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Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso-or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period. Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations.

The Land surface Interactions with the Atmosphere over the Iberian Semi‐arid Environment (LIAISE) campaign was conducted in July 2021, primarily to investigate the role of irrigation in modulating the boundary‐layer evolution in the Catalan region of northeastern Spain. Contrasts in near‐surface meteorological parameters and boundary‐layer thermodynamic profiles at an irrigated and rainfed (arid) site were established during the morning transition. Evapotranspriation dominated the flux partitioning at the irrigated site (Bowen ratio of 0.07–1.1), whilst sensible heat flux dominated at the rainfed (arid) site (Bowen ratio greater than 10.0). The cumulative evapotranspiration during July 2021 was a factor of 10 greater at the irrigated site than at the rainfed (arid) site. The presence of irrigation was shown to modulate the vertical gradients of turbulence, temperature, and moisture. Irrigation is shown to have a significant effect on the development of the boundary layer, including during the morning transition. The morning transition mean buoyancy flux was 2.8 times smaller at the irrigated site (1.1 m2·$$ {}^2\cdotp $$s−2$$ {}^{-2} $$) compared with the rainfed (arid) site (3.1 m2·$$ {}^2\cdotp $$s−2$$ {}^{-2} $$), with a resultant delay in the near‐surface buoyancy‐flux crossover time (30–90 min) at the irrigated site. At the start of the morning transition (sunrise), the average screen‐level (50‐m) temperature was −1.2$$ -1.2 $$ K (−1.9$$ -1.9 $$ K) colder at the irrigated site relative to the rainfed (arid) site. The colder temperatures at sunrise at the irrigated site are predominantly the result of a colder boundary‐layer thermodynamic profile from the previous day. At the end of the morning transition (convective onset), temperature differences between the two sites extend through much of the boundary layer and increase in magnitude. The average screen‐level (50‐m) temperature difference was −3.6$$ -3.6 $$ K (−2.4$$ -2.4 $$ K) colder at the irrigated site relative to the rainfed (arid) site. There was considerable day‐to‐day variability in temperature contrasts at a regional level (−2.4$$ -2.4 $$ to −6.0$$ -6.0 $$ K). [ABSTRACT FROM AUTHOR]

A steady-state low-level jet observed in the Stable Atmospheric Boundary Layer Experiment in Spain1998 (SABLES-98) campaign is simulated using a large-eddy simulation model. The radiation is taken into account since its contribution to the budget of the tendency of temperature is as important as the contribution of the turbulence in the surface layer. The resulting regime is a two-layer flow separated by a change of the temperature gradient at the level of maximum speed. The upper part has turbulence originated by shear production extending up to 3 times the height of wind maximum and of the same order of magnitude as in the layer below. The jet nose is seen by the model as a decoupling layer, and the use of passive scalars allows for the inspection of the amount of transport across it, which is very small. The lower layer is very sensitive to the surface boundary conditions, but the principal patterns in the upper layer do not change significantly. The runs are compared to the observations through mean profiles, time series, and probability density functions.