  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 >=10^3 km^2. 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
20km-high optically-thick cloud. Ice grains grow to 200 microns radius and fall
near (or in) the lake at mean rates up to 1.5 mm/hr water equivalent (maximum
rates up to 6 mm/hr water equivalent). Because atmospheric temperatures outside
the surface layer are always well below 273K, 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. Convection does not reach above the planetary boundary layer for lakes
<<10^3 km^2 or for atmospheric pressure >O(10^2) 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 Laskar et al., 2004. 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 (6K) greater than today.
