Observations of precipitable water vapor fluctuations in convective boundary layer via microwave interferometry

At microwave frequencies, each centimeter of precipitable water vapor (PWV) causes about 6.45 cm of extra electrical path length relative to the "dry" air. The fluctuations of the water vapor dominate the changes of the effec tive path length through the atmosphere in a relatively short time period o f a few hours. In this paper we describe a microwave interferometer develop ed for water vapor investigations and present the observation results. The interferometer consists of 10 antennas along two orthogonal 400-m arms that form many baselines (antenna pairs) ranging from 100 to 400 m. All the ant ennas receive a common CW signal (11.7 GHz) from a geostationary television satellite, and phase differences between pairs of antennas are measured. T he phase differences reflect the column-integrated water vapor differences from the top of the atmosphere to the spatially separated antennas at the g round. The interferometric, baseline-differential measurements allow us to study the statistical properties of the PWV fluctuations, as well as the tu rbulent activity of the convective boundary layer (CBL). Structure function analysis of the interferometer measurements shows good agreement with resu lts obtained from the Very Large Array (VLA) and with a theoretical model d eveloped for radio astronomical very long baseline interferometry (VLBI), r eported previously by other investigators. The diurnally varying structure constant correlates remarkably well with the combination of the latent and sensible heat fluxes measured simultaneously from a 10-m meteorological tow er. The average drift velocity of the PWV over the interferometer was also derived from the measurements. The derived velocity agrees well during the morning hours with the wind measured by an anemometer at the center of the interferometer.