SYNOPTIC STRUCTURE SYNOPTIC STRUCTURE AND EVOLUTION AND EVOLUTION OF A KONA LOWOF A KONA LOW

Steven Busingerand

Ian Morrison University of Hawaii

Kona LowKona 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 HazardsWeather Hazards

Kona Low of 1997Kona 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).

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Animation of Water Vapor Imagery

Numerical PredictionNumerical 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 TracesAVN Forecast Pressure Traces

Reanalysis Case StudyReanalysis 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 TrackObserved Kona Low Track

Observed Surface Pressure TraceObserved Surface Pressure Trace

Surface AnalysesSurface Analyses

Incipient Stage 18 UTC 23 Feb. 1997Incipient Stage 18 UTC 23 Feb. 1997

Intensifying Stage 12 UTC 24 Feb. 1997Intensifying Stage 12 UTC 24 Feb. 1997

Mature Stage 12 UTC 25 Feb. 1997Mature Stage 12 UTC 25 Feb. 1997

Weakening Stage 12 UTC 26 Feb. 1997Weakening Stage 12 UTC 26 Feb. 1997

Dissipating Stage 12 UTC 27 Feb. 1997Dissipating Stage 12 UTC 27 Feb. 1997

Satellite SignaturesSatellite Signatures

250 mb Height Analyses250 mb Height Analyses

Incipient Stage WV ImageIncipient Stage WV Image

Intensifying Stage WV ImageIntensifying Stage WV Image

Mature Stage WV ImageMature Stage WV Image

Weakening Stage WV ImageWeakening Stage WV Image

Dissipating Stage WV ImageDissipating Stage WV Image

Quasi-geostrophic DynamicsQuasi-geostrophic Dynamics

Incipient Stage – 250-mb isotachs, Incipient Stage – 250-mb isotachs, ageostrophic wind, and divergenceageostrophic wind, and divergence

Intensifying Stage – 250-mb isotachs, Intensifying Stage – 250-mb isotachs, ageostrophic wind, and divergenceageostrophic wind, and divergence

Mature Stage – 250-mb isotachs, Mature Stage – 250-mb isotachs, ageostrophic wind, and divergenceageostrophic wind, and divergence

Weakening Stage – 250-mb isotachs, Weakening Stage – 250-mb isotachs, ageostrophic wind, and divergenceageostrophic wind, and divergence

Dissipating Stage – 250-mb isotachs, Dissipating Stage – 250-mb isotachs, ageostrophic wind, and divergenceageostrophic wind, and divergence

Vorticity Advection by Thermal WindVorticity Advection by Thermal Wind

Intensifying Stage – TrenberthIntensifying Stage – Trenberth500-mb vorticity and 700-300-mb Thickness500-mb vorticity and 700-300-mb Thickness

Weakening Stage – TrenberthWeakening Stage – Trenberth500-mb vorticity and 700-300-mb Thickness500-mb vorticity and 700-300-mb Thickness

IPV and Low-level Advection of Moist AirIPV and Low-level Advection of Moist Air

Intensifying Stage – PV and Advection of Intensifying Stage – PV and Advection of e e

Mature Stage – PV and Advection of Mature Stage – PV and Advection of e e

Intensifying Stage – Best Lifted IndexIntensifying Stage – Best Lifted Index

Weakening Stage – Best Lifted IndexWeakening Stage – Best Lifted Index

Vertical StructureVertical Structure

Intensifying Stage: Vertical Cross SectionIntensifying Stage: Vertical Cross SectionHeight and Temperature AnomaliesHeight and Temperature Anomalies

Weakening Stage: Vertical Cross SectionWeakening Stage: Vertical Cross SectionHeight and Temperature AnomaliesHeight and Temperature Anomalies

Kona Low PropagationKona Low Propagation

Rossby Wave Propagation PredictionRossby Wave Propagation Prediction

ConclusionsConclusions

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.

ConclusionsConclusions

Water vapor imagery (6.7 m) 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 e).

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.

ConclusionsConclusions

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 ChallengeForecast 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.

ReferencesReferencesBusinger, 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/TR- 95/001,

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.

Questions?Questions?