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Univ. of Hawaii: "The dynamic structure and evolution of a Kona Low"

Final Report

1.1 Introduction:

Kona systems are subtropical cyclones that occur during the cool season in the North Central Pacific. The Hawaiian adjective, "kona", meaning leeward, is used to describe southwesterly winds that replace the normal trade wind regime. Historically, Kona Lows have resulted in heavy rains, hailstorms, flash floods, landslides, high winds, large surf and swell, waterspouts, and severe thunderstorms.

A strong Kona Low affected the Hawaiian Islands from February 24 - 27, 1997. The Island of Hawaii (Big Island) experienced record winds at Hilo, winter storm conditions prevailed on the summits of Mauna Kea, Mauna Loa, and Haleakala, and high surf (4 m) buffeted all islands. Later, as the low passed across the Big Island from east to west, thunderstorms produced large hail (125-mm diameter) and heavy rain on both the Big Island and Maui. Damage estimates due to crop loss, property damage, and electricity and phone outages exceeded four million dollars.

The overall project objectives were:

The February 1997 Kona Low was investigated using all available observational data and reanalysis data from the NCEP/NCAR Reanalysis Project. This study focuses on the synoptic spatial scale since the resolution of the reanalysis data is 2.5°. The relationship between synoptic scale and convection is investigated through surface observations and satellite data.

Project participants include Dr. Steven Businger (UH Professor), Ian Morrison (MS student), and NWS lead forecaster Robert Farrell. Constructive comments regarding the research and results were received from Kevin Kodama and Paul Jendrowski at the NWS, and from Drs. Gary Barnes and Tom Schroeder at UH. The research was undertaken by Ian Morrison under the direction of Dr. Businger. NWS staff provided data access and historical insight into the forecast challenges associated with Kona Lows. As the project progressed, the final conclusions and conceptual model were evaluated by Robert Farrell for relevance to forecast concerns. The conceptual model and track tools will be applied to future kona-type disturbances affecting Hawaii. We will provide a summary of the success of the application of the forecast tools described in Section 1.2 in a future appendix to this report.

1.2 Description of research/development accomplishments

The Kona Low was found to be a deep, baroclinic system characterized by warm, moist air rising on the east side and cold, dry air sinking on the west side. Convective cells were located within cloud bands in the eastern side of the low, and a tight pressure gradient created strong surface winds in the northeast quadrant of the low. Consistent with a synoptic-scale baroclinic system, the divergence field aloft was closely linked with the ageostrophic component of the wind field. At the surface the isotherms showed a lack of frontal boundaries, likely due to enhanced surface fluxes from the subtropical ocean and to entrainment of subsiding warm air into the marine boundary layer on the west side of the low.

Summary of Significant Research Results

Forecast Tools Developed by the Project


Because of the collocation of the NWSFO-HNL and the University of Hawaii (UH) Meteorology Department, NWS staff and UH students and faculty have a close working relationship. Students and faculty participate in the daily weather briefings given by the NWS forecasters. NWS staff attends weekly seminars sponsored by the Meteorology Department. These seminars are presented by UH faculty, students, and visiting scientists from around the world. A number of collaborative papers have been published by NWS forecasters and UH faculty over the past several years.


Morrison, I. J., and S. Businger, 1999: The Dynamic Structure and Evolution of a Kona Low. Manuscript in preparation is to be submitted to Monthly Weather Review.

Morrison, I. J., and S. Businger, 1999: The Dynamic Evolution of a Kona Low. Presented at the 1999 Pacific Northwest Workshop held in Seattle, Washington in February 1999.

Morrison, I. J., 1998: The Structure and Evolution of a Kona Low. Presented at the University of Hawaii Seminar Series.


4.1 University's perspective

Examples of concrete improvements in operational forecasting resulting from the research sponsored by the COMET Program are specifically requested for inclusion in Section 4.2. These examples are important in demonstrating the effectiveness of the Outreach Program, and justifying continued funding for it. A discussion of significant problems encountered will also help us improve the program.

A better understanding of the track and the life cycle of the wind and precipitation structures in Kona Lows have resulted from the work. The development of a conceptual model of the life cycle of a Kona Low is a valuable contribution to the forecast process when Kona Low conditions impact Hawaii. Four forecast tools are outlined in Section 1.2 above. We will provide a summary of the success of the application of these forecast tools in a future appendix to this report. A problem that was encountered in undertaking our investigation of the Kona Low was the inability to get a good NWP simulation of the cyclone. We worked with Ron Gelaro of the Naval Post Graduate School in Monterey. Ron performed a custom run of the Navy's NOGAPS model for our case. However, just as the AVN was unable to maintain the storm depth and circulation, the NOGAPS model also failed. This general inability of global NWP models to simulate Kona Lows is striking and is a topic of future research at UH.

Section 4.2 NWS Perspective

The meteorological problems presented by "Kona Lows" (subtropical lows) have always been one of the least understood of the weather patterns that affect Hawaii. These systems can produce flash flooding and locally very strong winds and their movement is most erratic, thus increasing the forecast and watch/warning process. Ian Morrison's research and the paper he has produced enhance our knowledge of these weather systems by providing a life-cycle progression for these storms and a statistical basis for storm track forecasting. This should lead to a better understanding of kona-low behavior and provide NWS forecasters important forecast tools, leading to better forecasts and warnings related to their associated weather phenomena.