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lmd_Hourdin1996_abstracts.html

1996 .

(2 publications)

M. Collins, S. R. Lewis, P. L. Read, and F. Hourdin. Baroclinic Wave Transitions in the Martian Atmosphere. Icarus, 120:344-357, April 1996. [ bib | DOI | ADS link ]

Surface pressure data from the Viking Lander mission and from GCM simulations of the martian atmosphere have been analyzed using singular systems analysis. Very regular oscillations are found with frequencies that are distributed bimodally with peaks corresponding to periods of approximately 2-4 days and 5-7 days, respectively. Reconstructions of the amplitudes of the two oscillations are often negatively correlated; i.e., when the amplitude of one oscillation is large, that of the other is small. The GCM simulations show that the negative correlation in the amplitudes of the two oscillations can be explained as a flipping between two different wavenumber modes. In the absence of diurnal forcing in the model, transition from an unrealistically regular high frequency mode to a similarly unrealistic regular low frequency mode occurs at most once during the northern winter season. The diurnal cycle in the model, however, acts in a non-linear sense to stimulate the transitions between the two wavenumbers and thus increases the frequency of mode flipping events. The corresponding simulations bear a closer resemblance to the observations.

W. T. Hutzell, C. P. McKay, O. B. Toon, and F. Hourdin. Simulations of Titan's Brightness by a Two-Dimensional Haze Model. Icarus, 119:112-129, January 1996. [ bib | DOI | ADS link ]

We have used a 2-D microphysics model to study the effects of atmospheric motions on the albedo of Titan's thick haze layer. We compare our results to the observed variations of Titan's brightness with season and latitude. We use two wind fields; the first is a simple pole-to-pole Hadley cell that reverses twice a year. The second is based on the results of a preliminary Titan GCM. Seasonally varying wind fields, with horizontal velocities of about 1 cm sec-1at optical depth unity, are capable of producing the observed change in geometric albedo of about 10% over the Titan year. Neither of the two wind fields can adequately reproduce the latitudinal distribution of reflectivity seen byVoyager. At visible wavelengths, where only haze opacity is important, upwelling produces darkening by increasing the particle size at optical depth unity. This is due to the suspension of larger particles as well as the lateral removal of smaller particles from the top of the atmosphere. At UV wavelengths and at 0.89 μm the albedo is determined by the competing effects of the gas and the haze material. Gas is bright in the UV and dark at 0.89 μm. Haze transport at high altitudes controls the UV albedo and transport at low altitude controls the 0.89-μm albedo. Comparisons between the hemispheric contrast at UV, visible, and IR wavelengths can be diagnostic of the vertical structure of the wind field on Titan.

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