Peatlands often experience turbulent sheltering from their surrounding upland forests, which results in spatially variable surface–atmosphere exchanges of momentum, heat, and moisture produced by flow-separation dynamics, which suppresses the transport of such scalars in the sheltered region while promoting transport in the reattachment zone. With evapotranspiration being the dominant source of water loss in the Boreal Plains, it is necessary to understand the dynamics and controls on evapotranspiration within these peatlands. We used the regional atmospheric forest large-eddy simulation (RAFLES) model to study the impact of flow separation and surface roughness on microclimates leeward of a forest-to-peatland roughness transition. We parametrized our simulation with observed vegetation characteristics and meteorological data from three natural peatlands to accurately estimate natural ranges of peatland roughnesses and energy dynamics. Our simulations show that changes to peatland roughness do not affect the distances required for flow reattachment, and therefore the size of the sheltered region. However, increasing the surface roughness produces greater surface turbulence, quicker flow recovery, and decreased flow reversals within the sheltered region. Further, substantial microclimatic differences are observed throughout the flow regions of the roughness transition. Our results show that turbulence, aerodynamic resistance, and the microclimate vary throughout the backward-facing step transition and should be taken into account when estimating the spatial dynamics of evaporative demand leeward of a roughness transition. Furthermore, increasing the surface roughness of a peatland minimizes the spatial variability of turbulent drivers of evapotranspiration across a roughness transition. That is, flow separation and the surface roughness of the peatland should be accounted for when estimating the spatial variability and total evaporative potential across a peatland.

中文翻译：

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半湿润北方平原森林到泥炭地过渡对空气动力阻力水汽转移的小气候影响

泥炭地经常受到周围高地森林的湍流遮蔽，这导致由流动分离动力学产生的动量、热量和水分在空间上发生变化的地表-大气交换，这抑制了这些标量在遮蔽区域的传输，同时促进了在该区域的传输。再附着区。由于蒸发蒸腾是北方平原水分流失的主要来源，因此有必要了解这些泥炭地内蒸散的动态和控制。我们使用区域大气森林大涡模拟 (RAFLES) 模型来研究流动分离和表面粗糙度对森林到泥炭地粗糙度过渡的小气候背风的影响。我们使用来自三个天然泥炭地的观测植被特征和气象数据对模拟进行参数化，以准确估计泥炭地粗糙度和能量动态的自然范围。我们的模拟表明，泥炭地粗糙度的变化不会影响流动重新附着所需的距离，因此不会影响遮蔽区域的大小。然而，增加表面粗糙度会产生更大的表面湍流，更快的流动恢复，并减少遮蔽区域内的流动逆转。此外，在粗糙度过渡的整个流动区域中观察到显着的小气候差异。我们的结果表明，湍流、空气动力阻力、小气候在整个向后阶梯过渡期间变化，在估计粗糙度过渡的背风蒸发量的空间动态时应考虑在内。此外，增加泥炭地的表面粗糙度可以最大限度地减少粗糙度过渡期间蒸散的湍流驱动因素的空间变异性。也就是说，在估算整个泥炭地的空间变异性和总蒸发潜力时，应考虑泥炭地的流动分离和表面粗糙度。