Surface-layer wind shear and momentum transport from clear-sky to cloudy
weather regimes over land
Abstract
This study investigates how wind shear and momentum fluxes in the
surface- and boundary layer vary across wind and cloud regimes. We use a
nine-year-long data set from the Cabauw tower of the Ruisdael
Observatory (NL) complemented by (8.2 x 8.2 km^2) daily LES
hindcasts. An automated algorithm classifies observed and simulated days
into different cloud regimes: 1) clear-sky days, 2) days with convective
clouds (cumulus) rooted in the surface layer, with three ranges of cloud
cover, and 3) days with clouds not rooted near the surface. Categorized
days in observations and LES do not fully match, with a tendency of the
LES to develop convective clouds on clear-sky days and less frequently
produce non-rooted clouds, whose scales are far larger than the LES
domain. Even so, the climatology and diurnal cycle of winds are for each
regime very similar in LES and observations, strengthening our
confidence in LES’ skill to reproduce certain clouds for an atmospheric
state. Wind shear is smallest in clear-sky and cumulus regimes with
limited cloud cover (CLCC), which also have the weakest 200 m wind speed
and largest surface buoyancy flux. They have notably larger cross-wind
fluxes, although along-wind momentum flux profiles are similar across
all regimes. Cloudy days have larger momentum fluxes distributed over
deeper layers, sustaining up to 20% of the surface flux value at cloud
base. Compared to clear-sky, the CLCC regimes have stronger updrafts and
deeper mixed-layers. At similar atmospheric stability, surface friction
is larger and underestimated by Monin-Obukhov Similarity Theory.