Aggeliki Dandou

Development and evaluation of an urban parameterization scheme in the Penn State/NCAR Mesoscale Model (MM5)

[1] In the present study, the Penn State/NCAR Mesoscale Model (MM5) was modified by considering recent advances in the urban boundary layer. In particular, the modifications were carried out in two directions: (1) With respect to the thermal properties of an urban surface the surface energy balance was modified by taking into account the anthropogenic heat released in urban areas and the urban heat storage term to account for urban/building mass effects, including hysteresis; and (2) the surface stress and fluxes of heat and momentum were modified following recent advances in the atmospheric boundary layer over rough surfaces under unstable conditions. The whole process was supplemented by detailed information on land use cover, derived from satellite image analysis. The modifications were applied to the high-resolution nonlocal medium-range forecast planetary boundary layer parameterization scheme, based on work by Troen and Mahrt (1986). The improvements seen with the modified model, after comparison with available measurements of temperature and fluxes, refer to (1) the strengthening of the nocturnal urban heat island; (2) the changes in the temperature, which proved to be favorable through the whole diurnal cycle, resulting in decreasing the temperature amplitude wave; (3) the decrease of turbulence and fluxes during the daytime; and (4) the diffusion coefficient and potential temperature profiles that are reduced during daytime and are increased at the lower levels during the night and thus affect accordingly the mixing height.

A Comparative Study and Evaluation of Mixing-Height Estimation Based on Sodar-RASS, Ceilometer Data and Numerical Model Simulations

A comparative study and evaluation of mixing-layer height estimation was conducted, using data from remote sensing and in-situ instrumentation, radiosondes, synoptic analyses and model simulations. The data were collected during an experimental campaign conducted at the Athens International Airport, Greece, from 15 to 26 September 2007. Mixing-layer height from the sodar dataset was estimated taking into account the backscatter signal, temperature, Richardson number profiles and surface-based measurements, while for the ceilometer data, the optical attenuated aerosol backscatter intensity first derivative was utilized. Numerical simulations using the Penn State/NCAR MM5 numerical mesoscale model and the Weather Research and Forecast numerical model were also performed. Comparative results under different meteorological conditions (local flows, moderate to strong background flows) are presented and discussed. According to our results under moderate to strong winds the existing mechanical turbulence creates good signal conditions for the two remote systems leading to a good overall agreement between the two methodologies, while both models give reliable estimations of the mixing height. The sodar-RASS system is more suitable under low to moderate winds or when local flows are developed with weak stability, while the ceilometer system is more suitable for moderate to strong winds, which is associated with a homogeneous atmosphere and weaker low-level temperature inversions. In the models, the existing approach for atmospheric boundary-layer depth simulation usually provides higher compared to remote sensing values, especially during local flow events. An alternative approach for the estimation of mixing height by the models, the estimation and use of the diffusion coefficient profiles, is a promising methodology regarding the comparison with the sodar-RASS mixing-height estimations.

THE IMPORTANCE OF MIXING HEIGHT IN CHARACTERISING POLLUTION LEVELS FROM AEROSOL OPTICAL THICKNESS DERIVED BY SATELLITE

In the present study the horizontal distribution of columnar aerosol optical thicknessderived at high spatial resolution from Earth observation satellite data in the Lombardy area (Italy) was converted to the horizontal distribution of optically effective aerosols concentration at the ground level. This was achieved by incorporating information on atmospheres mixing height, at which pollutants released at ground level are vertically dispersed by convection or mechanical turbulence. The resulted fields compared favourably to pollutant concentration measurements provided by the ground stations. These results show that it is possible to calculate mean concentration fields by using the spatial distribution of aerosol optical thickness (AOT) measured by satellite normalized by the atmospheric mixing height. The advantage of satellites in measuring AOT is that they can capture all actual emissions compared to the models, which are based on inventoried data.

Physical and Chemical Processes of Polluted Air Masses During Etesians: Aegean-Game Airborne Campaign – An Outline

Gaseous species and aerosol size distribution and chemical composition within the boundary layer during the Etesians is investigated, based upon airborne measurements, over the Aegean Sea, from Crete to Limnos islands (29/8–8/9 2011, Aircraft_BAe146–FAAM). Three flights of a similar route covered the eastern and western parts of the Aegean Sea. Two flights were performed on the same day to study the impact of the diurnal cycle. The sorties involved horizontal tracks mainly at 150 m a.s.l. and above the aerosol layer, at 2.5 km a.s.l., and profiles up to 4.5 km near the ground stations of Crete and Limnos and the Central Aegean Sea. Marked variations were detected in the vertical structure of aerosols and thermodynamic variables between the eastern and western segments flown around the Aegean. Several discrete aerosol layers, separated by a clean slot, containing particles of different chemical composition were observed, with sulfates and organics being the dominant components. CO concentrations ranged from 80 ppb above the mixing layer, up to 140 ppb near the surface. O3 ranged between 50 and 75 ppb, with higher values observed at surface upwind of Finokalia and in the mixing layer in Central and Northern Aegean Sea.

Evaluation of CALPUFF modelling system performance: an application over the Greater Athens Area, Greece

This study has been carried out in order to evaluate the performance of CALPUFF modelling system in an area of complex topography. For this reason a comparison was performed between numerical results produced by this modelling system and those derived by two Eulerian models (UAM-V and REMSAD), together with experimental measurements. The meteorological input parameters, produced by the meteorological model CALMET are also examined through comparison with observations and to MM5 model results. Emphasis is given to the mixing height calculation, due to its importance in the prediction of pollutant concentrations. It was found that the overall performance of the CALPUFF is satisfactory and the results derived are compatible with these produced by the Eulerian models, especially under unstable atmospheric conditions. The CALPUFF models performance is improved significantly when vertical meteorological profiles produced by MM5 are additionally employed.

Comparison of data fusion methods for satellite-assisted determination of PM10 ambient concentration

Our recent work has demonstrated the feasibility of using satellite-derived data to draw quantitative maps of particulate loading within the planetary boundary layer. Our method, when used in conjunction with atmospheric dispersion models and ground data, can provide a comprehensive estimate of tropospheric pollution from particulate matter. Information filtering techniques are used to reduce the error of the information fusion algorithm and, consequently, produce the best possible estimate of tropospheric aerosol. Two data filtering methods have been used and their effectiveness with regard to overall error reduction is determined in this work. The first one is based on a weight scheme to take into account an empirical estimate of local error and/or uncertainty in input data. The second uses a modified Kalman filter for error reduction. The effectiveness of each of the filtering techniques depends on factors such as relative error variance across the computational domain, and precision of model input, i.e. on the accuracy of the ground emissions inventory and the reliability of measured ambient aerosol concentrations. The ICAROS NET fusion method was applied in the greater area of Athens, Greece over several days of observation in order to assess conclusively the adequacy of the information fusion filters employed.

Issues related to aircraft take-off plumes in a mesoscale photochemical model

The physical and chemical characteristics of aircraft plumes at the take-off phase are simulated with the mesoscale CAMx model using the individual plume segment approach, in a highly resolved domain, covering the Athens International Airport. Emission indices during take-off measured at the Athens International Airport are incorporated. Model predictions are compared with in situ point and path-averaged observations (NO, NO₂) downwind of the runway at the ground. The influence of modeling process, dispersion properties and background air composition on the chemical evolution of the aircraft plumes is examined. It is proven that the mixing properties mainly determine the plume dispersion. The initial plume properties become significant for the selection of the appropriate vertical resolution. Besides these factors, the background NOx and O₃ concentration levels control NOx distribution and their conversion to nitrogen reservoir species.

Solar Irradiance Prediction over the Aegean Sea: Shortwave Parameterization Schemes and Aerosol Radiation Feedback

In order to study the solar irradiance’s prediction over Greece, WRF-Chem model is applied, using three shortwave radiation parameterization schemes: Dudhia, Goddard and RRTMG which simulate differently the aerosol-radiation interaction. This study focuses on a typical summertime wind pattern, the Etesian outbreaks, during which polluted air masses are transported in Greek territory and therefore they affect incoming solar irradiance. The results indicate that schemes overall overestimate solar irradiance reaching the ground; Dudhia scheme by 9%, RRTMG by 13%, and Goddard by 17%. The performance of all schemes is improved when the aerosol-radiation interaction is considered at least by 1.5%, while local temperature changes, by up to 1.5°, are noticed.

Sensitivity Tests in the Dynamical and Thermal Part of the MRF-Urban PBL Scheme in the MM5 Model

In the present study, sensitivity tests were carried out in the ‘dynamical’ and ‘thermal’ part of a meteorological model in urban environment. The numerical simulations were performed by the PSU/NCAR Mesoscale Model (MM5), by applying the non-local Medium-Range Forecast (MRF) Planetary Boundary Layer (PBL) parameterisation scheme, plus the MRF-urban scheme, whereby urban features are considered. An unrealistic run was also performed by the MRF scheme, where the city of Athens was replaced by dry cropland and pasture surface, as in the surrounding area. The model results were compared with sonic anemometer measurements of turbulence and routine meteorological data. Modifications in both the ‘dynamical’ and ‘thermal’ parts seem to play an important role and improve the model’s results. In addition, a delay in the sea breeze front was found, and a reasonable frictional retard concerning its penetration, as well as an inland displacement of the heat island, as the air moved over the city of Athens.