Sabina Stefan

Precipitation scavenging coefficient: influence of measured aerosol and raindrop size distributions.

Precipitation scavenging coefficients, widely used in pollution studies, are derived from microphysical parameterisations of aerosol particles and raindrop populations and parameterisations of their interactions. The present study investigates the effects of measured aerosol and raindrop size distributions in a microphysical polydisperse framework. The interactions between aerosol and raindrops parameterised as collision efficiency are explicitly included to account for Brownian diffusion, inertial impaction and interception. Estimated values of the polydisperse scavenging coefficients exhibit variations of orders of magnitude depending on the aerosol type and almost no variation with the raindrop size distributions. For practical use, linear relationships between the scavenging coefficients and rain intensity for different aerosol types are derived.

Impacts of the North Atlantic Oscillation and the El Niño–Southern Oscillation on Danube river flow variability

The impact of the North Atlantic Oscillation (NAO) and El Nino-Southern Oscillation (ENSO) on the Danube river streamflow variability is investigated for the period 1840 to 1998. A composite analysis reveals that positive streamflow anomalies are related to a large scale atmospheric circulation pattern that contains elements of the positive phase of the Pacific North American (PNA) pattern and negative phase of the NAO. The corresponding sea surface temperature (SST) pattern shows positive anomalies over most of the tropical region. Opposite atmospheric circulation and SST patterns are associated to negative streamflow anomalies. Significant decadal variations of the NAO and ENSO impact on the Danube streamflow are detected for the observational period. A lag-correlation analysis reveals that winter SST from tropical Pacific and some regions from the North Atlantic are significantly correlated with the streamflow variations from spring and summer suggesting a possible predictive skill of the Danube streamflow anomalies in these seasons using winter SST as a predictor.

A theoretical study of the microphysical parameterization of the scavenging coefficient as a function of precipitation type and rate

Abstract For practical reasons, numerical modelling of transport, transformation and deposition of atmospheric pollutants requires the parameterization of wet removal in terms of scavenging coefficients. In this paper, the scavenging coefficient–precipitation intensity relationships for different types of precipitation are derived. The variety of the obtained scavenging coefficients shows the importance of microphysical parameterizations.

Quasi-Decadal Variability in the Atlantic Basin Involving Tropics–Midlatitudes and Ocean–Atmosphere Interactions

Observational data are used to emphasise a 5- to 7-yr lag between the main modes of variability in the midlatitudes and in the Tropics. Considering this finding a mechanism for quasidecadal variability based on Tropics‐midlatitudes and ocean‐atmosphere interaction is described. It appears that the signal associated with the SST anomalies in the northern region of the tropical Atlantic is transferred in midlatitudes through the atmosphere and it will modify the thermal conditions of the ocean upper layers. In 5‐7 years, thermal conditions will affect the SST anomalies in the northern Tropics reversing their sign. The results suggest that the Tropics get a negative feedback from midlatitudes so that the Tropics‐midlatitudes system is capable of generating an oscillatory mode.

Evaluation of the boundary layer morning transition using the CL-31 ceilometer signals

The morning transition of the atmospheric boundary layer from nighttime to daytime conditions was investigated using the Vaisala’s CL-31 ceilometer, located at Magurele, Romania (44.35°N, 26.03°E). Based on the 5-days backward trajectories, we rejected those measurements which were related to the intrusions of long-range transported particles. In the several discussed cases, which are representative for the morning transition in spring and summer seasons over Magurele, the increasing depth of the boundary layer related to the local aerosol load was well discernible. The dynamic change of its depth was estimated with errors using a simple method based on finding the minimum of the first derivative of the ceilometer signal. In the summer, the increase of the boundary layer depth due to the morning transition from the nighttime to daytime conditions starts on average of about 80 min earlier and the growth rate of this depth is 143 ± 6 m/h and about 37% slower than in the spring case.

Annual air pollution level of major primary pollutants in Greater Area of Bucharest

Abstract In the international context of incomplete information on air pollution in East Europe, we made a review–image of the air pollution problem in Bucharest metropolitan area, Romania, by assessing concentrations, variability, and compliance with the EU regulations of the primary pollutant levels (PM 10 , PM 2.5 , SO 2 , CO, and NO x ) from eight sampling stations for six years of continuous sampling (2005–2010) and analyzing for factors affecting the seasonal and spatial variations of PM levels. Investigation of temporal and spatial variation of PM 10 and PM 2.5 concentrations, as well as their relationships with the measured gaseous air pollutants and with meteorological variables includes correlation analysis, trend analysis, multiple linear regression analysis, and atmospheric back–trajectory analysis. Starting with systematic exceedances of the limit values in 2005 for PM 10 and NO x , we observed negative trends for all main pollutants at majority of sites. Exception appears in the background levels where no major improvement was seen. SO 2 and CO were found in compliance with the EU regulations in 2010, but PM10 and NO x still remain a problem. Over the years, PM 2.5 represents a significant fraction (70–80%) of PM 10 , irrespective the type of monitoring site. PM levels are higher than those registered for other cities over the Western, Central, and Northern Europe. Combustion–related PM 10 fraction varies from 73% (cold season) to 59% (warm season) and is higher with 22–26% than in similar polluted area in Southeast Europe. The contributions are site dependent but the background sites experience comparative combustion–related PM 10 contributions to that of sites within Bucharest ring. Wind speed and temperature are the main factors that influence the PM levels; atmospheric pressure and humidity play a secondary role. Pollution events in Bucharest area are mainly caused by local anthropogenic emissions and not by advections from long distances.

The variability of winter high temperature extremes in Romania and its relationship with large-scale atmospheric circulation

The frequency variability of extreme high winter temperature as recorded at 85 meteorological stations from Romania during 1962–2010 period and its relationship with large-scale atmospheric circulation was investigated. An Empirical Orthogonal Function analysis reveals that large part of the extreme temperature frequency variability is common to all stations suggesting a strong influence of large-scale circulation anomalies. The North Atlantic Oscillation, West Pacific, East Atlantic, and Scandinavian patterns are related with extreme temperature frequency variability. We show that the East Atlantic Oscillation controls a significant part of interannual extreme high temperature variability over Romania via advection of warm air from the west. In addition, a strong relationship between blocking activity and frequency of extreme high temperature events in Romania was found. High blocking activity in the (20°W-70°E) sector is related with relatively strong advection of cold air over the country during winter. On the other hand, low blocking activity in the same sector is related with weak advection of relatively cold air in the region. Moreover, the blocking frequency in this sector is modulated mainly by the East Atlantic Oscillation.

Interdecadal variability generated by interactions between Pacific and Atlantic oceans

Recent studies suggest that, interannual, decadal, and quasibidecadal periodicities dominate global sea surface temperature and sea level pressure variability over past hundred years. In this study, we present evidence that the quasibidecadal mode of variability can be considered in a first order approximation as resulting from a linear interaction between the dominant modes of variability in the Tropical Pacific (associated with interannual variability) and North Atlantic ocean (associated with decadal variability). A two coupled delayed action oscillators model is proposed to qualitatively explain this result.

Analytical formulas for the below-cloud scavenging coefficient of an irreversibly soluble gas: a quantitative evaluation for HNO3

In the present paper, analytical expressions are derived for the below-cloud gas scavenging coefficient, considering various functions that can be found in the literature to describe raindrop populations and terminal velocities. Three primary cases have been identified where an analytical solution exists: (i) a raindrop size distribution described by a gamma function and raindrop terminal velocity by a power function of raindrop size; (ii) a raindrop size distribution described by a gamma function and raindrop terminal velocity by an exponential function; and (iii) a raindrop size distribution described by a log–normal function and raindrop terminal velocity by a power function. In addition, in case (i), the gas scavenging coefficient is expressed analytically as a function of rain intensity. The derived analytical expressions are subsequently used to compute the scavenging coefficients of HNO3 for different parameterizations of the raindrop size distribution and terminal velocity functions. In order to broaden the sensitivity test of HNO3 scavenging coefficients on parameterizations, scavenging coefficients of HNO3 are also computed numerically for the raindrop terminal velocity as a function of raindrop size and atmospheric conditions. The results show that there is a relative variation in the values of the gas scavenging coefficient up to 50% for the parameterization considered in this study, and that the overestimation given by the analytical formulas is not important for cut-off limits of raindrop spectra below 0.2 mm.

Analysis of Convective Thunderstorm Split Cells in South-Eastern Romania

The mesoscale configurations are analysed associated withthesplitting process of convective cells responsible for severe weather phenomena in the south-eastern part of Romania. The analysis was performed using products from the S-band Doppler weather radar located in Medgidia. The cases studied were chosen to cover various synoptic configurations when the cell splitting process occurs. To detect the presence and intensity of the tropospheric jet, the Doppler velocity field and vertical wind profiles derived from radar algorithms were used. The relative Doppler velocity field was used to study relative flow associated with convective cells. Trajectories and rotational characteristics associated with convective cells were obtained from reflectivity and relative Doppler velocity fields at various elevations. This analysis highlights the main dynamic features associated with the splitting process of convective cells: the tropospheric jet and vertical moisture flow associated with the configuration of the flow relative to the convective cells for the lower and upper tropospheric layers. These dynamic characteristics seen in the Doppler based velocity field and in the relative Doppler velocity field to the storm can indicate further evolution of convective developments, with direct implications to very short range forecast (nowcasting).