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Pressure oscillations and fallout downdraughts
Author(s) -
Clarke R. H.
Publication year - 1962
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49708837808
Subject(s) - oscillation (cell signaling) , geology , wavelength , shear (geology) , wind shear , attenuation , altitude (triangle) , range (aeronautics) , mechanics , physics , meteorology , seismology , geodesy , wind speed , optics , geometry , materials science , petrology , genetics , mathematics , composite material , biology
The generation of widespread pressure oscillations in southern Australia on 26–27 January 1960 is examined in some detail, by means of available barograms, and spectral analysis of a Melbourne Dines microbarogram. Photographs of mamma cloud taken at Adelaide give some insight into the processes at work producing both cloud form and oscillations. It is concluded that the oscillations were due to wave motions on an interface with large density contrast, at about 1,500 m altitude, and that these motions propagated approximately in the direction of vertical shear, with the velocity of the wind at about 2·5 km. Their phase and group velocities were similar, and their wavelengths in the range 10–70 km. These waves appear to have been transmitted over great distances without marked attenuation. An estimate of the energy of oscillation was compared with that which could be imparted to a horizontal interface with strong shear by the passage of downdraughts through it, and the conclusion is reached that the combination of downdraughts and shear could have provided the energy. On the other hand, there is good evidence that the waves were unstable, and could therefore have drawn their energy from that of the mean motion. Examination of other occasions on which marked pressure oscillations occurred suggested that fallout and strong shear were both necessary conditions.