Investigation and amelioration of long-term instrumental drifts in water vapor...

Livesey, N., W. G. Read, L. Froidevaux, A. Lambert, M. Santee, M. J. Schwartz, L. F. Millán, R. F. Jarnot, P. A. Wagner, D. Hurst, K. A. Walker, P. Sheese, and G. Nedoluha (2021), Investigation and amelioration of long-term instrumental drifts in water vapor and nitrous oxide measurements from the Aura Microwave Limb Sounder (MLS) and their implications for studies of variability and trends, Atmos. Chem. Phys., 21, 15409-15430, doi:10.5194/acp-21-15409-2021.

The Microwave Limb Sounder (MLS), launched on NASA’s Aura spacecraft in 2004, measures vertical profiles of the abundances of key atmospheric species from the upper troposphere to the mesosphere with daily near-global coverage. We review the first 15 years of the record of H2 O and N2 O measurements from the MLS 190 GHz subsystem (along with other 190 GHz information), with a focus on their long-term stability, largely based on comparisons with measurements from other sensors. These comparisons generally show signs of an increasing drift in the MLS “version 4” (v4) H2 O record starting around 2010. Specifically, comparisons with v4.1 measurements from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) indicate a ∼ 2 %–3 % per decade drift over much of the stratosphere, increasing to as much as ∼ 7 % per decade around 46 hPa. Larger drifts, of around 7 %–11 % per decade, are seen in comparisons to balloon-borne frost point hygrometer measurements in the lower stratosphere. Microphysical calculations considering the formation of polar stratospheric clouds in the Antarctic winter stratosphere corroborate a drift in MLS v4 water vapor measurements in that region and season. In contrast, comparisons with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on NASA’s Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission, and with ground-based Water Vapor Millimeter-wave Spectrometer (WVMS) instruments, do not show statistically significant drifts. However, the uncertainty in these comparisons is large enough to encompass most of the drifts identified in other comparisons. In parallel, the MLS v4 N2 O product is shown to be generally decreasing over the same period (when an increase in stratospheric N2 O is expected, reflecting a secular growth in emissions), with a more pronounced drift in the lower stratosphere than that found for H2 O. Comparisons to ACE-FTS and to MLS N2 O observations in a different spectral region, with the latter available from 2004 to 2013, indicate an altitude-dependent drift, growing from 5 % per decade or less in the mid-stratosphere to as much as 15 % per decade in the lower stratosphere. Detailed investigations of the behavior of the MLS 190 GHz subsystem reveal a drift in its “sideband fraction” (the relative sensitivity of the 190 GHz receiver to the two different parts of the microwave spectrum that it observes). Our studies indicate that sideband fraction drift accounts for much of the observed changes in the MLS H2 O product and some portion of the changes seen in N2 O. The 190 GHz sideband fraction drift has been corrected in the new “version 5” (v5) MLS algorithms, which have now been used to reprocess the entire MLS record. As

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Upper Atmosphere Research Program (UARP)