The 15 January 2017 a strong northerly synoptic flow lead to the generation of mountain waves and heavy snowfall over the eastern Pyrenees, particularly over the Cerdanya valley near the border between France, Spain and Andorra. Measurements from several instruments deployed during the Cerdanya-2017 field campaign and satellite imagery revealed the presence of mountain waves and the formation of an associated rotor underneath the first mountain wave crest. The evolution and location of the mountain waves were studied using high temporal resolution data from a UHF wind-profiler and a vertically pointing K-band Doppler radar, separated a few kilometres in horizontal distance. A mountain wave with a wavelength about 18 km was detected in the morning and shortened slightly in the afternoon when a transient rotor, elevated approximately 140 m above the ground, was formed, disconnected from the surface flow. A strong turbulence zone was identified at the upper edge of the mountain wave, above the rotor, a feature observed in previous studies. The mountain wave and rotor induced circulation was favoured by the valley shape and the second mountain ridge location, in addition to the weak and variable winds, established during the sunset close to the valley surface.

Turbulent and intermittent characteristics of the nocturnal boundary layer play an important role in determining the transport, diffusion, and storage of momentum, heat, and atmospheric pollutants. Here we use micrometeorological observations and numerical simulations to analyze two coherent turbulent structures-a density current and a low-level jet (LLJ)-that enhance mixing and intermittent turbulence in the nocturnal boundary layer. We analyze in detail their impact on the distribution of simulated pollutant concentrations. The Weather Research and Forecast (WRF) model is coupled with the Community Multiscale Air Quality photochemical model to simulate the dispersion of pollutants for the density-current and LLJ cases. The terms of the pollutant-concentration budget are then analyzed to quantify the contribution of different atmospheric processes on the net NO2 and O-3 concentrations. The results show that the WRF model reproduces successfully the essential characteristics observed in the density-current and LLJ cases. The density-current simulation shows turbulent-mixing enhancement in the frontal zone of the density current, leading to significant vertical pollutant diffusion. Results also indicate that the LLJ is able to transport pollutants from the residual layer to the surface layer due to vertical diffusion. In both cases, the main mechanism responsible for pollutant transport is the horizontal advection.

A night-time turbulence regime classification, the so-called “HOckey-Stick Transition ” (HOST) theory, proposed by Sun et al. (2012) from the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) is explored using data from the Boundary-Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign which took place during summer 2011 in the central French Pyrenean foothills. Results show that the HOST turbulence relationships for the BLLAST field campaign data are strongly dependent on both the meteorological and orographic features. The HOST pattern only appears for nights when a stably stratified boundary layer can be developed, corresponding to fair-weather and clear-sky nights, when the flow is generated by the nearby orography, from the south and the south-east. Those flows strongly influenced by the orography may generate intermittent or enhanced turbulence. When considering the whole nocturnal dataset for these flow directions, several enhanced turbulence points are found to be associated with sudden wind speed and directional shear transitions. In contrast, flows from other directions do not reproduce the HOST relationships and the turbulence relationship is almost linear, independent of vertical temperature gradients, corresponding to flows driven by synoptic scales. In addition we identify examples of gravity waves and top-down turbulent events that lead to transitions between the turbulence regimes.

Different types of land use (LU) have different physical properties which can change local energy balance and hence vertical fluxes of moisture, heat and momentum. This in turn leads to changes in near-surface temperature and moisture fields. Simulating atmospheric flow over complex terrain requires accurate local-scale energy balance and therefore model grid spacing must be sufficient to represent both topography and land-use. In this study we use both the Corine Land Cover (CLC) and United States Geological Survey (USGS) land use databases for use with the Weather Research and Forecasting (WRF) model and evaluate the importance of both land-use classification and horizontal resolution in contributing to successful modelling of surface temperatures and humidities observed from a network of 39 sensors over a 9day period in summer 2013. We examine case studies of the effects of thermal inertia and soil moisture availability at individual locations. The scale at which the LU classification is observed influences the success of the model in reproducing observed patterns of temperature and moisture. Statistical validation of model output demonstrates model sensitivity to both the choice of LU database used and the horizontal resolution. In general, results show that on average, by a) using CLC instead of USGS and/or b) increasing horizontal resolution, model performance is improved. We also show that the sensitivity to these changes in the model performance shows a daily cycle.

Mountain wave breaking, and the mechanisms by which turbulence may be triggered by directional wind shear, are investigated using numerical simulations of idealized, nearly hydrostatic, atmospheric flows over an axisymmetric isolated mountain. These simulations, which use the WRF-ARW model, differ in degree of flow nonlinearity and shear intensity. The aim is to diagnose the conditions for mountain wave breaking in terms of the orography elevation and wind shear, quantified by the dimensionless mountain height and the Richardson number of the background flow, respectively. The simulation results have been used to produce a regime diagram describing the wave breaking behaviour in Richardson number–dimensionless mountain height parameter space. By selecting flow overturning occurrence as a discriminating factor, it was possible to split the regime diagram into sub-regions with and without wave breaking. When mountain waves break, the associated convective instability leads to turbulence generation (which is one of the known forms of clear air turbulence (CAT)). Thus, regions within the simulation domain where wave breaking and the development of CAT are expected have been identified. The extent of these regions increases with terrain elevation and background wind shear intensity. Analysis of the model output, supported by theoretical arguments, suggests the existence of a link between wave breaking and the relative orientations of the incoming wind vector and the horizontal velocity perturbation vector. More specifically, in a wave breaking event, due to the effect of critical levels, the background wind vector and the wavenumber vector of the dominant mountain waves are perpendicular. It is shown that, at least for the wind profile employed in the present study, this corresponds to a situation where the background wind vector and the velocity perturbation vector are also approximately perpendicular.

Ground-level and vertical measurements (performed using tethered and non-tethered balloons), coupled with modelling, of ozone (O3), other gaseous pollutants (NO, NO2, CO, SO2) and aerosols were carried out in the plains (Vic Plain) and valleys of the northern region of the Barcelona metropolitan area (BMA) in July 2015, an area typically recording the highest O3 episodes in Spain. Our results suggest that these very high O3 episodes were originated by three main contributions: (i) the surface fumigation from high O3 reservoir layers located at 1500–3000 m a.g.l. (according to modelling and non-tethered balloon measurements), and originated during the previous day(s) injections of polluted air masses at high altitude; (ii) local/regional photochemical production and transport (at lower heights) from the BMA and the surrounding coastal settlements, into the inland valleys; and (iii) external (to the study area) contributions of both O3 and precursors. These processes gave rise to maximal O3 levels in the inland plains and valleys northwards from the BMA when compared to the higher mountain sites. Thus, a maximum O3 concentration was observed within the lower tropospheric layer, characterised by an upward increase of O3 and black carbon (BC) up to around 100–200 m a.g.l. (reaching up to 300 μg m−3 of O3 as a 10 s average), followed by a decrease of both pollutants at higher altitudes, where BC and O3 concentrations alternate in layers with parallel variations, probably as a consequence of the atmospheric transport from the BMA and the return flows (to the sea) of strata injected at certain heights the previous day(s). At the highest altitudes reached in this study with the tethered balloons (900–1000 m a.g.l.) during the campaign, BC and O3 were often anti-correlated or unrelated, possibly due to a prevailing regional or even hemispheric contribution Published by Copernicus Publications on behalf of the European Geosciences Union. 2818 X. Querol et al.: Phenomenology of high-ozone episodes in NE Spain of O3 at those altitudes. In the central hours of the days a homogeneous O3 distribution was evidenced for the lowest 1 km of the atmosphere, although probably important variations could be expected at higher levels, where the high O3 return strata are injected according to the modelling results and non-tethered balloon data. Relatively low concentrations of ultrafine particles (UFPs) were found during the study, and nucleation episodes were only detected in the boundary layer. Two types of O3 episodes were identified: type A with major exceedances of the O3 information threshold (180 μg m−3 on an hourly basis) caused by a clear daily concatenation of local/regional production with accumulation (at upper levels), fumigation and direct transport from the BMA (closed circulation); and type B with regional O3 production without major recirculation (or fumigation) of the polluted BMA/regional air masses (open circulation), and relatively lower O3 levels, but still exceeding the 8 h averaged health target. To implement potential O3 control and abatement strategies two major key tasks are proposed: (i) meteorological forecasting, from June to August, to predict recirculation episodes so that NOx and VOC abatement measures can be applied before these episodes start; (ii) sensitivity analysis with high-resolution modelling to evaluate the effectiveness of these potential abatement measures of precursors for O3 reduction.

Following Sun et al. (J Atmos Sci 69(1):338–351, 2012), vertical variations of turbulent mixing in stably stratified and neutral environments as functions of wind speed are investigated using the large-eddy simulation capability in the Weather Research and Forecasting model. The simulations with a surface cooling rate for the stable boundary layer (SBL) and a range of geostrophic winds for both stable and neutral boundary layers are compared with observations from the Cooperative Atmosphere–Surface Exchange Study 1999 (CASES-99). To avoid the uncertainty of the subgrid scheme, the investigation focuses on the vertical domain when the ratio between the subgrid and the resolved turbulence is small. The results qualitatively capture the observed dependence of turbulence intensity on wind speed under neutral conditions; however, its vertical variation is affected by the damping layer used in absorbing undesirable numerical waves at the top of the domain as a result of relatively large neutral turbulent eddies. The simulated SBL fails to capture the observed temperature variance with wind speed and the observed transition from the SBL to the near-neutral atmosphere with increasing wind speed, although the vertical temperature profile of the simulated SBL resembles the observed profile. The study suggests that molecular thermal conduction responsible for the thermal coupling between the surface and atmosphere cannot be parameterized through the Monin–Obukhov bulk relation for turbulent heat transfer by applying the surface radiation temperature, as is common practice when modelling air–surface interactions.

A trapped lee-wave mountain event in the southern part of the Pyrenees area is analyzed using the Weather Research and Forecasting (WRF) Model. Model experiments are designed to address the WRF predictability of such an event and to explore the influence of the model parameters in resolving the mountain waves. The results show that the model is able to capture a trapped lee-wave event using the 1-km horizontal grid model outputs. Different initial conditions, the vertical grid resolution, and the resolved topography lead to changes in the wave field distribution and the wave amplitude meaning that an ensemble of different model settings may be able to quantify the uncertainty of the numerical solutions. However, the model experiments do not significantly change the wavelength of the generated mountain waves, which is shorter in the three-dimensional real simulations than the one derived from satellite imagery. Comparison with observational data from the surface stations and a wind profiler upstream of the mountain range shows that the model underestimates the horizontal wind speed and this can be the reason for the underestimation of the wavelength. In addition, the valley circulations and the formation of a rotor near the surface are explored. The formation of a low-level rotor in the model is intermittent and brief, and it interacts with other flows coming from multiple directions. The first strong wave updraft is located over the valley aligned with the highest mountain peaks and strong vorticity is captured from the surface up to the first wave crest.

The aim of this research was to better understand the dynamics of air pollutants and to forecast the air quality over regional areas in order to develop emission abatement strategies for air pollution and adverse health effects. To accomplish this objective, we developed and applied a high resolution Eulerian system named ARAMIS (A Regional Air Quality Modelling Integrated System) over the north-east of Spain (Catalonia), where several pollutants exceed threshold values for the protection of human health. The results indicate that the model reproduced reasonably well observed concentrations, as statistical values fell within Environmental Protection Agency (EPA) recommendations and European (EU) regulations. Nevertheless, some hourly O3 exceedances in summer and hourly peaks of NO2 in winter were underestimated. Concerning PM10 concentrations less accurate model levels were obtained with a moderate trend towards underestimation during the day.

The aim of this research was to better understand the dynamics of air pollutants and to forecast the air quality over regional areas in order to develop emission abatement strategies for air pollution and adverse health effects. To accomplish this objective, we developed and applied a high resolution Eulerian system named ARAMIS (A Regional Air Quality Modelling Integrated System) over the north-east of Spain (Catalonia), where several pollutants exceed threshold values for the protection of human health. The results indicate that the model reproduced reasonably well observed concentrations, as statistical values fell within Environmental Protection Agency (EPA) recommendations and European (EU) regulations. Nevertheless, some hourly O-3 exceedances in summer and hourly peaks of NO2 in winter were underestimated. Concerning PM10 concentrations less accurate model levels were obtained with a moderate trend towards underestimation during the day.

A sequence of eight atmospheric density current fronts occurred in consecutive days are identified and analyzed using micrometeorological time series and numerical simulations. Observations were collected in the context of the INTERCLE project, which took place from September 2002 to November 2003 at the CIBA (Research Centre for the Lower Atmosphere) site located over the northern Spanish plateau. Numerical simulations used the Weather Research and Forecast (WRF) model with fine horizontal resolution (1 km). Both observations and simulations agree that the arrival of the density currents are characterized by a sharp change in temperature, wind velocity, wind direction and specific humidity and a source of intermittent turbulence. However, comparison between model and observations shows that the model predicts the intrusion of the density currents earlier than is observed. In addition, wavelet techniques applied to the data help distinguish the different scales present in the events, and therefore can reveal traces of gravity waves induced by the arrival of the density currents.

Using data collected at the Spanish low troposphere research centre CIBA (Centro de Investigacion de la Baja Atmosfera) and at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands, we analyzed the most significant features of different coherent structures occurring in the stable atmospheric boundary layer. In particular, we used both the Reynolds and wavelet methods to analyze a solitary wave, a gravity wave, a density current and a low-level jet. For each of these structures, we found that wavelet analysis had the capacity to distinguish the different scales involved in these events due to the different timing and heights of the thermal instabilities and downdrafts associated with the disturbances. In addition, the wavelet method highlights the different roles of turbulence and coherent structures in the transfer of heat, moisture and CO2 in the nocturnal boundary layer. (C) 2013 Elsevier B.V. All rights reserved.

In this study, starting from an observational case of internal gravity waves (IGWs) generated at the top of a drainage flow during the SABLES2006 field campaign, we aim to reproduce the IGWs and their origin through mesoscale meteorological modelling. We used the Weather Research and Forecast (WRF) model with fine horizontal resolution (1 km), testing the model capabilities to simulate the IGWs through a WRF fixed physics package option but two different planetary boundary layer schemes, the Mellor-Yamada-Janji (MYJ) and the Yonsei University (YSU). The comparison between model simulations and measurements from a 100 m meteorological tower reveals that the MYJ scheme simulation gives much better results, as it better represents the main features of the density current measured by the tower instruments, although the event is predicted to occur sooner than it is observed to occur. The study has also shown the capacity of this scheme to detect the oscillations in temperature and specific humidity generated by the arrival of the density current. In contrast, the YSU scheme captures the arrival of the current on time but it fails to correctly track its properties and therefore it does not reproduce the gravity waves with the current arrival. In addition, wave parameters calculated from model outputs (MYJ) using the wavelet method reveal waves with longer periods and longer wavelengths (T = 2022 min and = 810 km) than those calculated from measurements (T = 9.2 min and = 3.5 km) using the same technique. Copyright (c) 2012 Royal Meteorological Society

We present a detailed evaluation of the seasonal performance of the Community Multiscale Air Quality (CMAQ) modelling system and the PSU/NCAR meteorological model coupled to a new Numerical Emission Model for Air Quality (MNEQA). The combined system simulates air quality at a fine resolution (3 km as horizontal resolution and 1 h as temporal resolution) in north-eastern Spain, where problems of ozone pollution are frequent. An extensive database compiled over two periods, from May to September 2009 and 2010, is used to evaluate meteorological simulations and chemical outputs. Our results indicate that the model accurately reproduces hourly and 1-h and 8-h maximum ozone surface concentrations measured at the air quality stations, as statistical values fall within the EPA and EU recommendations. However, to further improve forecast accuracy, three simple bias-adjustment techniques—mean subtraction (MS), ratio adjustment (RA), and hybrid forecast (HF)—based on 10 days of available comparisons are applied. The results show that the MS technique performed better than RA or HF, although all the bias-adjustment techniques significantly reduce the systematic errors in ozone forecasts.

The Community Multiscale Air Quality (CMAQ) model was used along with the Weather Research and Forecasting (WRF) model to study air quality modelling sensitivity to various Planetary Boundary Layer (PBL) schemes and Land Surface Models (LSMs). The performance is assessed and quantified by comparing results with surface observations and the outputs provided by the fifth generation Mesoscale Model (MM5) when coupled to the CMAQ model. The evaluated meteorological variables include temperature, wind speed and direction and mixing ratio, while the CMAQ evaluation focuses on ozone concentrations. Differences were observed in circulatory patterns and meteorological variables, which influence spatial ozone distributions and concentrations.

For air quality models, it is important that the meteorological fields derived from meteorological models reflect the best characterisation of the atmosphere. Under strongly thermally-stratified conditions, the determining factors are turbulent kinetic energy and surface layer parameterisations. In this study, the 5th generation Mesoscale Model (MM5) and the Weather Research and Forecasting (WRF) meteorological models, with different Planetary Boundary Layer (PBL) schemes, were run under very stable conditions and checked against available information over the Duero basin in the Iberian Peninsula. WRF seemed to produce results closer to the observations, as statistical parameters indicate. Models were also coupled to the Community Multiscale Air Quality (CMAQ) model in order to analyse the influence of the parameterisations on air pollutant distribution.

The structure and evolution of the sea breeze in the north-west part of the Mediterranean (Catalonia, north-east Spain) is studied both experimentally and, predominantly, using numerical models to increase understanding of sea-breeze structure and three-dimensional (3D) pollution distributions in coastal environments. Sea-breeze components are modelled and analyzed using the fifth-generation Pennsylvania State University–National Centre for Atmospheric Research Mesoscale Model (MM5). The results show that the growth and structure of the sea-breeze circulation is modulated by the synoptic flow and especially by the complex topography of the area. 3D pollution transport in a sea breeze is modelled by coupling the MM5 to the Community Multiscale Air Quality (CMAQ) model, with results indicating that topography and synoptic flow are the main factors modulating horizontal and vertical pollutant transport in sea-breeze episodes. In this way, horizontal dispersion is limited by the complex topography of the area, whilst the sea-breeze flow is intensified by anabatic upslope winds that contribute to vertical pollutant transport. The numerical model results also indicate that the sea-breeze circulation with a weak return flow at upper levels grows due to a synoptic onshore wind component. However, such a sea-breeze circulation is capable of transporting pollutants towards the coast.

After evaluating some meteorological simulations, we detect an error of over 5°C in surface temperature. In order to assess the effect of this anomaly in temperature, we perform new simulations of an emission model and a photochemical model using the correct surface temperature in the meteorological data. Our goal is not to solve the problem in surface temperature estimates, but to reveal how it affects the air quality model. Our results show an increase in some maximum daily ozone levels, but they do not reveal any important change in the global pattern of ozone concentrations.

Using data from a case study performed during summer 2009 in the north-east of Spain, the WRF/Chem and WRF-CMAQ models were statistically evaluated and compared, focusing on ozone concentrations. It is shown that the WRF-CMAQ model provides a better forecast of O3 than WRF/Chem, although both models underestimate ozone concentrations. The major under-prediction of the WRF/Chem could be due to the direct effects of particulate matter on net short-wave radiation and semi-direct effects on air temperature and NO2 photolysis rate. The three-dimensional air quality models developed over the last 15-20 years consisted of a meteorological driver, an emissions module and a chemical transport model with the chemistry off-line solved. However, in the last few years, new generations of air quality models have been developed with the chemistry on-line solved with the meteorology because the simulation and prediction of air quality involves both meteorological factor and chemical processes. In the real atmosphere chemical and physical processes are coupled, as several factors such as wind speed and direction, turbulence, radiation, clouds, and precipitation interact with chemical processes such as deposition, and transformations. The chemistry can affect the meteorology, for example, through its effect on the radiation budget, as well as the interaction of aerosols with cloud condensation nuclei. Likewise, clouds and precipitation strongly influence chemical transformation and removal processes, and localized changes in the wind or turbulence fields continuously affect the chemical transport (Grell et al., 2005). In this contribution we present results from one case study using two different air quality modelling systems. The first system is an off-line matured modelling system based on the Weather Research and Forecasting (WRF) (Skamarock et al., 2005), using the mass coordinate version of the model, called Advanced Research Weather (ARW), coupled to the Community Model for Air Quality (CMAQ) (Byun and Ching, 1999). WRF is a mesoscale non-hydrostatic meteorological model, and the dispersion and chemical transport model CMAQ is one of the most up-to-date air quality dispersion chemical models. The second system is an on-line (one code, one system) tool to simulate air concentrations based on the WRF meteorological driver. In WRF/Chem the chemistry transport and transformations are embedded into WRF as part of the code (Grell et al., 2005), so the interactions between many meteorological variables and the chemistry properties can be explored and analyzed. The same emission model, MNEQA (Numerical Emission Model for Air Quality) (Ortega et al., 2009), was used in both models to test the performance of both modelling systems in forecasting high ozone concentrations that occurred during a case study corresponding to summer 2009 in north-eastern Spain (Catalonia). Comparison of WRF/CMAQ with observations as well as with WRF/Chem for the former pollutant provides the basis of our model evaluation. 2. MODELLING APPROACH Meteorological numerical simulations were performed using the WRF-ARW version 3.1.1. The model was configured with three nested domains that have grids of 27, 9 and 3 km (Fig. 1), with a two-way interface with the smallest grid. The innermost domain, D1, covers 69x45 grid cells; D2, 70x70 cells; and D3, the inner domain corresponding to Catalonia (NE Spain) covers 94x94 grid cells. The vertical grid is common to all the domains, with 31 vertical levels and a resolution of 15 m close to the surface, decreasing gradually with height, thus enabling low-level flow details to be captured; the top of the domain was at 100 hPa. Initial and boundary conditions were taken from the European Centre for Medium Range Weather Forecasts (ECMWF) with a 1.5oX1.5oresolution every 6 hours, and so boundary conditions could be updated at this time interval.