Effect of Dewfall and Frostfall on Nighttime Cooling in a Small, Closed Basin

Abstract

Series of tethered balloon soundings of temperature and humidity in Austria?s Gruenloch basin (floor elevation 1270 m MSL) on two June days showed that the water vapor mixing ratio fell by 2?3 g kg?1 overnight as dew or frost formed in the basin. After sunrise, the basin atmosphere remoistened as higher humidity was brought down into the basin from above and as evapotranspiration occurred from the basin floor and sidewalls. The latent heat released at night by the dewfall/frostfall was 33%?53% of the overall observed basin sensible heat loss, illustrating the important role of dew and frost formation on the nighttime heat budget of the basin atmosphere. An energy budget equation illustrates the decreasing importance of the latent heat release on the overall basin heat budget as ambient temperatures fall from summer to winter. Because the diurnal temperature range is frequently larger than the late-afternoon dewpoint depression, fog and clouds often form in this basin. The extreme temperature minima that have been previously observed in this basin are expected to be attained only if such cloud moisture is removed. Calculations show that several cloud moisture removal processes may be effective in removing this moisture.