The Effect of Land Surface Heterogeneity and Background Wind on Shallow Cumulus Clouds and the Transition to Deeper Convection
The Effect of Land Surface Heterogeneity and Background Wind on Shallow Cumulus Clouds and the Transition to Deeper Convection
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DOI：
10.1175/jas-d-18-0196.1
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发表时间：
2019-02
影响因子：
3.1
通讯作者：
J. Lee;Yunyan Zhang;S. Klein
中科院分区：
文献类型：
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作者：
J. Lee;Yunyan Zhang;S. Klein
采用非均匀地表热通量的理想化大涡模拟(LESS)，研究了非均匀长度、尺度和背景风速对浅层积云发展和随后向密集/深对流过渡的影响。我们研究了在有利于浅对流的大气中陆面非均质性的影响，但对于深层对流也是有条件的不稳定的。我们发现，在对流转换之前，更大更厚的浅积云附着在边界层顶部附近的蒸发分数较低的斑块上(称为“干”)。这一特征可归因于地表诱发的二次环流，其发展取决于非均匀大小和背景风速。在无环境风的情况下，对于大的斑块(≥5公里)，次级中尺度环流促进了水汽的垂直输送，形成了干燥斑块上的水汽池，而对于较小的斑块，则不会发展这种环流。背景风对二次环流的影响较大，任何大于2m的S−1风都能有效地消除地面非均质性对边界层的影响，不会产生二次环流。这是因为触发的次级环流不够强，不足以承受所施加的背景风。在此基础上，我们提出了两个对流转换的判据，即非均匀长度尺度大于5公里和背景风速小于UC0，其中UC0是在给定斑块大小为零背景风情况下次级环流的近地表横斑风速，在我们的例子中约为1.5m S−1。
Idealized large-eddy simulations (LESs) with prescribed heterogeneous land surface heat fluxes are performed to study the impact of the heterogeneity length scale and background wind speed on the development of shallow cumulus and the subsequent transition to congestus/deep convection. We study the impact of land surface heterogeneity in an atmosphere that favors shallow convection but is also conditionally unstable with respect to deeper convection. We find that before the convection transition, larger and thicker shallow cumulus clouds are attached to moisture pools near the PBL top over patches with low evaporative fraction (referred to as “DRY”). This feature is attributable to a surface-induced secondary circulation whose development depends on the heterogeneity size and the background wind speed. With large patches (≥5 km) under zero ambient wind, the secondary mesoscale circulation promotes the vertical transport of moisture forming a moisture pool over DRY patches, while with smaller patches, no such circulation develops. The influence of the background wind on the secondary circulation is strong such that any wind stronger than 2 m s−1 effectively eliminates the impact of surface heterogeneity on the PBL and brings no secondary circulation. This is because the triggered secondary circulation is not strong enough to withstand the imposed background wind. Based on these, we propose two criteria for the convection transition, namely, that the heterogeneity length scale is greater than 5 km and that the background wind speed is less than Uc0, where Uc0 is the near-surface cross-patch wind speed of the secondary circulation under zero background wind for a given patch size and is about 1.5 m s−1 in our cases.