Experimental evaluation of the effect of presence of obstacles in the vicinity of sites hosting near surface meteorological measurement. The case of the road.

The accuracy of near surface measurements of meteorological variables is influenced by the environmental characteristics of the site where the instruments are placed. WMO guide #8 establishes a qualitative/quantitative classification, by itemizing different site conditions.In the framework of the MeteoMet2 project, to deliver a validated analysis aiming at possibly improving the WMO siting classification, a one-year lasting experiment has been devised for evaluating the effect of obstacles on near surface air temperature measurements. The experiment consists in a 100 m long array of identical thermometers equipped with aspirated solar shields, placed on a flat grass field at increasing distances from an obstacle, such that the farthest station fulfils current WMO requirements for a Class 1 site.Thermometers are Pt100 calibrated against reference standards and other quantities of influence are also measured: humidity, solar radiation, wind speed and direction. Three identical experimental setups have been designed, built and characterized to separately identify the effect of three different kind of obstacles: asphalt roads (Italy), trees (Czech Republic) and buildings (Spain).The work here presented focuses only on the road siting experiment.A statistical analysis based on Generalized Additive Model (GAM) was performed to understand the effect of each quantity of influence on the temperature measurements. The model was instrumental in understanding the best combination of environmental factor that would boost the effect. The largest temperature biases (extremes) have been then modelled through Extreme Values Analysis (EVA), which allowed for an evaluation of the asymptotic behaviour of these biases, and an estimation of theroad siting effect.Results show that the roads influence temperature readings more intensely during nights and when wind is absent. The magnitude of the effect has been evaluated at 1.7±0.4 °C for a return period of 100 year

On‐ and off‐line evaluation of the single‐layer urban canopy model in London summertime conditions

Urban canopy models are essential tools in forecasting weather and air quality in cities. However, they require many surface parameters, which are uncertain and can reduce model performance if inappropriately prescribed. Here, we evaluate the model sensitivity of the Single-Layer Urban Canopy Model (SLUCM) in the Weather Research and Forecasting model (WRF) to surface parameters in two different configurations, one coupled to the overlying atmosphere (on-line) in a 1D configuration and one without coupling (off-line). A 2-day summertime period in London is used as a case study, with clear skies and low wind speeds. Our sensitivity tests indicate that SLUCM reacts differently, when coupled to the atmosphere. For certain surface parameters, atmospheric feedback effects can outweigh the variations caused by surface parameter settings. Hence to fully understand model sensitivity atmospheric feedbacks should be considered

Quality of wind characteristics in recent wind atlases over the North Sea

Offshore wind energy production is rapidly growing as an essential element in the sustainable energy share. Wind energy siting studies require accurate wind data, and in particular the knowledge of extreme wind events (low-level jets, wind ramps, extreme shear and high wind speeds) is crucial for resource and load assessment. This study evaluates the skill of three relatively new wind atlases, i.e. ERA-5, DOWA and NEWA on the representation of extreme wind events using observations taken at the Met Mast IJmuiden over the North Sea. Overall, DOWA appears to best represent the wind speed profile with virtually no bias. ERA-5 underestimates the mean wind speed profile though the wind shear is well represented, while NEWA correctly represents the near surface wind but underestimates the wind shear. The frequency of low-level jets are also best represented by DOWA. Wind speed ramps and direction ramps are best represented by ERA-5, while DOWA appears to outperform the others concerning wind shear.</p

Boundary-Layer Meteorology Surface temperature and surface layer turbulence in a convective boundary layer --Manuscript Draft-- Manuscript Number: Full Title: Surface temperature and surface layer turbulence in a convective boundary layer Surface temperat

Abstract: Previous laboratory and atmospheric experiments have shown that turbulence influences the surface temperature in a convective boundary layer. The main objective of this study is to examine land-atmosphere coupled heat transport mechanism for different stability conditions. High frequency infrared imagery and sonic anemometer measurements were obtained during the Boundary Layer Late Afternoon and Sunset Turbulence experimental campaign. Temporal turbulence data in the surface layer are then analyzed jointly with spatial surface temperature imagery. The surface temperature structures are strongly linked to atmospheric turbulence as manifested by several findings. The surface temperature coherent structures move at an advection speed similar to the upper surface layer or mixed layer wind speed with a decreasing trend with stability. Also, with increasing instability the streamwise surface temperature structure size decreases and the structures become more circular. The sequencing of surface and air temperature patterns is further examined through conditional averaging. Surface heating causes the initiation of warm ejection events followed by cold sweep events that result in surface cooling. The ejection events occur about 25% of the time, but account for 60 to 70% of the total sensible heat flux and cause fluctuations of up to 30% in the ground heat flux. Cross-correlation analysis between air and ground temperature confirms the validity of scalar footprint models. Previous laboratory and atmospheric experiments have shown that turbulence influences the 11 surface temperature in a convective boundary layer. The main objective of this study is to examine 12 land-atmosphere coupled heat transport mechanism for different stability conditions. High frequency 13 infrared imagery and sonic anemometer measurements were obtained during the Boundary Layer Late 14 Afternoon and Sunset Turbulence experimental campaign. Temporal turbulence data in the surface 15 layer are then analyzed jointly with spatial surface temperature imagery. 16 The surface temperature structures are strongly linked to atmospheric turbulence as 17 manifested by several findings. The surface temperature coherent structures move at an advection 18 speed similar to the upper surface layer or mixed layer wind speed with a decreasing trend with 19 stability. Also, with increasing instability the streamwise surface temperature structure size decreases 20 and the structures become more circular. The sequencing of surface and air temperature patterns is 21 further examined through conditional averaging. Surface heating causes the initiation of warm 22 ejection events followed by cold sweep events that result in surface cooling. The ejection events occur 23 about 25% of the time, but account for 60 to 70% of the total sensible heat flux and cause fluctuations 24 of up to 30% in the ground heat flux. Cross-correlation analysis between air and ground temperature 25 confirms the validity of scalar footprint models. 2

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

Simulaciones de transiciones de la capa límite atmosférica con el Modelo WRF: casos de estudio de la Campaña BLLAST

Ponencia presentada en: XXXIII Jornadas Científicas de la AME y el XIV Encuentro Hispano Luso de Meteorología celebrado en Oviedo, del 7 al 9 de abril de 2014.[ES]Usando simulaciones numéricas con el modelo WRF se analizan las transiciones vespertinas en la capa límite atmosférica, tomando para la validación casos de estudio de la campaña internacional BLLAST.[EN]WRF numerical simulations are employed to analyze atmospheric boundary layer evening transitions. BLLAST field campaign case studies are considered for validation

Exploring the formation and dissipation of radiation fog from observational data and numerical model results (WRF and HARMONIE)

Póster presentado en: 16th EMS Annual Meeting & 11th European Conference on Applied Climatology (ECAC) celebrado del 12 al 16 de septiembre de 2016 en Trieste, Italia.This research has been funded by the Spanish Government (MINECO project CGL2012-37416-C04-02 and grant BES-2013064585). The GR3/14 program (supported by UCM and Banco Santander) has also partially financed this work through the Research Group Micrometeorology and Climate Variability (No.910437). The contribution by G-J. Steeneveld has partly been sponsored by the NOW contract 863.10.010 (Liftingthefog)

Cool city mornings by urban heat

The urban heat island effect is a phenomenon observed worldwide, i.e. evening and nocturnal temperatures in cities are usually several degrees higher than in the surrounding countryside. In contrast, cities are sometimes found to be cooler than their rural surroundings in the morning and early afternoon. Here, a general physical explanation for this so-called daytime urban cool island (UCI) effect is presented and validated for the cloud-free days in the BUBBLE campaign in Basel, Switzerland. Simulations with a widely evaluated conceptual atmospheric boundary-layer model coupled to a land-surface model, reveal that the UCI can form due to differences between the early morning mixed-layer depth over the city (deeper) and over the countryside (shallower). The magnitude of the UCI is estimated for various types of urban morphology, categorized by their respective local climate zones