The Impact of Sudden Stratospheric Warmings and Elevated Stratopause Events on the VLF signal in high latitudes

Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events are atmospheric wave driven winter phenomena, which lead to significant changes in wind, temperatures and vertical mass transport, especially in stratospheric and mesospheric altitudes. Likely, SSW and ES induced changes also cause modifications in the sensitive D-region ionization (~60-90 km). This bottom side of the Ionosphere behaves together with the Earth-surface as a reflection boundary for the Very Low Frequency Transmission, used for long distance communication. Here we want to study the impact of SSW/ES events on the VLF signal in high latitudes. For the identification of SSW/ES induced perturbations of the VLF signal, the typical seasonal variation and outlier caused by noise, technical adjustments or solar events need to be removed. A quiet time curve, representing the seasonal VLF signal variation under undisturbed conditions, was developed with a polynomial fit of the composite. In preparation for the composite, the VLF data needed to be leveled due to artificial amplitude steps with technical origin in the timeseries. The leveling was done with help of the Pruned Exact Linear Time method. Additionally, outlier have been removed using the Median Absolute Deviation, a method from robust statistics. With help of the developed quiet time curve, VLF signal perturbations could be identified, caused by the SSW and ES events. Here we want to describe and discuss those VLF signal perturbations for multiple links in high latitudes, considering the different pathways between Transmitter and Receiver as the ES events vary strongly with longitude

Findings on the October Effect

Very Low Frequency (VLF) radio signals provide a unique possibility of continuously monitoring the lower ionosphere and their dynamics since these signals are reflected at the ionospheric D region between 60-90 km. Recent investigations have shown a very sharp decrease in signal amplitude at the beginning of October which deviates from the actual symmetric course of solar zenith angle variation over the year. The effect is developed differently depending on latitude, longitude and frequency, as we will present. In investigation for the cause of this phenomenon, first comparisons suggest a close correlation with the sudden reversal from easterly to westerly zonal flow, the asymmetric peak in semidiurnal solar tide S2, and the progression of the lower mesospheric temperature. Independent of the solar zenith angle mostly in high latitudes, a strong warming of the lower mesosphere during fall can be observed, confirming dominating atmospheric inner dynamics. Further studies are ongoing

The “polar vortex” winter of 2013/14

The term “polar vortex” remained largely a technical term until early January 2014 when the United States (US) media used it to describe an historical cold air outbreak in eastern North America. Since then, “polar vortex” has been used more frequently by the media and the public, often conflating circulation features and temperatures near the surface with only partially related features at the tropopause and in the stratosphere. The polar vortex in its most common scientific usage refers to a hemispheric-scale stratospheric circulation over the Arctic that is present during the Northern Hemisphere cold season. Reversal of the zonal mean zonal winds circumnavigating the stratospheric polar vortex (SPV), termed major sudden stratospheric warmings, can be linked to mid-latitude cold air outbreaks. However, this mechanism does not explain the cold US winter of 2013/2014. This study revisits the winter of 2013/2014 to understand how SPV variability may still have played a role in the severe winter weather. Observations indicate that anomalously strong vertical wave propagation occurred throughout the winter and disrupted, but did not fully break, the SPV. Instead, vertically propagating waves were reflected back downward, amplifying a blocking high near Alaska and downstream troughing across central North America, a classic signature for extreme cold air outbreaks across central and eastern North America. Thus, the association of the term “polar vortex” with winter 2013/2014, while not justified by the most common usage of the term, serves as a case study of the wave-reflection mechanism of SPV influence on mid-latitude weather

Relatório de estágio em farmácia comunitária

Relatório de estágio realizado no âmbito do Mestrado Integrado em Ciências Farmacêuticas, apresentado à Faculdade de Farmácia da Universidade de Coimbr

Stratospheric downward wave reflection events modulate North American weather regimes and cold spells

The Arctic stratospheric polar vortex is an important driver of mid-latitude winter cold spells. One proposed coupling mechanism between the stratospheric polar vortex and the troposphere is upward-propagating planetary waves being reflected downward by the polar vortex. However, while the wave reflection mechanism is well-documented, its role in favouring cold spells is still under-explored. Here, we analyse such stratospheric wave reflections and their impact on the tropospheric circulation and surface temperatures over North America in winter. We present a physically interpretable regional stratospheric wave reflection detection metric and identify the tropospheric circulation anomalies associated with prolonged periods of wave reflection, which we term reflection events. In particular, we characterise the tropospheric anomalies through the lens of North American weather regimes. Stratospheric reflection events show a systematic evolution from a Pacific Trough regime – associated on average with positive temperature anomalies and a near-complete absence of anomalously cold temperatures in North America – to an Alaskan Ridge regime, which favours low temperatures over much of the continent. The most striking feature of the stratospheric reflection events is thus a rapid, continental-scale decrease in temperatures. These emerge as continental-scale cold spells by the end of the reflection events. Stratospheric reflection events are thus highly relevant in a tropospheric predictability perspective

Impact of Sudden Stratospheric Warmings and Elevated Stratopause events on the VLF signal in high latitudes

Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events are atmospheric wave driven winter phenomena, which lead to significant changes in wind, temperatures and vertical mass transport, especially in stratospheric and mesospheric altitudes. Presumably, SSW and ES induced changes also cause modifications in the sensitive D-region ionization (~60-90 km). This bottom side of the Ionosphere behaves together with the Earth-surface as a reflection boundary for Very Low Frequency (VLF) transmission, used for long distance communication. Here we want to study the impact of SSW/ES events on the VLF signal amplitude in high latitudes. For the identification of SSW/ES induced perturbations of the VLF signal, the typical seasonal variation and outlier caused by noise, technical adjustments or solar events need to be removed. A quiet time curve, representing the seasonal VLF signal variation under undisturbed conditions, was developed with a polynomial fit of the composite. In preparation for the composite, the VLF data needed to be leveled due to artificial amplitude steps with technical origin in the timeseries. The leveling was done with help of the Pruned Exact Linear Time method. Additionally, outlier have been removed using the Median Absolute Deviation, a method from robust statistics. With help of the developed quiet time curve, VLF signal perturbations could be identified, caused by the SSW and ES events. Here we want to describe and discuss those VLF signal perturbations for multiple links in high latitudes, considering the different pathways between transmitter and receiver as the ES events vary strongly with longitude

The connection between the October effect in VLF observations and neutral atmosphere dynamics

The October effect is long known as a strong decrease in VLF amplitudes occurring every October. However, neither it’s formation mechanism nor the characteristic of the October effect are known. Simultaneously with the October effect we observe a regional temperature increase between 55 and 75 km, i.e. in the region where VLF radio waves are reflected. In theory an increase in temperature can lead to an increased collision frequency and thus an increased VLF signal absorption resulting in a decreased VLF amplitude. Thus, there seems to be a connection between the October effect and the neutral atmosphere dynamics. Here we want to investigate the characteristics of the October effect in VLF signals in connection to neutral atmosphere temperature using different transmitter and receiver combiations as well as MLS satellite observations. We found that the October effect is strongest in polar latitudes and vanishes in low latitudes for temperature and VLF observations. Finally a possible formation mechanism for the October effect will be discussed

The Impact of Sudden Stratospheric Warmings and Elevated Stratopause Events on the VLF signal

Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events are atmospheric wave driven winter phenomena, which lead to significant changes in atmospheric dynamics and temperatures. SSWs are characterized by a sudden warming in the stratosphere by up to 90K and a mesospheric cooling by up to 30K. At the same time the background wind decelerates and can even revers which modifies the vertical mass transport. Occasionally SSW are followed by an ES where the stratopause at 50-60 km vanishes and subsequently reforms in elevated altitude ranges of 70-85 km. This leads to a temperature increase of up to 50 K in mesospheric heights. The temperature changes during a SSW and ES event can have an impact on the VLF signal absorption. In theory a decrease/increase in temperature can lead to a decreased/enhanced VLF signal absorption due to a decreased/increased collision frequency. Here we want to investigate the impact of a SSW and ES on VLF signals using the VLF signals of three transmitter and receiver combinations, with reflections points located in high latitudes. For the winter 2008/2009 we found an anticorrelation between VLF amplitude perturbations and the temperature at 70 km at midpoint location confirming the theory. We aim to discuss possible mechanisms leading to those observed perturbations in the VLF signals during SSW and ES event

Why does the October Effect not occur at nighttime?

Radar waves with very low frequency (VLF) are reflected at the lower edge of the ionosphere, in the D-region. The D-region (60 - 90km) is influenced by the solar zenith angle and space weather from above as well as by dynamical and chemical processes in the mesosphere. During October there is a well-known sharp decrease of the daytime VLF amplitude between transmitter and receiver combinations whose great circle paths lie mainly in polar latitudes. Until now we do not know what causes the October effect. Space weather phenomena can be ruled out as a cause since their time scales are either too short or too long. The solar zenith angle, strongly influencing the seasonal variation of the VLF amplitude can also be ruled out as a similar behavior is not observed in spring. Thus, there is a strong assumption that neutral dynamical processes in the mesosphere play a major role. We assume and confirm that a regional warming in the lower mesosphere, occurring simultaneously and with similar characteristics as the October effect, plays a major role in the formation process of the October effect. The VLF reflection height is about 15km higher during nighttime than during daytime. This difference in combination with the location of the regional warming explains, why the October effect can not be observed during nighttime