Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6303.pdf

INVESTIGATING AN ORBIT-DRIVEN WATER ICE CLOUD GREENHOUSE WITH NASA AMES LEGACY MARS GLOBAL CLIMATE MODEL. M. . Kahre1, R. M. Haberle1, J. L. Hollingsworth1, and R. J. Wilson1, 1NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, [email protected]@nasa.gov.

Introduction: Clouds in planetary atmospheres The conditions required for a water ice can either cool or warm the surface. When they reside cloud greenhouse depend both on microphysical pro- over relatively dark surfaces, clouds raise the albedo cesses and on the characteristics of the global atmos- and provide negative forcing to the system. Clouds pheric circulation and transport processes. Significant also absorb infrared radiation from the surface and greenhouse warming from water ice clouds results emit a fraction of it back to the surface, thus providing when the clouds are optically thick, the cloud particles a positive forcing to the system. The balance of these are large enough to efficiently interact with infrared two opposing effects determines the net effect of radiation, and the clouds form at (or are transported to) clouds on the climate system. Cloud optical depths, high altitudes where the atmosphere is cold. As shown altitudes, and particle sizes influence this balance. In and discussed in Figures 1-3, all three of these condi- Earth’s atmosphere, low clouds tend to cool the sur- tions are met. face, and high clouds tend to warm the surface. The Annual and Zonal Average Cloud Optical Depth same trends are true on Mars. Under current condi- 20 tions, water ice clouds in Mars’ atmosphere provide

weak annual warming of less than 1 Kelvin. However, 15 recent climate modeling studies suggest that water ice clouds could have provide significant greenhouse warming during Mars’ recent past [1,2]. 10

Optical Depth Methods: use the NASA Ames Legacy Mars 5 Global Climate Model, which is supported by the Agency’s Mars Climate Modeling Center, to investi- 0 -90 -60 -30 0 30 60 90 gate how water ice clouds could have affected the Mar- Latitude tian climate throughout the Amazonian. We initially Figure 1: Annual and zonal mean water ice cloud infrared optical present two simulations at 35 obliquity: one with ra- depth for the case with radiatively active clouds (solid line) and the ° case with radiatively inert clouds (dashed line). Clouds are optically diatively active clouds and one with radiatively inert thick at all latitudes in the radiatively active simulation. clouds. With the exception of modifying the obliquity, the version of the model used here is identical to that Annual Mean Zonal Average Cloud Radius presented in [3]. We note that the dust forcing in both 1 4 6 of these simulations is based on observations from MY

24 [4]. 6 8 10 10

Results: As summarized in Table 1, clouds signif- 12 6

icantly impact the climate at 35° obliquity. While ra- 14

8 diatively active clouds increase the planetary albedo Pressure (mb) 100 8

over the case with inert clouds, they also increase the 10 10 24 12 annual mean surface temperature by more than 20 K. 10 22 14 8 24 201816 16 1000 6 8 1412 -90 -60 -30 0 30 60 90 Latitude Ap (K) (K) Te-Tse (K) Tg (K) Active 0.35 201 233 32 226 Figure 2: Annual and zonal mean water ice cloud effective particle radius (µm) for the case with radiatively active clouds. Cloud parti- Inert 0.26 208 216 8 203 cles are significantly larger than those in the present day aphelion A-1 .09 -7 17 24 23 cloud belt.

Table 1: Planetary Albedo (Ap), effective temperature at the top of the atmosphere (Te), effective surface temperature, greenhouse pow- (Te-Tse), and surface temperature (Tg) for the case with radiatively active clouds and the case with radiatively inert clouds.

Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6303.pdf

Annual and Zonal Average Temperature and Cloud (x106) MMR 1 100 Residual Cap (NPRC) is stable and the water cycle is 160

170 closed. These results suggest that it is likely that a 200

170 cloud greenhouse persisted throughout much of 300 100 10 170 the Amazonian. If that is true, there are im- 160 400 portant implications of these results for the distribution 100 180 200 of ice on the surface and in the subsurface over time. 200 190 300 Pressure (mb) 100 160 200 6 170 300 Clouds 10 MMR and MSF: Ls 90 100 210 180 210 1 400190 170180 220 200500 600100300120150130500140200400160 220 230 200190 210 140 600 1000 20 -90 -60 -30 0 30 60 90 10 Latitude 40 60 Figure 3: Annual and zonal mean temperature (color) and water ice 80 120

cloud mixing ratio (red contours) for the case with radiatively active 100 80 Pressure (Pa) 100 140 clouds. The τ = 1 location is shown in the red dashed line. Clouds 160180 form aloft at low to middle latitudes and nearer the surface at the 20 40 60 poles, but radiate to space at temperatures colder than the surface at 1000 -90 -60 -30 0 30 60 90 all latitudes. Latitude

6 Clouds 10 MMR and MSF: Ls 90 It is notable that the radiatively active cloud case is 1 wetter and cloudier than the radiatively inert cloud case (Figure 4). This is due to strong radiative- 10 dynamic feedbacks that occur in the presence 20 of radiatively active clouds. Clouds that form over 40

Pressure (Pa) 100 the north residual cap during summer warm the surface 60 and increase the water sublimation rates, which - 1000 hances the cloudiness over the cap. Atmospheric -90 -60 -30 0 30 60 90 warming by clouds aloft at lower latitudes drives an Latitude enhanced Hadley (.., mean meridional, zonally sym- Figure 5: Zonal mean cloud mixing ratio (color fill; ppm) and mass stream function (white contours; 108 kg s-1) at L metric) circulation, which in turn produces more s 90° for the case with radiatively active clouds (top) and the clouds in the tropics and subtropics and transports case with radiatively inert clouds (bottom). those clouds up higher where it is cold (Figure 5). Radiative-dynamic feedbacks play a critical role in producing the conditions needed for a strong cloud Global Average Water Content 600 greenhouse. These feedbacks will critically depend on the details of cloud microphysical processes, such as 500 the availability of ice nuclei and possibly the inclusion 400 of coagulation. Understanding how the dust cycle op-

300 erates at moderate obliquities will particularly im- portant because dust provides the ice nuclei for cloud Water (pr-um) 200 formation and will also affect the circulation and how

100 much water is sublimated from the surface. We plan to focus on understanding the sensitivity of our results to 0 30 60 90 120 150 180 210 240 270 300 330 360 the details of the cloud microphysics and the couplings Ls to the dust cycle in order to fully assess the feasibility Figure 4: Global mean water vapor (solid lines) and water ice of a substantial water ice cloud greenhouse on Mars. (dashed lines) for the case with radiatively active clouds (orange lines) and the case with radiatively inert clouds (black lines). References: [1] Haberle R. M. et al. (2012) LPSC. [2] Conclusions and Future Work: We have shown Kahre M. A. et al. (2015) CCTP2. [3] Haberle R. M. that water ice clouds can generate more than 25 K of et al. (2019) Icarus, in press. [4] Montabone L. et warming at 35° obliquity. Warming from clouds oc- al. (2015) Icarus, 251, pp.65-95 curs because they are thick, composed of large parti- cles, and reside up high where they radiate to space at temperatures colder than the surface. In these simula- tions, permanent ice reservoirs do not form outside of the north polar region, indicating that the North Polar