Comparative Studies Of Titan's And Earth's Atmospheres By Numerical Simulations

Titan's and Earth's atmospheres are studied comparatively by numerical simulations from the perspective of comparative planetology. Firstly, climatological circulation features of Titan's atmosphere are simulated by PGCM, and the phenomenon of zonal wind sink'and superrotation are emphasized; Meanwhile, changes of Titan's atmospheric circulation and'zonal wind sink'are studied under different rotation speeds. Furthermore, Earth's atmosphere is researched under different rotation velocities in comparison. In addition, the Earth's atmospheric circulations at different orbital parameters, such as rotation periods, orbital obliquity, solar constant and eccentricity, are simulated by CAM3.1. Besides, changes of climate systems like the monsoon and the meridional circulation to these parameters on Earth are also discussed. The main conclusions are as follows:1. The climatological circulation features of Titan's atmosphere are as follows:zonal wind is very strong especially at the upper stratosphere, whose largest velocity reaches 108 m s-1 so called supperrotation; Meridional and vertical velocities are veiy small at the whole levels, which means that the meiridional mean wind and circulation are very weak; Meridional circulation has the characteristic of two cells and distributes disorderly near surface; At the height between 30 and 100 hPa, there is a sink on the vertical wind profile named'zonal wind sink', which is more obvious at low latitude than high latitude.2. The features of'zonal wind sink'on Titan under rotation rates are as follow:the phenomenon of'zonal wind sink'happens at all rotation velocities except the rotation period of 1 day, which mainly occurs between the latitude of 30°S and 30°N. The'zonal wind sink'and superrotation become more obvious with the rotation speed increasing when the rotation period is less than 50 days, which also happens at 243 days but the wind becomes weaker.3. By comparing Titan's and Earth's atmospheres, it can be seen that some factors are necessary for'zonal wind sink'occurring. Firstly, the rotation period is between 5 and 50 days. Secondly, the difference of horizontal temperature gradient at levels upper and lower the sink zone, which means that the gradient is larger at upper but smaller at lower. Finally, wet convection may be one of the reasons why'zonal wind sink'doesn't happen on Earth.4. The characteristics of atmospheric circulation under different rotation periods on Earth are as follow:as rotation period gets longer, the temperature becomes smaller at low latitude but larger at high latitude. However, the meridional gradient of horizontal temperature decreases at one height. Zonal west wind at high latitude becomes stronger and then weaker, but weaker all along for east wind at low latitude. Blow 100 hPa, meridional and latitudinal winds get faster. The meridional circulation is hemispheric at slow rotation condition. The features of atmospheric circulation under different orbital inclinations on Earth are as follow:as the orbital inclination gets larger, the temperature becomes lower at equator and subtropic but higher at high latitude. It's warmer at polar and cooler at equator when the orbital inclination is very large. Zonal west wind gets weaker at middle and high latitude but east wind becomes weaker and wider meridionally. Meridional and latitudinal speeds decrease especially near surface. The characteristics of atmospheric circulation under different solar constants on Earth are as follow:as the solar constant gets larger, the temperature increases and zonal wind gets stronger. Both of meridional and latitudinal velocities decrease, which decrease greatly at north hemisphere.5. Monsoon distributes differently under different Earth's parameters. Horizontally, monsoon area is like a band latitudinally at fast rotation condition but distributes regionally at slow rotation condition. As the rotation period increases from 1/10 day to 1 day, monsoon area becomes smaller and smaller. As it increases from 1 day to 243 days, monsoon area becomes larger and larger. Vertically, monsoon area distributes disorderly near surface at fast rotation condition, and it is regularly at slow rotation condition which is mainly above the equator and subtropic at north hemisphere. At the levels between 100 and 500 hPa, monsoon area distributes between 30°S and 30°N, whose meridional width becomes larger and intensity center becomes weaker. Below 100 hPa, monsoon areas at north and south hemispheres are symmetrical at fast rotation condition, which distribute mainly at equator and subtropic. The intensity center becomes stronger and then weaker with rotation period increasing. Monsoon areas decrease obviously at slow rotation condition.6. Meridional circulation under different rotation periods has features as follow:meridional circulation is three-cell and distributes disorderly under fast rotation condition, whose intensity is small. While it's one-cell at hemisphere with large intensity under slow rotation condition, which has two cells vertically at the center. Meridional circulation under different orbital inclinations has features as follow:when the orbit inclination angular is small, meridional circulation is three-cell and strong. When orbital inclination is large, meridional circulation has more than three cells and the intensity is weak with upward flow at polar and downward flow at equator. Meridional circulation under different solar constant has features as follow:when the solar constant is small, meridional circulation is three-cell and strong. When solar constant is large, meridional circulation has less than three cells and the intensity is weak with Hadley circulation on south hemisphere across the equator whose scale is very large.