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  • Author or Editor: David L. Priegnitz x
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Richard H. Johnson
and
David L. Priegnitz

Abstract

An observational study of the thermodynamic and kinematic structure and properties of monsoon convective systems and their large-scale environment over the southern South China Sea during the field phase (December 1978) of the Winter Monsoon Experiment (WMONEX) has been carried out. The primary observations used are from three Soviet research vessels positioned in a triangular array off the north coast of Borneo during the period 6–28 December. Computations of thermodynamic fields, divergence and vertical motion have been made for the duration of the field phase based on six-hourly rawinsonde releases at the ship sites.

Analysis of the data indicates that the degree of convective activity over the southern South China Sea is modulated by long-period synoptic forcing (monsoon surges, easterly waves) and also significantly by diurnal forcing (land-sea breeze circulations). A diurnal cycle of convection persists in the region whether the synoptic-scale forcing is weak or strong. Convection over water to the north of Borneo regularly evolves on a diurnal basis from a small group of cumulonimbus clouds into a uniform mesoscale precipitation area having the characteristic structure of those observed in recent years over the tropical eastern Pacific and Atlantic oceans. In their mature stage the precipitation systems contain mesoscale anvil clouds commonly extending from near 500 mb to the tropopause covering a 104–105 km2 area. The ship observations provide direct evidence of mesoscale updraft motion within the anvil clouds and mesoscale down-draft below extending to near the surface.

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AndréA. Doneaud
,
James R. Miller Jr.
,
David L. Priegnitz
, and
Lakshmana Viswanath

Abstract

Two mesoscale case studies in the semi-arid climate of southeastern Montana were carried out on 1 May and 3 June 1980. I May was an unstable, rainy day with two rain periods over the mesonet area, and 3 June was a potentially unstable day, with a cold frontal passage in the afternoon producing a very intense convective event.

Data from an instrumented mesoscale network (supporting the HIPLEX Montana experiment located between Miles City and Baker), a 5 cm radar, soundings, satellite (GOES), and synoptic maps were considered. The mesonet wind, temperature and moisture data were processed, computed every 15 min, and compared with radar rain patterns.

The study confirmed that convergence cell development within the surface kinematic fields precedes radar echoes and is directly related to the convective event. The areas involved in the vertical motions generating storms are much larger compared to those reported in humid climates. The “areal convergence” is a better storm predictor than the maximum convergence point value. A cloud merging effect related to the storm intensity and reduced rain efficiencies were also found.

The structure of the divergence field over the whole network experienced a cyclic evolution in both cases. This cyclic evolution is identified as a potential predictor for rain beginning 25–70 min after the last cycle before the rain phase.

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