[1] Coalescence of cloud droplets is essential for the production of small raindrops at a given vertical distance above the cloud base (D p ). The rate of droplet coalescence is determined mainly by droplet size, spectrum width and concentrations. The droplet condensational growth is determined by the number of activated CCN (N a ) and height above cloud base. Here we show that when the droplet mean volume radius, r v , exceeds $13 mm, or when droplet effective radius (r e ) exceeds $14 mm, considerable precipitation mass (>0.03 g kg À1 ) is likely to be present in growing convective clouds. This is because the rate of droplet coalescence is proportional to $r v 5 which practically implies the existence of a threshold r v above which efficient warm rain formation can occur, and also because the vertical profile of r v , even in diluted clouds, nearly follows the theoretical adiabatic condensational growth curve. The small observed deviations are mainly caused by deviations from purely inhomogeneous mixing which cause partial droplet evaporation. Consequently, D p must theoretically change nearly linearly with N a . This is confirmed here observationally, where increasing N a by 100 per milligram (≈cm 3 at cloud base) of air, resulted in an increase of $280 m in D p for both Israeli and Indian deep convective clouds. This means that in highly polluted clouds or where strong cloud-base updrafts occur, clouds have to grow well above the freezing level, even in tropical atmosphere, before precipitation forms either by warm or by mixed-phase processes.Citation: Freud, E., and D. Rosenfeld (2012), Linear relation between convective cloud drop number concentration and depth for rain initiation,

Abstract. The Arctic environment has an amplified response to global climatic change. It is sensitive to human activities that mostly take place elsewhere. For this study, a multi-year set of observed aerosol number size distributions in the diameter range of 10 to 500 nm from five sites around the Arctic Ocean (Alert, Villum Research Station -Station Nord, Zeppelin, Tiksi and Barrow) was assembled and analysed.A cluster analysis of the aerosol number size distributions revealed four distinct distributions. Together with Lagrangian air parcel back-trajectories, they were used to link the observed aerosol number size distributions with a variety of transport regimes. This analysis yields insight into aerosol dynamics, transport and removal processes, on both an intraand an inter-monthly scale. For instance, the relative occurrence of aerosol number size distributions that indicate new particle formation (NPF) event is near zero during the dark months, increases gradually to ∼ 40 % from spring to summer, and then collapses in autumn. Also, the likelihood of Arctic haze aerosols is minimal in summer and peaks in April at all sites.The residence time of accumulation-mode particles in the Arctic troposphere is typically long enough to allow tracking them back to their source regions. Air flow that passes at low altitude over central Siberia and western Russia is associated with relatively high concentrations of accumulationmode particles (N acc ) at all five sites -often above 150 cm −3 .

Abstract. In-situ measurements in convective clouds (up to the freezing level) over the Amazon basin show that smoke from deforestation fires prevents clouds from precipitating until they acquire a vertical development of at least 4 km, compared to only 1-2 km in clean clouds. The average cloud depth required for the onset of warm rain increased by ∼350 m for each additional 100 cloud condensation nuclei per cm 3 at a super-saturation of 0.5% (CCN 0.5% ). In polluted clouds, the diameter of modal liquid water content grows much slower with cloud depth (at least by a factor of ∼2), due to the large number of droplets that compete for available water and to the suppressed coalescence processes. Contrary to what other studies have suggested, we did not observe this effect to reach saturation at 3000 or more accumulation mode particles per cm 3 . The CCN 0.5% concentration was found to be a very good predictor for the cloud depth required for the onset of warm precipitation and other microphysical factors, leaving only a secondary role for the updraft velocities in determining the cloud drop size distributions.The effective radius of the cloud droplets (r e ) was found to be a quite robust parameter for a given environment and cloud depth, showing only a small effect of partial droplet evaporation from the cloud's mixing with its drier environment. This supports one of the basic assumptions of satellite analysis of cloud microphysical processes: the ability to look at different cloud top heights in the same region and regard their r e as if they had been measured inside one well developed cloud. The dependence of r e on the adiabatic fraction decreased higher in the clouds, especially for cleaner conditions, and disappeared at r e ≥∼10 µm. We propose thatCorrespondence to: E. Freud (eyal.freud@mail.huji.ac.il) droplet coalescence, which is at its peak when warm rain is formed in the cloud at r e =∼10 µm, continues to be significant during the cloud's mixing with the entrained air, cancelling out the decrease in r e due to evaporation.