R. Michael Gomez

Increases in middle atmospheric water vapor as observed by the Halogen Occultation Experiment and the ground‐based Water Vapor Millimeter‐Wave Spectrometer from 1991 to 1997

Water vapor measurements made by the Halogen Occultation Experiment (HALOE) from 1991 to 1997 are compared with ground-based observations by the Water Vapor Millimeter-wave Spectrometers (WVMS) taken from 1992 to 1997 at Table Mountain, California (34.4°N, 242.3°E), and at Lauder, New Zealand (45.0°S, 169.7°E). The HALOE measurements show that an upward trend in middle atmospheric water vapor is present at all latitudes. The average trend in the HALOE water vapor retrievals at all latitudes in the 40–60 km range is 0.129 ppmv/yr, while the average trend observed by the WVMS instruments in this altitude range is 0.148 ppmv/yr. This trend is occurring below the altitude where changes in Lyman α associated with solar cycle variations should produce a significant increase in water vapor during this period, and is much larger than the ∼0.02 ppmv/yr trend in water vapor associated with increases in methane entering the stratosphere. In addition to the water vapor increase, HALOE measurements show that there is a temporal decrease in methane at altitudes between 40 and 70 km. This indicates an increase in the conversion of the available methane to water vapor, thus contributing to the observed increase in water vapor. The increase in water vapor observed by both instruments is larger than that which would be expected from the sum of all of the above effects. We therefore conclude that there has been a significant increase in the amount of water vapor entering the middle atmosphere. A temperature increase of ∼0.1 K/yr in regions of stratosphere-troposphere exchange could increase the saturation mixing ratio of water vapor by an amount consistent with the observed increase.

Ground‐based measurements of water vapor in the middle atmosphere

We present measurements of the middle atmospheric water vapor mixing ratio profile obtained using the ground-based Naval Research Laboratory water vapor millimeter-wave spectrometer (WVMS) instrument at the Jet Propulsion Laboratory Table Mountain Observatory. The measurements cover a period of 262 days from January 23, 1992, to October 13, 1992. During this campaign it was possible to retrieve useful daily mixing ratio profiles for 186 days. We thus have a nearly continuous record of water vapor mixing ratios for altitudes from ≈35 to 75 km. The retrievals are obtained using the optimal estimation method. Details of the error analysis are presented, and a technique is introduced that reduces baseline effects and helps to estimate the baseline error. The high-altitude (≳65 km) data show a sharp rise prior to the expected maximum near the summer solstice and a gradual decline in the following months. The mixing ratios generally peak between 55 and 65 km, at which point the mixing ratios are 6–7 parts per million by volume. The highest peaks occur in January, May, and October.

Measurements of water vapor in the middle atmosphere and implications for mesospheric transport

We present data obtained during more than 3 years of nearly continuous measurements of middle atmospheric water vapor. The data are obtained from ground-based measurements at 22 GHz taken at two sites, one in each hemisphere, using the Naval Research Laboratory water vapor millimeter-wave spectrometer (WVMS). With the construction of a second instrument, it has been possible to maintain continuous monitoring from both sites since January 1994. The measurements from both instruments show significant seasonal variability. There is a clear annual cycle, with the water vapor above ∼60 km increasing in summer and decreasing in winter. The observed amplitude of the annual oscillation is larger at 45.0°S than at 34.4°N, a result which is qualitatively consistent with the higher latitude of the southern hemisphere site. There is also an indication of a semiannual cycle, particularly at altitudes near 80 km. The annual cycle is consistent with transport due primarily to advection, while the weaker semiannual cycle may be indicative of the effect of gravity waves on diffusive transport.

A comparative study of mesospheric water vapor measurements from the ground-based water vapor millimeter-wave spectrometer and space-based instruments

We compare water vapor measurements from the Naval Research Laboratory ground-based Water Vapor Millimeter-wave Spectrometer (WVMS) instruments with measurements taken by five space-based instruments. For coincident measurements the retrievals from all of the instruments show qualitatively similar altitude profiles. The retrieved mixing ratios from most instruments generally differ from an average calculated using retrievals from all of the instruments by <1 ppmv at most altitudes from 40 km to 80 km. Comparisons with the Microwave Limb Sounder (MLS) and the Halogen Occultation Experiment (HALOE) allow for the validation of observed temporal variations. The observed variations show similar annual and semiannual cycles. A comparison of several years of data from HALOE and WVMS also shows that the instruments are detecting similar interannual variations. A regression analysis of the WVMS and HALOE data sets shows that the observed variability is consistent within the estimated errors in the mesosphere and that in the upper stratosphere, where the natural variability is small, there is a positive correlation between the WVMS and the HALOE data.

Water vapor measurements in the mesosphere from Mauna Loa over solar cycle 23

[1] The Water Vapor Millimeter-wave Spectrometer (WVMS) system has been making measurements from the Network for the Detection of Atmospheric Composition Change site at Mauna Loa, Hawaii (19.5N, 204.4E), since 1996, covering nearly the complete period of solar cycle 23. The WVMS measurements are compared with Halogen Occultation Experiment (HALOE) (1992–2005), Microwave Limb Sounder (MLS) (2004 to present), and Atmospheric Chemistry Experiment (ACE) Fourier transform spectrometer (2004 to present) measurements in the mesosphere. In the upper mesosphere Lyman a radiation photodissociates water vapor; hence, water vapor in the upper mesosphere varies with the solar cycle. We calculate fits to the WVMS and HALOE water vapor data in this region using the Lasp Interactive Solar Irradiance Datacenter Lyman a data set. This is, to our knowledge, the only published validation of the sensitivity of HALOE water vapor measurements to the solar cycle, and the HALOE and WVMS water vapor measurements show a very similar sensitivity to the solar cycle. Once the solar cycle variations are taken into account, the primary water vapor variations at all of these altitudes from 1992 to the present are an increase from 1992 to 1996, a maximum in water vapor in 1996, and small changes from 1997 to the present. Measurements from 2004 to 2008, which are available from WVMS, MLS, and ACE, show not only good agreement in interannual variations but also excellent agreement in their absolute measurements (to within better than 3%) of the water vapor mixing ratio from 50 to 80 km.

A comparison of middle atmospheric water vapor as measured by WVMS, EOS‐MLS, and HALOE

[1] We compare middle atmospheric water vapor measurements from the Halogen Occultation Experiment (HALOE), Water Vapor Mm-wave Spectrometer (WVMS), and Earth Observing System (EOS) Microwave Limb Sounder (MLS) instruments from 40 to 70 km. The ground-based WVMS measurements shown here were taken at Network for the Detection of Atmospheric Composition Change (NDACC) sites at Mauna Loa, Hawaii (19.5°N, 204.4°E), and Lauder, New Zealand (45.0°S, 169.7°E). A comparison of measurements where HALOE, MLS, and WVMS are all available shows that the average HALOE water vapor retrievals are lower than those from MLS at all altitudes from 40 to 70 km and lower than the WVMS retrievals everywhere except above 64 km at Lauder. The average difference between all coincident WVMS and MLS water vapor profiles is within 0.2 ppmv over almost the entire 40-70 km altitude range, both at Lauder and Mauna Loa. The standard deviation of the difference between weekly WVMS retrievals and coincident MLS retrievals is ∼0.2 ppmv at Mauna Loa and ∼0.3-0.4 ppmv at Lauder. The interannual correlation between water vapor observed by MLS and WVMS is slightly improved by the use of MLS temperature measurements in the WVMS retrievals. The MLS and WVMS profiles at Mauna Loa show particularly good interannual agreement, including a clear QBO signature.

Measurements of middle atmospheric water vapor from low latitudes and midlatitudes in the northern hemisphere, 1995–1998

We present middle atmospheric water vapor measurements made at 22 GHz using two Naval Research Laboratory Water Vapor Millimeter-wave Spectrometers (WVMS2 and WVMS3). We include measurements from an intercomparison campaign at Table Mountain, California (34.4°N, 242.3°E), and measurements obtained since March 1996 from Table Mountain and from Mauna Loa, Hawaii (19.5°N, 204.4°E). The mesospheric data from Mauna Loa show both a mixing ratio peak at a higher altitude and smaller seasonal variations than those from the WVMS instrument at Table Mountain. These differences are qualitatively consistent with the changes in mixing ratio and the increase in mesospheric variability expected with decreasing latitude. The latitudinal variation in the mixing ratio profile observed in the WVMS data is very similar to that observed by the Halogen Occultation Experiment (HALOE) in the upper stratosphere and lower mesosphere. The WVMS measurements generally show higher water vapor mixing ratios than HALOE does in the upper stratosphere and lower mesosphere but smaller mixing ratios in the upper mesosphere. The winter of 1997–1998 shows an unusually large decrease in mesospheric water vapor, a result that is consistent with measurements from HALOE throughout the Northern Hemisphere for this winter. There appears to be a slight overall decrease in water vapor for all seasons over the period of the WVMS observations from Mauna Loa. The relatively small seasonal variations, combined with the small amount of tropospheric water vapor above Mauna Loa, make this an ideal site for the monitoring of multiyear trends in water vapor.

Validation of long-term measurements of water vapor from the midstratosphere to the mesosphere at two Network for the Detection of Atmospheric Composition Change sites

We present measurements from the Water Vapor Millimeter-wave Spectrometer (WVMS) instruments at Table Mountain, California (34.4°N, 242.3°E), and Mauna Loa, Hawaii (19.5°N, 204.4°E), and highlight the extended altitude range of the measurements at these sites, which now provide measurements down to 26 km. We show that this extended altitude range has been acquired without disturbing the existing long-term WVMS data set at Mauna Loa. Validation of the successful transition is provided by comparing WVMS measurements with coincident satellite measurements from the Aura Microwave Limb Sounder (MLS), the Atmospheric Chemistry Experiment, and the Michelson Interferometer for Passive Atmospheric Sounding. At the lowest altitudes where WVMS measurements are possible, we also compare with frost-point hygrometer balloon measurements. The water vapor mixing ratios measured at 50 km over Mauna Loa are the highest ever reported in the WVMS (since 1996) or MLS (since 2004) time series. Particularly encouraging for the new 26 km WVMS measurements is that they indicate an increase between 2010 and 2011 that is comparable to that observed by other instruments. This shows that these measurements are sensitive to variations at this altitude and that the instrumental baseline remains stable.