Wind speed response of marine non-precipitating stratocumulus clouds over a diurnal cycle in cloud-system resolving simulations

Abstract

Abstract. Observed and projected trends in large-scale wind speed over the oceans prompt the question: how do marine stratocumulus clouds and their radiative properties respond to changes in large-scale wind speed? Wind speed drives the surface fluxes of sensible heat, moisture, and momentum and thereby acts on cloud liquid water path (LWP) and cloud radiative properties. We present an investigation of the dynamical response of non-precipitating, overcast marine stratocumulus clouds to different wind speeds over the course of a diurnal cycle, all else equal. In cloud-system resolving simulations, we find that higher wind speed leads to faster boundary layer growth and stronger entrainment. The dynamical driver is enhanced buoyant production of turbulence kinetic energy (TKE) from latent heat release in cloud updrafts. LWP is enhanced during the night and in the morning at higher wind speed, and more strongly suppressed later in the day. Wind speed hence accentuates the diurnal LWP cycle by expanding the morning-afternoon contrast. The higher LWP at higher wind speed does not, however, enhance cloud top cooling because in clouds with LWP 50 g m −2 , longwave emissions are insensitive to LWP. This leads to the general conclusion that in sufficiently thick stratocumulus clouds, additional boundary layer growth and entrainment due to a boundary layer moistening arises by stronger production of TKE from latent heat release in cloud updrafts, rather than from enhanced longwave cooling. We find that large-scale wind modulates boundary layer decoupling. At nighttime and at low wind speed during daytime, it enhances decoupling in part by faster boundary layer growth and stronger entrainment and in part because shear from large-scale wind in the sub-cloud layer hinders vertical moisture transport between the surface and cloud base. With increasing wind speed, however, in decoupled daytime conditions, shear-driven circulation due to large-scale wind takes over from buoyancy-driven circulation in transporting moisture from the surface to cloud base and thereby reduces decoupling and helps maintain LWP. The total (shortwave + longwave) cloud radiative effect (CRE) responds to changes in LWP and cloud fraction, and higher wind speed translates to a stronger diurnally averaged total CRE. However, the sensitivity of the diurnally averaged total CRE to wind speed decreases with increasing wind speed.

Mesoscale organization, entrainment, and the properties of a closed‐cell stratocumulus cloud

“…Possible explanations for the absence of a domain size dependence in previous studies are: (a) The dependence of entrainment and cloud water path on the grid spacing aspect ratio [ Kazil et al ., ; Pedersen et al ., ]. Duynkerke et al .…”

“…The surface fluxes of momentum, heat, and moisture were calculated from the total (residual + geostrophic) wind speed based on similarity theory and contributed to the residual circulation by production of TKE due to buoyancy and shear. This is akin to moving the domain with the geostrophic wind, although its geographic location remains fixed [ Kazil et al ., ]. This setup differs from the CGILS specification in which the mean wind speed is nudged toward prescribed profiles and the surface heat and moisture flux are calculated with a dedicated formula [ Blossey et al ., , Appendix A3].…”

“…This implies vertical motion as a driver of entrainment. Kazil et al [2016] identified cloud layer updrafts and TKE w associated with them as a key dynamical driver of entrainment in a growing boundary layer. In the quasi-steady state boundary layer investigated here, the correlation of w e with TKE w of updrafts is only imperceptibly higher than the correlation of w e with TKE w of downdrafts.…”

Evaluation of WRF SCM Simulations of Stratocumulus‐Topped Marine and Coastal Boundary Layers and Improvements to Turbulence and Entrainment Parameterizations

“…Here these coefficients are treated as constants for computational efficiency. Instead of formulating the cloud velocity scale as function of radiative divergence (as in both Lock et al, ; Wilson, ), we have chosen the buoyancy flux within the cloud layer, as longwave emission becomes insensitive to LWP changes for thick clouds (LWP > 50 g m −2 ; Kazil et al, ). Thus, by formulating as function of radiative divergence the parameterization fails to account for additional turbulence generated by latent heat releases in updrafts within in the cloud layer.…”

“…In order to improve the parameterization of the STBL we propose a revision to the YSU scheme (YSU‐BUOY) that accounts for the skewness of top‐driven convection and parameterizes entrainment based on the in‐cloud buoyancy flux rather than radiative flux divergence as in the Wilson () YSU update. This revision results in a more robust entrainment model since longwave emission becomes insensitive to changes in LWP for thick clouds (Kazil et al, ). Similar modifications were made to the ACM2 scheme (ACM2‐BUOY) in order to explore the impact of explicitly modeling these effects.…”