It is well known that subtropical shallow convection transports heat and water vapor upwards from the surface. It is less clear if it also transports horizontal momentum upwards to significantly affect the trade winds in which it is embedded. We utilize unique multiday large-eddy simulations run over the tropical Atlantic with ICON-LEM to investigate the character of shallow convective momentum transport (CMT). For a typical trade-wind profile during boreal winter, CMT acts as an apparent friction to decelerate the north-easterly flow. This effect maximizes below the cloud base while in the cloud layer, friction is very small, although present over a relatively deep layer. In the cloud layer, the zonal component of the momentum flux is counter-gradient and penetrates deeper than reported in traditional shallow cumulus LES cases. The transport through conditionally sampled convective updrafts and downdrafts explains a weak friction effect, but not the counter-gradient flux near the cloud tops. The analysis of the momentum flux budget reveals that, in the cloud layer, the counter-gradient flux is driven by convectively triggered nonhydrostatic pressure-gradients and horizontal circulations surrounding the clouds. A model set-up with large domain size and realistic boundary conditions is necessary to resolve these effects.

中文翻译：

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ICON-LEM 模拟中副热带浅对流的反梯度动量传输


众所周知，副热带浅对流将热量和水蒸气从地表向上输送。目前尚不清楚它是否也会向上输送水平动量以显着影响其所嵌入的信风。我们利用 ICON-LEM 在热带大西洋上运行的独特的多日大涡模拟来研究浅层对流动量传输 (CMT) 的特征。对于北方冬季典型的信风剖面，CMT 起到明显的摩擦作用，使东北风流减速。这种效应在云底以下最大化，而在云层中，摩擦力非常小，尽管存在于相对较深的层上。在云层中，动量通量的纬向分量是反梯度的，并且比传统浅积云 LES 情况中报道的穿透更深。通过条件采样的对流上升气流和下降气流的传输解释了弱摩擦效应，但不能解释云顶附近的反梯度通量。动量通量预算分析表明，在云层中，反梯度通量是由对流触发的非静水压力梯度和云周围的水平环流驱动的。为了解决这些影响，需要建立具有大域尺寸和现实边界条件的模型。