To study the influence of convective momentum transport (CMT) on wind, boundary layer and cloud evolution in a marine cold air outbreak (CAO) we use large‐eddy simulations subject to different baroclinicity (wind shear) but similar surface forcing. The simulated domain is large enough, ?km2), to develop typical mesoscale cellular convective structures. We find that a maximum friction induced by momentum transport (MT) locates in the cloud layer for an increase of geostrophic wind with height (forward shear, FW) and near the surface for a decrease of wind with height (backward shear, BW). Although the total MT always acts as a friction, the interaction of friction‐induced cross‐isobaric flow with the Coriolis force can develop supergeostrophic winds near the surface (FW) or in the cloud layer (BW). The contribution of convection to MT is evaluated by decomposing the momentum flux by column water vapor and eddy size, revealing that CMT acts to accelerate subcloud layer winds under FW shear and that mesoscale circulations contribute significantly to MT for this horizontal resolution (250 m), even if small‐scale eddies are nonnegligible and likely more important as resolution increases. Under FW shear, a deeper boundary layer and faster cloud transition are simulated, because MT acts to increase surface fluxes and wind shear enhances turbulent mixing across cloud tops. Our results show that the coupling between winds and convection is crucial for a range of problems, from CAO lifetime and cloud transitions to ocean heat loss and near‐surface wind variability. Plain Language Summary The vertical mixing of wind speed by shallow convection and clouds (called convective momentum transport, CMT) may play an important role in explaining boundary layer winds in midlatitude weather systems. In this study we use high‐resolution simulations to study the influence of CMT on the evolution of winds and clouds in a typical high‐latitude weather system: a cold air outbreak. In a cold air outbreak, strong surface fluxes and strong winds lead to extensive cloud decks that evolve as the system travels over increasingly warmer waters. To exemplify the role of wind mixing on surface winds and clouds we run simulations that are subject to different wind shear: from an increase of wind with height (Forward shear; FW) to a decrease of wind with height (backward shear; BW). We find that wind mixing always acts to slow down winds in the main flow direction, but the height where drag maximizes depends on the direction of shear. Whereas small‐scale turbulence always acts as a drag, the mesoscale circulations and clouds themselves can speed up winds under FW shear. Enhanced turbulent mixing across cloud top and faster surface winds under FW shear also lead the clouds to evolve faster from closed‐deck stratocumulus to broken cumulus fields, which is important for their radiative impact. Our results show that CMT has a significant influence on surface winds and is thus important for understanding air‐sea interaction and near‐surface wind variability, and as such, wind power generation.
展开▼
