How does Background Air Pressure Influence The Inner Edge Of The Habitable Zone for Tidally Locked Planets in a 3D View?

Effects of pN2 on the climate of a tidally locked aqua-planet. (a) air temperature, (b) relative humidity (RH), (c) water vapor concentration, (d) shortwave heating rate (QRS), (e) vertical velocity (W, solid line is zero velocity), (f) radial velocity (Vr), (g) cloud water content, and (h) cloud fraction in tidallylocked (TL) coordinates, for pN2 of 0.25, 0.5, 1.0, 2.0, 4.0, and 10.0 bar from left to right columns. The substellar point (SP) and anti-stellar point (AP) are at 0◦ and 180◦ , respectively. The contour lines in (f) are mass streamfunction with intervals of 1011 kg s−1 (solid lines: clockwise; dashed lines: anti-clockwise). The vertical dashed lines in (g-h) mask the region where the cloud fraction is relatively low. The numbers in the right corner of each panel is global-mean surface temperature in (a), total relative humidity in ((b), defined as the percentage of water vapor by mass contained in the whole atmosphere compared with the vapor mass that the atmosphere could theoretically hold if saturated everywhere, following Wolf & Toon (2015)), vertically integrated water vapor amount in (c), total atmospheric heat capacity in ((d), defined as Cpm where Cp is the specific heat capacity and m is the vertically integrated air mass per unit area), maximum vertical velocity below σ = 0.1 in (e), vertically integrated cloud water path in (g), and total cloud water fraction in ((h), assuming maximum–random overlap). The stellar flux is 1700 W m−2 , star temperature is 3700 K, and rotation period is 60 Earth days in all these experiments.

We examine the effect of varying background N2 surface pressure (labelled as pN2) on the inner edge of the habitable zone for 1:1 tidally locked planets around M dwarfs, using the three-dimensional (3D) atmospheric general circulation model (AGCM) ExoCAM. In our experiments, the rotation period is fixed when varying the stellar flux, in order to more clearly isolate the role of pN2.

We find that the stellar flux threshold for the runaway greenhouse is a non-monotonous function of pN2. This is due to the competing effects of five processes: pressure broadening, heat capacity, lapse rate, relative humidity, and clouds. These competing processes increase the complexity in predicting the location of the inner edge of the habitable zone.

For a slow rotation orbit of 60 Earth days, the critical stellar flux for the runaway greenhouse onset is 1700--1750, 1900--1950, and 1750--1800 W m−2 under 0.25, 1.0, and 4.0 bar of pN2, respectively, suggesting that the magnitude of the effect of pN2 is within ~13%. For a rapid rotation orbit, the effect of varying pN2 on the inner edge is smaller, within a range of ~7%. Moreover, we show that Rayleigh scattering effect as varying pN2 is unimportant for the inner edge due to the masking effect of cloud scattering and to the strong shortwave absorption by water vapor under hot climates. Future work using AGCMs having different cloud and convection schemes and cloud-resolving models having explicit cloud and convection are required to revise this problem.

Yixiao Zhang, Jun Yang
Comments: 14 Pages, 4 Figures, accepted for publication in ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
DOI: 10.3847/2041-8213/abb87f
Cite as: arXiv:2010.01466 [astro-ph.EP] (or arXiv:2010.01466v1 [astro-ph.EP] for this version)
Submission history
From: Yixiao Zhang
[v1] Sun, 4 Oct 2020 02:23:20 UTC (2,781 KB)

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