Abstract. 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 H2O and N2O 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) H2O 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
N2O product is shown to be generally decreasing over the
same period (when an increase in stratospheric N2O is
expected, reflecting a secular growth in emissions), with a more
pronounced drift in the lower stratosphere than that found for
H2O. Comparisons to ACE-FTS and to MLS N2O
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 H2O product and some
portion of the changes seen in N2O. 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 a result of this correction, the MLS v5 H2O
record shows no statistically significant drifts compared to
ACE-FTS. However, statistically significant drifts remain between
MLS v5 and frost point measurements, although they are reduced.
Drifts in v5 N2O are about half the size of those in v4 but
remain statistically significant. Scientists are advised to use MLS
v5 data in all future studies. Quantification of interregional and
seasonal to annual changes in MLS H2O and N2O will
not be affected by the drift. However, caution is advised in
studies using the MLS record to examine long-term (multiyear)
variability and trends in either of these species, especially
N2O; such studies should only be undertaken in consultation
with the MLS team. Importantly, this drift does not affect any of
the MLS observations made in other spectral regions such as
O3, HCl, CO, ClO, or temperature.
Seasonal to decadal variations of water vapor in the tropical lower stratosphere observed with balloon-borne cryogenic frost point hygrometers
