Long-term Changes of Water Vapor in the Stratosphere

Water vapor in the stratosphere plays an important role in the radiative and chemical balance of this region of the atmosphere. Although the basic processes controlling the water vapor distribution are known, details of the processes and the degree to which they are subject to change are less well understood. Longer term measurements of water vapor in the stratosphere are very limited. The only continuing measurement of water vapor in the stratosphere with a multi-decadal record is the balloon profile measurements from Boulder, Colorado that began in 1980. These soundings done on an approximately monthly basis until recently when the frequency has been increased to twice a month are made with a cryogenically cooled frost point hygrometer. Although the instrument has undergone a number of changes since its adaptation from an earlier design by John Mastenbrook, the basic principle and calibration scale have remained consistent. An important feature of the Boulder time series is its overlap with a number of satellite, balloon, and airborne measurements that have been made over the past two decades. These include satellite instruments such as SAGE II, HALOE and MLS on UARS, MLS and HRDLS on Aura, ACE-FTS, and MIPAS. Airborne and balloon sensors have included the Lyman á, TDL, and other chilled mirror instruments. A number of these observations have been compared in intercomparison exercises. Here several recent satellite observations from MLS and MIPAS over Boulder are compared with the balloon profiles. The time series of balloon water vapor observations at Boulder has shown a significant increase over the 25+ years of soundings. These increases moderated significantly beginning with the dramatic drop in lower stratospheric water vapor beginning in 2000 that has been noted in the satellite as well as the balloon data. More recently the balloon soundings suggest that these dryer conditions have begun to turnaround. Recent satellite observations over Boulder are investigated to determine if this recent change is confirmed in these data.