How Wind Shear Affects Trade‐wind Cumulus Convection. (28th November 2020)

• Record Type: Journal Article
• Authors: Helfer, K. C.; Nuijens, L.; de Roode, S. R.; Siebesma, A. P.
• Is Part Of: Journal of advances in modeling earth systems. Volume 12:Number 12(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-28
• Subjects: shallow convection -- wind shear -- cumulus -- trade wind -- large‐eddy simulation; Geological modeling -- Periodicals; Climatology -- Periodicals; Geochemical modeling -- Periodicals; 551.5011
• DOI: 10.1029/2020MS002183
• Languages: English
• ISSNs: 1942-2466

Abstract

Abstract: Motivated by an observed relationship between marine low cloud cover and surface wind speed, this study investigates how vertical wind shear affects trade‐wind cumulus convection, including shallow cumulus and congestus with tops below the freezing level. We ran large‐eddy simulations for an idealized case of trade‐wind convection using different vertical shears in the zonal wind. Backward shear, whereby surface easterlies become upper westerlies, is effective at limiting vertical cloud development, which leads to a moister, shallower, and cloudier trade‐wind layer. Without shear or with forward shear, shallow convection tends to deepen more, but clouds tops are still limited under forward shear. A number of mechanisms explain the observed behavior: First, shear leads to different surface wind speeds and, in turn, surface heat and moisture fluxes due to momentum transport, whereby the weakest surface wind speeds develop under backward shear. Second, a forward shear profile in the subcloud layer enhances moisture aggregation and leads to larger cloud clusters, but only on large domains that generally support cloud organization. Third, any absolute amount of shear across the cloud layer limits updraft speeds by enhancing the downward oriented pressure perturbation force. Backward shear—the most typical shear found in the winter trades—can thus be argued a key ingredient at setting the typical structure of the trade‐wind layer. Plain Language Summary: We used a high‐resolution weather model to investigate the influence of the shape of the wind profile (i.e., whether the wind blows faster, slower, or with the same velocity at greater altitudes compared to the surface) on shallow cumulus clouds typical of the North Atlantic trade‐wind region. In this region, easterly winds that decrease with height (and eventually turn westerly) are most common. Generally, the surface winds are also affected by how the wind blows further aloft, influencing what kind of clouds form. But even when we eliminate this effect in our study, we find that when the wind blows faster or slower at greater heights, clouds are not only tilted but also wider and both effects increase the overall cloud cover. Furthermore, if the wind speed changes with height, the updraft speed within clouds is diminished, which potentially decreases the height of clouds. However, if the wind speed increases with height (which only rarely occurs in the trades), clouds tend to cluster more, which "offsets" the weaker updrafts and thus still allows for deeper clouds. Key Points: Shear in the zonal wind influences cloud‐top heights via the effect of momentum transport on the surface wind and surface fluxes Backward shear (surface easterlies turn westerlies) lowers cloud tops and shallows and moistens the trade‐wind layer Any absolute amount of wind shear limits in‐cloud updraft speeds and enhances low‐level cloud fraction