Plane‐parallel biases computed from inhomogeneous Arctic clouds and sea ice
Plane‐parallel biases computed from inhomogeneous Arctic clouds and sea ice
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根据不均匀的北极云和海冰计算的平面平行偏差
DOI：
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发表时间：
2002
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通讯作者：
Robert F. Cahalan
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作者：
A. Rozwadowska;Robert F. Cahalan
提出了云结构不均匀性和地表反照率对北极大气短波辐射通量预期影响的蒙特卡罗模拟。特别地，研究了海冰上非吸收、低空、全液态层云的反照率和透射率的平面平行偏差。“绝对偏差”定义为均匀或平面平行情况下的云反照率或透射率与具有相同平均云光学厚度和相同平均表面反照率的非均匀条件下的反照率或透射率在给定区域内的平均值之间的差值（即偏差> 0表示平面平行高估）。北极条件典型输入参数的均值和标准差范围由第一次国际卫星云气候学项目（ISCCP）区域实验北极云实验(FIRE/ACE)/北冰洋表面热预算(SHEBA)/大气辐射测量计划（ARM）实验确定，该实验是美国能源部、美国国家航空航天局、美国国家科学基金会、美国国家海洋和大气管理局、美国海军研究办公室、以及大气环境服务局。我们根据以下因素确定偏差的灵敏度：云光学厚度和表面反照率的域平均平均值和空间方差、表面反射函数的形状、云下散射层的存在以及太阳天顶角。模拟结果表明，北极条件下的偏差一般小于亚热带层积云。对于反照率，绝对偏差的大小不可能超过0.02，对于透射率，绝对偏差的大小不可能超过0.05。“相对偏倚”表示绝对偏倚占实际云层反照率或透过率的百分比。在反射性的北极表面，反照率的相对偏差幅度通常在2%以下，而透射率的相对偏差幅度可以超过10%。
[1] Monte Carlo simulations of the expected influence of nonuniformity in cloud structure and surface albedo on shortwave radiative fluxes in the Arctic atmosphere are presented. In particular, plane-parallel biases in cloud albedo and transmittance are studied for nonabsorbing, low-level, all-liquid stratus clouds over sea ice. The “absolute bias” is defined as the difference between the cloud albedo or transmittance for the uniform or plane-parallel case, and the albedo or transmittance for nonuniform conditions with the same mean cloud optical thickness and the same mean surface albedo, averaged over a given area (i.e., bias > 0 means plane-parallel overestimates). Ranges of means and standard deviations of input parameters typical of Arctic conditions are determined from the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment Artic Cloud Experiment (FIRE/ACE)/Surface Heat Budget of the Arctic Ocean (SHEBA)/Atmospheric Radiation Measurement Program (ARM) experiment, a cooperative effort of the Department of Energy, NASA, NSF, the National Oceanic and Atmospheric Administration, the Office of Naval Research, and the Atmospheric Environment Service. We determine the sensitivity of the bias with respect to the following: domain averaged means and spatial variances of cloud optical thickness and surface albedo, shape of the surface reflectance function, presence of a scattering layer under the clouds, and solar zenith angle. The simulations show that the biases in Arctic conditions are generally lower than in subtropical stratocumulus. The magnitudes of the absolute biases are unlikely to exceed 0.02 for albedo and 0.05 for transmittance. The “relative bias” expresses the absolute bias as a percentage of the actual cloud albedo or transmittance. The magnitude of the relative bias in albedo is typically below 2% over the reflective Arctic surface, while the magnitude of the relative bias in transmittance can exceed 10%.