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第１回極域科学シンポジウム「極域大気圏を通して探る地球規模環境変動」12月2日（木） 国立極地研究所 ２階大会議室

第１回極域科学シンポジウム「極域大気圏を通して探る地球規模環境変動」ポスター発表

An analysis of the static stability and ozone vertical gradient in the ozone tropopause based (OTB) coordinate is applied to the ozonesonde data at 10 stations in the Southern Hemisphere (SH) extratropics. The tropopause inversion layer (TIL) with a static stability maximum just above the tropopause shows similar seasonal variations at two Antarctic stations, which are latitudinally far from each other. Since the sunshine hour varies with time in a quite different way between these two stations, it implies that the radiative heating due to solar ultraviolet absorption of ozone does not contribute to the seasonal variation of the TIL. A meridional section of the static stability in the OTB coordinate shows that the static stability just above the tropopause has a large latitudinal gradient between 60° S and 70° S in austral winter because of the absence of the TIL over the Antarctic. It is accompanied by an increase of westerly shear with height above the tropopause, so that the polar-night jet is formed above this latitude region. This result suggests a close relationship between the absence of the TIL and the stratospheric polar vortex in the Antarctic winter. A vertical gradient of ozone mixing ratio, referred to as ozone vertical gradient, around the tropopause shows similar latitudinal and seasonal variations with the static stability in the SH extratropics. In a height region above the TIL, a small ozone vertical gradient in the midlatitudes associated with the Antarctic ozone hole is observed in a height region of the subvortex but not around the polar vortex. This is a clear evidence of active latitudinal mixing between the midlatitudes and subvortex.

[1] We analyzed neutral winds, ambipolar diffusion coefficients, and neutral temperatures observed by the Nippon/Norway Tromso Meteor Radar (NTMR) and ion temperatures observed by the European Incoherent Scatter (EISCAT) UHF radar at Tromso (69.6°N, 19.2°E), during a major stratospheric sudden warming (SSW) that occurred in January 2009. The zonal winds at 80–100 km height reversed approximately 10 days earlier than the zonal wind reversal in the stratosphere and the neutral temperature at 90 km decreased simultaneously with the zonal wind reversal at the same altitude. We found different variations between geomagnetically quiet nighttime ion temperatures at 101–110 km and 120–142 km for about 10 days around the SSW. Our results from the ground-based observations agree well with the satellite observations shown in an accompanying paper. Thus, this study indicates that a SSW is strongly linked to thermal structure and dynamics in the high-latitude mesosphere, lower thermosphere, and ionosphere.

[1] Using hourly data from a three-year simulation based on a gravity-wave resolving general circulation model, we have first inferred a global view of gravity wave sources and propagation affecting significantly the momentum balance in the mesosphere. The meridional cross section of momentum fluxes suggests that there are a few dominant propagation paths originating from the subtropics in summer and the middle to high latitudes in winter. These gravity waves are focused into the mesospheric jets in their respective seasons, acting effectively to decelerate the jets. The difference in the source latitudes likely contributes to the hemispheric asymmetries of the jets. The horizontal distribution of the momentum fluxes indicates that the dominant sources are steep mountains and tropospheric westerly jets in winter and vigorous monsoon convection in summer. The monsoon regions are the most important window to the middle atmosphere in summer because of the easterlies associated with the monsoon circulation.

[1] The eastward moving 4-day wave in the Antarctic winter mesosphere is investigated using a high-resolution middle atmosphere general circulation model that directly simulates the spontaneous generation, propagation, and dissipation of gravity waves. The results are also compared with the simulated seasonal march of the meridional structures of the westerly jet streams in the Southern Hemisphere upper stratosphere and mesosphere in order to investigate baroclinic/barotropic instability as a possible excitation mechanism for the 4-day wave. The model successfully reproduces the dynamically unstable double-jet structure of the mesospheric westerly winds. The simulated 4-day wave develops in association with the baroclinic and barotropic instability of the mesospheric mean flows and has similar characteristics to those observed at 50–60°S near the stratopause. The 4-day wave has strong equatorward heat flux associated with strong baroclinicity in the Antarctic winter mesosphere, mainly attributable to poleward overturning circulation driven by gravity wave forcing. Eastward forcing due to the 4-day wave occurs within the double-jet structure and offsets part of the westward forcing due to gravity waves. Such an effect partially acts to stabilize the unstable mean flow structure in the Antarctic winter mesosphere.

[1] A wintertime temperature maximum can be observed at the subtropical stratopause but does not appear in the radiative equilibrium temperature distribution. This structure was well simulated using a high-resolution general circulation model and examined in detail. The stratopause temperature is latitudinally maximized in the winter subtropics because of a strong downwelling of the meridional circulation from summer tropics to winter subtropics passing above the stratopause. Its strong meridional flow consists of two parts: a strong poleward flow in the winter subtropics with nearly vertically aligned contours of absolute angular momentum (M) and a strong cross-equatorial flow toward winter hemisphere above the tropical stratopause with nearly horizontally aligned M contours. The strong poleward flow in the winter subtropics is driven by a large Eliassen-Palm (E-P) flux convergence in a small absolute vorticity region. The large E-P flux convergence is due to extratropical planetary waves and asymmetric inertial instability. On the other hand, the strong cross-equatorial flow is induced along the M contour to satisfy mass continuity with the strong poleward flow in the winter subtropics and exists in the easterly phase of S-SAO with a small latitudinal gradient of M. The easterly of S-SAO during its mature phase acts as a corridor for cross-equatorial meridional circulation, while cross-equatorial meridional circulation contributes to driving the easterly of S-SAO in its development phase.

[1] A high-resolution middle atmosphere general circulation model (GCM) developed for studying small-scale atmospheric processes is presented, and the general features of the model are discussed. The GCM has T213 spectral horizontal resolution and 256 vertical levels extending from the surface to a height of 85 km with a uniform vertical spacing of 300 m. Gravity waves (GWs) are spontaneously generated by convection, topography, instability, and adjustment processes in the model, and the GCM reproduces realistic general circulation in the extratropical stratosphere and mesosphere. The oscillations similar to the stratopause semiannual oscillation and the quasi-biennial oscillation (QBO) in the equatorial lower stratosphere are also spontaneously generated in the GCM, although the period of the QBO-like oscillation is short (15 months). The relative roles of planetary waves, large-scale GWs, and small-scale GWs in maintenance of the meridional structures of the zonal wind jets in the middle atmosphere are evaluated by calculating Eliassen-Palm diagnostics separately for each of these three groups of waves. Small-scale GWs are found to cause deceleration of the wintertime polar night jet and the summertime easterly jet in the mesosphere, while extratropical planetary waves primarily cause deceleration of the polar night jet below a height of approximately 60 km. The meridional distribution and propagation of small-scale GWs are shown to affect the shape of the upper part of mesospheric jets. The phase structures of orographic GWs over the South Andes and GWs emitted from the tropospheric jet stream are discussed as examples of realistic GWs reproduced by the T213L256 GCM.

The on-board small vacuum gauge developed in this study is the quartz friction gauge, which is based on the principle that the resonance impedance of a quartz oscillator varies with the pressure of ambient gas. The gauge has a wide measuring range (105-10-2 Pa), which corresponds to the atmospheric pressure from the ground to an altitude of about 100 km. The sensor part and data processing part weigh 136 g and 210 g, respectively. In addition, the gauge can operate at a power consumption level of less than 1W. The gauge is suited for high altitude balloon experiments where the weight of on-board instruments must be kept to a minimum. The tuning-fork-shaped quartz oscillator in the gauge is widely used for wrist watches and has high resistance to vibration and shock, and thus the gauge is applicable to sounding rocket experiments that require on-board instruments with high resistance to the environment.

[1] Turbulent diffusivity and turbulent ozone fluxes in the tropo-stratosphere are studied employing simultaneous observations with the Middle and Upper (MU) Atmosphere radar and ozonesondes in Shigaraki, Japan during April 16–24, 1998. A broad region around the tropopause was dynamically active. Maxima of turbulent diffusivity were observed at 8–14 km altitude. Such maxima may produce vertical turbulent ozone fluxes across the tropopause with magnitudes comparable to those required for the global ozone budget. Meso-meteorological ozone intrusions may enhance the fluxes.

[1] In order to examine gravity wave characteristics over the ocean, we conducted a radiosonde observation campaign to scan the stratosphere meridionally over the middle Pacific with Hakuho-maru research vessel operated by the Ocean Research Institute, University of Tokyo. Seventy vertical profiles of temperature, winds, and humidity were successfully obtained over a wide latitude range of 28°N to 48°S with an interval of 1 degree. Gravity wave energy and vertical wavenumber spectra were examined as a function of latitude. Gravity wave energy is maximized in the tropical region where the convection is active and in the middle latitude region where the subtropical westerly jet is situated. The wave energy in the tropical region is greater than the climatology based on operational radiosonde observation over land regions. The gravity wave amplitude does not necessarily increase with altitude, in contradiction to simple vertical propagation. The latitudinal distribution of wave energy rather suggests that gravity waves propagate meridionally toward higher latitudes. An interesting feature is that a wave packet having large horizontal wind and temperature amplitudes is observed continuously over 7 days in the latitudinal region extending 26 degrees. Results of hodograph analysis indicate that the wave packet is due to an inertia-gravity wave with horizontal and vertical wavelengths of about 1800 km and 4–5km, respectively. This inertia-gravity wave is likely generated in association with vigorous convection in the Intertropical Convergence Zone (ITCZ) in the Northern Hemisphere and propagated southward interhemispherically. The results in this study show that there are strong nonstationary gravity wave sources over the ocean. The importance of meridionally propagating gravity waves is discussed.

[1] An intensive observation with ozonesondes, radiosondes, and the Middle and Upper atmosphere (MU) radar was conducted at Shigaraki (34.9°N, 136.1°E), Japan, from 16 through 24 April 1998 to investigate the formation mechanism of ozone laminae in the midlatitude lower stratosphere. In these observations the temporal variation of an ozone lamina at Shigaraki was continuously observed through a short time interval of 8 hours. During the observation period an ozone lamina with a thickness of 2–3 km appeared and ascended with a speed of ∼1 km day−1 in the height region of 18–21 km. It is deduced that inertia gravity waves existing in this height region did not contribute to formation of the ozone lamina, because their amplitudes were too small and their vertical phase propagation was in the opposite direction. The reverse domain filling method was used to reconstruct the “high-resolution” modified potential vorticity field using backward trajectory calculations. The results indicate that this ozone lamina was caused by differential advection due to the vertical shear associated with the subtropical westerly jet and embedded stationary Rossby wave therein. This means that large-scale motions concurrent with vertical wind shear can produce much smaller-scale structures in material fields than those of the large-scale motions themselves.

A grating spectrometer for hydroxyl (OH) airglow installed at Syowa Station (69°S, 39.6°E) by the 49th Japanese Antarctic Research Expedition (JARE49) has been in operation since late February, 2008. A dataset of 153 nights was acquired at this location in the austral winter season of 2008. This dataset shows variations in the rotational temperature over a range of temporal scales. The rotational temperature around the polar mesopause region is high in winter and decreases toward summer, which is a pattern similar to that observed at Davis Station, located at almost the same latitude as Syowa Station. A large temperature variation with a period of several days was observed in early May, 2008. Based on a comparison with a simultaneous dataset obtained by the SABER instrument onboard the TIMED satellite, it is inferred that this rotational temperature variation was due to the modulation of vertical motions around the mesopause.

[1] A global distribution and seasonal variability of short-period (