A. Clappier

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.

A Metamodeling approach for integrated assessment of air quality policies

A method is proposed to build integrated models (also called “Metamodels”) aimed at quantifying the economic efficiency of air quality policies. This “Metamodeling” approach is based on the coupling of two complementary models, that operate at different scales in space and time, and which represent the economic and the physical and chemical processes, respectively. The joint consideration of the physico-chemical and techno-economic structure of the pollution control problems permits a comprehensive evaluation of air pollution abatement strategies. The motivating pollution control problems include urban-regional air quality management through efficient energy and traffic control policies. A pilot study, exploiting data collected in the Geneva canton (Switzerland), is used to demonstrate the potential of the approach.

Total VOC reactivity in the planetary boundary layer 2. A new indicator for determining the sensitivity of the ozone production to VOC and NOx

A new indicator is proposed to det. whether tropospheric O3 prodn. in a specific area is limited by volatile org. compds. (VOC) or NOx. This indicator, tVOCOH/tNOxOH, describes the ratio of OH- lifetime vs. losses by reaction with VOC and NOx. Whereas tNOxOH can be obtained by conventional measurements, the new pump and probe OH approach makes it possible to also obtain tVOCOH. Indicator values above a threshold value of 0.2 +- 50% are representative of NOx-satd. conditions, where an increase of NOx emissions causes lower O3 prodn. For values <0.01, O3 prodn. is very insensitive to changes of VOC emissions. The robustness of this indicator against several parameters (temp., humidity, photolysis, initial O3 concn.) was tested in a box model and compared to the robustness of other earlier proposed indicators. In contrast to earlier proposed indicators, this new one is not based on photochem. produced long-lived species, but describes the instantaneous regime of an air parcel. Three-dimensional modeling showed this indicator is quite successful for estg. the impact of increased or reduced emissions on O3 concns. for each location in the modeled area. This will make it a very helpful tool for developing O3 abatement strategies.

Episode selection for ozone modelling and control strategies analysis on the Swiss Plateau

An episode selection procedure was developed and applied to select sets of days representing characteristic meteorological conditions leading to high ozone episodes over the Swiss Plateau. The selection procedure was applied to data extending from January 1991 through December 1998, and is comprised of two steps: First, days were classified according to observed air quality and meteorological characteristics using classification and regression trees analysis (CART). Second, the CART results were used in conjunction with observed air quality data to identify sets of days characteristic of those leading to elevated ozone. These sets of days were selected to optimise how well a limited number of days represented seasonal air quality, and that formed longer episodes for use in the air quality modelling. CART analysis was performed for three zones of the Swiss Plateau that have different air quality and meteorological characteristics. The results for two zones were used together in the episode selection procedure in order to identify days representative for the whole Plateau. Meteorological analysis for a third zone suggested that it would be strongly impacted by pollutants transported in from outside the country. One thousand and eight hundred optimisation runs were performed to minimise the likelihood that the set of days was a local optimum, increasing the robustness for use in air quality modelling analysis. Fifteen days, grouped in four episodes ranging from 3 to 5 days were selected along with their calculated representativeness (or weight) to recreate a seasonal metric. The variety of local as well as regional meteorological characteristics showed that the episode selection procedure chose days representing a diverse set of meteorological situations which are associated with elevated ozone. This set of episodes can now be used to test air quality strategies.

Total VOC reactivity in the planetary boundary layer 1. Estimation by a pump and probe OH experiment

OH- radical reactivity with total volatile org. compds. (VOC), SkVOC[VOC], is directly obtained in the planetary boundary layer (PBL). A method is based on a perturbation technique where high initial OH- concns. are created by flash photolysis of O3 and subsequent reaction of O(1D) with H2O. Laser-induced fluorescence measures residual OH- concn. at different time delays after the perturbation (<100 ms) to obtain a direct est. of OH- lifetime (tOH) under different atm. conditions. For specific exptl. conditions, transport by wind may be neglected, and chem. processes governing OH- decay may be expressed using a detailed box model. With a simple chem. equation derived using tOH and complementary measurements of CO, O3, and NOx, an in-situ est. of SkVOC[VOC] in the PBL is retrieved with an uncertainty <20% vs. detailed box model calcns. This anal. was applied to lab. measurements with 3 synthetic NOx/VOC/O3 gas mixts.; retrieved OH- lifetimes and total VOC reactivity are discussed against model predictions.

An inter-comparison exercise of mesoscale flow models applied to an ideal case simulation

Abstract An exercise is described aiming at the comparison of the results of seven mesoscale models used for the simulation of an ideal circulation case. The exercise foresees the simulation of the flow over an ideal sea–land interface including ideal topography in order to verify model deviations on a controlled case. All models involved use the same initial and boundary conditions, circulation and temperature forcings as well as grid resolution in the horizontal and simulate the circulation over a 24-h period of time. The model differences at start are reduced to the minimum by the case specification and consist mainly of the parameterisation and numerical formulation of the fundamental equations of the atmospheric flow. The exercise reveals that despite the reduction of the differences in the case configuration, the differences in model results are still remarkable. An ad hoc investigation using one model of the original seven identifies the treatment of the boundary conditions, the parameterisation of the horizontal diffusion and of the surface heat flux as the main cause for the model deviations. The analysis of ideal cases represents a revealing and interesting exercise to be performed after the validation of models against analytical solution but prior to the application to real cases.

Atmospheric pollution transport: the parallelization of a transport & chemistry code

Abstract We discuss a model for computing the chemical reactions occurring among air pollutants and predict their dispersion in the atmosphere. We consider a parallel implementation on a CRAY T3D, using up to 128 processors. A regular subdomain decomposition is considered to optimize inter-processor communications, while load balancing – the main technical difficulty to obtain a good efficiency – is achieved by re-assigning temporarily pieces of work to other processors, according to a global strategy. The performance of our implementation is described by an analytical relation whose validity is checked on realistic data sets. A speedup of 100 is obtained with 128 processors.

An Innovative High Repetition Rate Shot Per Shot Acquisition System and its Application to a New Kind of Tropospheric Lidar Measurements

Analog to Digital converters and scalers have been integrated into a VME data acquisition system operated at a repetition trigger rate up to 250 Hz with a shot per shot data transfer of 16 Kbytes. Data are stored and treated online for each single shot. This gives access to informations which are lost when hardware accumulating devices are employed to follow such a repetition rate. The system has been first used in the field of tropospheric LIDAR measurements. It provides original informations on both statistical and systematic effects on those measurements, allowing a new approach for the study of the dynamical properties of the atmosphere.

Ozone Flux Measurements in the Troposphere with a Shot per Shot DIAL Instrument Operated in the UV Spectral Region During the MEDCAPHOT Campaign

The first combined reconstruction of range-resolved wind velocities and O3 concns. is presented from the anal. of a single DIAL data set analyzed with a correlation method to det. wind speeds. The system can be used for the measurement of a unique component of the wind velocity with an accuracy comparable to that of stand-alone fixed beam correlation lidar instruments. A photochem. model is also developed to predict the spatio-temporal evolution of O3 concns., originated by both chem. and transport effects.

Photo-oxidant formation in the Milan metropolitan area

During the Limitation of Oxidant Prodn. (LOOP) campaign, different instruments were deployed, providing a complete data set. This will be used to evaluate the model and its ability to reproduce pollution events. Then, the complete data set will be used for a better definition of the model inputs. The final aim of the LOOP project is to use the model to test different abatement strategies, particularly by defining the region where ozone prodn. is NOx or VOC sensitive.