Abstract
AbstractTo 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.
Highlights
The influence of convective momentum transport (CMT) by shallow moist convection on large-scale atmospheric circulations is not well understood
We find that a maximum friction induced by momentum transport (MT) locates in the cloud layer for an increase of geostrophic wind with height and near the surface for a decrease of wind with height
We study this problem by running a Langrangian large eddy simulation (LES) of a well-observed cold air outbreak (CAO) case developed by the Working Group on Numerical Experimentation (WGNE) Greyzone project (Field et al, 2014), which we subject to different vertical shear in one of the wind components
Summary
The influence of convective momentum transport (CMT) by shallow moist convection on large-scale atmospheric circulations is not well understood. One of the reasons is that studies of shallow convection have traditionally focused on first-order effects of shallow convection, such as vertical mixing of moisture and cloud formation, as well as their influence on the energy budget. Another reason is that the turbulence-resolving models, which we use to study shallow convection, are run on domain sizes much smaller than that of atmospheric weather systems, so that the large-scale wind is traditionally prescribed. Recent sensitivity tests with the European Center of Medium-range Weather Forecasting (ECMWF) IFS model show that long-standing biases in near-surface wind speed and direction over global oceans (Sandu et al, 2013) may be linked to momentum transport by shallow convection. We wish to better understand the importance of CMT in the momentum budget of cloud-dominated atmospheres that are subject to a different baroclinicity, for example, vertical shear in the large-scale horizontal wind
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