various observing platforms document a general increase in water vapor
that, at least over the oceans, is consistent with the rate of 7% per
degree Celsius of tropospheric warming that is expected from the
Clausius-Clapeyron Equation. Many of these analyses utilize
observations that, while global in coverage, are limited to ten or 20
years of record or rely on atmospheric reanalyses that are affected by
inhomogeneities resulting from changes in the types of observations
being assimilated. To complement these findings, we used the longest
available record of tropospheric temperature and humidity
measurements, viz. that from radiosondes, to diagnose trends in
surface-to-500-hPa precipitable water (PW) and their relationship to
temperature changes within the same atmospheric layer. Monthly means
of PW and surface-to-500-hPa layer-mean temperature were calculated
from Version 2 of the Integrated Global Radiosonde Archive, passed
through the latest version of the Pairwise Homogenization Algorithm to
remove detectable artificial discontinuities such as those associated
with changes in instrumentation or observing practices, and then
aggregated into 5x5-degree gridboxes for the computation of area
averages. Key findings of our analysis are that 1) for the time series
of PW averaged over all available Northern Hemisphere land areas, the
trend for the period 1973-2017 is 0.09 mm per decade; 2) the variations
and trend of this time series between 2002 and 2016 is largely
consistent with variations in PW anomalies derived from the
Atmospheric Infrared Sounder; and 3) consistent with
earlier studies of the PW-temperature relationship over land, the
radiosonde-based monthly PW anomalies vary with temperature anomalies
at a rate that is lower than that expected from the Clausius-Clapeyron
Equation. Implications of these findings in the context of other
published analyses will be discussed.
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