Title:
The residence times and transport pathways of water vapor in the atmosphere
Abstract:
Water in the atmosphere is a key ingredient of the climate system. The time water vapor stays in the atmosphere between evaporation and precipitation is termed the residence time of water vapor. Commonly, the global average residence time has been estimated to be about 8-10 days. In a recent study, we suggested that the residence time may be shorter, on the order of 4.5 days, which has received both attention and criticism from later studies. Traditional calculation methods based on an exponential decay process tacitly assume a homogeneous and stationary water cycle. I show that both of these conditions are commonly not fulfilled, with substantial consequences for the estimated residence time. Using a conceptual framework and ERA-Interim reanalysis data, I show that in an inhomogeneous and non-stationary water cycle, both longer and shorter residence times are possible, while maintaining the observed mass balance. From a sensitivity study with a moisture tracking algorithm, I update our recent estimate of the global mean residence time to 5.2±0.5 days. A simple, independent plausibility argument using humidity-weighted wind speed shows that such a short residence time is more plausible than the commonly used 8-10 days. In a further analysis, I present an investigation of the residence time of water vapour during a major cold-air outbreak in the Norwegian Sea with water vapour tracers. The cold-air outbreak sets up an intense, local water cycle, with moisture turnover times on the order of 1-2 days. Regional topography appears to play a key role in this local water cycle. That case study for a major cold-air outbreak provides an important backdrop for forthcoming field activity in the framework of recently initiated research projects to study the residence time and transport patterns of atmospheric water with the help of its stable isotope composition.