The PECAN field campaign assembled a rich array of observations from lower-tropospheric profiling systems, mobile radars and mesonets, and aircraft over the Great Plains during June-July 2015 to better understand nocturnal mesoscale convective systems and their relationship with the stable boundary layer, the low-level jet, and atmospheric bores.

Abstract. In this study, a detailed model of an urban landscape has been re-constructed in the wind tunnel and the flow structure inside and above the urban canopy has been investigated. Vertical profiles of all three velocity components have been measured with a Laser-Doppler velocimeter, and an extensive analysis of the measured mean flow and turbulence profiles carried out. With respect to the flow structure inside the canopy, two types of velocity profiles can be distinguished. Within street canyons, the mean wind velocities are almost zero or negative below roof level, while close to intersections or open squares, significantly higher mean velocities are observed. In the latter case, the turbulent velocities inside the canopy also tend to be higher than at street-canyon locations. For both types, turbulence kinetic energy and shear stress profiles show pronounced maxima in the flow region immediately above roof level.Based on the experimental data, a shear-stress parameterization is proposed, in which the velocity scale, u s , and length scale, z s , are based on the level and magnitude of the shear stress peak value. In order to account for a flow region inside the canopy with negligible momentum transport, a shear stress displacement height, d s , is introduced. The proposed scaling and parameterization perform well for the measured profiles and shear-stress data published in the literature.The length scales derived from the shear-stress parameterization also allow determination of appropriate scales for the mean wind profile. The roughness length, z 0 , and displacement height, d 0 , can both be described as fractions of the distance, z s − d s , between the level of the shear-stress peak and the shear-stress displacement height. This result can be interpreted in such a way that the flow only feels the zone of depth z s − d s as the roughness layer. With respect to the lower part of the canopy (z < d s ) the flow behaves as a skimming flow. Correlations between the length scales z s and d s and morphometric parameters are discussed.The mean wind profiles above the urban structure follow a logarithmic wind law. A combination of morphometric estimation methods for d 0 and z 0 with wind velocity measurements at a reference height, which allow calculation of the shear-stress velocity, u * , appears to be the most reliable and easiest procedure to determine mean wind profile parameters. Inside the roughness sublayer, a local scaling approach results in good agreement between measured and predicted mean wind profiles.

[1] In previous studies, the Yonsei University (YSU) planetary boundary layer (PBL) scheme implemented in the Weather Research and Forecasting (WRF) model was reported to perform less well at night, while performing better during the day. Compared to observations, predicted nocturnal low-level jets (LLJs) were typically weaker and higher. Also, the WRF model with Chemistry (WRF/Chem) with the YSU scheme was reported to sometimes overestimate near-surface ozone (O 3 ) concentration during the nighttime. The updates incorporated in WRF version 3.4.1, include modifications of the nighttime velocity scale used in the YSU boundary layer scheme. The impacts of this update on the prediction of nighttime boundary layers and related implications for wind resource assessment and air quality simulations are examined in this study. The WRF/Chem model with the updated YSU scheme predicts smaller eddy diffusivities in the nighttime boundary layer, and consequently lower and stronger LLJs over a domain focusing on the southern Great Plains area, showing a better agreement with the observations. As a result, related overestimation problems for near-surface temperature and wind speeds appear to be resolved, and the nighttime minimum near-surface O 3 concentrations are better captured. Simulated vertical distributions of meteorological and chemical variables for weak wind regimes (e.g., in the absence of LLJ) are less impacted by the YSU updates.Citation: Hu, X.-M., P. M. Klein, and M. Xue (2013), Evaluation of the updated YSU planetary boundary layer scheme within WRF for wind resource and air quality assessments,

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A grand challenge from the wind energy industry is to provide reliable forecasts on mountain winds several hours in advance at microscale (∼100 m) resolution. This requires better microscale wind-energy physics included in forecasting tools, for which field observations are imperative. While mesoscale (∼1 km) measurements abound, microscale processes are not monitored in practice nor do plentiful measurements exist at this scale. After a decade of preparation, a group of European and U.S. collaborators conducted a field campaign during 1 May–15 June 2017 in Vale Cobrão in central Portugal to delve into microscale processes in complex terrain. This valley is nestled within a parallel double ridge near the town of Perdigão with dominant wind climatology normal to the ridges, offering a nominally simple yet natural setting for fundamental studies. The dense instrument ensemble deployed covered a ∼4 km × 4 km swath horizontally and ∼10 km vertically, with measurement resolutions of tens of meters and seconds. Meteorological data were collected continuously, capturing multiscale flow interactions from synoptic to microscales, diurnal variability, thermal circulation, turbine wake and acoustics, waves, and turbulence. Particularly noteworthy are the extensiveness of the instrument array, space–time scales covered, use of leading-edge multiple-lidar technology alongside conventional tower and remote sensors, fruitful cross-Atlantic partnership, and adaptive management of the campaign. Preliminary data analysis uncovered interesting new phenomena. All data are being archived for public use.