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Lightning Assimilation Techniques

This study explores the use of lightning assimilation techniques to improve convective rainfall forecasts. The lightning strike data is converted into a lightning rate, which is then used to estimate convective rainfall rates. The assimilated convective rainfall data is incorporated into a weather model to enhance rainfall predictions.

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Lightning Assimilation Techniques

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  1. Lightning Assimilation Techniques • Non-thinned Lightning Experiment • NLDN/LR lightning strike is detected • Lightning strike is converted into 30 minute lightning rate from nearby LTNG observations • Lightning rate converted into “observation” of convective rainfall rate using Pessi/Businger convective rain rate/lightning rate relationship • Convective rainfall is scaled up to 6 hour cumulative total to match model background forecasted 6 hour convective rainfall • Convective rainfall (mm) is assimilated into WRF-EnKF

  2. Lightning Assimilation Techniques Pessi/Businger Lightning rate/Convective rainfall rate relationship

  3. Lightning Assimilation Techniques • Thinned Lightning Experiment • NLDN/LR lightning strike is detected • Lightning strike is converted into 30 minute lightning rate from nearby LTNG observations • Any lightning strikes used in the density calculation are no longer allowed to be an assimilation point, resulting in a thinning out of the lightning “observations” (although strikes will be used to calculate nearby densities) • Lightning rate converted into “observation” of convective rainfall rate using Pessi/Businger convective rain rate/lightning rate relationship • Convective rainfall is scaled up to 6 hour cumulative total to match model background forecasted 6 hour convective rainfall • Convective rainfall (mm) is assimilated into WRF-EnKF

  4. Lightning Assimilation Techniques • Thinned Lightning Experiment – 1 hr rain • NLDN/LR lightning strike is detected • Lightning strike is converted into 30 minute lightning rate from nearby LTNG observations • Any lightning strikes used in the density calculation are no longer allowed to be an assimilation point, resulting in a thinning out of the lightning “observations” (although strikes will be used to calculate nearby densities) • Lightning rate converted into “observation” of convective rainfall rate using Pessi/Businger convective rain rate/lightning rate relationship • Convective rainfall is NO LONGER scaled up as model background forecasted convective rainfall is a 1 hour cumulative total • Convective rainfall (mm) is assimilated into WRF-EnKF

  5. Pacific Ocean Low observation density; location of important storm tracks; errors propagate downstream to mainland United States North America High observation area; potential of forecast improvement; Similar studies with similar domain Pessi/Businger previously studied domain for lightning assimilation Experiment Design

  6. Observations Control case Radiosondes Surface stations (ASOS, ship, buoy) ACARS Cloud drift winds (no sat. radiances) Experimental cases Control observations Lightning Experiment Design

  7. WRF Setup WRF 2.1.2 (Jan 27, 2006) 100 by 86 grid 45-km horizontal resolution 33 vertical levels 270 second timestep Shortwave: Dudhia Longwave: Rrtm Surface: Noah land-sfc PBL: MYJ TKE scheme Cumulus: Kain-Fritsch (new Eta) Experiment Design

  8. EnKF Setup 90 ensemble members 6-hr Analyses 24-hr Forecasts (starting every 12 hours) 8 assimilations of “spin-up” before lightning assimilations Square root filter (Whitaker and Hamill, 2002) Horizontal localization – Gaspari and Cohn 5th order piecewise Fixed covariance perturbations to lateral boundaries Zhang covariance inflation method Localization radius – 2000 km Experiment Design

  9. Test Cases • Test Case #1 • December 16-21, 2002 • Test Case #2 • October 4-8, 2004 • Test Case #3 • November 8-12, 2006

  10. Experiment observations exampleACARS observations spatial distribution

  11. Experiment observations exampleCloud track wind observations spatial distribution

  12. Experiment observations exampleRadiosonde, surface station and buoy observations • Radiosonde Obs • Surface Stations • Buoys

  13. Experiment observations exampleLightning Observations

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