Quadrennial Ozone Symposium Preparing Final Camera Ready high-occurrence season0

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					Irreversible vertical mixing of ozone caused by inertio-gravity

wave breaking in the lower stratosphere
K. Noguchi, T. Imamura and K. -I. Oyama
Japan Aerospace Exploration Agency, Tokyo, Japan

Abstract. The irreversible vertical mixing of ozone due                                           Shear instability region   Altitude
                                                                 Group velocity     Wave front
to the breaking of inertio-gravity waves is investigated
based on a case study with the aid of ozone density and
meteorological data obtained by ozonesondes at Santa Cruz        Phase velocity
(28.46˚N). The altitude regions where the conditions of
shear instability and convective instability were satisfied
had locally flat vertical distributions of ozone mixing ratio
                                                                                                                                 Wind velocity in the
and potential temperature. A hodograph analysis showed                                                                           direction of horizontal
                                                                         Material surface    Convective instability region
that a gravity wave observed simultaneously caused such                                                                          propagation
instabilities and the vertical mixing of potential tempera-
ture and ozone. Statistical analysis is also conducted to        Figure 1. Schematic of the regions of shear instability and con-
show how often the condition of shear instability is satis-      vective instability in the field of a gravity wave whose group ve-
                                                                 locity is upward.
fied in the lower stratosphere.
                                                                 Case study of vertical mixing
                                                                    The ozonesonde data which are used in the present study
    It has been recognized that the Brewer-Dobson circula-       are supplied by World Ozone and Ultraviolet Radiation
tion governs the transport of trace gases in the stratosphere    Data Centre (WOUDC), which contains ozonesonde data
[Holton et al., 1995]. Besides the large-scale vertical          obtained at over 400 stations in the world. We pick up
transport associated with the Brewer-Dobson circulation,         Santa Cruz (28.46˚N) stations since the data of Santa Cruz
the vertical mixing by small-scale sporadic turbulence due       have high vertical resolution (~20 m).
to gravity wave breaking also causes the transport across           Figure 2 shows an example of the case where tempera-
isentropic surfaces [Lindzen, 1981; Garcia and Solomon,          ture and ozone distributions seem to be strongly affected by
1983]. In the present study, the contribution of gravity wave    the vertical advection and mixing associated with gravity
breaking to the vertical mixing of ozone is discussed using      wave breaking. At altitudes of 15-25 km, the zonal and me-
ozone density data and meteorological data obtained by           ridional wind velocity profiles have distinct wavelike
ozonesondes.                                                     structures, which strongly suggest that a gravity wave ex-
Conditions of instabilities
    Small-scale vertical mixing is caused by the breaking of
gravity waves through shear instability and convective in-
stability [Fritts and Rastogi, 1985]. The condition for
convective instability is given by θ/z < 0, where θ is po-
tential temperature and z the altitude, and the condition for                                                                    Meridional
shear instability is Ri < 0.25, where Ri is the Richardson                                                                       wind
number defined by
                                       2                                  Ozone
                                 u 
                       Ri  N 2   .
                                 z 
Here N is the Brunt Väisälä frequency defined by
                                  ln 
                        N2  g          ,
where g is the gravitational acceleration and u the horizon-                                                         Zonal
tal wind. In the middle atmosphere, these instabilities are                                                          wind
usually initiated by gravity waves as illustrated in Figure 1.
Taking the coordinate of wind velocity fluctuation u’ in the
direction of horizontal propagation, convective instability
occurs around the regions where u’ peaks, while shear in-
stability occurs around the regions where u’ ≈ 0. The re-
gions where these instabilities occur should have vertical-
ly-smoothed distributions of tracers and potential tempera-
ture due to vigorous vertical mixing. Such regions are easi-     Figure 2. Ozonesonde data on August 11, 1999 at Santa Cruz
ly found in ozonesonde data, as shown later.                     (28.46˚N). (Left) Ozone mixing ratio and temperature. (Right)
                                                                 Zonal and meridional wind velocity.
   To investigate the relationship between these small-scale           shear or convective instability at Santa Cruz (28.46˚N). The
structures and the instabilities which are illustrated in Fig-         altitude where the probability of ~0.25 occurs varies with
ure 1, ozone mixing ratio and potential temperature are                season similarly to that of the tropopause, below which the
compared with N2 and Ri in Figure 3. The vertical distribu-            instability occurs more frequently than above. This result is
tions of ozone mixing ratio and potential temperature tend             consistent with the fact that the background atmosphere is
to be locally flat in the regions where N2 ~ 0 and/or Ri <             less stable in the troposphere than in the stratosphere.
0.25, i.e., the condition of convective and/or shear instabil-              The probability decreases above the tropopause more
ity is satisfied. Then, it is suggested that vertical mixing has       sharply in summer than in winter. The activity of gravity
occurred in those regions due to the instabilities. Consi-             waves is inferred from temperature data. As the index of
dering that these regions have thickness of 200-300 m and              gravity wave activity, the potential energy
typically include 10-15 data points in each layer, the fea-                                         1 g2 T                    (1)
                                                                                              Ep          
                                                                                                    2 N 2  T0 
tures should not be artificial.
                                                                                                           
                                                                       is adopted, where T’ is the fluctuation component of tem-
                                                                       perature and T0 the background component of temperature.
                                                                       Comparison of the probability distribution (Figure 4a) with
                                                                       the Ep distribution (Figure 4b) suggests that the gravity
                                                                       wave enhancement around the tropopause cause more in-
                                                                       stability in winter than in summer. This is consistent with
                                                                       the results of the radar observations of the eddy diffusivity,
                                                                       which were shown to have a maximum around the winter
                                                                       tropopause jetstream [Fukao et al., 1994]. The present
                                                                       study suggests that such an enhancement of the eddy diffu-
                                                                       sivity is attributed to an active occurrence of turbulence by
                                                                       shear instability due to gravity waves.

         Potential                                                          (a)                               (b)

Figure 3. An altitude section of the ozonesonde observation
shown in Figure 2. (a) Ozone mixing ratio and potential tempera-
                                                                                  Month                              Month
ture. (b) Static stability. (c) Richardson number. (d) Wind velocity
fluctuation with λz ≤ 3 km along the direction of horizontal prop-     Figure 4. (a) Probability of the occurrence of shear or convective
agation. Shaded regions indicate the regions where shear instabil-     instability at Santa Cruz (28.46˚N). (b) Potential energy of gravity
ity or convective instability is expected to occur.                    waves with λz ≤ 2 km defined by (1) in unit of J kg-1. White cir-
                                                                       cles in (a) and (b) indicate tropopause.
  In order to relate the regions of instability to the gravity
wave field, a hodograph analysis is applied to the data                    Acknowledgments       The ozonesonde data are supplied by
adopting the procedure described by Hamilton [1991]. The               WOUDC. I thank Dr. Yoshihiro Tomikawa of National Institute of
result suggests that the observed fluctuations of wind and             Polar Research (NIPR), Japan for his useful comments and dis-
                                                                       cussion. The global ozonesonde data are supplied by World
temperature are attributed to an inertio-gravity wave with             Ozone and Ultraviolet Radiation Data Centre.
horizontal wavelength λx = 410 km, vertical wavelength λz =
1.0 km and the intrinsic period of 19.8 hours. The propaga-            References
tion direction was also determined, and u’ in this direction
is compared with other variables in Figure 3. Considering              Fritts, D. C. and Rastogi, P. K., Convective and dynamical insta-
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while the region D, convective instability.                            Fukao, S., M. D. Yamanaka, N. Ao, W. K. Hocking, T. Sato, M.
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occur can be answered by evaluating the ratio of the num-              Hamilton, K., Climatological statistics of stratospheric iner-
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                                                                       Holton, J. R., P. H. Haynes, M. E. McIntyre, A. R. Douglass, R. B.
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Description: Quadrennial Ozone Symposium Preparing Final Camera Ready high-occurrence season0