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Ann. Geophys., 22, 3937-3950, 2004
www.ann-geophys.net/22/3937/2004/
© European Geosciences Union 2004
An observational study of a shallow gravity current triggered by katabatic flow
A. Adachi1,2, W. L. Clark2,3, L. M. Hartten2,3, K. S. Gage2, and T. Kobayashi1
1Meteorological Research Institute, Nagamine 1-1, Tsukuba, Japan
2NOAA Aeronomy Laboratory, 325 Broadway, Boulder, Colorado, USA
3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
Abstract. Observations from a wind profiler and a meteorological tower are utilized to
study the evolution of a gravity current that passed over the Meteorological
Research Institute's (MRI) field site in Tsukuba, Japan. The gravity current
was created by katabatic flow originating on the mountainous slopes west of
the field site. The passage of the shallow current was marked by a
pronounced pressure disturbance and was accompanied by vertical circulations
seen in the tower and profiler data. Direct vertical-beam measurements are
difficult, especially at low heights during high-gradient events like
density currents. In this study vertical velocities from the profiler are
derived from the four oblique beams by use of the Minimizing the Variance of
the Differences (MVD) method. The vertical velocities derived from the MVD
method agree well with in situ vertical velocities measured by a sonic anemometer
on the tower.
The gravity current is analyzed with surface observations, the wind
profiler/RASS and tower-mounted instruments. Observations from the
profiler/RASS and the tower-mounted instruments illustrate the structure of
the gravity current in both wind and temperature fields. The profiler data
reveal that there were three regions of waves in the vertical velocity
field: lee-type waves, a solitary wave and Kelvin-Helmholtz waves. The
lee-type waves in the head region of the gravity current seem to have been
generated by the gravity current acting as an obstacle to prefrontal flow.
The solitary wave was formed from the elevated head of the gravity current
that separated from the feeder flow. Profiler vertical-motion observations
resolve this wave and enable us to classify it as a Benjamin-Davis-Ono (BDO)
type solitary wave. The ducting mechanism that enabled the solitary wave to
propagate is also revealed from the wind profiler/RASS measurements. The
combination of high-resolution instruments at the MRI site allow us to
develop a uniquely detailed picture of a shallow gravity current structure.
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