Simulation of the Martian Polar Climate with the LMD General Circulation Model: CO2 Snow Falls and Upper Atmosphere Polar Warming

Details Simulation of the Martian Polar Climate with the LMD General Circulation Model: CO2 Snow Falls and Upper Atmosphere Polar Warming Simulation of the Martian Polar Climate with the LMD General Circulation Model: CO2 Snow Falls and Upper Atmosphere Polar Warming

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0310f&link_type=abstract

American Astronomical Society, DPS meeting #28, #03.10; Bulletin of the American Astronomical Society, Vol. 28, p.1066

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The available observations of the Martian polar regions during the polar night have revealed some interesting climatic processes. On the one hand, Mariner 9 and Viking recorded anomalously cold brightness temperatures, thought to be created by CO_2 snow falls [Forget et al., JGR,100 ,p21119 ,1995]. On the other hand, Viking IRTM measured a strong warming of the northern polar night upper atmosphere during the global dust storms, caused by atmospheric dynamical processes [Jakosky and Martin, Icarus, 72, p528, 1987]. Both processes strongly decreased the amount of CO_2 that condensed into the polar caps [Forget and Pollack, JGR,101,p16865,1996]. We have successfully simulated these two processes in the LMD Martian atmosphere general circulation model (GCM). First, we have modeled the effect of the CO_2 snow falls on the infrared emission by decreasing the local emissivities when atmospheric condensation was predicted by the model. This parametrization was based on physical considerations (radiative transfer through the CO_2 ice particles, snow metamorphism on the ground). Without tuning the model parameters, we have been able to reproduce the general behavior of the observed low emission zones. Second, by extending the top of the GCM above 80 km, and by using a visible dust optical depth above 2, we have found that a polar warming similar to the observed phenomenon was simulated by the model. Until recently, most Martian GCMs were unable to simulate this behavior: this was a major weakness of these models. In our model, preliminary analyses suggest that the strong dust loading enhance the Hadley circulation at high altitude ( ~ 70 km) increasing there the poleward circulation and the adiabatic heating below. Overall, the LMD general circulation model is able to simulate the CO_2 condensation-sublimation cycle realistically. In particular, the seasonal variations of the surface pressure recorded by the Viking Lander can now be reproduced without artificially decreasing the condensation rate as done before. This work was supported in part by ESA (contract 11369/95/NL/JG SC)

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