Graduate Thesis Or Dissertation
 

Diurnal and subdiurnal variability in the Mars Pathfinder Presidential meteorology sessions

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  • The Mars Pathfinder (MPF) arrived on the Martian surface on 4 July 1997 to become only the third successful landed mission to Mars, recording surface meteorological data intermittently over a period of 83 Martian days ("sols"). The in situ observations made by the MPF meteorology (MET) experiment were recorded at much greater precision than those of the previous missions, Viking Landers 1 and 2. These observations have been analyzed, focusing primarily upon the four so-called "Presidential" sessions, which each covered a complete diurnal cycle. The signature of very strong convective activity was seen in the temperature data, beginning soon after sunrise with temperatures changing as much as 14.39 K over the four-second interval between observations, and ceasing in late afternoon at the collapse of the boundary layer. Less extreme variability occurred at most other times of day and night. Examination of the first ten tidal pressure harmonic amplitudes for each Presidential session revealed strong diurnal and semidiurnal amplitudes and smaller, yet significant, amplitudes at the higher tidal frequencies. The normalized diurnal amplitude was slightly more than 1.7% for one session and averaged ~2.5% for the other three sessions. The semidiurnal amplitude averaged ~1.3%. A pattern in the tidal pressure harmonic amplitudes exists, in which odd-numbered harmonics (excluding the diurnal frequency) have smaller amplitudes than those of the next lower and next higher, even-numbered harmonics. Wind direction data for one Presidential session show very high variability throughout most of the diurnal cycle, the most intense activity occurring during the daytime convective period. A generally clockwise rotation of the mean wind direction was observed throughout the session. Temperature and wind data were examined closely for evidence of contamination of the temperature data by thermal effects of the lander itself. No evidence was found for such "lander interference" in the morning, but lander interference may have occurred in the afternoon of the session examined. A study of a numerical simulation by the NASA Ames Mars General Circulation Model (MGCM) showed prominent minima and maxima, resembling those observed by MPF, in the diurnal pressure cycles of simulated sols corresponding to the Presidential sessions. Also well simulated in each sol is the very rapid increase in surface pressure immediately after the daily minimum. Maps of diurnal and semidiurnal tidal amplitudes for the simulated Presidential sols show that tidal harmonic amplitudes are very spatially dependent, and that large changes in the harmonic amplitudes at any given location are likely to result if the global amplitude pattern for one or more frequencies undergoes small shifts in areographic location. Simulated temperature has a classic "red" power spectrum, while simulated pressure power is concentrated in the tidal frequency range. These spectral shapes are roughly consistent with those computed from the MPF Presidential sessions. The ratios of simulated to observed temperature power spectral estimates for frequencies from 1 to 50 cycles/sol show that the MGCM's simulated temperature variability is too low at all frequencies and especially so at higher frequencies.
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