How does Background Air Pressure Influence the Inner Edge of the Habitable Zonefor Tidally Locked Planets in a 3D View?

Abstract:

We examine the effect of varying background N$_2$ surface pressure (labelled as $p$N$_2$) on the inner edge of the habitable zone for 1:1 tidally locked planets around M dwarfs, using the three-dimensional (3D) global atmospheric circulation model (AGCM) ExoCAM. In all these experiments, the occurring of runaway greenhouse is associated with the onset of temperature inversion followed by cloud collapse. We find that the stellar flux threshold for the runaway greenhouse is a non-monotonous function of $p$N$_2$. This is due to the competing effects of five processes: pressure broadening, heat capacity, lapse rate, relative humidity, and clouds. For a slow rotation orbit of 60 earth days, the critical stellar flux for runaway greenhouse onset is 1700--1750, 1900--1950, and 1750--1800~W\,m$^{-2}$ under 0.25, 1.0, and 4.0 bar of $p$N$_2$, respectively, suggesting that the magnitude of the effect of $p$N$_2$ is within $\approx$13\%. For 1.0 and 4.0 bar, the difference is mainly due to pressure broadening and relative humidity, both of which increase with $p$N$_2$ and act to raise greenhouse effect, air temperature and water vapor amount, increase shortwave heating rate, and promote temperature inversion and cloud collapse, making the planet enter the runaway greenhouse at a lower stellar flux. For 1.0 and 0.25 bar, the difference is mainly due to lapse rate and heat capacity, which decrease with reducing $p$N$_2$ and act to increase shortwave heating rate and promote temperature inversion and cloud collapse, making the planet enter the runaway greenhouse at a lower stellar flux with reducing $p$N$_2$. For a rapid rotation orbit, the effect of air mass on the inner edge is smaller, within a rang of $\approx$7\%. Moreover, we show that Rayleigh scattering effect as varying $p$N$_2$ is unimportant for the inner edge due to the masking of cloud scattering and the strong shortwave absorption by water vapor under hot climates. All these competing processes increase the complexity in predicting the width of the habitable zone. Future work using 3D AGCMs having different cloud and convection schemes and cloud resolving models having explicit clouds and convection are required to revise this problem.