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A. P. Ingersoll, H. G. Roe, E. L. Schaller, M. E. Brown (Caltech)
A fundamental number for convective activity is the time tau needed to re-humidify the atmosphere. This is M/E, where M is the mass of condensate per unit area (50 kg/m2 for Earth) and E is the evaporation rate (1.5 m of liquid water/yr). Alternately, tau is ML/F, where L is the latent heat of vaporization and F is the surface heat flux (125 W/m2 for Earth). With these numbers, tau = 12 days for Earth. This number also controls the time that a parcel spends in the descending branch of the Hadley cell, since the parcel must radiate away the heat of vaporization that it gained in the ascending branch. Tropical convective activity fluctuates on comparable time scales. Equatorial wave disturbances propagate to the west with a period of 4-5 days (Holton, 2004, p. 375). The equatorial intraseasonal oscillation, also known as the Madden-Julian oscillation (MJO), propagates to the east on a timescale of 30-60 days (Holton, 2004, p. 385). These bracket the time scale tau. Other oscillations like El Nino Southern Oscillation (ENSO) and the quasi-biennial oscillation (QBO) involve the oceans and the stratosphere, and are less relevant to the timescales of tropical convection. The value of tau for Titan is hundreds of times greater than for Earth, because the surface heat flux is much less and the latent heat content is about the same as on Earth. The polar cloud outbreaks at intervals of months around Titan's southern summer solstice are then a puzzle, as are the short-term variations of mid-latitude clouds observed after the solstice. One possibility (Roe et al., 2005, submitted; Schaller et al., 2005, submitted) is that the clouds originate from eruptions at the surface, which is controlling the variability of the atmosphere.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.