid intensification of tropical cyclones. We consider a possible influence on severe weather occurrence through solar wind coupling to the magnetosphere-ionosphere-atmosphere system, mediated by aurorally excited atmospheric gravity waves. Solar wind disturbances, including high-speed streams, high-density plasma adjacent to the heliospheric current sheet, and interplanetary coronal mass ejections, cause intensifications of ionospheric currents at high latitudes launching gravity waves globally propagating in the atmosphere [1]. While these gravity waves reach the troposphere with much attenuated amplitudes, they are subject to amplification when encountering opposing winds and vertical wind shears. They may contribute to release of conditional symmetric instabilities [2] leading to slantwise convection, latent heat release and intensification of storms. The ERA5 re-analysis is used to evaluate slantwise convective available potential energy (SCAPE) that is of importance in the development of storms. It has been shown that significant weather events, including explosive extratropical cyclones [3,4], rapid intensification of tropical cyclones [5], and heavy rainfall causing floods and flash floods [6,7] tend to occur following arrivals of solar wind high-speed streams from coronal holes. Further evidence is provided by superposed-epoch analysis of high-rate precipitation occurrence obtained from satellite-based precipitation data sets. To support the published results, the occurrence of heavy-rainfall-induced floods and cool season precipitation events in Canada, as well as large tornado outbreaks in the United States are studied in the context of solar wind.  [1] Mayr H.G., et al., Space Sci. Rev. 54, 297–375, 1990. doi:10.1007/BF00177800 [2] Chen T.-C., et al., J. Atmos. Sci. 75, 2425–2443. doi:10.1175/JAS-D-17-0221.1[3] Prikryl P., et al., J. Atmos. Sol.-Terr. Phys. 149, 219–231. doi:10.1016/j.jastp.2016.04.002[4] Prikryl P., et al., J. Atmos. Sol.-Terr. Phys. 171, 94–10, 2018. doi:10.1016/j.jastp.2017.07.023[5] Prikryl P., et al., J. Atmos. Sol.-Terr. Phys. 183, 36-60, 2019. doi:10.1016/j.jastp.2018.12.009[6] Prikryl P., et al., Ann. Geophys. 39 (4), 769–93, 2021. doi:10.5194/angeo-39-769-2021[7] Prikryl P., et al., Atmosphere 12 (9), 2021. doi:10.3390/atmos12091186. 

<p>Tropical and extratropical cyclones can intensify into the most destructive weather systems that have significant societal and economic impacts. Rapid intensification of such weather systems has been examined in the context of solar wind coupling to the magnetosphere-ionosphere-atmosphere (MIA) system. It has been shown [1,2] that explosive extratropical cyclones and rapid intensification of tropical cyclones tend to follow arrivals of high-speed solar wind when the MIA coupling is strongest. The coupling generates atmospheric gravity waves (AGWs) that propagate from the high-latitude lower thermosphere both upward and downward [3,4]. In the upper atmosphere, AGWs are observed as traveling ionospheric disturbances. In the lower atmosphere, they can reach the troposphere and be ducted [4] to low latitudes. Despite significantly reduced wave amplitude, but subject to amplification upon over-reflection in the upper troposphere, these AGWs can trigger/release moist instabilities leading to convection and latent heat release, which is the energy driving the storms. The release of conditional symmetric instability is known to initiate slantwise convection producing rain/snow bands in extratropical cyclones. Severe weather, including severe winter storms, heavy snowfall and rainfall events, have been examined in the context of MIA coupling [5]. The results indicate a tendency of significant weather events, particularly if caused by low pressure systems in winter, to follow arrivals of solar wind high-speed streams from coronal holes. In the present paper we review the published results and provide further evidence to support them. This includes the occurrence of heavy rainfall events and flash floods, as well as the rapid intensification of recent hurricanes and typhoons, with the goal to identify sources of AGWs at high latitudes that may play a role in triggering convective bursts potentially leading to such events.</p><p>[1] Prikryl P., et al., J. Atmos. Sol.-Terr. Phys., 149, 219–231, 2016.</p><p>[2] Prikryl P., et al., J. Atmos. Sol.-Terr. Phys., 183, 36–60, 2019.</p><p>[3] Prikryl P., et al., Ann. Geophys., 23, 401–417, 2005.</p><p>[4] Mayr H.G., et al., J. Geophys. Res., 89, 10929–10959, 1984.</p><p>[5] Prikryl P., et al., J. Atmos. Sol.-Terr. Phys., 171, 94–110, 2018.</p>

توفان‌های تندری نوعی از توفان است که عموما با ابرهای همرفتی و معمولا با سیلاب‌های لحظه‌ای و گاهی تگرگ و باد شدید همراه هستند. ابرهای مربوط به توفان‌های همرفتی در بیشتر مناطق مشاهده می‌شوند، اما درصد کوچکی از این توفان‌های همرفتی تولید شرایط هوای سخت و سیل‌های ناگهانی را می‌کنند و خسارات زیادی به بار می‌آورند. یکی از این توفان‌های تندری مرگ‌بار، توفان تندری 28 تیرماه 1394 است که دارای خسارات مالی و جانی فراوانی بود. در این پژوهش به بررسی شرایط سینوپتیکی و ترمودینامیکی این توفان تندری پرداخته شده است. هدف از انجام این پژوهش پیش‌بینی احتمال وقوع توفان تندری، تعیین شدت توفان احتمالی، تعیین مکان توفان همرفتی و ارتباط آن با سامانه‌های سینوپتیکی بوده است. در این راستا از داده‌های NCEP/NCAR، تصاویر ماهواره‌ای NOVA/AVHRR، داده‌های جو بالا و نرم‌افزارهای GRADS، ENVI، RAOB و ArcGIS برای رسیدن به اهداف فوق استفاده شد. نتایج این پژوهش نشان داد که شرایط سینوپتیکی مساعد برای وقوع توفان تندری ازجمله کم‌فشار تراز دریا، ناوه تراز میانی، همگرایی رطوبت و وجود رطوبت در لایه‌های پایینی جو وجود دارد. همچنین هسته اصلی توفان که بین کرج و قزوین قرار دارد با مرکز بیشینه امگای منفی تراز 500 هکتوپاسکال منطبق است. نتایج شاخص‌های ناپایداری برای ساعت 00UTC نشان داد که شاخص‌های KO، KI، JI و VT شدت ناپایداری را قوی و توفان همرفتی شدید را پیش‌بینی کرده‌اند. 6 شاخص نیز ناپایداری(توفان همرفتی) متوسط و فقط دو شاخص توفان همرفتی ضعیف را پیش‌بینی کرده‌اند. نرم‌افزار RAOB حداکثر سرعت قائم در این ساعت را 30 متر بر ثانیه برآورد کرده است که نشان‌دهنده صعود شدید و درنتیجه وقوع توفان تندری شدید است.

Abstract. Heavy rainfall events causing floods and flash floods are examined in the context of solar wind coupling to the magnetosphere–ionosphere–atmosphere system. The superposed epoch (SPE) analyses of solar wind variables have shown the tendency of severe weather to follow arrivals of high-speed streams from solar coronal holes. Precipitation data sets based on rain gauge and satellite sensor measurements are used to examine the relationship between the solar wind high-speed streams and daily precipitation rates over several midlatitude regions. The SPE analysis results show an increase in the occurrence of high precipitation rates following arrivals of high-speed streams, including recurrence with a solar rotation period of 27 d. The cross-correlation analysis applied to the SPE averages of the green (Fe XIV; 530.3 nm) corona intensity observed by ground-based coronagraphs, solar wind parameters, and daily precipitation rates show correlation peaks at lags spaced by solar rotation period. When the SPE analysis is limited to years around the solar minimum (2008–2009), which was dominated by recurrent coronal holes separated by ∼ 120∘ in heliographic longitude, significant cross-correlation peaks are found at lags spaced by 9 d. These results are further demonstrated by cases of heavy rainfall, floods and flash floods in Europe, Japan, and the USA, highlighting the role of solar wind coupling to the magnetosphere–ionosphere–atmosphere system in severe weather, mediated by aurorally excited atmospheric gravity waves.

Extreme weather events, such as heavy rainfall causing floods and flash floods continue to present difficult challenges in forecasting. Using gridded daily precipitation datasets in conjunction with solar wind data it is shown that high-rate precipitation occurrence is modulated by solar wind high-speed streams. Superposed epoch analysis shows a statistical increase in the occurrence of high-rate precipitation following arrivals of high-speed streams from coronal holes, including their recurrence with the solar rotation period of 27 days. These results are consistent with the observed tendency of heavy rainfall leading to floods and flash floods in Japan, Australia, and continental United States to follow arrivals of high-speed streams. A possible role of the solar wind–magnetosphere–ionosphere–atmosphere coupling in weather as mediated by globally propagating aurorally excited atmospheric gravity waves triggering conditional moist instabilities leading to convection in the troposphere that has been proposed in previous publications is highlighted.

Abstract. Cases of mesoscale cloud bands in extratropical cyclones are observed a few hours after atmospheric gravity waves (AGWs) are launched from the auroral ionosphere. It is suggested that the solar-wind-generated auroral AGWs contribute to processes that release instabilities and initiate slantwise convection thus leading to cloud bands and growth of extratropical cyclones. Also, if the AGWs are ducted to low latitudes, they could influence the development of tropical cyclones. The gravity-wave-induced vertical lift may modulate the slantwise convection by releasing the moist symmetric instability at near-threshold conditions in the warm frontal zone of extratropical cyclones. Latent heat release associated with the mesoscale slantwise convection has been linked to explosive cyclogenesis and severe weather. The circumstantial and statistical evidence of the solar wind influence on extratropical cyclones is further supported by a statistical analysis of high-level clouds (<440 mb) extracted from the International Satellite Cloud Climatology Project (ISCCP) D1 dataset. A statistically significant response of the high-level cloud area index (HCAI) to fast solar wind from coronal holes is found in mid-to-high latitudes during autumn-winter and in low latitudes during spring-summer. In the extratropics, this response of the HCAI to solar wind forcing is consistent with the effect on tropospheric vorticity found by Wilcox et al. (1974) and verified by Prikryl et al. (2009). In the tropics, the observed HCAI response, namely a decrease in HCAI at the arrival of solar wind stream followed by an increase a few days later, is similar to that in the northern and southern mid-to-high latitudes. The amplitude of the response nearly doubles for stream interfaces associated with the interplanetary magnetic field BZ component shifting southward. When the IMF BZ after the stream interface shifts northward, the autumn-winter effect weakens or shifts to lower (mid) latitudes and no statistically significant response is found at low latitudes in spring-summer. The observed effect persists through years of low and high volcanic aerosol loading. The similarity of the response in mid-to-high and low latitudes, the lack of dependence upon aerosol loading, and the enhanced amplitude of the effect when IMF BZ component shifts southward, favor the proposed AGW link over the atmospheric electric circuit (AEC) mechanism (Tinsley et al., 1994). The latter requires the presence of stratospheric aerosols for a significant effect and should produce negative and positive cloud anomalies in mid-to-high and low latitudes, respectively. However, if the requirement of aerosols for the AEC mechanism can be relaxed, the AGW and AEC mechanisms should work in synergy at least in mid-to-high latitudes.

Tropospheric weather influenced by solar wind through atmospheric vertical coupling downward control

The National Oceanic and Atmospheric Administration National Weather Service database of tornadoes provided by the Storm Prediction Center is used to investigate the occurrence of tornado outbreaks in the United States from 1963 to 2021 in the context of solar wind that impacts the Earth’s magnetosphere. A link between the solar wind and large tornado outbreaks is found. Superposed epoch analysis of tornado occurrence reveals a peak in the cumulative number of tornadoes near the interplanetary magnetic field sector boundary (heliospheric current sheet) crossings. The latter often closely precede or coincide with co-rotating interaction regions at the leading edge of high-speed streams from coronal holes. Most of the large tornado outbreaks (20 or more tornadoes per 24 hours) are associated with high-density plasma adjacent to heliospheric current sheets and co-rotating interaction regions. Other large tornado outbreaks followed impacts of interplanetary coronal mass ejections or occurred in a declining phase of major high-speed streams. We consider the role of the solar wind coupling to the magnetosphere-ionosphere-atmosphere system in severe weather development, mediated by globally propagating aurorally excited atmospheric gravity waves. While these gravity waves reach the troposphere with attenuated amplitudes, when over-reflecting in regions of low-level wind shear and opposing winds, they can contribute to conditional symmetric instability release in frontal zones of extratropical cyclones leading to intensification of supercells that spawn tornado outbreaks. The ERA5 meteorological re-analysis is used to evaluate slantwise convective available potential energy (SCAPE) to assess conditional symmetric instability and slantwise convection in cases of large tornado outbreaks.

A link between high-speed solar wind streams and explosive extratropical cyclones

Slantwise convection, the process by which moist symmetric instability is released, has often been linked to banded clouds and precipitation, especially in frontal zones within extratropical cyclones. Studies also suggest that the latent heat release associated with slantwise convection can lead to a spinup of surface frontogenesis, which can enhance the rapid intensification of extratropical cyclones. However, most of these studies considered only local areas or short time durations. In this study, we provide a novel statistical investigation of the global climatology of the potential occurrence of slantwise convection, in terms of conditional symmetric instability, and its relationship with precipitating systems. Using the 6-hourly ERA-Interim, two different indices are calculated, namely, slantwise convective available potential energy (SCAPE) and vertically integrated extent of realizable symmetric instability (VRS), to assess the likelihood of occurrence of slantwise convection around the globe. The degree of association is quantified between these indices and the observed surface precipitation as well as the cyclone activity. The susceptibility of midlatitude cyclones to slantwise convection at different stages of their life cycle is also investigated. As compared to the nonexplosive cyclone cases, the time evolution of SCAPE and VRS within rapidly deepening cyclones exhibit higher values before, and a more significant drop after, the onset of rapid intensification, supporting the idea that the release of symmetric instability might contribute to the intensification of storms.

The relationship between two classes of coronal holes and high-speed quasi-stationary streams of solar wind at the Earth’s orbit is investigated. “Open” coronal holes, whose area is invariable or increases with the height over the solar surface, are rated in the first class, and “closed” coronal holes with areas decreasing with the height are referred to as second-class holes. The parameters of the coronal holes are determined from IR and EUV images and spectroheliograms. It is shown that most open coronal holes can be associated with high-speed solar-wind streams, while most closed coronal holes exhibit a much lower correlation with such streams.

Since the 1970s it has been empirically known that the area of solar coronal holes affects the properties of high-speed solar wind streams (HSSs) at Earth. We derive a simple analytical model for the propagation of HSSs from the Sun to Earth and thereby show how the area of coronal holes and the size of their boundary regions affect the HSS velocity, temperature, and density near Earth. We assume that velocity, temperature, and density profiles form across the HSS cross section close to the Sun and that these spatial profiles translate into corresponding temporal profiles in a given radial direction due to the solar rotation. These temporal distributions drive the stream interface to the preceding slow solar wind plasma and disperse with distance from the Sun. The HSS properties at 1 AU are then given by all HSS plasma parcels launched from the Sun that did not run into the stream interface at Earth distance. We show that the velocity plateau region of HSSs as seen at 1 AU, if apparent, originates from the center region of the HSS close to the Sun, whereas the velocity tail at 1 AU originates from the trailing boundary region. Small HSSs can be described to entirely consist of boundary region plasma, which intrinsically results in smaller peak velocities. The peak velocity of HSSs at Earth further depends on the longitudinal width of the HSS close to the Sun. The shorter the longitudinal width of an HSS close to the Sun, the more of its “fastest” HSS plasma parcels from the HSS core and trailing boundary region have impinged upon the stream interface with the preceding slow solar wind, and the smaller is the peak velocity of the HSS at Earth. As the longitudinal width is statistically correlated to the area of coronal holes, this also explains the well-known empirical relationship between coronal hole areas and HSS peak velocities. Further, the temperature and density of HSS plasma parcels at Earth depend on their radial expansion from the Sun to Earth. The radial expansion is determined by the velocity gradient across the HSS boundary region close to the Sun and gives the velocity-temperature and density-temperature relationships at Earth their specific shape. When considering a large number of HSSs, the assumed correlation between the HSS velocities and temperatures close to the Sun degrades only slightly up to 1 AU, but the correlation between the velocities and densities is strongly disrupted up to 1 AU due to the radial expansion. Finally, we show how the number of particles of the piled-up slow solar wind in the stream interaction region depends on the velocities and densities of the HSS and preceding slow solar wind plasma.

Numerical forecasts from the National Centers for Environmental Prediction’s mesoscale version of the η coordinate–based model, hereafter referred to as MESO, have been analyzed to study the roles of conditional symmetric instability (CSI) and frontogenesis in copious precipitation events. A grid spacing of 29 km and 50 layers are used in the MESO model. Parameterized convective and resolvable-scale condensation, radiation physics, and many other physical processes are included. Results focus on a 24-h forecast from 1500 UTC 1 February 1996 in the region of a low-level front and associated deep baroclinic zone over the southeastern United States. Predicted precipitation amounts were close to the observed, and the rainfall in the model was mainly associated with the resolvable-scale condensation. During the forecast deep upward motion amplifies in a band oriented west-southwest to east-northeast, nearly parallel to the mean tropospheric thermal wind. This band develops from a sloping updraft in the low-level nearly saturated frontal zone, which is absolutely stable to upright convection, but susceptible to CSI. The updraft is then nearly vertical in the middle troposphere where there is very weak conditional instability. We regard this occurrence as an example of model-produced deep slantwise convection (SWC). Negative values of moist potential vorticity (MPV) occur over the entire low-level SWC area initially. The vertical extent of SWC increases with the lifting upward of the negative MPV area. Characteristic features of CSI and SWC simulated in some high-resolution nonhydrostatic cloud models also develop within the MESO. As in the nonhydrostatic SWC, the vertical momentum transport in the MESO updraft generates a subgeostrophic momentum anomaly aloft, with negative absolute vorticity on the baroclinically cool side of the momentum anomaly where outflow winds are accelerated to the north. Contribution of various processes to frontogenesis in the SWC area is investigated. The development of indirect circulation leads to low-level frontogenesis through the tilting term. The axis of frontogenesis nearly coincides with the axis of maximum vertical motion when the SWC is fully developed. Results suggest that strong vertical motions in the case investigated develop due to release of symmetric instability in a moist atmosphere (CSI), and resultant circulations lead to weak frontogenesis in the SWC area.

In this study, we introduce a parameterization scheme for slantwise convection (SC) to be considered for models that are too coarse to resolve slantwise convection explicitly (with a horizontal grid spacing coarser than 15 km or less). This SC scheme operates in a locally defined 2D cross section perpendicular to the deep-layer-averaged thermal wind. It applies momentum tendency to adjust the environment toward slantwise neutrality with a prescribed adjustment time scale. Condensational heating and the associated moisture loss are also considered. To evaluate the added value of the SC scheme, we implement it in the Weather Research and Forecasting (WRF) Model to supplement the existing cumulus parameterization schemes for upright convection and test for two different numerical setups: a 2D idealized, unforced release of conditional symmetric instability (CSI) in an initially conditionally stable environment, and a 3D real-data precipitation event containing both CSI and conditional instability along the cold front of a cyclonic storm near the United Kingdom. Both test cases show significant improvements for the coarse-gridded (40-km) simulations when parameterizing slantwise convection. Compared to the 40-km simulations with only the upright convection scheme, the counterparts with the additional SC scheme exhibit a larger extent of CSI neutralization, generate a stronger grid-resolved slantwise circulation, and produce greater amounts of precipitation, all in better agreement with the corresponding fine-gridded reference simulations. Given the importance of slantwise convection in midlatitude weather systems, our results suggest that there exist potential benefits of parameterizing slantwise convection in general circulation models.

The generation of internal gravity waves from an initially geostrophically balanced flow is diagnosed in nonhydrostatic numerical simulations of shear instabilities for varied dynamical regimes. A nonlinear decomposition method up to third order in the Rossby number (Ro) is used as the diagnostic tool for a consistent separation of the balanced and unbalanced motions in the presence of their nonlinear coupling. Wave emission is investigated in an Eady-like and a jet-like flow. For the jet-like case, geostrophic and ageostrophic unstable modes are used to initialize the flow in different simulations. Gravity wave emission is in general very weak over a range of values for Ro. At sufficiently high Ro, however, when the condition for symmetric instability is satisfied with negative values of local potential vorticity, significant wave emission is detected even at the lowest order. This is related to the occurrence of fast ageostrophic instability modes, generating a wide spectrum of waves. Thus, gravity waves are excited from the instability of the balanced mode to lowest order only if the condition of symmetric instability is satisfied and ageostrophic unstable modes obtain finite growth rates. Significance Statement We aim to understand the generation of internal gravity waves in the atmosphere and ocean from a flow field that is initially balanced, i.e., free from any internal gravity waves. To examine this process, we use simulations from idealized numerical models and nonlinear flow decomposition method to identify waves. Our results show that a prominent mechanism by which waves can be generated is related to symmetric or ageostrophic instabilities of the balanced flow possibly occurring during frontogenesis. This process can be a significant mechanism to dissipate the energy of the geostrophic flow in the ocean.