As part of the second SPARC (Stratosphere–troposphere Processes And their
Role in Climate) water vapor assessment (WAVAS-II), we present measurements
taken from or coincident with seven sites from which ground-based
microwave instruments measure water vapor in the middle atmosphere. Six of
the ground-based instruments are part of the Network for the Detection of
Atmospheric Composition Change (NDACC) and provide datasets that can be
used for drift and trend assessment. We compare measurements from these
ground-based instruments with satellite datasets that have provided
retrievals of water vapor in the lower mesosphere over extended periods
since 1996.
We first compare biases between the satellite and ground-based instruments
from the upper stratosphere to the upper mesosphere. We then show a number
of time series comparisons at 0.46 hPa, a level that is sensitive to changes
in H2O and CH4 entering the stratosphere but, because almost all
CH4 has been oxidized, is relatively insensitive to dynamical
variations. Interannual variations and drifts are investigated with
respect to both the Aura Microwave Limb Sounder (MLS; from 2004 onwards) and
each instrument's climatological mean. We find that the
variation in the interannual difference in the mean H2O measured by any
two instruments is typically ∼ 1%. Most of the datasets
start in or after 2004 and show annual increases in H2O of
0–1 % yr−1. In particular, MLS shows a trend of between 0.5 % yr−1 and
0.7 % yr−1 at the comparison sites. However, the two longest measurement
datasets used here, with measurements back to 1996, show much smaller trends
of +0.1 % yr−1 (at Mauna Loa, Hawaii) and −0.1 % yr−1 (at Lauder, New
Zealand)