Mesoscale Fluctuations of Columnar Water Vapor

Abstract

During a 7-week period in the winter of 1986, a six-channel ground-based microwave radiometer was operated at a site near Halifax, Nova Scotia, as part of the Canadian Atlantic Storms Program. One of the quantities measured by this instrument was the vertically integrated water-vapor content of the atmosphere, called the columnar vapor. Time histories of the columnar vapor show variability on many scales. This paper focuses on the fluctuations with frequencies between 0.2 and 2 cycles per hour (periods between 30 min and 5 h), which we denote as mesoscale fluctuations. Power spectra of the columnar vapor fluctuations were calculated for seven records of at least 30 h length. Three of these records were found to have well-defined spectral peaks in the mesoscale range; three had weaker peaks; and one had no significant peak in this range, the spectrum failing off smoothly with approximately an f?2.5 power law. In all cases analyzed, the weather was generally fair, with a stable layer extending approximately between 900 and 700 mb and with Ins stable conditions from the surface to 900 mb. An investigation of the average temperature profiles during the seven periods analyzed showed that the mesoscale spectral peaks tended to be more prominent in cases with stronger low-level instability. From this association between the existence of the spectral peaks and the temperature profile, we tentatively conclude that the mesoscale fluctuations in columnar vapor arise from convection-induced waves in the stable layer between 900 and 700 mb. Thew waves modulate the thickness of the mixed layer and show up as time variations in the columnar vapor as the waves propagate and are advected over the observing site.