Improved dynamic geomagnetic rigidity cutoff modeling: testing predictive accuracy

. In the polar atmosphere, significant chemical and ionization changes occur during solar proton events (SPE). The access of solar protons to this region is limited by the dynamically changing geomagnetic field. In this study we have used riometer absorption observations to investigate the accuracy of a model to predict Kp-dependent geomagnetic rigidity cutoffs, and hence the changing proton fluxes. The imaging riometer at Halley, Antarctica is ideally situated for such a study, as the rigidity cutoff sweeps back and forth across the instrument's field of view, providing a severe test of the rigidity cutoff model. Using observations from this riometer during five solar proton events, we have confirmed the basic accuracy of this rigidity model. However, we find that the model can be improved by setting a lower Kp limit (i.e., Kp=5 instead of 6) at which the rigidity modeling saturates. We also find that for L>4.5 the apparent L-shell of the beam moves equatorwards. In addition, the Sodankyla Ion and Neutral Chemistry model is used to determine an empirical relationship between integral proton precipitation fluxes and nighttime ionosphere riometer absorption, in order to allow consideration of winter time SPEs. We find that during the nighttime the proton flux energy threshold is lowered to include protons with energies of >5 MeV in comparison with >10 MeV for the daytime empirical relationships. In addition, we provide an indication of the southern and northern geographic regions inside which SPEs play a role in modifying the neutral chemistry of the stratosphere and mesosphere

Seasonal variations of gravity wave activity in the lower stratosphere over an Antarctic Peninsula station

An 8 year series of 965 high-resolution radiosonde soundings over Rothera (67 degrees S, 68 degrees W) on the Antarctic Peninsula are used to study gravity wave characteristics in the lower stratosphere. The gravity wave energy is shown to have a seasonal variation with peaks at the equinoxes; the largest peak is around the spring equinox. During the winter months and extending into the spring, there is both an enhancement in the downward propagating wave activity and a reduction in the amount of critical-level filtering of upward propagating mountain waves. The horizontal propagation directions of the gravity waves were determined using hodographs. It was found that there is a predisposition toward northward and westward propagating waves above Rothera. This is in agreement with previous observations of gravity wave momentum flux in the wintertime mesosphere over Rothera. These results are consistent with a scenario whereby the stratospheric gravity wavefield above Rothera is determined by a combination of wind flow over topography-generating waves from below, and sources such as the edge of the polar stratospheric vortex-generating waves from above, especially during winter and spring

Winds and tides of the Extended Unified Model in the mesosphere and lower thermosphere validated with meteor radar observations

The mesosphere and lower thermosphere (MLT) is a critical region that must be accurately reproduced in general circulation models (GCMs) that aim to include the coupling between the lower and middle atmosphere and the thermosphere. An accurate representation of the MLT is thus important for improved climate modelling and the development of a whole atmosphere model. This is because the atmospheric waves at these heights are particularly large, and so the energy and momentum they carry is an important driver of climatological phenomena through the whole atmosphere, affecting terrestrial and space weather. The Extended Unified Model (ExUM) is the recently developed version of the Met Office s Unified Model which has been extended to model the MLT. The capability of the ExUM to model atmospheric winds and tides in the MLT is currently unknown. Here, we present the first study of winds and tides from the ExUM. We make a comparison against meteor radar observations of winds and tides from 2006 between 80 and 100 km over two radar stations - Rothera (68° S, 68° W) and Ascension Island (8° S, 14° W). These locations are chosen to study tides in two very different tidal regimes - the equatorial regime, where the diurnal (24 h) tide dominates, and the polar regime, where the semi-diurnal (12 h) tide dominates. The results of this study illustrate that the ExUM is capable of reproducing atmospheric winds and tides that capture many of the key characteristics seen in meteor radar observations, such as zonal and meridional wind maxima and minima, the increase in tidal amplitude with increasing height, and the decrease in tidal phase with increasing height. In particular, in the equatorial regime some essential characteristics of the background winds, tidal amplitudes and tidal phases are well captured but with significant differences in detail. In the polar regime, the difference is more pronounced. The ExUM zonal background winds in austral winter are primarily westward rather than eastward, and in austral summer they are larger than observed above 90 km. The ExUM tidal amplitudes here are in general consistent with observed values, but they are also larger than observed values above 90 km in austral summer. The tidal phases are generally well replicated in this regime. We propose that the bias in background winds in the polar regime is a consequence of the lack of in situ gravity wave generation to generate eastward fluxes in the MLT. The results of this study indicate that the ExUM has a good natural capability for modelling atmospheric winds and tides in the MLT but that there is room for improvement in the model physics in this region. This highlights the need for modifications to the physical parameterization schemes used in the model in this region - such as the non-orographic spectral gravity wave scheme - to improve aspects such as polar circulation. To this end, we make specific recommendations of changes that can be implemented to improve the accuracy of the ExUM in the MLT. </p

Coordinated Observations of 8- and 6-hr Tides in the Mesosphere and Lower Thermosphere by Three Meteor Radars Near 60^°S Latitude

Atmospheric 8‐ and 6‐hr tides are observed for the first time in the zonal and meridional winds at ~82–97 km altitudes simultaneously at Tierra del Fuego (TDF; 53.7°S, 67.7°W), King Edward Point (KEP; 54.3°S, 36.5°W), and Rothera (ROT; 67.5°S, 68.0°W) at Southern Hemisphere (SH) middle‐to‐high latitudes during long time spans, allowing to reveal climatology and migrating nature. The monthly averaged amplitudes vary between ~1 and 8 m/s for the 8‐hr tides while the amplitudes of 6‐hr tides are smaller ~0.5–4 m/s. Both tides exhibit an annual pattern having the amplitude maxima during SH winter and minima in SH summer. The tidal phases are smaller (earlier) in the zonal wind than in the meridional wind by about 90°. The phase differences observed between TDF and KEP, which are located at similar latitudes but different longitudes suggest the propagation of migrating tides. The study finds that 8‐ and 6‐hr tides are correlated

Simulation study for measurement of horizontal wind profiles in the polar stratosphere and mesosphere using ground-based observations of ozone and carbon monoxide lines in the 230–250 GHz region

Meteorological and atmospheric models are being extended up to 80 km altitude but there are very few observing techniques that can measure stratospheric–mesospheric winds at altitudes between 20 and 80 km to verify model datasets. Here we demonstrate the feasibility of horizontal wind profile measurements using ground-based passive millimetre-wave spectroradiometric observations of ozone lines centred at 231.28, 249.79, and 249.96 GHz. Vertical profiles of horizontal winds are retrieved from forward and inverse modelling simulations of the line-of-sight Doppler-shifted atmospheric emission lines above Halley station (75°37′ S, 26°14′ W), Antarctica. For a radiometer with a system temperature of 1400 K and 30 kHz spectral resolution observing the ozone 231.28 GHz line we estimate that 12 h zonal and meridional wind profiles could be determined over the altitude range 25–74 km in winter, and 28–66 km in summer. Height-dependent measurement uncertainties are in the range 3–8 m s−1 and vertical resolution  ∼  8–16 km. Under optimum observing conditions at Halley a temporal resolution of 1.5 h for measuring either zonal or meridional winds is possible, reducing to 0.5 h for a radiometer with a 700 K system temperature. Combining observations of the 231.28 GHz ozone line and the 230.54 GHz carbon monoxide line gives additional altitude coverage at 85 ± 12 km. The effects of clear-sky seasonal mean winter/summer conditions, zenith angle of the received atmospheric emission, and spectrometer frequency resolution on the altitude coverage, measurement uncertainty, and height and time resolution of the retrieved wind profiles have been determined

Horizontal phase velocity distributions of Antarctic mesospheric gravity waves observed by airglow imager network (ANGWIN)

第6回極域科学シンポジウム分野横断型セッション：[IM] 横断　中層大気・熱圏11月17日（火）　統計数理研究所 セミナー室2（D304

Polar stratospheric clouds initiated by mountain waves in a global chemistry-climate model: A missing piece in fully modelling polar stratospheric ozone depletion

An important source of polar stratospheric clouds (PSCs), which play a crucial role in controlling polar stratospheric ozone depletion, is the temperature fluctuations induced by mountain waves. These enable stratospheric temperatures to fall below the threshold value for PSC formation in regions of negative temperature perturbations or cooling phases induced by the waves even if the synoptic-scale temperatures are too high. However, this formation mechanism is usually missing in global chemistry–climate models because these temperature fluctuations are neither resolved nor parameterised. Here, we investigate in detail the episodic and localised wintertime stratospheric cooling events produced over the Antarctic Peninsula by a parameterisation of mountain-wave-induced temperature fluctuations inserted into a 30-year run of the global chemistry–climate configuration of the UM-UKCA (Unified Model – United Kingdom Chemistry and Aerosol) model. Comparison of the probability distribution of the parameterised cooling phases with those derived from climatologies of satellite-derived AIRS brightness temperature measurements and high-resolution radiosonde temperature soundings from Rothera Research Station on the Antarctic Peninsula shows that they broadly agree with the AIRS observations and agree well with the radiosonde observations, particularly in both cases for the “cold tails” of the distributions. It is further shown that adding the parameterised cooling phase to the resolved and synoptic-scale temperatures in the UM-UKCA model results in a considerable increase in the number of instances when minimum temperatures fall below the formation temperature for PSCs made from ice water during late austral autumn and early austral winter and early austral spring, and without the additional cooling phase the temperature rarely falls below the ice frost point temperature above the Antarctic Peninsula in the model. Similarly, it was found that the formation potential for PSCs made from ice water was many times larger if the additional cooling is included. For PSCs made from nitric acid trihydrate (NAT) particles it was only during October that the additional cooling is required for temperatures to fall below the NAT formation temperature threshold (despite more NAT PSCs occurring during other months). The additional cooling phases also resulted in an increase in the surface area density of NAT particles throughout the winter and early spring, which is important for chlorine activation. The parameterisation scheme was finally shown to make substantial differences to the distribution of total column ozone during October, resulting from a shift in the position of the polar vortex

Energetic outer radiation belt electron precipitation during recurrent solar activity

Transmissions from three U.S. VLF (very low frequency) transmitters were received at Churchill, Canada, during an event study in May to November, 2007. This period spans four cycles of recurrent geomagnetic activity spaced similar to 27 days apart, with daily Sigma Kp reaching similar to 30 at the peaks of the disturbances. The difference in the amplitude of the signals received during the day and during the night varied systematically with geomagnetic activity, and was used here as a proxy for ionization changes caused by energetic electron precipitation. For the most intense of the recurrent geomagnetic storms there was evidence of electron precipitation from 3 300 keV and similar to 1 MeV trapped electrons, and also consistent with the daily average ULF (ultralow frequency) Pc1-2 power (L = 3.9) from Lucky Lake, Canada, which was elevated during the similar to 1 MeV electron precipitation period. This suggests that Pc1-2 waves may play a role in outer radiation belt loss processes during this interval. We show that the > 300 keV trapped electron flux from POES is a reasonable proxy for electron precipitation during recurrent high-speed solar wind streams, although it did not describe all of the variability that occurred. While energetic electron precipitation can be described through a proxy such as Kp or Dst, careful incorporation of time delays for different electron energies must be included. Dst was found to be the most accurate proxy for electron precipitation during the weak recurrent-activity period studied

Gravity waves in the winter stratosphere over the Southern Ocean: high-resolution satellite observations and 3-D spectral analysis.

Atmospheric gravity waves play a key role in the transfer of energy and momentum between layers of the Earth's atmosphere. However, nearly all Global Circulation Models (GCMs) seriously under-represent the momentum fluxes of gravity waves at latitudes near 60° S. This can result in modelled winter stratospheres that are unrealistically cold – a significant bias known as the "cold-pole problem". There is thus a need for measurements of gravity-wave fluxes near 60S to test and constrain GCMs. Such measurements are notoriously difficult, because they require 3-D observations of wave properties if the fluxes are to be estimated without using significant limiting assumptions. Here we use 3-D satellite measurements of stratospheric gravity waves from NASA's AIRS/Aqua instrument. We present the first extended application of a 3-D Stockwell transform (3DST) method to determine localised gravity-wave amplitudes, wavelengths and directions of propagation around the entire region of the Southern Ocean near 60° S during austral winter 2010. We first validate our method using a synthetic wave field and two case studies of real gravity waves over the Southern Andes and the island of South Georgia. A new technique to overcome wave amplitude attenuation problems in previous methods is also presented. We then characterise large-scale gravity-wave occurrence frequencies, directional momentum fluxes and short-timescale intermittency over the entire Southern Ocean. Our results show that highest wave-occurrence frequencies, amplitudes and momentum fluxes are observed in the stratosphere over the mountains of the Southern Andes and Antarctic Peninsula. However, we find that around 60–80 % of total zonal-mean momentum flux is located over the open Southern Ocean during June–August, where a large "belt" of increased wave-occurrence frequencies, amplitudes and fluxes is observed. Our results also suggest significant short-timescale variability of fluxes from both orographic and non-orographic sources in the region. A particularly striking result is a widespread convergence of gravity-wave momentum fluxes towards latitudes around 60° S from the north and south. We propose that this convergence, which is observed at nearly all longitudes during winter, accounts for a significant part of the under-represented flux in GCMs at these latitudes

Wind variations in the mesosphere and lower thermosphere near 60°S latitude during the 2019 Antarctic sudden stratospheric warming

Sudden Stratospheric Warmings (SSWs) could act as an important mediator in the vertical coupling of atmospheric regions and dramatic variations in the mesosphere and lower thermosphere (MLT) in response to SSWs have been documented. However, due to rare occurrences, SSWs in the Southern Hemisphere (SH) and their impacts on the MLT dynamics are not well understood. This study presents an analysis of MLT winds at ∼80‐98 km altitudes measured by meteor radars located at Tierra del Fuego (53.7°S, 67.7°W), King Edward Point (54.3°S, 36.5°W) and King Sejong Station (62.2°S, 58.8°W) near 60°S latitude during the Antarctic winter. Eastward zonal winds from these stations are observed to decrease significantly near the peak date of the 2019 Antarctic SSW, and both zonal and meridional winds in 2019 exhibit considerable differences to the mean winds averaged over other non‐SSW years. A quasi 6‐day oscillation is observed at all three radar locations, being consistent with the presence of the westward propagating zonal wave‐1 planetary wave. The vertical wavelength of this wave is estimated to be ∼55 km, and the enhancement of the wave amplitude during this SSW is noticeable. Evidence of the interaction between the 6‐day wave and the semidiurnal diurnal tide is provided, which suggests a possible mechanism for SSWs to impact the upper atmosphere. This study reports the large‐scale variations in winds in the MLT region at SH mid‐to‐high latitudes in a key dynamic but largely unexplored latitudinal band in response to the 2019 Antarctic SSW