179 research outputs found
Meteorology applied to urban air pollution problems: concepts from COST 715
International audienceThis selective review of the COST 715 considers simple descriptive concepts in urban meteorology with particular attention to air pollution assessment. It is shown that these are helpful for understanding the complex structure of the urban boundary layer, but that simple concepts only apply under a limited number of occasions. However such concepts are necessary for insight into how both simple and complex air pollution models perform. Wider considerations are needed when considering routine air quality assessments involving an air quality model's formulation and pedigree. It is argued that there is a reluctance from model developers to move away from familiar concepts of the atmospheric boundary layer even if they are not appropriate to urban areas. An example is given from COST 715 as to how routine urban meteorological measurements of wind speed may be used and adapted for air quality assessments. Reference to the full COST 715 study is made which provides further details
Meteorology applied to urban air pollution problems: concepts from COST 715
International audienceThe outcome of COST 715 is reviewed from the viewpoint of a potential user who is required to consider urban meteorology within an air pollution assessment. It is shown that descriptive concepts are helpful for understanding the complex structure of the urban boundary layer, but that they only apply under a limited number of conditions. However such concepts are necessary to gain insight into both simple and complex air pollution models. It is argued that wider considerations are needed when considering routine air quality assessments involving an air quality model's formulation and pedigree. Moreover there appears to be a reluctance from model developers to move away from familiar concepts of the atmospheric boundary layer even if they are not appropriate to urban areas. An example is given from COST 715 as to how routine urban meteorological measurements of wind speed may be used and adapted for air quality assessments. Reference to the full COST 715 study is made which provides further details
Implementation of the 64-meter-diameter Antennas at the Deep Space Stations in Australia and Spain
The management and construction aspects of the Overseas 64-m Antenna Project in which two 64-m antennas were constructed at the Tidbinbilla Deep Space Communications Complex in Australia, and at the Madrid Deep Space Communications Complex in Spain are described. With the completion of these antennas the Deep Space Network is equipped with three 64-m antennas spaced around the world to maintain continuous coverage of spacecraft operations. These antennas provide approximately a 7-db gain over the capabilities of the existing 26-m antenna nets. The report outlines the project organization and management, resource utilization, fabrication, quality assurance, and construction methods by which the project was successfully completed. Major problems and their solutions are described as well as recommendations for future projects
A simple two-dimensional parameterisation for Flux Footprint Prediction (FFP)
Flux footprint models are often used for interpretation of flux tower
measurements, to estimate position and size of surface source areas, and the
relative contribution of passive scalar sources to measured fluxes. Accurate
knowledge of footprints is of crucial importance for any upscaling exercises
from single site flux measurements to local or regional scale. Hence,
footprint models are ultimately also of considerable importance for improved
greenhouse gas budgeting. With increasing numbers of flux towers within large
monitoring networks such as FluxNet, ICOS (Integrated Carbon Observation System), NEON (National Ecological Observatory Network), or AmeriFlux, and with increasing temporal range of observations from such
towers (of the order of decades) and availability of airborne flux
measurements, there has been an increasing demand for reliable footprint
estimation. Even though several sophisticated footprint models have been
developed in recent years, most are still not suitable for application to
long time series, due to their high computational demands. Existing fast
footprint models, on the other hand, are based on surface layer theory and
hence are of restricted validity for real-case applications.
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To remedy such shortcomings, we present the two-dimensional parameterisation for Flux
Footprint Prediction (FFP), based on a novel scaling approach for the
crosswind distribution of the flux footprint and on an improved version of
the footprint parameterisation of Kljun et al. (2004b). Compared to the latter,
FFP now provides not only the extent but also the width and shape of
footprint estimates, and explicit consideration of the effects of the surface
roughness length. The footprint parameterisation has been developed and
evaluated using simulations of the backward Lagrangian stochastic particle
dispersion model LPDM-B (Kljun et al., 2002). Like LPDM-B, the parameterisation
is valid for a broad range of boundary layer conditions and measurement
heights over the entire planetary boundary layer. Thus, it can provide
footprint estimates for a wide range of real-case applications.
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The new footprint parameterisation requires input that can be easily determined from, for example, flux tower
measurements or airborne flux data. FFP can be applied to data of long-term
monitoring programmes as well as be used for quick footprint estimates in the
field, or for designing new sites
The performance of RAMS in representing the convective boundary layer structure in a very steep valley
Data from a comprehensive field study in the Riviera Valley of Southern Switzerland are used to investigate convective boundary layer structure in a steep valley and to evaluate wind and temperature fields, convective boundary layer height, and surface sensible heat fluxes as predicted by the mesoscale model RAMS. Current parameterizations of surface and boundary layer processes in RAMS, as well as in other mesoscale models, are based on scaling laws strictly valid only for flat topography and uniform land cover. Model evaluation is required to investigate whether this limits the applicability of RAMS in steep, inhomogeneous terrain. One clear-sky day with light synoptic winds is selected from the field study. Observed temperature structure across and along the valley is nearly homogeneous while wind structure is complex with a wind speed maximum on one side of the valley. Upvalley flows are not purely thermally driven and mechanical effects near the valley entrance also affect the wind structure. RAMS captured many of the observed boundary layer characteristics within the steep valley. The wind field, temperature structure, and convective boundary layer height in the valley are qualitatively simulated by RAMS, but the horizontal temperature structure across and along the valley is less homogeneous in the model than in the observations. The model reproduced the observed net radiation, except around sunset and sunrise when RAMS does not take into account the shadows cast by the surrounding topography. The observed sensible heat fluxes fall within the range of simulated values at grid points surrounding the measurement sites. Some of the scatter between observed and simulated turbulent sensible heat fluxes are due to sub-grid scale effects related to local topograph
Uncertainty propagation for flood forecasting in the Alps: different views and impacts from MAP D-PHASE
D-PHASE was a Forecast Demonstration Project
of theWorldWeather Research Programme (WWRP) related
to the Mesoscale Alpine Programme (MAP). Its goal was to
demonstrate the reliability and quality of operational forecasting
of orographically influenced (determined) precipitation
in the Alps and its consequences on the distribution of
run-off characteristics. A special focus was, of course, on
heavy-precipitation events.
The D-PHASE Operations Period (DOP) ran from June
to November 2007, during which an end-to-end forecasting
system was operated covering many individual catchments
in the Alps, with their water authorities, civil protection organizations
or other end users. The forecasting system’s core
piece was a Visualization Platform where precipitation and
flood warnings from some 30 atmospheric and 7 hydrological
models (both deterministic and probabilistic) and corresponding
model fields were displayed in uniform and comparable
formats. Also, meteograms, nowcasting information
and end user communication was made available to all the
forecasters, users and end users. D-PHASE information was
assessed and used by some 50 different groups ranging from
atmospheric forecasters to civil protection authorities or water
management bodies.
In the present contribution, D-PHASE is briefly presented
along with its outstanding scientific results and, in particular,
the lessons learnt with respect to uncertainty propagation. A
focus is thereby on the transfer of ensemble prediction information
into the hydrological community and its use with
respect to other aspects of societal impact. Objective verification
of forecast quality is contrasted to subjective quality
assessments during the project (end user workshops, questionnaires) and some general conclusions concerning forecast
demonstration projects are drawn
AN URBAN SURFACE EXCHANGE PARAMETERISATION FOR MESOSCALE MODELS
A scheme to represent the impact of urban buildings on airflow in mesoscale atmospheric
models is presented. In the scheme, the buildings are not explicitly resolved, but their effects on the grid-averaged variables are parameterised. An urban quarter is characterised by a horizontal building size, a street canyon width and a building density as a function of height. The module computes the impact of the horizontal (roof and canyon floor) and vertical (walls) surfaces on the wind speed, temperature and turbulent kinetic energy. The computation of the shortwave and longwave radiation, needed to compute the temperature of the urban surfaces, takes into account the shadowing and radiation trapping effects induced by the urban canyons. The computation of the turbulent length scales in the TKE equation is also modified to take into account the presence of the buildings. The parameterisation is introduced into a mesoscale model and tested in a bidimensional case
of a city over flat terrain. The new parameterisation is shown to be able to reproduce the most important features observed in urban areas better than the traditional approach which is based only on the modification of the roughness length, thereby retaining the Monin–Obukhov similarity theory. The new surface exchange parameterisation is furthermore shown to have a strong impact on the dispersion characteristics of air pollutants in urban areas
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In-street wind direction variability in the vicinity of a busy intersection in central London
We present results from fast-response wind measurements within and above a busy intersection between two street canyons (Marylebone Road and Gloucester Place) in Westminster, London taken as part of the DAPPLE (Dispersion of Air Pollution and Penetration into the Local Environment; www.dapple.org.uk) 2007 field campaign. The data reported here were collected using ultrasonic anemometers on the roof-top of a building adjacent to the intersection and at two heights on a pair of lamp-posts on opposite sides of the intersection. Site characteristics, data analysis and the variation of intersection flow with the above-roof wind direction (θref) are discussed. Evidence of both flow channelling and recirculation was identified within the canyon, only a few metres from the intersection for along-street and across-street roof-top winds respectively. Results also indicate that for oblique rooftop flows, the intersection flow is a complex combination of bifurcated channelled flows, recirculation and corner vortices. Asymmetries in local building geometry around the intersection and small changes in the background wind direction (changes in 15-min mean θref of 5–10 degrees) were also observed to have profound influences on the behaviour of intersection flow patterns. Consequently, short time-scale variability in the background flow direction can lead to highly scattered in-street mean flow angles masking the true multi-modal features of the flow and thus further complicating modelling challenges
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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
Validation of an Urban Surface Exchange Parameterization for Mesoscale Models—1D Case in a Street Canyon
A detailed urban parameterization scheme is used in and above a street canyon. To validate this new scheme, the model is run offline on a vertical column (one-dimensional simulations), using measurements from a 30-m-high tower for upper boundary conditions. Measurements were obtained during the intensive observation period of the Basel Urban Boundary Layer Experiment (BUBBLE). Vertical profiles of meteorological variables are simulated in the street canyon. The validation of the parameterization is made with measurements from the tower in the street canyon and directly above roof height. The results show that the urban parameterization scheme is able to catch most of the typical processes that are induced by an urban surface near the ground. The fit to measured profiles is improved in comparison with a model using the traditional approach for urban parameterization (variation of z0 to take into account the presence of a city)
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