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

Impacts of afternoon and evening sea-breeze fronts on local turbulence, and on CO2 and radon-222 transport

We investigated sharp disruptions of local turbulence and scalar transport due to the arrival of sea-breeze fronts (SBFs). To this end, we employed a comprehensive 10-year observational database from the Cabauw Experimental Site for Atmospheric Research (CESAR, the Netherlands). Sea-breeze (SB) days were selected using a five-filter algorithm, which accounts for large-scale conditions and a clear mesoscale-frontal signal associated with the land-sea contrast. Among those days (102 in all, 8.3%), based on the value of the sensible-heat flux at the onset of SB, we identified three atmospheric boundary-layer (ABL) regimes: convective, transition and stable. In the convective regime, the thermally driven convective boundary layer is only slightly altered by a small enhancement of the shear when the SBF arrives. Regarding the transition regime, we found that the ABL afternoon transition is accelerated. This was quantified by estimating the contributions of shear and buoyancy to the turbulent kinetic energy. Other relevant disruptions are the sharp reduction in ABL depth (similar to 250 m/hr) and the sudden increase in average wind speed (> 2 m/s). In the stable regime, the arrival of the SB leads to disturbances in the wind profile at the surface layer. We observed a deviation of more than 1 m/s in the observed surface-layer wind profile compared with the profile calculated using Monin-Obukhov Similarity Theory (MOST). Our findings furthermore reveal the determinant role of the SB direction in the transport of water vapour, CO2 and Rn-222. The return of continental air masses driven by the SB circulation generates sharp CO2 increases (up to 14 ppm in half an hour) in a few SB events. We suggest that the variability in Rn-222 evolution may also be influenced by other non-local processes such as the large-scale footprint from more remote sources

Flujos térmicos mesoescalares y su interacción con la turbulencia en la capa límite atmosférica

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra y Astrofísica, leída el 11-01-2019Los movimientos atmosféricos se clasifican de acuerdo a su escala espacial y temporal de mayor a menor en macroescalares, mesoescalares y microescalares. Entre los procesos mesoescalares encontramos los flujos térmicos, producidos por contrastes térmicos horizontales en escalas que abarcan entre 1 y 100–200 km aproximadamente, y que ocurren en forma de circulaciones cerradas. Estos flujos juegan un papel críticoen la transferencia de energía entre la macroescala y microescala. En esta tesis investigamos las brisas marinas (SBs) y de montaña (MBs). Además de ser ubicuas, juegan un papel fundamental en el complejo decaimiento de la turbulencia que tiene lugar durante la transición vespertina (AET) de la capa límite atmosférica (ABL). Íntimamenterelacionado con la interacción entre los flujos mesoescalares y la turbulencia en la ABL, la concentración de escalares relevantes como el CO2 puede verse significativamente alterada por el establecimiento de circulaciones térmicas. Modelos mesoescalares tales como el Weather Research & Forecasting (WRF) muestran deficiencias en reproducir el decaimiento de turbulencia durante la AET y la influencia de los flujos mesoescalares en el balance de CO2...Atmospheric motions are classified according to their spatial and time scales from largest to smallest into macroscales, mesoscales and microscales. Among mesoscale motions we find the thermally-driven flows, which are driven by horizontal thermal contrasts in scales between around 1 and 100–200 km, and occur in the form of closed circulations. They have a critical role in transferring energy between the macroscales and microscales. In this thesis we investigate sea breezes (SBs) and mountain breezes(MBs). Apart from being ubiquitous, they play a fundamental role in the complex turbulent decay during the afternoon and evening transition (AET) of the atmospheric boundary layer (ABL). Intimately linked with the interaction between mesoscale flowsand ABL turbulence, relevant scalars such as CO2 can also be significantly affected by the onset of thermally-driven circulations. Mesoscale models such as Weather Research& Forecasting (WRF) have shown deficiencies in reproducing the turbulent decay during the AET and the influence of mesoscale flows on the CO2 budget...Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEunpu

10 años de simulaciones con WRF en Cabauw (2001-2010)

These data, obtained from numerical simulations from the WRF model, complement a robust observational meteorological database in Cabauw (The Netherlands), spanning a 10-year period (2001-2010). The uploaded data contain the variables used in the analysis of the different atmospheric scales that influence the sea-breeze formation and their frontal characteristics. This study is in press to be published in Journal of the Geophysical Research: Atmospheres, under the title ''Analyzing the Synoptic, Meso and Local Scales Involved in Sea-Breeze Formation and Frontal Characteristics''.Estos datos, obtenidos a partir a simulaciones numéricas con el modelo WRF, son complementarios a la base meteorológica observacional robusta de Cabauw (Países Bajos), abarcando un periodo de 10 años (2001-2010). Los datos aquí subidos contienen las variables empleadas en el análisis de las diferentes escalas atmósfericas que influyen en la formación de las brisas marinas y sus características frontales. Este estudio está para ser publicado en Journal Geophysical Research: Atmospheres, bajo el título 'Analyzing the Synoptic, Meso and Local Scales Involved in Sea-Breeze Formation and Frontal Characteristics'.Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasMinisterio de Economía y Competitividad (MINECO)/FEDERWageningen UniversityNational Center for Atmospheric Researchinpres

A characterisation of sea-breeze events in the eastern Cantabrian coast (Spain) from observational data and WRF simulations

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 behaviour of the sea breeze along the north coast of Spain is investigated using observations of two topographically contrasting sites together with simulations from the Weather Research and Forecasting (WRF) model. An objective and systematic selection method is used to detect sea-breeze days from a database of two summer months. The direction and intensity of the sea breeze are significantly affected by the topography of the area; indeed, the estimated sea-breeze intensity shows an opposite relationship with the cross-shore temperature gradient for both sites. WRF simulations reproduce the onset of the sea breeze, but some characteristics are not adequately simulated: they generally overestimate the wind speed, smooth the temperature evolution and they do not represent the correct interaction with the terrain-induced flows. Additionally, four sensitivity experiments are performed with the WRF model varying the Planetary Boundary Layer (PBL) scheme, as well as the grid analysis nudging for an anomalous case study which is incorrectly filtered. As the two simulations considering nudging reproduce an unreal (not observed) sea breeze, this day turns out to be of great interest: it allows to evaluate the influence of the passage of the sea-breeze front (SBF) in other variables mainly related to turbulence. Furthermore, the best model scores are obtained for the PBL scheme that does not use a TKE closure. (C) 2016 Elsevier B.V. All rights reserved.Gobierno de EspañaUCMBanco SantanderDepto. de Física de la Tierra y AstrofísicaCAI Ciencias de la Tierra y ArqueometríaTRUEpu

From weak to intense downslope winds: origin, interaction with boundary-layer turbulence and impact on CO2 variability

[eng] The interconnection of local downslope flows of different intensities with the turbulent characteristics and thermal structure of the atmospheric boundary layer (ABL) is investigated through observations. Measurements are carried out in a relatively flat area 2 km away from the steep slopes of the Sierra de Guadarrama (central Iberian Peninsula). A total of 40 thermally driven downslope events are selected from an observational database spanning the summer 2017 period by using an objective and systematic algorithm that accounts for a weak synoptic forcing and local downslope wind direction. We subsequently classify the downslope events into weak, moderate and intense categories, according to their maximum 6 m wind speed. This classification enables us to contrast their main differences regarding the driving mechanisms, associated ABL turbulence and thermal structure, and the major dynamical characteristics. We find that the strongest downslope flows (U > 3.5 m s−1) develop when soil moisture is low ( < 0.07 m3 m−3) and the synoptic wind not so weak (3.5 m s−1 < V850 < 6 m s−1) and roughly parallel to the direction of the downslope flow. The latter adds an important dynamical input, which induces an early flow advection from the nearby steep slope, when the local thermal profile is not stable yet. Consequently, turbulence driven by the bulk shear increases up to friction velocity (u*) ≃ 1 m s−1, preventing the development of the surface-based thermal inversion and giving rise to the so-called weakly stable boundary layer. On the contrary, when the dynamical input is absent, buoyancy acceleration drives the formation of a katabatic flow, which is weak (U < 1.5 m s−1) and generally manifested in the form of a shallow jet below 3 m. The relative flatness of the area favours the formation of very stable boundary layers marked by very weak turbulence (u* < 0.1 m s−1). In between, moderate downslope flows show intermediate characteristics, depending on the strength of the dynamical input and the occasional interaction with down-basin winds. On the other hand, by inspecting individual weak and intense events, we further explore the impact of downslope flows on CO2 variability. By relating the dynamics of the distinct turbulent regimes to the CO2 budget, we are able to estimate the contribution of the different terms. For the intense event, indeed, we infer a horizontal transport of 67 ppm in 3 h driven by the strong downslope advection

Investigating mountain breezes characteristics and their effects on CO2 concentration at three different sites

International audienceThe characteristics of daytime and nighttime mountain breezes have been analysed and compared at three different sites: a) in the foothills of the Guadarrama Mountain range (El Escorial, Spain); b) on a plateau close to The Pyrenees (Lannemezan, France); and c) in the Salt Lake Valley (SLV, Utah, US). A systematic algorithm, based on synoptic and local meteorological conditions, has been used to detect automatically numerous events at each site. On the one hand, the wind characteristics of these mountain breezes depend on the scale of the breeze detected at each site. Their arrivals are observed approximately when the sensible heat flux changes sign, but they are delayed in the sites that are farther away from the mountains. On the other hand, the effects of these breezes on CO2 mixing ratios have been investigated. The typical increases and decreases of CO2 mixing ratios observed around the afternoon and morning transition do not always occur at the same time of the breeze arrival to the tower site, which unlinks these drastic changes in CO2 from the direct horizontal advection produced by the breezes. However, the CO2 mixing ratio is sensitive to changes in wind direction in highly heterogeneous sites, like the SLV site. Besides, the changes in surface turbulence produced by the breezes have an important effect on CO2. Indeed, a clear relationship is found for CO2 mixing ratio and the turbulent kinetic energy in the lowest atmospheric layers during the nighttime events