Localized precipitation and runoff on Mars

We use the Mars Regional Atmospheric Modeling System (MRAMS) to simulate lake storms on Mars, finding that intense localized precipitation will occur for lake size ≥10³ km². Mars has a low-density atmosphere, so deep convection can be triggered by small amounts of latent heat release. In our reference simulation, the buoyant plume lifts vapor above condensation level, forming a 20 km high optically thick cloud. Ice grains grow to 200 μm radius and fall near (or in) the lake at mean rates up to 1.5 mm h^-1 water equivalent (maximum rates up to 6 mm h^-1 water equivalent). Because atmospheric temperatures outside the surface layer are always well below 273 K, supersaturation and condensation begin at low altitudes above lakes on Mars. In contrast to Earth lake-effect storms, lake storms on Mars involve continuous precipitation, and their vertical velocities and plume heights exceed those of tropical thunderstorms on Earth. For lake sizes 10^2.5 to 10^3.5 km, plume vertical velocity scales linearly with lake area. Convection does not reach above the planetary boundary layer for lakes $\ll$10³ km² or for atmospheric pressure >O(10²) mbar. Instead, vapor is advected downwind with little cloud formation. Precipitation occurs as snow, and the daytime radiative forcing at the land surface due to plume vapor and storm clouds is too small to melt snow directly (<+10 W m^-2). However, if orbital conditions are favorable, then the snow may be seasonally unstable to melting and produce runoff to form channels. We calculate the probability of melting by running thermal models over all possible orbital conditions and weighting their outcomes by probabilities given by long-term integrations of the chaotic diffusion of solar system orbital elements. With this approach, we determine that for an equatorial vapor source, sunlight 15% fainter than at present and snowpack with albedo 0.28 (0.35), melting may occur with 4% (0.1%) probability. This rises to 56% (12%) if the ancient greenhouse effect was modestly (6 K) greater than today.