Abstract. 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 tetheredballoon 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 seasonal dependence of Weather Research and Forecasting (WRF) model surface temperature biases and sensitivity to planetary boundary layer (PBL) schemes are jointly explored. For this purpose, the year 2001 was simulated using three different PBL schemes in a domain covering all Europe. The simulations were compared with gridded observations, upper-air data and high-frequency station data. Seasonal and daily cycles were analysed, aimed at providing a link between long-term biases and restricted case studies. The results show that the model mean bias significantly depends on the season, being warm in winter and cold in summer. The winter warm bias is related to misrepresented cold extremes, while a systematic cold bias dominates the whole temperature range in summer. Regarding PBL schemes, an overall underestimation of the entrainment is found, with the non-local Yonsei University scheme producing systematically warmer temperatures. It is shown that the opposite seasonal biases and systematic behaviour of the PBL schemes during the year lead to a different best-performing scheme in winter and summer. Moreover, the best-performing PBL scheme in an average sense is a result of the compensation of errors. The average summer results can be partially explained by a detailed case study. It is concluded that short-term studies should be used with caution to decide on the parametrizations to be used in long-term simulations.

Flow in a stably stratified environment is characterized by anisotropic and intermittent turbulence and wavelike motions of varying amplitudes and periods. Understanding turbulence intermittency and wave-turbulence interactions in a stably stratified flow remains a challenging issue in geosciences including planetary atmospheres and oceans. The stable atmospheric boundary layer (SABL) commonly occurs when the ground surface is cooled by longwave radiation emission such as at night over land surfaces, or even daytime over snow and ice surfaces, and when warm air is advected over cold surfaces. Intermittent turbulence intensification in the SABL impacts human activities and weather variability, yet it cannot be generated in state-of-the-art numerical forecast models. This failure is mainly due to a lack of understanding of the physical mechanisms for seemingly random turbulence generation in a stably stratified flow, in which wave-turbulence interaction is a potential mechanism for turbulence intermittency. A workshop on wave-turbulence interactions in the SABL addressed the current understanding and challenges of wave-turbulence interactions and the role of wavelike motions in contributing to anisotropic and intermittent turbulence from the perspectives of theory, observations, and numerical parameterization. There have been a number of reviews on waves, and a few on turbulence in stably stratified flows, but not much on wave-turbulence interactions. This review focuses on the nocturnal SABL; however, the discussions here on intermittent turbulence and wave-turbulence interactions in stably stratified flows underscore important issues in stably stratified geophysical dynamics in general.

No abstract

Nowadays, mesoscale meteorological models coupled to Urban Canopy Parameterizations (UCP) can be used to complement and interpret the information gathered from intensive meteorological campaigns on the behaviour of the Urban Boundary Layer (UBL). Moreover, the impact of the air conditioning (AC) systems on the air temperature, the relationships existing between energy consumption (EC) and meteorological conditions, and the evaluation of strategies to mitigate the Urban Heat Island (UHI) phenomenon can be evaluated using detailed UCP. In this work, a new UCP implemented in the Weather Research and Forecasting (WRF) model (version V3.2) has been tested over the city of Madrid using two different turbulent parameterisations of the Planetary Boundary Layer (PBL) under atmospheric conditions that were favourable for a large UHI. Two selected days were analysed coinciding with the Dual-use European Security IR Experiment (DESIREX) campaign that took place in the summer of 2008, and focused on Urban Heat Island (UHI) and Urban Thermography (UT) monitoring and assessment. For the two simulated days (30 June and 1 July) a high UHI intensity (5-6°C) was observed and modelled. Numerical results for the surface air temperature and wind speed were compared against measurements showing a global satisfactory performance of the model. Some differences in the air temperature predictions were observed within the two turbulent schemes. Subsequently, the impact of the AC systems and the EC were evaluated for the simulated period. The heat fluxes coming from AC systems were responsible of an increase in the air temperature up to 1.5-2°C in some dense urban areas. Effects of modifications in the roof albedo and building material properties reduced the total EC by 4.8 and 3.6%, respectively, affecting the intensity of the UHI. When AC systems were not ejecting the heat fluxes out in the atmosphere, the EC was reduced to 2.5%.