Thursday, January 21, 2016

Why Do We Need to Monitor Water Vapour Globally?

The need for accurate long-term measurements of water vapor in the upper troposphere and lower stratosphere with global coverage (20 page pdf, Rolf Muller, Anne Kunz, Dale F. Hurst, Christian Rolf, Martina Kramer, and Martin Riese1, Earth's Future, Dec. 30, 20015)

Today we review a journal article calling for the establishment of a global network of upper air balloons to measure water vapour in the upper troposphere and lower stratosphere. Although water vapour is the most important greenhouse gas it unlike CO2 has a lifetime of only a week or so because of the evaporation/condensation hydrological cycle, compared to a century for CO2 to accumulate in the atmosphere. Despite this water vapour acts as a positive feedback when the air has a higher humidity leading to more convective precipitation as a result of the warming of the earth’s surface.

 The global measurement of water vapour on a routine and operational basis lags the networks established earlier for CO2 and Ozone. Ideally, a dedicated upper air balloon network is recommended, augmented by satellite sensors with 2 km resolution to estimate Atmospheric water vapour. Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) is such a network made up of 30-40 sites. Following through on this ask will be important when estimating future climate impacts resulting from warming due to carbon emissions.

 water vapour  

Key Quotes:

“water vapor is the most important greenhouse gas in the Earth’s atmosphere; it accounts for about half of the present day greenhouse effect and is the most important gaseous source of infrared opacity in the atmosphere.. In contrast to CO2, water vapor in the atmosphere can condense and precipitate; therefore water vapor concentrations in the atmosphere are determined by condensation”

 “Increasing atmospheric CO2 also causes changes in the tropospheric Hadley circulation, with implications for convective ascent, cloud patterns and thus the tropospheric humidity distribution, particularly in the tropics” “water vapor changes in the lower stratosphere have been identified as an important driver of decadal global surface climate change”

“The Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) is an initiative …making progress in establishing a global network of 30 to 40 measurement sites…GRUAN sites are required to perform a water vapor sounding with a balloon-borne frost point hygrometer at least once per month.”

 “It would be extremely valuable to augment the long-term balloon network with satellite sensors, providing high vertical resolution (better than ≈ 2 km) measurements of water vapor in the UTLS on a time scale of decades.”

“We suggest that a global, long-term balloon-borne measurement program for UTLS water vapor should be established. Stations should cover the entire globe with an optimized distribution of sites, the launching strategy should take into account the local meteorology, and the intercalibration of the different stations should be ensured.”

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