"The correspondence of total ozone to isentropic pressure suggests the former as a diagnostic of vertical air motion in the lower stratosphere . . . . Ultimately, this application of total ozone measurements is limited by the once daily asynoptic sampling of TOMS, which is inadequate to resolve small scale structure continuously in time."
Salby and Callaghan, 1993
Triana does indeed fulfill this gap.
Upper atmosphere dynamics will be studied using ozone as a tracer together with data assimilation models and direct high time and space resolution observations from Triana. The first-time use of sunrise to sunset data will greatly improve the retrieval of winds and wave structure through data assimilation.
Waves can produce an uplift of stratospheric layers in certain regions and down drafts in others. When an air parcel goes up, its pressure diminishes and so does its ozone partial pressure (it is important to note that it is the partial pressure that decreases and not the ozone mixing ratio). When the layer where most of the ozone is concentrated is uplifted, the total ozone, i.e. the ozone content of a unit section column, diminishes. Thus from total ozone measurements one can detect atmospheric waves. This has been demonstrated by comparing TOMS or TOVS measurements with NMC or ECMWF analyses. However, the construction of TOMS or TOVS total ozone images requires a time lag of several hours, during which the spatial structures may vary; this will not be the case with TRIANA data, which will provide instantaneous views of the total ozone field.
The ozone fields retrieved from TOMS and TOVS have indeed been used up to now to detect planetary waves. This has been possible because the spatial extent of these waves is large and their motion relatively slow. We believe however that Triana will bring a better monitoring of planetary waves, due to its instantaneous planetary view associated with high temporal resolution.
Figure 24a displays the ozone field as Triana can view it ; the map has been constructed using total ozone observations from TOMS. The high ozone zones found around the Arctic region are the signatures of planetary waves (Teitelbaum et al., 1998). Figure 24b shows the corresponding geopotential field on the 475K isentropic surface, calculated from ECMWF analyses. Comparing Figure 24a and 24b, it is clear that high ozone zones correspond to downward motions of isentropic surfaces.
In addition, the space resolution of Triana will allow an almost continuous monitoring of gravity waves, whose small horizontal scale could not hitherto be resolved by TOMS or TOVS. In particular, Triana should be able to detect the variations of total ozone content induced by large vertical uplifts of air masses within localized areas, associated with orographic waves propagating much higher than the tropopause. The detection of other types of gravity waves, such as those triggered by deep convection, frontogenesis or jet instabilities, is still open to discussion. Knowing more about the distribution of gravity waves in the stratosphere (especially orographic waves) is an important input for general circulation models.
Figure 24 shows the southern ozone hole surrounded by a border where there is a strong gradient of total ozone. This border in general coincides with the vortex edge defined on an isentropic surface near 475K. This is illustrated in Figure 24c that depicts the corresponding Ertel potential vorticity (EPV) map at 475K calculated from ECMWF analyses. The Antarctic polar vortex appears on the Ertel¹s potential vorticity (EPV) map. The equatorward edge of the vortex region is shown as a thick line in the figure.
On the other side of the Earth, view centered at 45°W, the structure of ozone (Figure 25) is very different. Such a structure appears when an uplift of isentropic surfaces occurs in the vortex edge region. Then the edge dilates by separation of potential vorticity isolines producing what has been called "macrofilaments" (Teitelbaum et al., 1998). It is clear that Triana will see this type of structure with higher spatial and temporal resolution. Contrary to the filaments produced by horizontal velocity gradients, which lead to fine structures and mixing in an irreversible process, "macrofilaments" are partly due to elastic, meteorological reversible processes. It is important to study how such reversible processes do affect the mixing of air masses and diffusion across the vortex edge region.
Triana will allow the study of the existence of EPV anomalies, anticyclones and cyclones in the vicinity of the tropopause, and their displacements with a precision not yet attained. Ozone miniholes are localized regions (a few thousands of km2) of low total ozone content. The dynamical basis is explained in Hoskins et al. (1985). In the vicinity of the tropopause differential advection often produces a localized decrease (increase) of Ertel¹s potential vorticity; the EPV decrease (increase) appears together with an anticyclone (cyclone). The EPV anomaly extends its influence upward under the form of an uplift of air masses in the case of an anti-cyclonic wind, or a downward motion when the wind is cyclonic. The consequences on total ozone of this vertical movement are discussed by Salby and Callaghan (1993).
An example is shown in Figure 26a. The total ozone TOMS map shows two localized increases (+) and one decrease (-‹) of ozone. In Figure 26b we can see the corresponding EPV map on the 325K isentropic surface. EPV anomalies appear at the same geographical positions as the total ozone anomalies. Finally Figure 26c shows the wind at 300 mb; one anticyclone and two cyclones are seen in the wind field.
We can add another possibility although of some speculative character. Mini-holes are the signature of an uplift of isentropic surfaces and then of the cooling of air masses. When the season and the latitude indicate the possibility of low background temperature, this uplift decreases the temperature further and may induce the formation of a PSC. The relationship between uplifts, miniholes, and PSC has been shown in McKenna et al. (1989) and in Teitelbaum and Sadourny (1998).
Triana measurements may also be most useful for detecting the filaments induced by quasi-two-dimensional differential advection in the stratosphere.
Fine scale layering of the lower stratosphere is often observed in ozone vertical or horizontal profiles. It was demonstrated recently that those laminae in ozone profiles which cannot be explained by gravity waves are essentially associated to filamentary structures generated by differential advection along isentropic surfaces. Up to now, the existence of filaments has only been proven in numerical simulations by the means of contour dynamics (Dritschel and Saravanan, 1994); the only experimental support is partial and relies on aircraft observations and vertical soundings (Waugh et al., 1994). Triana has the potential to provide us for the first time a full two-dimensional view of the filaments and their evolution in time. Modeling and theoretical considerations suggest that, in absence of vigorous vertical mixing, these filaments should survive for more than two weeks until their vertical scale is reduced to a few tens of meters and horizontal scale to about ten kilometers. The production of such filaments at the vortex edge is critical for the exchanges and mixing of air masses between inside and outside the polar stratospheric vortex. In particular, during the polar night, they can induce transport of chemically perturbed vortex air to mid-latitudes, resulting in photochemical ozone destruction there; in late winter or spring, filaments can also transport ozone depleted vortex air to mid-latitudes. Present observations such as the ones by TOMS are unable to resolve such filamentary structures, and similarly the crude resolution of operational meteorological analyses produces filtered potential vorticity maps that do not resolve these filaments.
Although filaments are local structures both in the vertical and in the horizontal, high-resolution total ozone will be helpful to detect these structures when located near the altitude of ozone highest concentration (level of potential temperature about 475-500K). Calculations done with profiles with laminae show that the variation in the total ozone may be of the order of 5% to 20%, well within the accuracy of Triana instruments.
It is clear that the possibility to follow almost continuously the deformation of such structures will bring new information on lower stratosphere dynamics. Tracking these filaments will bring direct information on the winds. (All major weather forecasting centers are already preparing the assimilation of tracers such as ozone in their operational analysis systems.) Observation of the filamentary structures will bring valuable information on the evolution of small-scale structures and mixing processes in the lower stratosphere and allow studying their relationship with gravity and orographic waves. They will be very useful to validate high-resolution transport studies and chemical models.
Possible synergism of Triana with other space missions like UARS, POAM, and ENVISAT are being studied. We are also considering complementing ozone with other dynamics tracers like aerosols and possibly PSCs. In parallel, we plan to use our second generation atmospheric GCM (LMDZ-T) whose vertical resolution is currently being increased to 50 levels to simulate and eventually assimilate Triana data.
All fields are represented here from a Triana viewpoint. The season is close to the spring equinox, the most interesting period for investigating the polar vortex in connection to the ozone hole. We suppose that the phasing of Triana on the Lissajous orbit can be programmed in such a way that at the equinoxes the Earth can be seen at the maximum angle of about 15 degrees with respect to the Earth-Sun axis, allowing maximum visibility of the spring side polar region.
