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Modeling Infrasound Propagation in Realistic Atmospheres (“Silence of the Lamb Waves”)

Modeling Infrasound Propagation in Realistic Atmospheres (“Silence of the Lamb Waves”). Kenneth E. Gilbert Carrick L. Talmadge Roger Waxler The University of Mississippi, Oxford, MS USA. MODEL. STATUS. · Wavenumber Integration (FFP) ¾ Linear Finite Element ¾ Linear/Constant k 2 (z).

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Modeling Infrasound Propagation in Realistic Atmospheres (“Silence of the Lamb Waves”)

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  1. Modeling Infrasound Propagation in Realistic Atmospheres (“Silence of the Lamb Waves”) Kenneth E. Gilbert Carrick L. Talmadge Roger Waxler The University of Mississippi, Oxford, MS USA

  2. MODEL STATUS ·Wavenumber Integration (FFP) ¾Linear Finite Element ¾Linear/Constant k2(z) In Use In Development ·Parabolic Equation ¾Split-Step Fourier ¾Finite Difference (Padé) Testing Testing ·Normal Mode In Development NCPA INFRASOUND PROPAGATION MODELS

  3. ton explosion c(signal) = 353 m/s c(ground) = 339 m/s

  4. OCT. 30, 1961 TEST (USSR) C(z) GUIDED WAVES SOUND SPEED LAMB WAVE C0 58 MT DETONATION WASH D.C. (7000 km) NEW ORLEANS (8000 km) x

  5. THEORETICAL AND OBSERVED SIGNALS FROM A NUCLEAR EXPLOSION (OCT 30, 1961) THEO. (DGH) THEO. (DGH) THEORY 0.34 0.44 0.42 (a) (a) OBSER. 10 min. 10 min. EXP. 1.75 1.30 0.64 OBSER. PRESSURE (mbars) (b) (b) THEO. THEORY 0.57 THEO. 0.55 0.32 (c) (c) 58 MT AT 3.66 km 58 MT AT 3.66 km TIME TIME 7000 km (Washington D.C.) 8000 km (New Orleans)

  6. “LAMB WAVE” IN A REALISTIC ATMOSPHERE cH < c0 cH > c0 c(z) = cA(z) + wind H 50 km c(z) = cA(z) c0 > 340 m/s < 0 for guided waves cH < c0 cH > c0 f< 4103 Hz f  no limit ~

  7. ton explosion c(signal) = 353 m/s c(ground) = 339 m/s

  8. Characterization of Wavenumber Integration We let • where p(,z) is the solution to the wave equation for a fixed horizontal wavenumber . This wave equation is based on Pierce’s theory and incorporates: • a z-stratified medium • exact treatment of horizontal but vertically varying winds • viscosity can be incorporated with ease.

  9. Effects of Winds on Propagation in a Realistic Atmosphere f = 0.1 Hz Sound speed and effective sound speed (westerly propagation) from Kulichkov data base (Oct 10, 1990). Green’s functions for no winds and with winds

  10. Effects of Winds on Propagation in a Realistic Atmosphere (ground-to-ground) f = 0.1 Hz Transmission Loss [dB] • Note without winds, that no significant upper-atmospheric arrivals occur until after 200 km. • With stratospheric duct, additional arrivals are observable for ranges as short as 30-40 km. • Tropospheric duct (“weather”) in this case has suppressed some of short-distances bounces.

  11. FFP Synthetic Pulse at 305 km (stratospherically ducted) c(signal) = 335 m/s c(ground) = 339 m/s

  12. ton explosion c(signal) = 353 m/s c(ground) = 339 m/s

  13. Conclusions • Propagation models are important for correct interpretation of experimental observations. • A gravitationally trapped Lamb wave does not exist for kiloton (and smaller) events (“silence of the Lamb waves”). • Stratospherically ducted modes can appear as strong early arrivalswith speeds near 340 m/s.

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