Abstract
Abstract. An accurate characterization of the vertical structure of the Arctic atmosphere is useful in climate change and attribution studies as well as for the climate modelling community to improve projections of future climate over this highly sensitive region. Here, we investigate one of the dominant features of the vertical structure of the Arctic atmosphere, i.e. water-vapour inversions, using eight years of Atmospheric Infrared Sounder data (2002–2010) and radiosounding profiles released from the two Arctic locations (North Slope of Alaska at Barrow and during SHEBA). We quantify the characteristics of clear-sky water vapour inversions in terms of their frequency of occurrence, strength and height covering the entire Arctic for the first time. We found that the frequency of occurrence of water-vapour inversions is highest during winter and lowest during summer. The inversion strength is, however, higher during summer. The observed peaks in the median inversion-layer heights are higher during the winter half of the year, at around 850 hPa over most of the Arctic Ocean, Siberia and the Canadian Archipelago, while being around 925 hPa during most of the summer half of the year over the Arctic Ocean. The radiosounding profiles agree with the frequency, location and strength of water-vapour inversions in the Pacific sector of the Arctic. In addition, the radiosoundings indicate that multiple inversions are the norm with relatively few cases without inversions. The amount of precipitable water within the water-vapour inversion structures is estimated and we find a distinct, two-mode contribution to the total column precipitable water. These results suggest that water-vapour inversions are a significant source to the column thermodynamics, especially during the colder winter and spring seasons. We argue that these inversions are a robust metric to test the reproducibility of thermodynamics within climate models. An accurate statistical representation of water-vapour inversions in models would mean that the large-scale coupling of moisture transport, precipitation, temperature and water-vapour vertical structure and radiation are essentially captured well in such models.
Highlights
Water vapour is a prominent greenhouse gas that plays a key role in the regional and global hydrological cycle, and its positive feedback in a warming world is physically well understood (Bony et al, 2006)
Radiosoundings from the North Slope of Alaska (NSA) Atmospheric Radiation Measurement (ARM) site are released from 71.3◦ N 156.6◦ W, and we examine profiles from January 2003 to October 2009
In the Arctic, the atmospheric boundary layer is partly decoupled from the free troposphere during large parts of the year. This decoupling is strongest during winter when solar radiation is absent and strong surface inversions often form in cloud-free conditions, and weakest during summer, when clouds are more frequent and solar radiation warms the surface (Kahl, 1990; Devasthale et al, 2010)
Summary
Water vapour is a prominent greenhouse gas that plays a key role in the regional and global hydrological cycle, and its positive feedback in a warming world is physically well understood (Bony et al, 2006). Water vapour has a well-defined zonal and vertical distribution, with a maximum in the tropics, secondary maxima at the mid-latitudes, while minima are observed in the polar regions. In the vertical it is typically at maximum near the surface. The variability in its zonal distribution is influenced by many factors but the temperature control of the saturation water-vapour partial pressure is the most important Sedlar et al (2011) report frequent specific-humidity inversions associated with low-level cloud cover from both central and pan-Arctic observational sites during all seasons and hypothesize on the importance of these for cloud formation and cloud geometric location.
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