A Method for Diagnosing the Sources of Infrasound in Convective Storm Simulations

This paper presents a convenient method for diagnosing the sources of infrasound in a numerical simulation of a convective storm. The method is based on an exact acoustic wave equation for the perturbation Exner function Π′. One notable source term (Suu) in the Π′ equation is commonly associated with adiabatic vortex fluctuations, whereas another (Sm) is directly connected to the heat and mass generated or removed during phase transitions of moisture. Scale estimates suggest that other potential sources are usually unimportant. Simple numerical simulations of a disturbed vortex and evaporating cloud droplets are carried out to illustrate the infrasound of Suu and Sm. Moreover, the diagnostic method is applied to a towering cumulonimbus simulation that incorporates multiple categories of ice, liquid, and mixed-phase hydrometeors. The sensitivity of Sm to the modeling of the hail-to-rain category conversion is briefly addressed.

VORTEX FLUCTUATIONS IN WEAK MAGNETIC FIELDS

Flux-noise and ac susceptibility measurements have been performed on epitaxial films of YBa 2 Cu 3 O 7. The validity of the fluctuation-dissipation-theorem is verified by the proportionality between flux noise and ac susceptibility results. The nominal zero-field flux-noise spectrum (Sϕ(f)) can be characterized as follows: Below a temperature dependent frequency f0(T), the flux-noise spectrum is frequency independent. For frequencies f>f0(T), the flux-noise spectrum follows f-x, with x≈1.7-1.8. f0 decreases by 4 orders of magnitude in a temperature interval of ΔT≈0.5 K . Moreover, the flux-noise spectrum scales as fSϕ=g(f/f0(T)), where g(·) is a scaling function. The influence of weak perturbing magnetic fields was investigated. While the general characteristics of the flux-noise spectrum remain, the characteristic frequency f0 increases by 3-4 orders of magnitude increasing the field from zero to 1 Oe. Flux-noise measurements performed using a compensating field to reduce the residual field in the experimental setup reveal new features of the measured noise spectra. The flux-noise is dominated by thermally generated vortex-antivortex pair fluctuations at temperatures below the mean-field transition temperature, while with decreasing temperature there is a transition to vortex fluctuations being dominated by field generated vortices.