Middle Atmospheric Water Vapor Measurements From Mauna Loa, 1996-1998
Gerald E. Nedoluha, Richard M. Bevilacqua, R. Michael Gomez, and Brian C. Hicks Naval Research Laboratory, Washington, D.C. 20375-5320
We present data obtained during nearly 2 years of continuous measurements of middle atmospheric water vapor from Mauna Loa, Hawaii (19.5°N, 204.4°E). The measurements are made at 22 GHz using a Naval Research Laboratory water vapor millimeter-wave spectrometer (WVMS). The data from Mauna Loa show less seasonal variation than those from WVMS instruments at Table Mountain, California (34.4°N, 242.3°E) and Lauder, New Zealand (45.0°S, 169.7°E), as is expected given the lower latitude of the Mauna Loa site. These relatively small seasonal variations, combined with the small tropospheric water vapor signal at the Mauna Loa site, make it ideal for the monitoring long term changes in water vapor.
Water vapor is the reservoir of odd hydrogen in the middle atmosphere and thus is important to ozone chemistry. Observations by the WVMS instruments have been providing nearly continuous measurements of water vapor since 1992. Nedoluha et al.,  showed that from 1992-1997 the WVMS instruments measured a significant increase in middle atmospheric water vapor. An average over the 40-60 km altitude range showed an increase of 0.144 ± 0.070 ppmv yr-1 at Table Mountain from 1993-1997 and an increase of 0.152 ± 0.070 ppmv yr-1 at Lauder from 1992-1997. This increase was similar to that measured by the Halogen Occultation Experiment (HALOE), which has been taking data nearly continuously since 1991. The HALOE measurements from 1991-1997 showed a global increase of 0.129 ± 0.022 ppmv yr-1. Thus the trends measured from 1991-1997 were all significantly larger than those observed by Oltmans and Hofmann  using midlatitude lower stratospheric water vapor measurements from 1981 to 1994.
The WVMS instrument at Mauna Loa is the third such instrument to be deployed. It is essentially identical to the WVMS2 instrument in operation at the Network for the Detection of Stratospheric Change (NDSC) site at Table Mountain since August 1993. These instruments are both very similar to the WVMS1 instrument that is deployed at the NDSC site at Lauder.
The retrieval of a vertical water vapor profile with ground-based microwave measurements relies upon the change in pressure as a function of altitude. The line width of the spectrum monotonically decreases with altitude due to the dependence on pressure broadening. Thus the resultant signal, which is the sum of the emission from all altitudes, can be deconvolved to retrieve a vertical profile. The primary difference between the WVMS2 and WVMS3 instruments and the WVMS1 instrument at the NDSC site in Lauder is the presence of an additional set of 50 kHz filters that improve the high altitude retrievals for the newer instruments. Details of the measurement technique and instrumentation are given by Nedoluha et al. [1995; 1996].
The 22.2 GHz transition used for these observations is optically thin in the troposphere permitting nearly continuous observation under most conditions even at low altitude sites such as Lauder. The smaller tropospheric optical depths through which observations are made from the Mauna Loa site reduces errors in the magnitude of the signal from the middle atmosphere and reduces baseline errors that can cause inaccurate stratospheric retrievals.
In Figure 1 we show the mixing ratios retrieved at several altitudes from 500 scan (~weekly) integrations. There is a summer peak in the mixing ratio at 70 and 80 km consistent with the upward motion of the atmosphere in the summer hemisphere. The annual oscillation magnitude is, however, smaller than that observed at Table Mountain or Lauder [cf. Nedoluha et al., 1997]. The smaller magnitude of this oscillation should help to facilitate the identification of any interannual changes, but it will take several more years of observations to clearly distinguish the effects of quasi-biennial oscillation (QBO) related changes from any longer term trends.
Fig. 1. Water vapor mixing ratios retrieved from WVMS3 measurements at Mauna Loa. Each scan represent approximately a week of continuous measurement.
Acknowledgments. We wish to thank S. McDermid and D. Walsh for their technical assistance with the Mauna Loa radiometer. This project is funded by NASA under the Upper Atmospheric Research Program.
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Nedoluha, G. E., et al., Measurements of water vapor in the middle atmosphere and implications for mesospheric transport, J. Geophys. Res., 101, 21,183-21,193, 1996.
Nedoluha, G. E., et al, A comparative study of mesospheric water vapor measurements from the ground-based Water Vapor Millimeter-wave Spectrometer and space-based instruments, J. Geophys. Res., 102, 16,647-16,661, 1997.
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Oltmans, S. J., and D. J. Hofmann, Increase in lower-stratospheric water vapor at a midlatitude northern hemisphere site from 1981 to 1994, Nature, 374, 146-149, 1995.
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