Koning A M, Nuijens L, Bosveld F C, Siebesma A P, van Dorp P A, Jonker H J J
Delft University of Technology Delft The Netherlands.
Dutch Royal Meteorological Insitute (KNMI) De Bilt The Netherlands.
J Geophys Res Atmos. 2021 Nov 16;126(21):e2021JD035087. doi: 10.1029/2021JD035087. Epub 2021 Oct 28.
This study investigates how wind shear and momentum fluxes in the surface- and boundary layer vary across wind and cloud regimes. We use a 9-year-long data set from the Cabauw observatory complemented by (8.2 × 8.2 ) daily Large Eddy Simulation (LES) hindcasts. An automated algorithm classifies observed and simulated days into different cloud regimes: (a) clear-sky days, (b) days with shallow convective clouds rooted in the surface layer, with two ranges of cloud cover, and (c) non-convective cloud days. Categorized days in observations and LES do not always match, particularly the number of non-convective cloud days are underestimated in the LES, which likes to develop convection. However, the climatology and diurnal cycle of winds for each regime are very similar in LES and observations, strengthening our confidence in LES' skill to reproduce certain clouds for certain atmospheric states. Along-wind momentum flux profiles are similar across all regimes, but large cloud cover (convective and non-convective) days have larger total momentum flux distributed over a deeper layer, with up to 30% of the surface flux still present near cloud base. The clear-sky and especially shallow cumulus regime with low cloud cover have notably larger crosswind momentum fluxes in the boundary layer. Surface-layer wind shear at daytime is smallest in the shallow cumulus regimes, having deeper boundary layers and a steady increase in surface layer wind speed during daytime. Compared to clear-sky days at a similar stability, convective cloud regimes have smaller surface-layer wind shear and larger surface friction than estimated by Monin-Obukhov Similarity Theory.
本研究调查了表层和边界层中风切变和动量通量如何随风况和云况而变化。我们使用了来自卡堡观测站的长达9年的数据集，并辅以（8.2×8.2）每日大涡模拟（LES）后报。一种自动算法将观测和模拟的日子分类为不同的云况：（a）晴空日，（b）表层有浅对流云的日子，有两个云量范围，以及（c）非对流云日。观测和LES中的分类日子并不总是匹配，特别是LES中低估了非对流云日的数量，因为LES倾向于发展对流。然而，LES和观测中每个云况下的风气候学和日变化周期非常相似，增强了我们对LES再现特定大气状态下特定云的能力的信心。所有云况下的沿风动量通量剖面相似，但大云量（对流云和非对流云）的日子在更深的层中分布有更大的总动量通量，在云底附近仍有高达30%的表面通量。晴空以及特别是低云量的浅积云况在边界层中有明显更大的侧风动量通量。浅积云况下白天表层风切变最小，边界层更深，白天表层风速持续增加。与类似稳定性的晴空日相比，对流云况下的表层风切变更小，表面摩擦力比莫宁-奥布霍夫相似理论估计的更大。
