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

Final Report

SECTION 1 PROJECT OBJECTIVES AND ACCOMPLISHMENTS
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:

• To investigate the synoptic-scale dynamic evolution of the Kona Low.
• To better understand the track and the life cycle of the wind and precipitation structures in Kona Lows.
• To develop a conceptual model of the life cycle of a Kona Low as a forecast tool that can be applied when Kona Low conditions impact Hawaii in the future.

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

• A north south oriented jet streak aloft was the mechanism for genesis of the Kona Low. The jet streak occurred in the trailing side of a developing upper-level trough and enhanced winds remained on the west side of the vortex during the Kona Low's life cycle.
• The 250-mb level had the strongest absolute vorticity values and the greatest height anomalies. This result is contrary to previous analyses that emphasized a 500-mb circulation.
• The greatest temperature anomalies occurred in the layer between 400 and 250 mb.
• The tilt of the axis of enhanced temperature anomaly was greatest in the early stages.
• The Kona Low was characterized by cloud bands with imbedded convective cells. The cloud bands formed on the low's eastern side and propagated eastward, eventually leaving the area of synoptic scale ascent and losing their convective signature in satellite imagery. Later in the life cycle, cloud bands were less organized and convection appeared more isolated.
• The best-lifted index was able to predict the core regions of convection, as inferred from satellite imagery.
  Horizontal vorticity advection and vorticity generated by divergence were the two major terms that contributed most to the absolute vorticity.
• The vorticity tendency in the lower troposphere (<500 mb) was dominated by the divergence term, especially during the incipient and intensifying stages. Horizontal vorticity advection played an equal role with the divergence term in the upper tropospheric vorticity tendency for all stages.
• Divergence profiles showed a well-defined level of non-divergence at 500 mb during the strengthening stages with low-level convergence and upper-level divergence. Later stages were characterized by smaller values of convergence and divergence, and the level of non-divergence is indistinct.
• The vorticity profiles of bombs had a much greater amount of vorticity in the lower troposphere (<500 mb) than the Kona Low. However, aloft the Kona Low was characterized by greater values of vorticity at 250 mb and a steep increasing gradient of vorticity from 600 - 250 mb.
• Global NWP models (both the AVN and NOGAPS) show significant difficulty in predicting the depth and track of Kona Lows.

Forecast Tools Developed by the Project

• A conceptual model of the dynamic evolution of a Kona Low was constructed based on the results of the case study. This model will aid forecasters to predicting the likely behavior of attendant heavy rains and high winds in future Kona Lows.
• Rossby wave propagation theory was able to predict the observed zonal propagation of the 500-mb vortex. The propagation of future Kona Lows can be evaluated with reference to a tool based on the planetary wave equation developed by this project.
• The best-lifted index coincided with core regions of deep convection, as inferred from satellite imagery. Given a good forecast by the AVN or future NWP model, the lifted index forecast can be used to diagnose and forecast the areal extent of deep convective activity.
• The importance of the 250-mb jet structure was revealed in this research. The signature of a strong N-S oriented jet streak on 250-mb prognostic charts can alert forecasters early to the potential for Kona Low development.

SECTION 2 SUMMARY OF UNIVERSITY/NWS EXCHANGES

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.

SECTION 3 PRESENTATIONS AND PUBLICATIONS

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.

SECTION 4 SUMMARY OF BENEFITS AND PROBLEMS ENCOUNTERED

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.