The Mediterranean Intensive Oxidant Study, performed in the summer of 2001, uncovered air pollution layers from the surface to an altitude of 15 kilometers. In the boundary layer, air pollution standards are exceeded throughout the region, caused by West and East European pollution from the north. Aerosol particles also reduce solar radiation penetration to the surface, which can suppress precipitation. In the middle troposphere, Asian and to a lesser extent North American pollution is transported from the west. Additional Asian pollution from the east, transported from the monsoon in the upper troposphere, crosses the Mediterranean tropopause, which pollutes the lower stratosphere at middle latitudes.

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Abstract. As part of a series of papers on the sources, distribution and potential impact of biological particles in the atmosphere, this paper introduces and summarizes the potential role of biological particles in atmospheric clouds. Biological particles like bacteria or pollen may be active as both cloud condensation nuclei (CCN) and heterogeneous ice nuclei (IN) and thereby can contribute to the initial cloud formation stages and the development of precipitation through giant CCN and IN processes. The paper gives an introduction to aerosol-cloud processes involving CCN and IN in general and provides a short summary of previous laboratory, field and modelling work which investigated the CCN and IN activity of bacterial cells and pollen. Recent measurements of atmospheric ice nuclei with a continuous flow diffusion chamber (CFDC) and of the heterogeneous ice nucleation efficiency of bacterial cells are also briefly discussed. As a main result of this overview paper we conclude that a proper assessment of the impact of biological particles on tropospheric clouds needs new laboratory, field and modelling work on the abundance of biological particles in the atmosphere and their CCN and heterogeneous IN properties.

Most atmospheric motions of different spatial scales and precipitation are closely related to phase transitions in clouds. The continuously increasing resolution of large-scale and mesoscale atmospheric models makes it feasible to treat the evolution of individual clouds. The explicit treatment of clouds requires the simulation of cloud microphysics. Two main approaches describing cloud microphysical properties and processes have been developed in the past four and a half decades: bulk microphysics parameterization and spectral (bin) microphysics (SBM). The development and utilization of both represent an important step forward in cloud modeling. This study presents a detailed survey of the physical basis and the applications of both bulk microphysics parameterization and SBM. The results obtained from simulations of a wide range of atmospheric phenomena, from tropical cyclones through Arctic clouds using these two approaches are compared. Advantages and disadvantages, as well as lines of future development for these methods are discussed.

[1] The size distribution and chemical composition of aerosol particles during a dust storm in the eastern Mediterranean are analyzed. The data were obtained from airborne measurements during the Mediterranean Israeli Dust Experiment (MEIDEX). The dust storm passed over the Mediterranean Sea and extended up to an altitude of about 2.5 km. The uniqueness of this dust storm is that approximately 35% of the coarse particles up to about 1 km in height were internally mixtures of mineral dust and sea salt. Just north of the dust storm, large convective clouds developed, and heavy rain was recorded by the radar on the Tropical Rainfall Measuring Mission satellite. The chemical and physical properties of the particles are used as initial conditions for conducting a sensitivity simulation study with the two-dimensional detailed spectral bin microphysical model of Tel Aviv University. The simulations show that ignoring the ice-nucleating ability of the mineral dust, but allowing the soluble component of the mixed aerosols to act as efficient giant cloud condensation nuclei (CCN), enhances the development of the warm rain process in continental clouds. In our simulations the rain amounts increased by as much as 37% compared to the case without giant CCN. Introducing similar coarse-mode particles into more maritime-type clouds does not have significant effect on the cloud or on the amount of rainfall. On the other hand, allowing the mineral dust particles to also act as efficient ice nuclei (IN) reduces the amount of rain on the ground compared to the case when they are inactive. The simulations also reveal that under the same profiles of meteorological parameters, maritime clouds develop precipitation earlier and reach lower altitudes than continental clouds. When the dust particles are active as both giant CCN and effective IN, the continental clouds become wider, while the effects on the more maritime clouds is very small. Citation: Levin, Z., A. Teller, E. Ganor, and Y. Yin (2005), On the interactions of mineral dust, sea-salt particles, and clouds: A measurement and modeling study from the Mediterranean Israeli Dust Experiment campaign,

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Measuring systems for atmospheric ice nuclei are undergoing development anew and are beginning to meet the needs for studies of aerosol effects on ice-containing clouds. U nderstanding and predicting the formation of ice in clouds and its possible relation to the changing state of atmospheric composition (aerosols and gas phase) remain enigmatic. Such knowledge and capabilities are critical to quantifying the role of aerosols and their changing compositions on clouds, precipitation, and climate (Denman et al. 2007;Levin and Cotton 2009). This challenge is a major motivation for renewed attempts to measure ice nucleation processes in general, and to design and deploy new portable systems for measuring ice nuclei (IN), the particles that are considered the only means for initiation of the ice phase at temperatures warmer than about −36°C in the atmosphere. The fundamental desire to understand ice nucleation remains the same as when such research began in earnest more than 60 yr ago. The search to identify atmospheric ice nuclei lapsed during the 1970s-80s

We explore the effects of increases in aerosol concentration on cloud lifetime for warm convective clouds using a two‐dimensional single cloud model and three‐dimensional large eddy simulations (LES). The models include size‐resolved treatment of drop size distributions and warm microphysical processes. It is shown using a variety of soundings representing marine trade cumulus, and continental convective clouds that contrary to expectation, an increase in aerosol concentration from very clean to very polluted does not increase cloud lifetime, even though precipitation is suppressed. Cloud lifetimes are statistically similar although individual clouds may experience decreases in lifetime of 10–40%. An evaporation‐entrainment feedback that tends to dilute polluted clouds more than clean clouds is identified. It is proposed that the small changes in cloud lifetime are due to competing effects of precipitation suppression and enhanced evaporation, with the latter tending to dominate in these shallow clouds.