CFC11 MEASUREMENTS BY CRISTA
R. Spang, M. Riese and D. Offermann
Physics Department, University at Wuppertal, Gauss Str. 20 D-42097 Wuppertal, F.R.Germany
ABSTRACT
CFC11 is a good tracer for dynamical processes in the lower stratosphere. The CRyogenic IR Spectrometers and Telescopes for the Atmosphere instrument (CRISTA) measured CFC11 global maps between 16 and 24 km for 7 days in November 1994. The CFC11 radiance maps show small and large scale structures in various geographic regions. This data can be used for qualitative studies of transport processes in the northern and southern hemisphere, especially for the fine structures and the polar vortex edges.
INTRODUCTION
With a chemical life time of around 4 years at 20 km, CFC11 is a good tracer for horizontal transport in the lower stratosphere. Due to photodissociation, the chemical life time (tc) decreases rapidly with altitude (e.g. tc = 20 days at 30 km), and it yields to a strong vertical gradient in the mixing ratio profile of CFC11. This implies that CFC11 is a very sensitive indicator for vertical transport as well. Such a vertical stratified species provides an excellent tracer for testing theoretical models of mass transport and diffusion in the middle atmosphere (e.g. Andrews et al. 1987). CRISTA measures CFC11 in the wave number region 840 - 855 cm-1 (for details Riese et al., this issue). There is some contamination due to the emissions from aerosol, HNO3 and ozone in this band (in the order of 20-30% at 18 km). The CFC11 emission starts to be identified at tangent heights around 24 km and becomes dominant between 22 and 20 km. At the current state of the CRISTA data processing only a small part of retrieved CFC11 mixing ratio profiles are available. The results indicate the influence of the atmospheric temperature field on the observed small and medium scale structures is moderate. The radiance field is more sensitive to vertical and horizontal transports due to the steep gradients of CFC11. Therefore, structures in the CFC11 radiance fields (Type B1) will be used as a proxy for structures in mixing ratio fields.
GLOBAL SITUATION
The CRISTA experiment was described by Riese et al. (this issue). Figure 1 gives a global CFC11 radiance map at 18 km for the 6th of November 1994 (Day 310). The triple trace of the CRISTA instrument is easily recognized. In this measurement mode, the along-track distance of two adjacent data points is 200 km. The vertical width of the field of view is in the order of 1.5 km. Due to the limb geometry and the strong vertical gradient in CFC11 profile, about 80% of the measured signal originates from a 2 km thick layer above the tangent point at an altitude of 18 km. The radiance error is in the order of 1-3% (mainly due to attitude uncertainties).
Upward transports appear to be indicated by enhanced radiation in a belt 20° north and south of the equator. These areas are not equally distributed around the equator, but are centered around the Amazons, the Kongo and the Indonesia areas. These are regions of a strong upwelling of warm air and a strong vertical convection. Analysis of the measured spectra shows that the emissions are not gaseous but originate from grey bodies (i.e. aerosols and clouds presumably). In the northern and southern polar region downward transport is indicated by low radiances. A streamer of low radiances (blue dots) draws from the Canadian east coast to mid-latitude in the Pacific. Such extrusions play an important role in understanding the transport and mixing processes of the isolated vortex air with the mid-latitudes (Juckes and McIntyre, 1987, Waugh et al. 1994). Only the northern part of the south polar vortex could be reached by CRISTA (57° south). The radiances show steep vortex edges and very low values inside the vortex. This suggests unmixed downward flow bounded by the vortex edge.
NORTH POLAR REGION
During November 1994 the northern polar vortex started to build up from the upper stratosphere down to the lower altitudes. This typical situation for autumn time in the northern polar region was accompanied by an unusual high wave number 1 forcing, which has been the strongest wave number 1 activity for November in the last 17 years (Bacmeister 1996, NMC analyses). For a better illustration of the vortex form and streamer evolution only the lowest values of the CFC11 radiances are plotted in Figure 2. The polar projection of CFC11 radiances in Figure 2(a) shows a slightly shifted (wave no. 1) and elongated vortex which yields a wave no. 2 like structure (low values over Siberia, high over Alaska, low over north east Canada, high over the north Atlantic). In the Pacific region the streamer does not look like a continous filament. The standard potential vorticity maps of the NASA/Goddard/Data Assimilation Office (not shown here) point out a less generated vortex shape and only some weak indications of the streamer elements in Figure 2(a) at this altitude.
Figure 2 (b) and (c) give the vortex form und evolutuion at the 16 and 22 km level for the same time period. At 16 km only a small belt over Siberia can be found. This indicates the lower vortex edge at this altitude. The 22 km and also the 20 km level (not shown here) show almost the same form and dimension of the vortex as at the 18 km level. But the higher altitudes shows a more generated vortex shape and streamer than the 18 km level. No clear tilting in the vortex with altitude could be found. We have also analysed the day to day varibility of the vortex streamer structure and found a small drift from west to east for the origin of the filament. An example is given in Figure 2 (d) for Day 315 compared to Day 310 in Figure 2 (a). There are some changes in the form of the elongated vortex and no clear wave no. 2 structure is visible. This is a hint to a reduced wave number 2 forcing in this period of the CRISTA measurement, which agrees very well with northern hemisphere wave analyses by Naujokat et al. (1995).
(a) CFC11 / Day 310 / 18 km | (b) CFC11 / Day 310 / 16 km
 |
(c) CFC11 / Day 310 / 22 km
 | (d) CFC11 / Day 315 / 18 km
 |
SOUTH POLAR REGION
The spring time antarctic region is characterized by the rapid ozone
depletion inside the polar vortex. During the end of the
CRISTA mission the vortex was no longer centered around the pole and
had moved towards South America. Consequently, it was possible
for CRISTA with its 57° inclination orbit, to measure inside
the south polar vortex (see Figure 1 for Day 310, low radiances / violet dots
inside the vortex).
In order to explore the vortex edge and its vertical structure, data from
Day 315 (16 to 24 km in 2 km steps) was analysed. Figure 3 shows the results
with the radiances of the -56.5° to -57° belt for different
tangent heights on Day 315 (16 to 24 km in 2 km steps).
The rapid decrease of the radiances looks very simillar for all tangent
heights. This suggests a vortex that does not tilt with height.
The vortex boundary is extremly steep. Radiances go down to very
low values at all tangent heights, especially for the 18-20 km level
(please note the logarithmic scale in Fig. 3).
It is surprising that the radiances of these levels fall down to the
values of the 22 km level, which indicates very low CFC11 mixing ratios
inside the polar vortex.
By the analysis of the radiance profiles between 16 and 24 km inside and
outside the vortex it is possible to compute a scale size for the vertical
transport of 
5-6 km.
Under the assumption that the downward transport works for 4-5 months,
a descent rate of around 1.2 km/month can be estimated. This result is in good
agreement with earlier detailed studies by Schoeberl et al. (1995)
or Bacmeister et al. (1995).
Prelimenary mixing ratios of CFC11 retrieved from the radiances show clearly
reduced values inside the vortex (e.g. factor 2-3 at 20 km). This results in
a vertical transport scale in the same order as for the radiance analysis.
The data implies that there exists a CFC11 ``hole'' inside the southern
hemisphere vortex, due to the rapid downward transport taking place during
the winter-spring period.
The very steep vortex boundary between -130° and -100° longitude
indicates additional very slow horizontal transport.
An analysis of the western vortex boundary gives an upper limit for horizontal
eddy diffusion rate Kyy, if there is horizontal diffusion at all.
200 km from dot to dot.
Under the assumptions: typical eddy size of 100 km, a transition region for diffusion at the vortex edge of 1000 km (here: -130° to -110° longitude) and a time scale of 5 months/10, an upper limit of Kyy to 104 m2/sec can be estimated. This is a factor of 10 smaller than in recent publications like Bacmeister et al. (1995). This indicates a strongly isolated south polar vortex.
CONCLUSIONS
The first results of CFC11 measurements by CRISTA indicates that CFC11 limb radiances are a good tracer for qualitative examinations. Streamer structures in the northern hemisphere can be confirmed in PV analyses or by the CRISTA ozone and ClONO2 results (Bittner et al., Riese et al., this issue).
We found a rapid decrease of the radiances inside the south polar vortex and a strongly isolated vortex resulting in a CFC11-hole like state. The estimation of vertical descent and horizontal diffusion rate yields an unexpected low Kyy. This has to be checked by the retrieved CFC11 mixing ratio profiles.
ACKNOWLEDGEMENT
The CRISTA experiment is funded by the Bundesministerium für Bildung und Forschung (BMBF, Bonn) through Deutsche Agentur für Raumfahrtangelegenheiten (DARA, Bonn).
REFERENCES
