Transcript KONALOW

SYNOPTIC STRUCTURE
AND EVOLUTION
OF A KONA LOW
Steven Businger
and
Ian Morrison
University of Hawaii
Kona Low

Kona lows are subtropical cyclones that
occur during the cool season in the north
central Pacific (Simpson 1952, Ramage
1962).
 The Hawaiian adjective "kona" means
leeward and is used to describe winds
with a southerly component that replace
the prevailing trade wind regime.
Weather Hazards
Kona Low of 1997

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.
 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).
Animation of Water Vapor Imagery
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Numerical Prediction

Global operational NWP models perform
poorly in forecasting kona low
development and track.
 A lack of data over the central Pacific
Ocean and insufficient model resolution
are likely factors in this deficiency.
AVN Forecast Pressure Traces
Reanalysis Case Study

A detailed case study of the kona low was
conducted using all available operational
data and data from the NCEP/NCAR
Reanalysis Data Set.
 Given the resolution (~2.5˚) of the Reanalysis
data, the focus of the investigation is the
synoptic-scale structure and evolution of the
low.
Observed Kona Low Track
Observed Surface Pressure Trace
Surface Analyses
Incipient Stage 18 UTC 23 Feb. 1997
Intensifying Stage 12 UTC 24 Feb. 1997
Mature Stage 12 UTC 25 Feb. 1997
Weakening Stage 12 UTC 26 Feb. 1997
Dissipating Stage 12 UTC 27 Feb. 1997
Satellite Signatures
250 mb Height Analyses
Incipient Stage WV Image
Intensifying Stage WV Image
Mature Stage WV Image
Weakening Stage WV Image
Dissipating Stage WV Image
Quasi-geostrophic Dynamics
Incipient Stage – 250-mb isotachs,
ageostrophic wind, and divergence
Intensifying Stage – 250-mb isotachs,
ageostrophic wind, and divergence
Mature Stage – 250-mb isotachs,
ageostrophic wind, and divergence
Weakening Stage – 250-mb isotachs,
ageostrophic wind, and divergence
Dissipating Stage – 250-mb isotachs,
ageostrophic wind, and divergence
Vorticity Advection by Thermal Wind
Intensifying Stage – Trenberth
500-mb vorticity and 700-300-mb Thickness
Weakening Stage – Trenberth
500-mb vorticity and 700-300-mb Thickness
IPV and Low-level Advection of Moist Air
Intensifying Stage – PV and Advection of qe
Mature Stage – PV and Advection of qe
Intensifying Stage – Best Lifted Index
Weakening Stage – Best Lifted Index
Vertical Structure
Intensifying Stage: Vertical Cross Section
Height and Temperature Anomalies
Weakening Stage: Vertical Cross Section
Height and Temperature Anomalies
Kona Low Propagation
Rossby Wave Propagation Prediction
Conclusions




Enhanced PVA associated with a north-south
oriented jet streak at 250 mb was the mechanism
for genesis.
Height anomalies and absolute vorticity were
greatest at 250 mb. Temperature anomalies were
greatest at 300-400 mb.
An enhanced pressure gradient on the western side
of the low was maintained as the circulation
propagated westward towards an upper-level ridge.
Enhanced low-level advection of warm, moist air
was linked to large potential vorticity aloft.
Conclusions



Water vapor imagery (6.7 mm) shows an intrusion
of dry air on the NW side of the low.
Regions of deep convection coincided with areas
where the Best-Lifted Index <0 and low-level
advection of warm, moist air (advection of qe).
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.
Conclusions


The low dissipated when the surface cyclone moved
westward relative to the upper level low and the
circulation became decoupled.
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.
Forecast Challenge


Rainbands associated with the kona low track
eastward, as rainbands do in midlatitude systems.
However, vigorous new rainbands form to the west
of older bands, as the storm center tracks westward,
creating a special forecast challenge.
References
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.
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.
Ramage, C. S., 1962: The subtropical cyclone. J. Geophys. Res., 67, 1401-1411.
____, 1995: Forecasters guide to tropical meteorology, AWS TR 240 Updated. AWS/TR95/001, AWS, USAF, 392 pp. [Available from Headquarters AWS, Scott AFB, IL
62225.
Schroeder, T. A., 1977a: Meteorological analysis of an Oahu flood. Mon. Wea. Rev.,
105, 458-468.
____, 1977b: Hawaiian waterspouts and tornadoes. Mon. Wea. Rev., 105, 1163-1170.
Simpson, R. H., 1952: Evolution of the Kona storm: A subtropical cyclone. J. Meteor.,
9, 24-35.
Acknowledgement
This research was supported by the UCAR COMET Outreach Program
under grants UCAR S97-86992 and S98-87020.
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