4.2.2. Stratospheric Water Vapor as Part of the Observations of the Middle Stratosphere (OMS) Program
The Observations of the Middle Stratosphere (OMS) project was established to investigate the transport of atmospheric tracers in the stratosphere at altitudes generally inaccessible to high altitude aircraft. A balloon package was developed that is capable of measuring longer-level compounds to altitudes near 35 km. We designed and developed a package for the balloon gondola to measure water vapor. In addition, we also flew free-flier balloons that were launched in conjunction with the main gondola launches. Campaigns were carried out from Fort Sumner, New Mexico, in October 1996; from Fairbanks, Alaska, in July 1997; and from Juazeiro do Norte, Brazil, in February and November 1997. Unfortunately useable data were not obtained from the Fairbanks campaign due to multiple instrument problems. In all of the campaigns the gondola water vapor instrument gave limited data because of contamination from the gondola. Several attempts at improving the performance were not successful and this instrument will not be used in future campaigns. The free-flier balloon packages, on the other hand, yielded valuable data.
The flight in February 1997 at Juazeiro do Norte, Brazil, came near the end of the northern hemisphere (NH) winter when water vapor concentrations in the tropical lowermost stratosphere are at a minimum. This corresponds to the time of coldest tropical tropopause temperatures. The cold tropopause temperature (particularly) in the western tropical Pacific impress a signal on the air passing through the tropopause by drying the air to the frost-point temperature. This mechanism is very nicely shown in several profiles obtained near the equator (Figure 4.8) including the two from Brazil. During the Central Equatorial Experiment (CEPEX) in 1993 water vapor profiles were obtained in the western and central Pacific. These profiles are similar in shape and mixing ratio except in the lowest part of the stratosphere between the 350 K and 400 K potential temperature surfaces (Figure 4.8). This is a result of the much colder tropopause temperatures in the western Pacific. The profile obtained in Brazil at roughly the same time of year (but 4 years later) is remarkably similar to the profile at Christmas Island. In particular, the minimum near 400 K and the maximum near 460 K are reproduced in each profile. The profile of November 1997 from Brazil, on the other hand, shows a different vertical structure, although a series of minima and maxima are clearly present. In November a minimum is developing near the tropopause reflecting the appearance of cold temperatures in the equatorial region. This does not reflect a local drying of the air parcels as they pass through the tropopause over Brazil, however, since the frost-point temperature is much colder than saturation at the local tropopause could produce. This is also true for February 1997 over Brazil and March 1993 over Christmas Island. In fact, only in the western Pacific do the tropopause temperatures reach the required low values. Outside of the western Pacific the mixing ratios do not maintain the very low values suggesting that the air is modified as it is mixed within the equatorial zone. During the NH summer when the equatorial tropopause temperatures are warmer, moister air passes through the tropopause. The rising air motion in the tropical stratosphere [Holton et al., 1995] propagates the seasonal tropopause signature upward [Mote et al., 1996]. Thus the minimum seen over Brazil at 400 K in February appears at about 460 K in November. The minima and maxima propagate upward at somewhat different rates because of the difference in the ascent rate of the air during different times of the year. These signatures can also be attenuated by lateral mixing in and out of the tropics. The ability to identify the features in individual profiles does suggest that this lateral mixing is somewhat limited, however [Volk et al., 1996].
Fig. 4.8. Water vapor mixing ratio profiles from three locations along the equator.