Atmospheric effects of radiation belt precipitation over Antarctica

第3回極域科学シンポジウム　横断セッション「中層大気・熱圏」 11月26日（月） 国立極地研究所 ２階大会議

Observational evidence of the influence of Antarctic stratospheric ozone variability on middle atmosphere dynamics

Modeling results have suggested that the circulation of the stratosphere and mesosphere in spring is strongly affected by the perturbations in heating induced by the Antarctic ozone hole. Here using both mesospheric MF radar wind observations from Rothera Antarctica (67°S, 68°W) as well as stratospheric analysis data, we present observational evidence that the stratospheric and mesospheric wind strengths are highly anti-correlated, and show their largest variability in November. We find that these changes are related to the total amount of ozone loss that occurs during the Antarctic spring ozone hole and particularly with the ozone gradients that develop between 57.5°S and 77.5°S. The results show that with increasing ozone loss during spring, winter conditions in the stratosphere and mesosphere persist longer into the summer. These results are discussed in the light of observations of the onset and duration of the Antarctic polar mesospheric cloud seaso

Characteristics and sources of gravity waves observed in noctilucent cloud over Norway

Four years of noctilucent cloud (NLC) images from an automated digital camera in Trondheim and results from a ray-tracing model are used to extend the climatology of gravity waves to higher latitudes and to identify their sources during summertime. The climatology of the summertime gravity waves detected in NLC between 64 and 74° N is similar to that observed between 60 and 64° N by Pautet et al. (2011). The direction of propagation of gravity waves observed in the NLC north of 64° N is a continuation of the north and northeast propagation as observed in south of 64° N. However, a unique population of fast, short wavelength waves propagating towards the SW is observed in the NLC, which is consistent with transverse instabilities generated in situ by breaking gravity waves (Fritts and Alexander, 2003). The relative amplitude of the waves observed in the NLC Mie scatter have been combined with ray-tracing results to show that waves propagating from near the tropopause, rather than those resulting from secondary generation in the stratosphere or mesosphere, are more likely to be the sources of the prominent wave structures observed in the NLC. The coastal region of Norway along the latitude of 70° N is identified as the primary source region of the waves generated near the tropopause

Seasonal variations in the horizontal wind structure from 0-100 km above Rothera station, Antarctica (67° S, 68° W)

A medium frequency spaced-antenna radar has been operating at Rothera station, Antarctica (67° S, 68° W) for two periods, between 1997-1998 and since 2002, measuring winds in the mesosphere and lower thermosphere. In this paper monthly mean winds are derived and presented along with three years of radiosonde balloon data for comparison with the HWM-93 model atmosphere and other high latitude southern hemisphere sites. The observed meridional winds are slightly more northwards than those predicted by the model above 80 km in the winter months and below 80 km in summer. In addition, the altitude of the summer time zero crossing of the zonal winds above the westward jet is overestimated by the model by up to 8 km. These data are then merged with the wind climatology obtained from falling sphere measurements made during the PORTA campaign at Rothera in early 1998 and the HWM-93 model atmosphere to generate a complete zonal wind climatology between 0 and 100 km as a benchmark for future studies at Rothera. A westwards (eastwards) maximum of 44 ms-1 at 67 km altitude occurs in mid December (62 ms-1 at 37 km in mid July). The 0 ms-1 wind contour reaches a maximum altitude of 90 km in mid November and a minimum altitude of 18 km in January extending into mid March at 75 km and early October at 76 km

Trends and variability of mesospheric temperature at high-latitudes

Using ground-based measurements of the hydroxyl (OH) Meinel (3,1) band nightglow near 1500 nm, nightly means of mesospheric temperature and OH radiance from 1991 to 1998 have been derived over Stockholm (59.5degreesN, 18.2degreesE). Time-series analysis techniques applied both to the eight-year data set as well as to an annual superposed epoch revealed several statistically significant periodic components. A trend analysis that included these periodic components revealed a small positive trend over the eight-year temperature time series. However, examining the trends on a mouth-to-mouth basis revealed positive trends during winter, small negative trends during equinox, and no significant trend during summer. This seasonal variability indicates that dynamic feedbacks, rather than radiative forcing of the mesosphere by infrared active gases, may dominate the response of the mesosphere to greenhouse gas emissions. In support of this an examination of the variability in the superposed epoch of OH temperature and radiance showed strong impulses near equinox. A simple gravity-wave transmission and dissipation model indicates that these are due in part to seasonal increases in the gravity-wave transmission of the lower atmosphere, and enhanced wave heating and mixing in the mesosphere. (C) 2002 Elsevier Science Ltd. All rights reserved

Equilibrium temperature of water-ice aerosols in the high-latitude summer mesosphere

Previous models of the equilibrium temperature and existence regions of mesospheric aerosols have shown significant radiative heating of the aerosols and, consequently, a substantially reduced existence region. We have developed an iterative model that extends this previous work by incorporating a complete collisional energy transfer algorithm, including the effects of vertical winds and particle fall velocity, that is appropriate for the free molecular flow conditions found in the mesosphere. We have also updated the ice refractive index used in the model and accounted for the dependence of the radiative heating and collisional cooling terms on particle tempe;ature. Finally, a radiation model has been used to calculate the solar, terrestrial and atmospheric radiative inputs including the effects of multiple scattering and atmospheric absorption. As with the previous models, the particle temperature is calculated under steady-state conditions, assuming the background gas temperature remains constant and the aerosol does not change size, state or altitude. Under these conditions, the largest differences from previous models occur as a result of the updated ice index of refraction, particularly in the visible, which produces significantly less aerosol heating. These temperatures are combined with the observed properties of mesospheric aerosols to place limits on the water vapour mixing ratio, vertical-wind speeds, and maximum particle sizes. It is found that H2O mixing ratios of 10 ppmv and vertical winds of order 0.02 m s(-1) are consistent with observed particle distributions, and these lead to a radiative limit on the maximum particle radius of 250 nm

Mean winds and tides in the mesosphere and lower thermosphere above Halley, Antarctica

An imaging Doppler interferometer (IDI) at Halley, Antarctica (76°S, 26°W) has been used to record near continuous mean winds in the mesosphere/lower thermosphere since December 1996. Monthly mean winds are calculated between 75 and 105 km for comparison with the HWM93 model winds. Below 95 km the zonal mean winds are 5–10 m s−1 eastward weakening to 0 m s−1 in summertime. Between 95 and 105 km a wintertime eastward wind (0–5 m s−1) strengthens to 17 m s−1 in summer. Above 95 km the meridional wind is northward (5–10 m s−1) in winter turning strongly southward (11 m s−1) in summertime, with weaker northward winds below this strengthening around equinox. Excellent agreement is found between the IDI-measured zonal winds and the model except for mid-summer when the model winds between 75 and 95 km are much stronger westwards than those observed. IDI meridional winds are found to agree well with the model across the entire height range in mid-summer and mid-winter with a bias of around 8 m s−1 northwards found around the equinoxes. A wavelet analysis of the entire data set shows the inter- and intra-annual variation in waves with periods between 12 h and 30 days as well as their relative strength as a function of height between 75 and 105 km. Strong 12 and 24 h components are observed in both the zonal and meridional winds increasing in amplitude with height and peaking in summer. The phase of the 24 h wave is seen to vary from one year to the next though on average it is evanescent during the summer months with increased phase scatter during winter. The 12 h wave is vertically propagating all seasons and demonstrates a strong phase transition around equinox in both the zonal and meridional components. The nature of the 12 h wave is more consistent with that seen at South Pole and Scott Base/McMurdo than at lower latitudes suggesting that the non-migrating zonal wavenumber s=1 12 h wave extends at least 15° north of pole in the southern hemisphere

Regional variations of mesospheric gravity-wave momentum flux over Antarctica

Images of mesospheric airglow and radar-wind measurements have been combined to estimate the difference in the vertical flux of horizontal momentum carried by high-frequency gravity waves over two dissimilar Antarctic stations. Rothera (67° S, 68° W) is situated in the mountains of the Peninsula near the edge of the wintertime polar vortex. In contrast, Halley (76° S, 27° W), some 1658 km to the southeast, is located on an ice sheet at the edge of the Antarctic Plateau and deep within the polar vortex during winter. The cross-correlation coefficients between the vertical and horizontal wind perturbations were calculated from sodium (Na) airglow imager data collected during the austral winter seasons of 2002 and 2003 at Rothera for comparison with the 2000 and 2001 results from Halley reported previously (Espy et al., 2004). These cross-correlation coefficients were combined with wind-velocity variances from coincident radar measurements to estimate the daily averaged upper-limit of the vertical flux of horizontal momentum due to gravity waves near the peak emission altitude of the Na nightglow layer, 90km. The resulting momentum flux at both stations displayed a large day-to-day variability and showed a marked seasonal rotation from the northwest to the southwest throughout the winter. However, the magnitude of the flux at Rothera was about 4 times larger than that at Halley, suggesting that the differences in the gravity-wave source functions and filtering by the underlying winds at the two stations create significant regional differences in wave forcing on the scale of the station separation

Seasonal variations in the horizontal wind structure from 0-100 km above Rothera station, Antarctica (67° S, 68° W)

A medium frequency spaced-antenna radar has been operating at Rothera station, Antarctica (67° S, 68° W) for two periods, between 1997-1998 and since 2002, measuring winds in the mesosphere and lower thermosphere. In this paper monthly mean winds are derived and presented along with three years of radiosonde balloon data for comparison with the HWM-93 model atmosphere and other high latitude southern hemisphere sites. The observed meridional winds are slightly more northwards than those predicted by the model above 80 km in the winter months and below 80 km in summer. In addition, the altitude of the summer time zero crossing of the zonal winds above the westward jet is overestimated by the model by up to 8 km. These data are then merged with the wind climatology obtained from falling sphere measurements made during the PORTA campaign at Rothera in early 1998 and the HWM-93 model atmosphere to generate a complete zonal wind climatology between 0 and 100 km as a benchmark for future studies at Rothera. A westwards (eastwards) maximum of 44 ms-1 at 67 km altitude occurs in mid December (62 ms-1 at 37 km in mid July). The 0 ms-1 wind contour reaches a maximum altitude of 90 km in mid November and a minimum altitude of 18 km in January extending into mid March at 75 km and early October at 76 km