Abstract
Abstract. We present two years of in situ measurements of water vapour (H2O) and its isotopologue ratio (δD, the standardized ratio between H216O and HD16O), made at two remote mountain sites on Tenerife in the subtropical North Atlantic. We show that the data – if measured during night-time – are well representative for the lower/middle free troposphere. We use the measured H2O-δD pairs, together with dust measurements and back trajectory modelling for analysing the moisture pathways to this region. We can identify four principally different transport pathways. The air mass transport from high altitudes and high latitudes shows two different scenarios. The first scenario brings dry air masses to the stations, as the result of condensation events occurring at low temperatures. The second scenario brings humid air masses to the stations, due to cross-isentropic mixing with lower-level and more humid air during transport since last condensation (LC). The third pathway is transportation from lower latitudes and lower altitudes, whereby we can identify rain re-evaporation as an occasional source of moisture. The fourth pathway is linked to the African continent, where during summer, dry convection processes over the Sahara very effectively inject humidity from the boundary layer to higher altitudes. This so-called Saharan Air Layer (SAL) is then advected westward over the Atlantic and contributes to moisten the free troposphere. We demonstrate that the different pathways leave distinct fingerprints on the measured H2O-δD pairs.
Highlights
In the subtropical free troposphere, in the region of the descending branch of the Hadley cell, the humidity is not conserved along the mean subsidence (Pierrehumbert, 1998)
The upward transport is caused by the combination of the thermally driven growth of the marine boundary layer (MBL) volume and the buoyant airflows caused by the heating of the air located just above the terrain (Rodríguez et al, 2009)
We assume that the measurements made in the second half of the night are only very weakly affected by the local circulation on the island and represent the free troposphere (FT) well
Summary
In the subtropical free troposphere, in the region of the descending branch of the Hadley cell, the humidity is not conserved along the mean subsidence (Pierrehumbert, 1998) Instead, this dry air is often moistened (Galewsky and Hurley, 2010; Couhert et al, 2010; Risi et al, 2012). The few studies based on atmospheric modelling points to the turbulent transport of water vapour from the surface upwards as the dominant moistening process, balancing the drying in the subtropical free troposphere (Couhert et al, 2010; Risi et al, 2012) Additional processes, such as evaporation of condensate and isentropic eddy transport of moist air from the tropics, are suggested to contribute to moisture in this region (Galewsky and Hurley, 2010, and references therein), but the sources and dynamics involved in this moistening are still unclear. Such observations can be used to distinguish between the different mechanisms as-
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