Abstract
Abstract. Water vapour is the strongest greenhouse gas in our atmosphere, and its strength and its dependence on temperature lead to a strong feedback mechanism in both the troposphere and the stratosphere. Raman water vapour lidars can be used to make high-vertical-resolution measurements on the order of tens of metres, making height-resolved trend analyses possible. Raman water vapour lidars have not typically been used for trend analyses, primarily due to the lack of long-enough time series. However, the Raman Lidar for Meteorological Observations (RALMO), located in Payerne, Switzerland, is capable of making operational water vapour measurements and has one of the longest ground-based and well-characterized data sets available. We have calculated an 11.5-year water vapour climatology using RALMO measurements in the troposphere. Our study uses nighttime measurements during mostly clear conditions, which creates a natural selection bias. The climatology shows that the highest water vapour specific-humidity concentrations are in the summer months and the lowest in the winter months. We have also calculated the geophysical variability of water vapour. The percentage of variability of water vapour in the free troposphere is larger than in the boundary layer. We have also determined water vapour trends from 2009 to 2019. We first calculate precipitable water vapour (PWV) trends for comparison with the majority of water vapour trend studies. We detect a nighttime precipitable water vapour trend of 1.3 mm per decade using RALMO measurements, which is significant at the 90 % level. The trend is consistent with a 1.38 ∘C per decade surface temperature trend detected by coincident radiosonde measurements under the assumption that relative humidity remains constant; however, it is larger than previous water vapour trend values. We compare the nighttime RALMO PWV trend to daytime and nighttime PWV trends using operational radiosonde measurements and find them to agree with each other. We cannot detect a bias between the daytime and nighttime trends due to the large uncertainties in the trends. For the first time, we show height-resolved increases in water vapour through the troposphere. We detect positive tropospheric water vapour trends ranging from a 5 % change in specific humidity per decade to 15 % specific humidity per decade depending on the altitude. The water vapour trends at five layers are statistically significant at or above the 90 % level.
Highlights
Water vapour is the atmosphere’s most important minor constituent
As far as we are aware, there have been only four publications on operational Raman water vapour lidars in the last 2 decades (Goldsmith et al, 1994; Dinoev et al, 2013; Hadad et al, 2018; Reichardt et al, 2012). While these lidars have been run operationally over the last 2 decades, Raman Lidar for Meteorological Observations (RALMO) is the only one which has presented a water vapour trend study, which we show here
The specific-humidity profiles used for the climatology and trend analysis are determined from the RALMO measurements using the optimal estimation method (OEM) retrieval introduced in Sica and Haefele (2016) for water vapour lidars
Summary
Water vapour is the atmosphere’s most important minor constituent. It plays a significant role in almost all aspects of the atmosphere, including dynamics, circulation, radiative processes, and chemistry with other molecular and particulate species. Since radiosonde measurements can require significant corrections, water vapour trend and climatology studies moved towards including other instruments such as microwave radiometers (Morland et al, 2009; Hocke et al, 2011), satellite-based instruments like the microwave limb sounders (Hegglin et al, 2014; Khosrawi et al, 2018), global positioning systems (GPS, Jin et al, 2007; Wang et al, 2016), and reanalyses models (Trenberth et al, 2005). While satellite measurements provide excellent global coverage, their vertical resolutions are typically on the order of kilometres, which limits their ability to capture water vapour’s large variability with altitude Unlike these instruments, a Raman water vapour lidar has the ability to measure high-resolution water vapour profiles in the troposphere (on the order of metres) as well as calculate column measurements (Melfi, 1972; Whiteman, 2003).
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