Overlap of fractional cloud for radiation calculations in GCMs: A global analysis using CloudSat and CALIPSO data
Overlap of fractional cloud for radiation calculations in GCMs: A global analysis using CloudSat and CALIPSO data
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DOI：
10.1029/2007jd009677
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发表时间：
2008-04
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通讯作者：
H. Barker
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文献类型：
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作者：
H. Barker
[1]全球气候模式（GCM）关于云的部分数量的垂直重叠的假设对模拟的辐射收支有重大影响。利用CloudSat-CALIPSO卫星测量得到的2个月的云掩模数据进行了一次分数云重叠特性的全球调查。云重叠被诊断为最大和随机重叠的组合，其特征在于垂直恒定的去相关长度cf*。通常情况下，云在最大和随机之间重叠，最小的cf*（中位数→ 0 km）与小的总云量相关，而最大的cf*（中位数> 3 km）往往发生在0.7附近。全球平均cf* 为1.22 km，冬季热带和极地地区有最大值的轻微趋势。通过粗略地从云掩模数据中删除近地面降水，cf* 通常减少<1 km。当云层掩模基于雷达和激光雷达数据时，太阳落下时cf* 的中值超过太阳升起时的中值近1 km;雷达的使用仅显示最小的日变化，但cf* 明显较大。这表明，日出的cf* 推断可能会受到激光雷达数据中的太阳噪声的影响。考虑了50、100、200、500和1000 km的云掩模横截面长度L。cf* 的分布对L稍微敏感，因此表明cf* 的GCM参数化可能在很大范围的分辨率上与分辨率无关。简单的参数化cf* 可能是可能的，如果过度的随机噪声，因此辐射通量，可以容忍。仅使用云掩模数据，并假设全球平均短波云辐射效应为−45 W m−2，则大气顶部短波辐射对cf* 的敏感度估计为2至3 W m−2 km−1。
[1] Assumptions made by global climate models (GCMs) regarding vertical overlap of fractional amounts of clouds have significant impacts on simulated radiation budgets. A global survey of fractional cloud overlap properties was performed using 2 months of cloud mask data derived from CloudSat-CALIPSO satellite measurements. Cloud overlap was diagnosed as a combination of maximum and random overlap and characterized by vertically constant decorrelation length cf*. Typically, clouds overlap between maximum and random with smallest cf* (medians → 0 km) associated with small total cloud amounts , while the largest cf* (medians ∼3 km) tend to occur at near 0.7. Global median cf* is ∼2 km with a slight tendency for largest values in the tropics and polar regions during winter. By crudely excising near-surface precipitation from cloud mask data, cf* were reduced by typically <1 km. Median values of cf* when Sun is down exceed those when Sun is up by almost 1 km when cloud masks are based on radar and lidar data; use of radar only shows minimal diurnal variation but significantly larger cf*. This suggests that sunup inferences of cf* might be biased low by solar noise in lidar data. Cloud mask cross-section lengths L of 50, 100, 200, 500, and 1000 km were considered. Distributions of cf* are mildly sensitive to L thus suggesting the convenient possibility that a GCM parametrization of cf* might be resolution-independent over a wide range of resolutions. Simple parametrization of cf* might be possible if excessive random noise in , and hence radiative fluxes, can be tolerated. Using just cloud mask data and assuming a global mean shortwave cloud radiative effect of −45 W m−2, top of atmosphere shortwave radiative sensitivity to cf* was estimated at 2 to 3 W m−2 km−1.