A Mechanism for the Maintenance of Sharp Tropical Margins
A Mechanism for the Maintenance of Sharp Tropical Margins
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维持热带尖锐边缘的机制
DOI：
10.1175/jas-d-19-0154.1
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发表时间：
2020
影响因子：
3.1
通讯作者：
Mapes Brian E.
中科院分区：
文献类型：
--
作者：
Masunaga Hirohiko;Mapes Brian E.
潮湿的深层热带地区通常与干燥的亚热带地区之间存在明显的水平水分梯度。利用A-Train卫星观测资料研究了这一热带边缘的物理性质，重建了它的合成平均准经向热力学结构和过程。在这里，裕度被定义为沿卫星轨迹的指定柱状水汽(CWV)阈值的最北端位置。多个CWV阈值的选择范围为35至60 mm，包括48 mm的全球热带直方图最小值。对于所有边缘阈值，CWV从副热带向赤道方向增加，最终在热带一侧逐渐接近48毫米，显然是综合平均值的重合，因为如上所述，48毫米的值很少见。对于所有边界阈值，降水在热带一侧达到峰值，然后向赤道方向渐近接近85W m−2，等于蒸发渐近线。对于48 mm的阈值，气柱的总非绝热强迫(辐射加热加上地面潜热和感热通量)从热带一侧的正号变为副热带的负号，主要是由于高云的长波效应而在辐射加热方面形成了主要对比。根据观测数据拟合出赤道流向气柱的双垂直模式解析模式，以解释拉格朗日气柱转变的过程。该模型捕捉到了复合体的关键特征，并表明边缘突然增湿的关键过程是在深部副热带下沉之下向上生长的底部沉重的上升过程。
The moist deep tropics are typically separated from the drier subtropics by a sharp horizontal gradient of moisture. The physical nature of this tropical margin is investigated by using A-Train satellite observations to reconstruct its composite mean quasi-meridional thermodynamic structure and processes. The margin is defined here as the most poleward position of a specified column water vapor (CWV) threshold along a satellite track. Multiple CWV thresholds are selected from 35 to 60 mm, bracketing the global tropics histogram minimum value of 48 mm. For all margin thresholds, CWV increases equatorward from the subtropics and eventually asymptotically approaches 48 mm far on the tropical side, apparently as a coincidence of composite averaging since values of 48 mm are infrequent as noted above. For all margin thresholds, precipitation peaks on the tropical side and then asymptotically approaches equatorward a value of 85 W m−2, equal to the evaporation asymptote. For the 48-mm threshold, total diabatic forcing of the air column (radiative heating plus surface latent and sensible heat fluxes) changes sign from positive on the tropical side to negative in the subtropics, with the main contrast in radiative heating, owing principally to the longwave effect of high clouds. An analytic two-vertical-mode model of equatorward-flowing air columns is fitted from the observations to elucidate the processes in a Lagrangian column transition. The model captures key features of the composite, and suggests that a key process in the abrupt moistening at the margin is bottom-heavy ascent growing upward beneath the deep subtropical subsidence.