Observed influence of moist convection and cloudiness on boundary layer wind and momentum flux profiles.

Convective momentum transport (CMT) measurements are scarce, but important to constrain the impacts of CMT on wind profiles, variability of the wind and possibly the large-scale circulation.

We investigate how wind profiles and momentum fluxes change with cloudiness and convection. With stronger convection, we expect that the wind shear in the lowest 200m, wherein wind turbines are located, reduces. Cumulus days are generally strongly convective and hence well mixed. They are expected to differ from clear-sky days: the boundary layer is deeper, and cumulus may induce a different (thermal) circulation in the sub-cloud layer. Comparing cumulus and other days fairly, we must be mindful of the changes in convection strength with cloud cover, time of the day, seasons, and the wind strength that impacts the wind shear magnitude.

This study uses nine years of data from the Cabauw observatory, The Netherlands, containing 10-minute averages of wind speed, wind direction, and momentum fluxes from a 200 m tall tower along with cloud-base heights from a ceilometer. Realistic fine-scale Large Eddy Simulation (LES) hindcasts over the same time period and a 5km³ domain over Cabauw provide insight into the processes at higher altitude. In both observations and LES, days with rooted clouds, which have strong connection to the sub-cloud layer, are separated from clear-sky days and days in which clouds only impact the convection through radiation effects. Days with rooted clouds are subsequently divided into three groups of increasing cloud cover: 5-30% (shallow clouds), 30-70% (somewhat deeper clouds) and >70% (overcast).

Both observations and LES show that shear in the near-surface wind speed (NSWS) reduces with stronger insolation, which is expected: more insolation causes a more unstable atmosphere, stronger convection, thus more mixing. In a weakly unstable atmosphere, rooted clouds (5-70% cloud cover) generally have better mixed winds (less normalised shear). The NSWS accelerates more from morning to afternoon on these days, indicating that not only the mixing is stronger, but also that downward mixing of higher momentum by the clouds affects the wind in the lowest 200m. If this is true, the assumption of Monin-Obukhov Similarity Theory (MOST) that large convective eddies are not important in the surface layer, does not hold. This possibly has a great impact on surface-flux parametrizations based on MOST, which are used by many numerical models, from local and mesoscale to global models. Analysing surface-layer scaling for momentum, we test whether this assumption is indeed violated in such cases.

Momentum transport profiles in LES show that when deeper clouds with larger cloud cover are present, transport in the cloud layer is larger. In the cross-wind component of the profile, the four categories show different deceleration in the mixed layer, and different acceleration near the top of the mixed layer. Likely, the stronger inversion-jump in the cross-wind causes this momentum flux character.

With this study, we provide an overview of the effects that have been observed in different cloudiness and convective conditions and gained understanding of the important processes and implications of the cloud effects on momentum transport.