Boundary layer moisture regimes in small closed basins

The correspondence between temperature and moisture structure evolution in small closed Alpine and Rocky Mountain basins is investigated using series of tethered balloon soundings, with the goal of determining the diurnal evolution of moisture in the basin atmosphere, its effect on basin heat budgets and its relationship to boundary layer development. Heat budgets in closed basins are much easier to compute than heat budgets in valleys because of the closed volume and the consequent elimination of large-scale advective effects.

The data show that air confined inside a closed basin dries during the night as dewfall and frost formation occur on the sidewalls and basin floor. The latent heat that is released by these processes keeps the nocturnal temperature from falling to its normal levels. The nighttime atmospheric moisture loss in Austria’s Gruenloch basin was about 2 g/kg. While the dewfall computed from the series of mixing ratio profiles would have amounted to a equivalent depth of only .048 mm, the heat released during the night by the condensation was fully 63% of the calculated nocturnal sensible heat loss from the basin. Thus, basin cooling was strongly affected by the latent heat release. In Utah’s Peter Sinks basin a similar series of soundings in a drier climate setting showed that a lower, but nonetheless, significant rate of dewfall also reduces the cooling of the stable boundary layer there at night. Following sunrise, the moisture content increases in the basins as the inversion descends, bringing down the higher moisture contents that remained above the basin during the night. Evaporation from the valley floor and sidewalls plays a role, as well, increasing the post-sunrise moisture contents. In the talk, we will present a conceptual model that relates the changing moisture profiles to basin boundary layer development, and will contrast boundary layer development over basins to the typical evolution over homogeneous and valley terrain where temperature and moisture profiles can be affected strongly by advection.