Departure from Clausius-Clapeyron scaling of water entering the stratosphere in response to changes in tropical upwelling
Departure from Clausius-Clapeyron scaling of water entering the stratosphere in response to changes in tropical upwelling
复制标题
进入平流层的水因响应热带上升流的变化而偏离克劳修斯-克拉佩龙比例
DOI：
10.1002/2013jd020772
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发表时间：
2014
期刊：
影响因子：
--
通讯作者：
Fueglistaler S
中科院分区：
文献类型：
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作者：
Fueglistaler S
进入平流层的水（[H2O]进入）受到热带对流层顶层（TTL）温度的强烈限制。热带对流层顶的温度比辐射平衡低 15–20 K。根据辐射传输计算，一般环流模型建议剩余环流加强，以应对不断增加的温室气体，据估计，上升流每变化 10%，温度就会下降约 2 K（对垂直尺度长度有一定敏感性）。对于内热带地区的均匀温度变化，[H2O]条目可能会发生变化，如蒸汽压的温度依赖性所预测的那样，此处称为“克劳修斯-克拉佩龙（CC）比例”。在 CC 缩放下，这对应于上升流变化 10% 时 [H2O] 条目变化约 1 ppmv。然而，上升流的变化也会改变空气在 TTL 中的停留时间。我们通过轨迹计算表明，这会影响 [H2O] 进入，使得 [H2O] 进入变化比 CC 缩放预期的小约 10%。水蒸气的停留时间效应是温度场时空变化的结果。我们表明，对于当今的 TTL，一半多一点的影响是由于流场和温度场之间的系统关系造成的。其余部分可以从随机游走问题的角度来理解，较慢的上升（较长的路径）会增加每个气团遇到异常低温的概率。我们的结果表明，即使所有脱水物理过程保持不变，大气中的水蒸气也可能会随着平均温度而偏离 CC 缩放。
Water entering the stratosphere ([H2O]entry) is strongly constrained by temperatures in the tropical tropopause layer (TTL). Temperatures at tropical tropopause levels are 15–20 K below radiative equilibrium. A strengthening of the residual circulation as suggested by general circulation models in response to increasing greenhouse gases is, based on radiative transfer calculations, estimated to lead to a temperature decrease of about 2 K per 10% change in upwelling (with some sensitivity to vertical scale length). For a uniform temperature change in the inner tropics, [H2O]entrymay be expected to change as predicted by the temperature dependence of the vapor pressure, referred here as “Clausius‐Clapeyron (CC) scaling.” Under CC scaling, this corresponds to ∼1 ppmv change in [H2O]entryper 10% change in upwelling. However, the change in upwelling also changes the residence time of air in the TTL. We show with trajectory calculations that this affects [H2O]entry, such that [H2O]entrychanges ∼10 % less than expected from CC scaling. Thisresidence timeeffect for water vapor is a consequence of the spatiotemporal variance in the temperature field. We show that for the present‐day TTL, a little more than half of the effect is due to the systematic relation between flow and temperature field. The remainder can be understood from the perspective of a random walk problem, with slower ascent (longer path) increasing each air parcel's probability to encounter anomalously low temperatures. Our results show that atmospheric water vapor may depart from CC scaling with mean temperatures even when all physical processes of dehydration remain unchanged.