Análisis observacional y numérico de las transiciones vespertinas en la capa límite atmosférica

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la tierra, Astronomía y Astrofísica I (Geofísica y Meterología), leída el 16-12-2015This PhD thesis focuses on the study of the physical processes occurring in the atmospheric boundary layer (ABL) during the transition from a convective, well-mixed diurnal situation, to a nocturnal stable one: the afternoon and evening transition. This is a challenging topic mainly due to the confluence of weak and frequently opposite forcings. Furthermore, the micrometeorological conditions in this time frame may play a crucial role in diverse and relevant events, such as fog onset and growth, the development of frost or health hazards linked to air quality. For these reasons, the interest in this area of research is currently increasing, and field experiments are designed to better understand the afternoon and evening transition processes.En esta tesis doctoral se estudian los procesos físicos que tienen lugar en la capa límite atmosférica (CLA) durante la transición desde una situación diurna, convectiva y bien mezclada, a una situación de estabilidad nocturna: la transición vespertina. Este tema supone un reto por tratarse de un periodo temporal en el que confluyen forzamientos débiles y habitualmente de signos opuestos. Las condiciones micrometerológicas durante dicha transición pueden influir de manera decisiva, entre otros procesos, sobre la formación y el desarrollo de nieblas, heladas o situaciones de alerta por contaminación atmosférica, hechos todos de gran relevancia. Por ello, esta área de investigación está ganando interés, diseñándose campañas experimentales de medidas para mejorar el conocimiento actual sobre los procesos de la transición vespertina.Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEunpu

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

Wind kinetic energy climatology and effective resolution for the ERA5 reanalysis

Producción CientíficaERA5 represents the state of the art for atmospheric reanalyses and is widely used in meteorological and climatological research. In this work, this dataset is evaluated using the wind kinetic energy spectrum. Seasonal climatologies are generated for 30° latitudinal bands in the Northern Hemisphere (periodic domain) and over the North Atlantic area (limited-area domain). The spectra are also assessed to determine the effective resolution of the reanalysis. The results present notable differences between the latitudinal domains, indicating that ERA5 is properly capturing the synoptic conditions. The seasonal variability is adequate too, being winter the most energetic, and summer the least energetic season. The limited area domain results introduce a larger energy density and range. Despite the good results for the synoptic scales, the reanalysis’ spectra are not able to properly reproduce the dissipation rates at mesoscale. This is a source of uncertainties which needs to be taken into account when using the dataset. Finally, a cyclone tropical transition is presented as a case study. The spectrum generated shows a clear difference in energy density at every wavelength, as expected for a highly-energetic status of the atmosphere.Ministerio de Asuntos Económicos y Transformación Digital y Universidad Complutense de Madrid (projects: (PID2019-105306RB-I00 (IBER - CANES), CGL2016-78702 (SAFEFLIGHT), PCIN-2016-080 and FEI-EU-17-16)Publicación en abierto financiada por el Consorcio de Bibliotecas Universitarias de Castilla y León (BUCLE), con cargo al Programa Operativo 2014ES16RFOP009 FEDER 2014-2020 DE CASTILLA Y LEÓN, Actuación:20007-CL - Apoyo Consorcio BUCL

Characterization of spread in a mesoscale ensemble prediction system: multiphysics versus initial conditions

In this research, uncertainty associated with initial and boundary conditions is evaluated for short-term wind speed prediction in complex terrain. The study area is the Alaiz mountain range, a windy region in the northern Iberian Peninsula. A multiphysics and multiple initial and boundary condition ensemble prediction system (EPS) was generated using the Weather Research and Forecasting model. Uncertainty of the EPS is analyzed using an index based on the spread between ensemble members, considering its behavior under different wind speed and direction events, and also during distinct atmospheric stability conditions. The results corroborate that physical parameterization uncertainty is greater for short-term forecasts (63.5%). However, it is also necessary to consider the uncertainty associated with initial conditions, not only for its quantitative importance (36.5%) but also for its behavior during thermal inversion conditions in the narrow valleys surrounded by mountains

Comparison of the WRF and HARMONIE models ability for mountain wave warnings

Mountain lee waves usually involve aircraft icing and turbulence events. These weather phenomena, in turn, are a threat to aviation safety. For this reason, mountain lee waves are an interesting subject of study for the scientific community. This paper analyses several mountain lee waves events in the south-east of the Guadarrama mountain range, near the Adolfo Suarez Madrid-Barajas airport (Spain), using the Weather Research and Forecasting (WRF) and the HARMONIE-AROME high-resolution numerical models. For this work, simulated brightness temperature from the optimum WRF parametrization schemes and from the HARMONIE are validated using satellite observations to evaluate the performance of the models in reproducing the lenticular clouds associated to mountain lee waves. The brightness temperature probability density shows interesting differences between both models. Following, a mountain wave characterization is performed simulating some atmospheric variables (wind direction, wind speed, atmospheric stability, liquid water content and temperature) in several grid points located in the leeward, windward and over the summit of the mountains. The characterization results are compared for both numerical models and a decision tree is developed for each to forecast and warn the mountain lee waves, lenticular clouds and icing events with a 24 to 48 h lead time. These warnings are validated using several skill scores, revealing similar results for both models

The BLLAST field experiment: Boundary-Layer late afternoon and sunset turbulence

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.publishedVersio

The BLLAST field experiment: Boundary-Layer late afternoon and sunset turbulence

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 Making of the New European Wind Atlas - Part 2: production and evaluation

This is the second of two papers that document the creation of the New European Wind Atlas (NEWA). In Part 1, we described the sensitivity experiments and accompanying evaluation done to arrive at the final mesoscale model setup used to produce the mesoscale wind atlas. In this paper, Part 2, we document how we made the final wind atlas product, covering both the production of the mesoscale climatology generated with the Weather Research and Forecasting (WRF) model and the microscale climatology generated with the Wind Atlas Analysis and Applications Program (WAsP). The paper includes a detailed description of the technical and practical aspects that went into running the mesoscale simulations and the downscaling using WAsP. We show the main results from the final wind atlas and present a comprehensive evaluation of each component of the NEWA model chain using observations from a large set of tall masts located all over Europe. The added value of the WRF and WAsP downscaling of wind climatologies is evaluated relative to the performance of the driving ERA5 reanalysis and shows that the WRF downscaling reduces the mean wind speed bias and spread relative to that of ERA5 from −1.50 ± 1.30 to 0.02 ± 0.78 m s^(−1) . The WAsP downscaling has an added positive impact relative to that of the WRF model in simple terrain. In complex terrain, where the assumptions of the linearized flow model break down, both the mean bias and spread in wind speed are worse than those from the raw mesoscale results