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Steven Businger
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Ian Morrison |
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University of Hawaii |
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Kona lows are subtropical cyclones that occur
during the cool season in the north central Pacific (Simpson 1952, Ramage
1962). |
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The Hawaiian adjective "kona" means
leeward and is used to describe winds with a southerly component that
replace the prevailing trade wind regime. |
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On 24 - 27 February 1997 a kona low affected the
island of Hawaii and brought with it record winds at Hilo, high surf, large
hail, and blizzard conditions at higher elevations. |
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Damage estimates for the storm due to crop loss,
property damage, and electricity and phone line outages exceed 4 million
dollars (NOAA Storm Data 1997). |
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Global operational NWP models perform poorly in
forecasting kona low development and track. |
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A lack of data over the central Pacific Ocean
and insufficient model resolution are likely factors in this deficiency. |
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A detailed case study of the kona low was
conducted using all available operational data and data from the NCEP/NCAR
Reanalysis Data Set. |
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Given the resolution (~2.5˚) of the
Reanalysis data, the focus of the investigation is the synoptic-scale
structure and evolution of the low. |
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Enhanced PVA associated with a north-south
oriented jet streak at 250 mb was the mechanism for genesis. |
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Height anomalies and absolute vorticity were
greatest at 250 mb. Temperature
anomalies were greatest at 300-400 mb. |
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An enhanced pressure gradient on the western
side of the low was maintained as the circulation propagated westward
towards an upper-level ridge. |
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Enhanced low-level advection of warm, moist air
was linked to large potential vorticity aloft. |
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Water vapor imagery (6.7 mm) shows an intrusion
of dry air on the NW side of the low. |
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Regions of deep convection coincided with areas
where the Best-Lifted Index <0 and low-level advection of warm, moist
air (advection of qe). |
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Cloud bands with embedded convective cells
formed on the low’s eastern side and propagated eastward, eventually
leaving the area of synoptic scale ascent and losing their convective
characteristics. Their life span was ~60 hours. |
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The low dissipated when the surface cyclone
moved westward relative to the upper level low and the circulation became
decoupled. |
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The Rossby-wave dispersion relationship was
found to provide a good estimate of the zonal propagation speed of the kona
low throughout its life cycle. |
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Rainbands associated with the kona low track
eastward, as rainbands do in midlatitude systems. |
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However, vigorous new rainbands form to the west
of older bands, as the storm center tracks westward, creating a special
forecast challenge. |
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Businger, S., T. Birchard Jr., K. Kodama, P. A.
Jendrowski, and J. Wang, 1998: A bow echo and severe weather associated
with a kona low in Hawaii. Wea. Forecasting, 13, 576-591. |
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Kodama, K. R. and G. M. Barnes, 1997: Heavy rain
events over the south-facing slopes of Hawaii: Attendant conditions. Wea.
Forecasting, 12, 347-367. |
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Ramage, C. S., 1962: The subtropical cyclone. J.
Geophys. Res., 67, 1401-1411. |
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____, 1995: Forecasters guide to
tropical meteorology, AWS TR 240 Updated. AWS/TR- 95/001, AWS, USAF, 392
pp. [Available from Headquarters AWS, Scott AFB, IL 62225. |
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Schroeder, T. A., 1977a: Meteorological analysis
of an Oahu flood. Mon. Wea. Rev., 105, 458-468. |
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____, 1977b: Hawaiian waterspouts and
tornadoes. Mon. Wea. Rev., 105, 1163-1170. |
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Simpson, R. H., 1952: Evolution of the Kona
storm: A subtropical cyclone. J. Meteor., 9, 24-35. |
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Acknowledgement |
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This research was supported by the UCAR
COMET Outreach Program under grants UCAR S97-86992 and S98-87020. |
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Notes