Humidity Structures in the Marine Atmospheric Boundary Layer

Abstract: The turbulence structure over the sea was studied with the emphasis on humidity. The data sets used came from the island of Östergarnsholm outside Gotland in the Baltic Sea. The study included spectral and quadrant analyses of the wind, temperature and humidity parameters from one measuring level. The wave state of the sea was deduced from data from a wave rider buoy anchored 4 km from the site.Two turbulence instruments for humidity were compared, the MIUU instrument (hot wire) and an open pass infrared gas analyser from LI-COR. The comparison showed that the LI-COR instrument resolved the high frequency fluctuations of the humidity better. The unstable cospectra of the sensible and latent heat fluxes were studied and categorised. It was found that many cospectra have two or more maxima. The higher frequency maxima gained influence when the stratification became near neutral.The quadrant analyses showed that the structures of humidity flux were similar to those of the heat flux. The sources of the flux were studied using different ratios. The ratio between events of moist updrafts and dry downdrafts were extensively studied. It was shown that the events of moist updrafts were more dominating during swell than during growing sea. When the results of the spectral and quadrant analyses were combined, it was shown that the smaller sized eddies of heat dominate the events of warm updrafts and that the large eddies dominate the cold downdrafts. The bulk transfer number for moisture, the Dalton number (CE), was found to be almost constant with stratification for unstable runs. The mean value was calculated to (1.0±0.3)·10-3.