Thursday, 31 August 2023: 11:00 AM
Great Lakes BC (Hyatt Regency Minneapolis)
Every day, approximately 217,000 fishermen go out on Lake Victoria (LV) in East Africa. Approximately 11% of the ~5.4 million people who live in Lake Victoria Basin (LVB) live on islands in the lake and rely on marine transport. An estimated 1500 fatalities occur on the lake each year, two-thirds of which (1000 deaths) are believed to be weather-related. This is due, in part, to the lake having one of the highest lightning frequencies in the world from nocturnal thunderstorms. The WMO-sponsored HIGH Impact Weather lAke sYstem (HIGHWAY) Project ran a field campaign during 2019 centered on LVB to collect an enhanced set of observations for scientific understanding of weather hazards over the lake. This presentation discusses radar-based methodologies to assist forecasters in nowcasting, detection, and warning of intense thunderstorms and strong winds over the lake.
The Tanzania Meteorological Authority (TMA) operates an S-Band polarimetric radar on the south shore of the lake by the city of Mwanza. This radar operated continuously as part of HIGHWAY Field Campaign during the rainy and dry seasons of Lake Victoria Basin (LVB). This data has been used in the identification of 6 primary modes of thunderstorm initiation and evolution over the lake. These storm systems can easily be tracked using storm extrapolation algorithms that provide nowcasts of storm future locations. An important component in the initiation of these storms is the setup of lake breezes in the afternoon and land breezes at night. Later in the evolution of the storms, gust fronts play a substantial role in the organization of multicellular storms and the maintenance and longevity of squall lines that propagate across the lake during the night.
Strong winds and wave heights are a major contributor to fatalities. Nowcasting and radar-detection of strong winds are particularly challenging yet very important over the lake. Our research documents the mesoscale-scale flow over LVB that is dominated by tropical easterly winds. We will present the near-surface, low-level wind magnitudes associated with thunderstorm downdraft region and thunderstorm outflows. Wind speeds as high as 20-25 ms-1 have been observed within the downdraft regions of these storms, with 8-15 ms-1 more typically associated with the gust front speeds. These strong winds are likely associated with higher wave heights on the surface of the lake that can capsize small fisher boats and cause fatalities. No direct surface water and wave height measurements were available during HIGHWAY due to lack of funding for deploying buoys or other water instrumentation. Thus in this presentation we are only able to compare the magnitudes of low-level Doppler velocities with wind thresholds documented in the literature that are critical for producing large wave heights. Techniques for nowcasting strong winds will be discussed.
The Tanzania Meteorological Authority (TMA) operates an S-Band polarimetric radar on the south shore of the lake by the city of Mwanza. This radar operated continuously as part of HIGHWAY Field Campaign during the rainy and dry seasons of Lake Victoria Basin (LVB). This data has been used in the identification of 6 primary modes of thunderstorm initiation and evolution over the lake. These storm systems can easily be tracked using storm extrapolation algorithms that provide nowcasts of storm future locations. An important component in the initiation of these storms is the setup of lake breezes in the afternoon and land breezes at night. Later in the evolution of the storms, gust fronts play a substantial role in the organization of multicellular storms and the maintenance and longevity of squall lines that propagate across the lake during the night.
Strong winds and wave heights are a major contributor to fatalities. Nowcasting and radar-detection of strong winds are particularly challenging yet very important over the lake. Our research documents the mesoscale-scale flow over LVB that is dominated by tropical easterly winds. We will present the near-surface, low-level wind magnitudes associated with thunderstorm downdraft region and thunderstorm outflows. Wind speeds as high as 20-25 ms-1 have been observed within the downdraft regions of these storms, with 8-15 ms-1 more typically associated with the gust front speeds. These strong winds are likely associated with higher wave heights on the surface of the lake that can capsize small fisher boats and cause fatalities. No direct surface water and wave height measurements were available during HIGHWAY due to lack of funding for deploying buoys or other water instrumentation. Thus in this presentation we are only able to compare the magnitudes of low-level Doppler velocities with wind thresholds documented in the literature that are critical for producing large wave heights. Techniques for nowcasting strong winds will be discussed.
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner