[1] Trend analyses are presented for 30 years of balloon-borne stratospheric water vapor measurements over Boulder, Colorado. The data record is broken into four multiple-year periods of water vapor trends, including two that span the wellexamined but unattributed 1980-2000 period of stratospheric water vapor growth. Trends are determined for five 2 km stratospheric layers (16-26 km) utilizing weighted, piecewise regression analyses. Stratospheric water vapor abundance increased by an average of 1.0 ± 0.2 ppmv (27 ± 6%) during 1980-2010 with significant shorter-term variations along the way. Growth during period 1 (1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989) was positive and weakened with altitude from 0.44 ± 0.13 ppmv at 16-18 km to 0.07 ± 0.07 ppmv at 24-26 km. Water vapor increased during period 2 (1990-2000) by an average 0.57 ± 0.25 ppmv, decreased during period 3 (2001-2005) by an average 0.35 ± 0.04 ppmv, then increased again during period 4 (2006-2010) by an average 0.49 ± 0.17 ppmv. The diminishing growth with altitude observed during period 1 is consistent with a water vapor increase in the tropical lower stratosphere that propagated to the midlatitudes. In contrast, growth during periods 2 and 4 is stronger at higher altitudes, revealing contributions from at least one mechanism that strengthens with altitude, such as methane oxidation. The amount of methane oxidized in the stratosphere increased considerably during 1980-2010, but this source can account for at most 28 ± 4%, 14 ± 4%, and 25 ± 5% of the net stratospheric water vapor increases during 1980-2000, 1990-2000, and 1980-2010, respectively.

Acquiring accurate measurements of water vapor at the low mixing ratios (< 10 ppm) encountered in the upper troposphere and lower stratosphere (UT/LS) has proven to be a significant analytical challenge evidenced by persistent disagreements between high-precision hygrometers. These disagreements have caused uncertainties in the description of the physical processes controlling dehydration of air in the tropical tropopause layer and entry of water into the stratosphere and have hindered validation of satellite water vapor retrievals. A 2011 airborne intercomparison of a large group of in situ hygrometers onboard the NASA WB-57F high-altitude research aircraft and balloons has provided an excellent opportunity to evaluate progress in the scientific community toward improved measurement agreement. In this work we intercompare the measurements from the Midlatitude Airborne Cirrus Properties Experiment (MACPEX) and discuss the quality of agreement. Differences between values reported by the instruments were reduced in comparison to some prior campaigns but were nonnegligible and on the order of 20% (0.8 ppm). Our analysis suggests that unrecognized errors in the quantification of instrumental background for some or all of the hygrometers are a likely cause. Until these errors are understood, differences at this level will continue to somewhat limit our understanding of cirrus microphysical processes and dehydration in the tropical tropopause layer.