Estudio de nubes convectivas ligadas a un sistema frontal con imágenes IR

En este trabajo se ha realizado un estudio de las propiedades de las nubes a diferentes niveles dentro de un frente, por medio de imágenes de Meteosat, utilizando, la banda de infrarrojo térmico (IR). Para ello, se definen umbrales apropiados de Temperatura de Brillo (TB) que permiten delimitar el área correspondiente a nubes de distinto origen. Por una parte, el área extensa con temperatura de brillo inferior a 248 K, que se identifica como frente, y por otra, el área de temperatura inferior a 221 K asociado a nubes de desarrollo vertical. Los centros de las nubes convectivas obtenidos de la posición geográfica de los centros de gravedad, presentan un notable alineamiento que ha sido caracterizado mediante regresión lineal. Se compara el alineamiento de los cúmulos con la orientación del propio frente, según la dirección principal del mismo. Esto permite desarrollar una metodología en el análisis comparativo de estructuras nubosas de diferentes orígenes, así como la mejora en el conocimiento de sus propiedades y evolución en el desplazamiento de los sistemas sinópticos, relativo a los paralelos geográficos

Influence of atmospheric stratification on the integral scale and fractal dimension of turbulent flows

In this work the relation between integral scale and fractal dimension and the type of stratification in fully developed turbulence is analyzed. The integral scale corresponds to that in which energy from larger scales is incoming into a turbulent regime. One of the aims of this study is the understanding of the relation between the integral scale and the bulk Richardson number, which is one of the most widely used indicators of stability close to the ground in atmospheric studies. This parameter will allow us to verify the influence of the degree of stratification over the integral scale of the turbulent flows in the atmospheric boundary layer (ABL). The influence of the diurnal and night cycles on the relationship between the fractal dimension and integral scale is also analyzed. The fractal dimension of wind components is a turbulent flow characteristic, as has been shown in previous works, where its relation to stability was highlighted. Fractal dimension and integral scale of the horizontal (u′) and vertical (w′) velocity fluctuations have been calculated using the mean wind direction as a framework. The scales are obtained using sonic anemometer data from three elevations 5.8, 13 and 32 m above the ground measured during the SABLES 98 field campaign (Cuxart et al., 2000). In order to estimate the integral scales, a method that combines the normalized autocorrelation function and the best Gaussian fit (R² ≥ 0.70) has been developed. Finally, by comparing, at the same height, the scales of u′ and w′ velocity components, it is found that the turbulent flows are almost always anisotropic

Estimating fog-top height through near-surface micrometeorological measurements

Fog-top height (fog thickness) is very useful information for aircraft maneuvers, data assimilation/validation of Numerical Weather Prediction models or nowcasting of fog dissipation. This variable is usually difficult to determine, since the fog-layer top cannot be observed from the surface. In some cases, satellite data, ground remote sensing instruments or atmospheric soundings are used to provide approximations of fog-top height. These instruments are expensive and their data not always available. In this work, two different methods for the estimation of fog-top height from field measurements are evaluated from the statistical analysis of several radiation-fog events at two research facilities. Firstly, surface friction velocity and buoyancy flux are here presented as potential indicators of fog thickness, since a linear correlation between fog thickness and surface turbulence is found at both sites. An operational application of this method can provide a continuous estimation of fog-top height with the deployment of a unique sonic anemometer at surface. Secondly, the fog-top height estimation based on the turbulent homogenisation within well-mixed fog (an adiabatic temperature profile) is evaluated. The latter method provides a high percentage of correctly-estimated fog-top heights for well-mixed radiation fog, considering the temperature difference between different levels of the fog. However, it is not valid for shallow fog (~ less than 50 m depth), since in this case, the weaker turbulence within the fog is not able to erode the surface-based temperature inversion and to homogenise the fog layer

Radiation and cloud-base lowering fog events: observational analysis and evaluation of WRF and HARMONIE

Most of the effects caused by fog are negative for humans. Yet, numerical weather prediction (NWP) models still have problems to simulate fog properly, especially in operational forecasts. In the case of radiation fog, this is partially caused by the large sensitivity to many aspects that contribute to its formation, evolution and dissipation, such as the synoptic and local conditions, the near-surface turbulence, the aerosol and droplet microphysics, or the surface characteristics, among others. This work focuses on an interesting 8-day period with several alternating radiation and cloud-base lowering (CBL) fog events observed at the Research Centre for the Lower Atmosphere (CIBA) in the Spanish Northern Plateau. The site was appropriately instrumented to characterize fog from the surface up to the height of 100 m. On the one hand, radiation fog events are associated with strong surface cooling leading to high stability close to the surface and low values of turbulence, giving rise to shallow fog. The evolution of this type of fog is markedly sensitive to the dynamical conditions close to the surface (i.e., wind speed and turbulence). On the other hand, CBL fog presents deeper thickness associated with higher values of turbulence and less stability. Subsequently, we evaluated the fog-forecasting skill of two mesoscale models (WRF and HARMONIE) configured as similar as possible. Both models present more difficulties simulating radiation fog events than CBL ones. However, the duration and vertical extension of the CBL fog events is normally overestimated. This extended-fog avoids the surface radiative cooling needed to simulate radiation fog events formed the following nights. Therefore, these periods with alternating CBL and radiation fog are especially challenging for NWP models

Vertical Structure of the Stable Boundary Layer Detected by RASS-SODAR and In-Situ Measurements in SABLES 2006 Field Campaign

Data from the SABLES 2006 field campaign are used in order to analyse some of the main processes present along the nocturnal periods: surface-based inversions, low level jets, katabatic winds, wave-like motions, pressure perturbations, etc. These processes have an important influence on the vertical structure (both thermal and dynamical) of the atmospheric boundary layer, and can be better described with the synergetic combination of RASS-SODAR data and in-situ measurements (such as sonic anemometer data and high-resolution pressure series from microbarometers). It is shown how the different air masses and their evolution are easily identified when pressure and RASS-SODAR wind and temperature data are presented together. Likewise, periodic pressure fluctuations observed in the surface array of microbarometers reveal the existence of gravity wave motions whose propagation is better understood after locating the wave ducting layers with the help of RASS-SODAR average wind ant temperature profiles

Influence of stability on the flux-profile relationships for wind speed, φ_m, and temperature, φ_h, for the stable atmospheric boundary layer

Data from SABLES98 experimental campaign have been used in order to study the influence of stability (from weak to strong stratification) on the flux-profile relationships for momentum, φ_m, and heat, φ_h. Measurements from 14 thermocouples and 3 sonic anemometers at three levels (5.8, 13.5 and 32 m) for the period from 10 to 28 September 1998 were analysed using the framework of the local-scaling approach (Nieuwstadt, 1984a; 1984b), which can be interpreted as an extension of the Monin-Obukhov similarity theory (Obukhov, 1946). The results show increasing values of φ_m and φ_h with increasing stability parameter ζ=z/3, up to a value of ζ≈1–2, above which the values remain constant. As a consequence of this levelling off in φm and φh for strong stability, the turbulent mixing is underestimated when linear similarity functions (Businger et al., 1971) are used to calculate surface fluxes of momentum and heat. On the other hand when φ_m and φ_h are related to the gradient Richardson number, R_i , a different behaviour is found, which could indicate that the transfer of momentum is greater than that of heat for high R_i . The range of validity of these linear functions is discussed in terms of the physical aspects of turbulent intermittent mixing

Atmospheric boundary-layer evening transitions: a comparison between two different experimental sites

El texto completo de este trabajo no se encuentra disponible por no haber sido facilitado aún por su autor, por restricciones de copyright, o por no existir una versión digitalThe planetary boundary-layer (PBL) afternoon and evening transition is investigated with measurements from two-month datasets, gathered at two experimental sites significantly different regarding heterogeneity, the degree of terrain wetness, and proximity to mountains. The period of 4 h prior to and after astronomical sunset is extensively analyzed. We show the mean evolution, average, maximum and minimum values of PBL variables, including wind speed, turbulent kinetic energy and potential temperature vertical gradient. Characteristic events, such as the wind minimum around sunset and a common pattern in the evolution of other variables, are identified. Results suggest that, for the establishment of the nocturnal stable boundary layer, moisture plays a more decisive role than turbulence. We also look into the occurrence of katabatic flows, finding more intense but less frequent events at the driest site. In contrast, at that location the crossover of the sensible heat flux takes place later. Time-scale evolution is investigated through case studies, and air humidity and soil moisture are found to have crucial importance explaining most of the site-to-site differences. Therefore, a humidity sensitivity experiment with the Weather Research and Forecasting model is performed, evaluating the role of moisture during the transition by increasing the soil humidity at the driest site and reducing it at the other location. The simulations reveal that humidity effects are more important until 1 h before sunset, both near the surface and at upper levels in the PBL. Furthermore, the moisture change is more relevant at the less humid and more homogeneous site, with intense and long-lasting effects after sunset.Ministerio de Economía y Competitividad (España)UCMBanco SantanderMicrometeorology and Climate VariabilityFPI-UCM fellowshipINSU-CNRS (Institut National des Sciences de l'Univers, Centre national de la Recherche Scientifique, LEFE-IDAO program), Meteo-FranceObservatoire Midi-Pyrenees (University of Toulouse)EUFAR (EUropean Facility for Airborne Research)COST ES0802 (European Cooperation in the field of Scientific and Technical)Centre National de la Recherche Scientifique (CNRS)University of Toulouse and European POCTEFA FluxPyrUnión EuropeaDepto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEpu

Evening transitions of the atmospheric boundary layer: characterization, case studies and WRF simulations

Micrometeorological observations from two months (July-August 2009) at the CIBA site (Northern Spanish plateau) have been used to evaluate the evolution of atmospheric stability and turbulence parameters along the evening transition to a Nocturnal Boundary Layer. Turbulent Kinetic Energy thresholds have been established to distinguish between diverse case studies. Three different types of transitions are found, whose distinctive characteristics are shown. Simulations with the Weather Research and Forecasting-Advanced Research WRF (WRF-ARW) mesoscale model of selected transitions, using three different PBL parameterizations, have been carried out for comparison with observed data. Depending on the atmospheric conditions, different PBL schemes appear to be advantageous over others in forecasting the transitions.Ministerio de Ciencia e Innovación (España)UCMResearch Group “Micrometeorology and Climate Variability”Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEpu

Forecasting radiation fog at climatologically contrasting sites: evaluation of statistical methods and WRF

A six-year climatology of radiation fog has been compiled at two sites: the Research Centre for the Lower Atmosphere (CIBA, Spain) and the Cabauw Experimental Site for Atmospheric Research (CESAR, The Netherlands). These sites are contrasted in terms of geographical situation, climate zone, altitude, humidity and soil water availability. Therefore, several climatological differences in fog abundance, onset, dissipation and duration have been quantified between the two sites. The more humid site (CESAR) is characterised by relatively short radiation fog events distributed throughout the year. However, radiation fog at the drier site (CIBA) is more persistent and appears during late autumn/winter months. In general, its formation requires more time after sunset (approximate to 2 h more), since further cooling is required to reach saturation. The forecast of these fog events has been evaluated through two different approaches. First, we extend the statistical method presented by Menut et al. (2014) (M14). This method uses statistics to define threshold values on key variables for fog formation (pre-fog) and verifies its predictability using observations and numerical model output. We present some of the most appropriate threshold values for the forecasting of pre-fog periods at both sites, which differ from those presented in M14 and depend on the optimisation of the hit rate or the false-alarm rate. Additionally, we also extend M14 by suggesting other variables as potential predictors for fog formation (friction velocity and visibility tendency). Finally, we focus on fog simulation by the Weather Research and Forecasting (WRF) model in terms of liquid water content. The WRF model was able to simulate radiation fog when configured with sophisticated physical options and high resolution. However it failed in simulating the onset, dissipation and the vertical extent of fog (which was overestimated). The model results were extremely sensitive to the spin-up time.Ministerio de Economía y Competitividad (España)Netherlands Organization for Scientific Research (NWO)Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEpu

Assessment of microscale spatio-temporal variation of air pollution at an urban hotspot in Madrid (Spain) through an extensive field campaign

El texto completo de este trabajo no se encuentra disponible por no haber sido facilitado aún por su autor, por restricciones de copyright, o por no existir una versión digitalPoor urban air quality is one of the main environmental concerns worldwide due to its implications for population exposure and health -related issues. However, the development of effective abatement strategies in cities requires a consistent and holistic assessment of air pollution processes, taking into account all the relevant scales within a city. This contribution presents the methodology and main results of an intensive experimental campaign carried out in a complex pollution hotspot in Madrid (Spain) under the TECNAIRE-CM research project, which aimed at understanding the microscale spatio-temporal variation of ambient concentration levels in areas where high pollution values are recorded. A variety of instruments were deployed during a three-week field campaign to provide detailed information on meteorological and micrometeorological parameters and spatio-temporal variations of the most relevant pollutants (NO2 and PM) along with relevant information needed to simulate pedestrian fluxes. The results show the strong dependence of ambient concentrations on local emissions and meteorology that turns out in strong spatial and temporal variations, with gradients up to 2 mu g m(-3) m(-1) for NO2 and 55 mu m(-3) min(-1) for PM10. Pedestrian exposure to these pollutants also presents strong variations temporally and spatially but it concentrates on pedestrian crossings and bus stops. The analysis of the results show that the high concentration levels found in urban hotspots depend on extremely complex dynamic processes that cannot be captured by routinely measurements made by air quality monitoring stations used for regulatory compliance assessment. The large influence from local traffic in the concentration fields highlights the need for a detailed description of specific variables that determine emissions and dispersion at microscale level. This also indicates that city-scale interventions may be complemented with local control measures and exposure management, to improve air quality and reduce air pollution health effects more effectively. (C) 2016 Elsevier Ltd. All rights reserved.Comunidad Autónoma de Madrid. Dirección General de Universidades e InvestigaciónDepto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEpu