The Sensitivity of Numerical Simulations of Cloud‐Topped Boundary Layers to Cross‐Grid Flow

Wyant, M. C.; Bretherton, Christopher S.; Blossey, Peter N.

doi:10.1002/2017ms001241

Cited by 16 publications

(27 citation statements)

References 25 publications

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“…Using a second-order scheme in the horizontal further increased the differences among the shear cases (in particular under free surface fluxes), which we attribute to the fact that the second-order scheme accumulates a lot of energy on the smallest length scales close to the grid size. To reduce horizontal advective errors and allow for a larger time step, the grid was horizontally translated using a velocity that is equal to the imposed wind at 3 km height (Galilean transform; see, e.g., Wyant et al, 2018).…”

Section: Windsmentioning

confidence: 99%

The role of shallow convection in the momentum budget of the trades from large‐eddy‐simulation hindcasts

Helfer

Nuijens

Dixit

2021

Quart J Royal Meteoro Soc

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Motivated by the abundance of low clouds in the subtropics, where the easterly trade winds prevail, we study the role of shallow convection in the momentum budget of the trades. To this end, we use ICON‐LEM hindcasts run over the North Atlantic for 12 days corresponding to the NARVAL1 (winter) and NARVAL2 (summer) flight campaigns. The simulation protocol consists of several nested domains, and we focus on the inner domains (≈100 × 100 km2) which have been run at resolutions of 150–600 m and are forced by analysis data, thus exhibiting realistic conditions. Combined, the resolved advection and the subgrid stresses decelerate the easterly flow over a frictional layer that balances the prevailing geostrophic wind forcing. Irrespective of the horizontal resolution, this layer is about 2 km deep in the strong winter trades and 1 km in summer, as winds and geostrophic forcing weaken and cloudiness reduces. The unresolved processes are strongest near the surface and are well captured by traditional K‐diffusion theory, but convective‐scale motions which are not considered in K‐diffusion theory contribute the most in the upper part of the mixed layer and are strongest just below cloud base. The results point out that convection in the mixed layer – the roots of trade‐wind cumuli and subcloud‐layer circulations – play an important role in slowing down easterly flow below cloud base (but little in the cloud layer itself), which helps make the zonal wind jet more distinct. Most of the friction within the clouds and near the wind jet stems from smaller‐scale turbulence stresses.

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Section: Windsmentioning

confidence: 99%

The role of shallow convection in the momentum budget of the trades from large‐eddy‐simulation hindcasts

Helfer

Nuijens

Dixit

2021

Quart J Royal Meteoro Soc

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“…1-2). To determine whether sensitivities to cross-grid flow like those highlighted by Wyant et al (2018) have compared various runs with a fixed and a translating grid (at the approximate average speed of the cloud-layer flow).…”

Section: Les Experimentsmentioning

confidence: 99%

Environmental sensitivities of shallow-cumulus dilution – Part 2: Vertical wind profile

Drueke¹,

Kirshbaum²,

Kollias³

2021

Preprint

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Abstract. This second part of a numerical study on shallow-cumulus dilution focuses on the sensitivity of cloud dilution to changes in the vertical wind profile. Insights are obtained through large-eddy simulations of maritime and continental cloud fields. In these simulations, the speed of the initially uniform geostrophic wind, and the strength of geostrophic vertical wind shear in the cloud and subcloud layer are varied. Increases in the cloud-layer vertical wind shear (up to 9 m/s/km) lead to 40–50 % larger cloud-core dilution rates compared to their respective unsheared counterparts. When the background wind speed, on the other hand, is enhanced by up to 10 m/s and subcloud-layer vertical wind shear develops or is initially prescribed, the dilution rate decreases by up to 25 %. The sensitivities of the dilution rate are linked to the updraft strength and the properties of the entrained air. Increases in the wind speed or vertical wind shear result in lower vertical velocities across all sets of experiments with stronger reductions in the cloud-layer wind shear simulation (27–47 %). Weaker updrafts are exposed to mixing with the drier surrounding air for a longer time period, allowing more entrainment to occur (i.e., the "core exposure effect"). However, reduced vertical velocities, in concert with increased cloud-layer turbulence, also assist in widening the humid shell surrounding the cloud cores, leading to entrainment of more humid air (i.e., the "core-shell dilution effect"). In the experiments with cloud-layer vertical wind shear, the core exposure effect dominates and the cloud-core dilution increases with increasing shear. Conversely, when the wind speed is increased and subcloud-layer vertical wind shear develops or is imposed, the core-shell dilution effect dominates to induce a purifying effect. The sensitivities are generally stronger in the maritime simulations, where weaker sensible heat fluxes lead to narrower, more tilted, and, therefore, more suppressed cumuli when cloud-layer shear is imposed. Moreover, in the experiments with subcloud wind shear, the weaker baseline turbulence in the maritime case allows for a larger turbulence enhancement, resulting in a widening of the transition zones between the cores and their environment, leading to the entrainment of more humid air.

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“…Using a second‐order scheme in the horizontal further increased the differences among the shear cases (in particular under free surface fluxes), which we attribute to the fact that the second‐order scheme accumulates a lot of energy on the smallest length scales close to the grid size. To reduce horizontal advective errors and allow for a larger time step, the grid was horizontally translated using a velocity that is equal to the imposed wind at 3 km height (Galilean transform; see, e.g., Wyant et al, 2018).…”

Section: Experimental Designmentioning

confidence: 99%

How Wind Shear Affects Trade‐wind Cumulus Convection

Helfer

Nuijens

Roode

et al. 2020

J Adv Model Earth Syst

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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.

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The Sensitivity of Numerical Simulations of Cloud‐Topped Boundary Layers to Cross‐Grid Flow

Cited by 16 publications

References 25 publications

The role of shallow convection in the momentum budget of the trades from large‐eddy‐simulation hindcasts

The role of shallow convection in the momentum budget of the trades from large‐eddy‐simulation hindcasts

Environmental sensitivities of shallow-cumulus dilution – Part 2: Vertical wind profile

How Wind Shear Affects Trade‐wind Cumulus Convection

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