4.1.6. Atmospheric Water Vapor
Water vapor profile measurements continued at Boulder, Colorado. Some difficulties were experienced in adapting several replacement components in the balloonborne frost-point hygrometer. This led to a number of months in 1997 in which profiles of acceptable quality were not obtained. As reported earlier [Oltmans and Hofmann, 1995; Hofmann et al., 1996] stratospheric water vapor amounts have increased significantly since the beginning of the measurement program in 1981. Earlier results suggested that the increase in the vicinity was somewhat larger than that seen below 20 km. With the inclusion of recent data (Figure 4.4), the increase seems to be more uniform with altitude between 14-28 km. The increase of about 1% per year is almost twice as large as expected from the long-term increase in atmospheric methane. A plausible cause for such an increase is an enhanced injection of water vapor into the stratosphere [Evans et al., 1998]. This could occur if the tropical tropopause cold-trap temperature was gradually increasing. At present, there is not strong evidence for such a change in tropopause temperatures, although this change needs to be relatively small and may be difficult to detect in the temperature record.
Fig. 4.4. The linear trend as a function of altitude of the water vapor mixing ratio over Boulder, Colorado, for 1981-1997. The trend is in percent per year. The shaded area is the 95% confidence interval of the trend.