火星上的局部降水和径流
Localized precipitation and runoff on Mars
摘要：
我们使用火星区域大气模拟系统（MRAMS）来模拟火星上的湖暴，发现当湖泊面积大于等于10^3平方公里时，会出现强烈的局部降水。火星有一个低密度的大气层，所以深层对流可以由少量的潜热释放引发。在我们的参考模拟中，浮力羽流将蒸汽提升到冷凝水平以上，形成20公里高的光学厚度云。冰粒生长到200微米半径，并以平均速率高达1.5毫米/小时水当量（最大速率高达6毫米/小时水当量）落在湖泊附近（或湖泊中）。由于火星表面层外的大气温度总是远低于273 K，因此过饱和和冷凝开始于火星湖泊上方的低海拔。与地球上的湖暴不同，火星上的湖暴涉及持续的降水，其垂直速度和羽流高度超过地球上的热带雷暴。对于小于10^3 km^2的湖泊或大于O（10^2）mbar的大气压力，对流不会到达行星边界层以上。相反，蒸汽被平流顺风，几乎没有云的形成。降水以降雪的形式出现，而陆面上由羽流蒸汽和风暴云产生的日间辐射强迫太小，不足以直接融化积雪（<+10 W/m^2）。然而，如果轨道条件是有利的，那么雪可能会季节性地不稳定融化，并产生径流形成通道。我们通过在所有可能的轨道条件下运行热模型，并根据Laskar等人给出的概率对结果进行加权，2004.我们确定，对于赤道水汽源，阳光比现在暗15%，积雪的温度为0.28（0.35），融化可能发生的概率为4%（0.1%）。如果古代的温室效应比今天稍大（6 K），这一比例将上升到56%（12%）。
英文摘要：
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.
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