Performance on the NASA ER-2 research aircraft

During the Stratospheric Tracers of Atmospheric Transport (STRAT) mission in 1995, the ER-2 resumed making stairstep ("stacked") flights, where the aircraft flew level flight tracks of approximately 1/2 hour duration at different altitudes in the troposphere and lower stratosphere. These provided near-ideal conditions for the comparison of water vapor measured independently by fluorescence and absorption. In the mid to upper troposphere, water vapor can vary by 10-100 ppmv at one altitude with other atmospheric conditions (temperature, pressure, etc.) nearly constant. Because absorption by water vapor cannot be neglected, a true I₀ cannot be calculated for absorption (unless the fluorescence measurement is assumed to be correct, and the absorption due to water vapor is removed). However, relative changes in water vapor by absorption and fluorescence can be compared during level flight tracks with large variations in water vapor. Segments near the beginning and end with similar average amounts of water vapor are selected, and relative changes in H₂O by both methods are calculated for data between the two segments. This approach has the advantage of being an interpolation, and in addition, mirror reflectivity and lamp output are nearly constant. Furthermore, O₂ number density changes very little on level flight tracks, so even large errors in the O₂ cross section have only a minor effect on these absorption measurements. Since 1995, we have been able to make these in-flight comparisons on seven ER-2 flights, with agreement between absorption and fluorescence averaging 1%.

During the POLARIS mission (April-September 1997), we compared our instrument with a new tunable diode laser (TDL) hygrometer from the Jet Propulsion Laboratory (JPL) [May, 1998]. The JPL instrument is an open-path spectrometer operating near 1.37 mm, mounted below the right wing superpod on the ER-2 during POLARIS. There is generally agreement to +/- 0.1 ppmv throughout, except near the start and end, where the Harvard instrument measures lower than the JPL instrument. These data correspond to ascent and descent (13-18.5 km). For the entire June-July deployment, average agreement was 1% from 0-200 ppmv H₂O with an offset of less than 0.2 ppmv and most measurements agreeing to within 5%.

Data from POLARIS and previous ER-2 campaigns have been used to calculate the hydrogen budget (H₂O + 2 · CH₄) of the lower stratosphere [Hurst et al., 1999]. Our H₂O data for older air masses encountered during the June-July POLARIS deployment, along with coincident measurements of methane by a TDL instrument [Webster et al., 1994], yield a mean H₂O + 2 · CH₄ value of 7.5 ppmv, within 0.1 ppmv of the result of Dessler et al. [1994] and independent of whether flights with changed calibrations are used or not. This value corresponds to a global average of 4.1 ppmv H₂O in air entering the overworld in the tropics, and is consistent with the earlier conclusions of Weinstock et al. [1995], drawn from data obtained in the tropics. In a more detailed study, Hurst et al. [1999] also find that measurements of water vapor with the Harvard and JPL instruments, combined with simultaneous CH₄ and H₂ data, lead to conservation of total hydrogen in the overworld. This is powerful additional evidence for the accuracy of our data and would not be the case if our measurements were biased by 10% or more.

Water Vapor Instrument

ER-2 Performance

WB-57 Performance

References