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Keywords:

  • atmospheric water vapour;
  • outflow boundary;
  • moisture convergence

Abstract

The time-varying three-dimensional water vapour field derived from mesoscale Global Positioning System (GPS) tomography data is used to describe the water vapour variability in relation to the dynamics of the atmosphere during convective initiation (CI). The paper presents the theoretical framework of GPS tomography at the mesoscale, including aspects related to the assimilation of independent data (e.g. water vapour profiles issued from meteorological balloon soundings). GPS tomography-derived water vapour density retrievals are validated against lidar, the Atmospheric Emitted Radiance Interferometer and radiosonde data, even if the short three-day period of the study prevents conclusions about the real accuracy of the GPS tomography technique.

GPS tomography products are used, in synergy with surface and sounding-derived meteorological variable measurements, satellite imagery and reflectivity composites from the WSR-88D network and S-POL radar, to study environmental conditions leading to the 12 June 2002 CI event during the International H2O Project. On this day, CI was triggered simultaneously, shortly after 2100 UTC, in two locations along an old outflow boundary lying east-west in the vicinity of the Oklahoma–Kansas border. The study focuses on CI to the east, which was triggered at the intersection of the outflow boundary with a distinct southwest–northeast-oriented moisture convergence line. The latter formed as the result of a cross-dryline circulation leading to the penetration of dry air meeting with the moister air mass associated with the southerly low-level flow east of the dryline. These intersecting boundaries appeared to have provided the necessary triggering mechanism for getting moist surface air parcels up to the level of free convection. Tomography-derived water vapour fields provided observational evidence of the vertical transport of water vapour above the lifting condensation level and the level of free convection to the south of and along the intersecting boundaries. Copyright © 2009 Royal Meteorological Society