WVMS: Measuring Water Vapor in the Middle Atmosphere

G.E. Nedoluha, R.M. Bevilacqua, R.M. Gomez, and B.C. Hicks
Remote Sensing Division

Introduction: The Naval Research Laboratory has a long history of measuring water vapor in the middle atmosphere. This is the region of the atmosphere above the troposphere (~0 to 10 km), which includes the stratosphere (~10 to 50 km) and the mesosphere (~50 to 90 km). In the early 1960s, there was still considerable uncertainty over how much water the stratosphere contained. Some measurements showed that it was "wet," with ~100 water vapor molecules for every million molecules (100 ppmv), while others showed that it was "dry," with ~4 to 5 ppmv. NRL's John Mastenbrook¹ launched frost-point hygrometers on balloons up to altitudes of ~28 km, and is credited with finally resolving this controversy in favor of the "dry" stratosphere. This program continued at the Naval Research Laboratory from 1964 until 1979.

Water vapor measurements in the middle atmosphere are important for several reasons. First, water vapor is the primary source of the OH radical and other hydrogen compounds, and is therefore important in ozone chemistry. In addition, water vapor entering the stratosphere is extremely sensitive to temperatures at the tropical tropopause, and is therefore relevant to our understanding of how and where air rises from the troposphere into the stratosphere. Finally, because water vapor is an important greenhouse gas, the amount of water vapor in the atmosphere is extremely relevant to the global warming problem.

WVMS: The Water Vapor Millimeter-wave Spectrometer (WVMS) instruments (Fig. 1) make spectral measurements near 22 GHz, and thereby provides measurements of the water vapor profile from 40 to 80 km. The goal of this project is to provide the first continuous record of water vapor in the middle atmosphere using ground-based radiometers. All of the instruments have provided nearly continuous data records during their period of operation. The data from Table Mountain, California, covers the period from May 1993 to November 1997. The measurements from Lauder, New Zealand, cover the period from November 1992 to May 1993, and from January 1994 to the present. A third instrument was installed at Mauna Loa, Hawaii, in February 1996, and has been providing continuous data since that time. The instruments are all operated remotely from the Naval Research Laboratory, with calibration and emergency support provided by on-site staff. Figure 2 shows the data record from all three sites. The instruments are installed at sites of the international Network for the Detection of Stratospheric Change (NDSC). NDSC is a set of high-quality remote- sounding research stations for observing and understanding the physical and chemical state of the stratosphere. The WVMS instruments provide the sole source of middle atmospheric water vapor data from these sites.

FIGURE 1
The WVMS instrument at Mauna Loa, Hawaii. The high altitude of this site makes it ideal for microwave measurements of the middle atmosphere.

FIGURE 2
Full data record from the three sites where WVMS instruments have been deployed. Note the seasonal cycles at all three sites. Because the Lauder, NZ, site is farthest from the equator, it shows the most pronounced annual variation.

Increasing Middle Atmospheric Water Vapor: There has been an observed increase in middle atmospheric water vapor both on decadal and multidecadal timescales. The WVMS measurements played an important role in documenting the large increase (~2%/year) in middle atmospheric water vapor that was observed in the early 1990s.² This increase is thought to be related to the eruption of Mount Pinatubo in 1991, but the precise mechanism that caused the increase is still not understood. In addition to the increase in the early 1990s, there are measurements that suggest a longer term but somewhat slower increase in middle atmospheric water vapor that dates back to the 1950s.³ Although some of this long-term increase can be attributed to the effects of global increases in methane on water vapor, the observed increase is too large to be attributed entirely to this mechanism. The magnitude of the observed increases in middle atmospheric water vapor, the difficulty of pinpointing the causes of these increases, and the importance of such increases to ozone chemistry and global warming all highlight the importance of maintaining these continuous measurements.

[Sponsored by NASA]
References
¹H.J. Mastenbrook, "Water Vapor Distribution in the Stratosphere and High Troposphere," J. Atmos. Sci. 25, 299-311 (1968).
²G.E. Nedoluha et al., "Increases in Middle Atmospheric Water Vapor as Observed by HALOE and the Ground-based Water Vapor Millimeter-wave Spectrometer from 1991-1997," J. Geophys. Res. 103, 3531-3542 (1998).
³ K.H. Rosenlof et al., "Stratospheric Water Vapor Increases over the Past Half-century," Geophys. Res. Let. 28, 1195-1198 (2001).