Martian Atmospheric Tides And Planetary Boundary Layer

Research on Martian atmosphere has been a topic of scientific curiosity for decades.Although Mars is smaller and farther from the Sun than the Earth,its climate has important similarities such as the seasonal changes and polar ice caps.So the comparative study between Mars and Earth is in favor for us to better understand the mechanisms on the climate evolution and the history and the future of our planet.On the other hand,the climate is of considerable relevance to the question of whether life is or was present on Mars.And study on Mars atmosphere has received more interest since the exploration based on advanced instruments such as Flyby and orbital spacecraft and a number of landers and rovers.In this thesis I focus on two aspects:atmospheric thermal tides and the planetary boundary layer(PBL)on Mars.Solar thermal tides are the dominant global-scale oscillations in the middle and upper atmosphere of a rapidly rotating planet like Earth and Mars.Tides on Mars are mainly excited by the heating from the widely distributed CO2,dust and water ice.Once propagating to the middle atmosphere,tides can modify the atmospheric circulation and it's important for the couple between the lower,middle and higher atmosphere.As the boundary layer on Mars is very high compared to Earth and it also controls the dust and water ice distribution,it's important to analyze its influence to the atmospheric tides as well.Previous studies have not connected these two factors.In this thesis,both atmospheric tides and PBL are investigated.The main works associated are as below:1.The increased local time coverage observed by Mars Climate Sounder(MCS)on board Mars Reconnaissance Orbiter(MRO)can enable direct extraction of thermal tides in Mars middle atmosphere with reduced aliasing.Using temperature profiles from Mars year(MY)30 to 32,we study the latitudinal and seasonal variations of tides and stationary planetary waves with zonal wave numbers s = 1-3.The amplitude of the migrating diurnal tide(DW1)has a strong semiannual variation both in the equatorial region and in the Southern Hemisphere(SH)middle latitudes.The migrating semidiurnal tide(SW2)shows clear semiannual variation in the Northern Hemisphere(NH)middle latitudes and equatorial region but annual variation in the SH.The spatial and temporal correlation between SW2 amplitude and density-scaled opacity of both water ice and dust in the equatorial region may provide a possible explanation for tidal forcing of SW2.Three Kelvin modes with zonal wave number 1,2 and 3(DE1,DE2,DE3)illustrate significant seasonal variations in the equatorial region.The DE1 seems to suggest a nearly semiannual variation,while both DE2 and DE3 show annual variation.For the first time,we extract the westward propagating diurnal tide with s = 2 and 3(DW2,DW3)and semidiurnal tide with s =1(SW1)in the Mars middle atmosphere using observational data.All three waves have an asymmetric latitudinal distribution,which should correspond to their possible excitation source,i.e.nonlinear interactions involving stationary planetary waves with s-1 and 2(SP1 and SP2).2.We use two Martian years' temperature data from Mars Climate Sounder(MCS)cross-track observations and investigate the spatial and seasonal variation of the migrating diurnal tide(MDT)from the perspective of classical tidal theory by Hough mode decomposition.The results suggest that during the equinox the vertically propagating(1,1)mode is dominant at all altitudes from the near surface to-0.1 Pa with the magnitude growing with height.The trapped modes(1,-2),(1,-4),(1,-6)are restricted to the lower altitudes with a similar vertical structure.Both the(1,1)and(1,-2)modes have clearly semiannual variations.The comparison between Hough modes components of MDT and dust and water ice heating rate indicates that both the dust and water ice contribute to tidal excitation.However,both the annual and semiannual variations of dust heating rate are out-of-phase with those in MDT while semiannual variation of the water ice heating rate is in-phase,indicating that the water ice may contribute to the semiannual variation of MDT.Using model wind results,we also find that the zonal mean zonal wind and its latitudinal shear at the low latitudes modify the vertical propagation condition of the MDT(1,1)mode and further affect its seasonal variation.The semiannual variations of equatorial MDT and its corresponding mechanism on Mars are comparable to those on Earth,suggests that the two planets may have more common characteristics.3.Large eddy simulations(LES)of meteorological process and dust transport in the Martian convective boundary layer(CBL)are performed employing a Mars version of the Weather Research and Forecasting model(WRF),adapted to use periodic boundary conditions.A constant threshold wind stress dust lifting scheme is used to determine dust lifting,and the lifted dust is entrained into the updraught.Surface albedo,thermal inertia and solar forcing are set uniform across the domain,using values obtained from the MarsWRF General Circulation Model(GCM)at the same locations.The model is validated by reproducing the major results of several cases of the previous study.Then we examine the effect of dust on the CBL process.The dusty atmosphere will shade the sunlight and weaken the convection process.However,the inhomogeneous dust distribution will enhance the thermal contrast both at the surface and in the atmosphere and induce more energy into the CBL by increasing the buoyant production.If the lifting rate is large enough,the extra energy due to the inhomogeneous effect may exceeds the decreased energy due to the shading effect and the overall effect results in the higher CBL height at the end of the afternoon.A proportional relation between the maximum inhomogeneity of dust distribution and the maximum CBL height of the Martian daytime is found.