The cloud droplet number concentration (N d) is of central interest to improve the understanding of cloud physics and for quantifying the effective radiative forcing by aerosol‐cloud interactions. Current standard satellite retrievals do not operationally provide N d, but it can be inferred from retrievals of cloud optical depth (τ c) cloud droplet effective radius (r e) and cloud top temperature. This review summarizes issues with this approach and quantifies uncertainties. A total relative uncertainty of 78% is inferred for pixel‐level retrievals for relatively homogeneous, optically thick and unobscured stratiform clouds with favorable viewing geometry. The uncertainty is even greater if these conditions are not met. For averages over 1° ×1° regions the uncertainty is reduced to 54% assuming random errors for instrument uncertainties. In contrast, the few evaluation studies against reference in situ observations suggest much better accuracy with little variability in the bias. More such studies are required for a better error characterization. N d uncertainty is dominated by errors in r e, and therefore, improvements in r e retrievals would greatly improve the quality of the N d retrievals. Recommendations are made for how this might be achieved. Some existing N d data sets are compared and discussed, and best practices for the use of N d data from current passive instruments (e.g., filtering criteria) are recommended. Emerging alternative N d estimates are also considered. First, new ideas to use additional information from existing and upcoming spaceborne instruments are discussed, and second, approaches using high‐quality ground‐based observations are examined.

Abstract. In this paper we describe and summarize the main achievements of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). EUCAARI started on 1 January 2007 and ended on 31 December 2010 leaving a rich legacy including: (a) a comprehensive database with a year of observations of the physical, chemical and optical properties of aerosol particles over Europe, (b) comprehensive aerosol measurements in four developing countries, (c) a database of airborne measurements of aerosols and clouds over Europe during May 2008, (d) comprehensive modeling tools to study aerosol processes fron nano to global scale and their effects on climate and air quality. In addition a new Pan-European aerosol emissions inventory was developed and evaluated, a new cluster spectrometer was built and tested in the field and several new aerosol parameterizations and computations modules for chemical transport and global climate models were developed and evaluated. These achievements and related studies have substantially improved our understanding and reduced the uncertainties of aerosol radiative forcing and air quality-climate interactions. The EUCAARI results can be utilized in European and global environmental policy to assess the aerosol impacts and the corresponding abatement strategies.

Abstract. We present GPS, radiosonde and microwave radiometer (MWR) estimates of precipitable water vapor (PW) at Cape Grim, Tasmania, during November and December 1995.The rms differences between GPS and radiosonde, MWR and radiosonde and GPS and MWR estimates of PW were 1.5 mm, 1.3 mm and 1.4 mm, respectively, whilst the biases between the three systems were -0.2 mm. However, there are occasions when the amount of PW was underestimated by GPS whilst at other times was over-estimated by MWR. The average overlap error of the GPS estimates of PW between adjacent daily solutions is related to the orbit overlap error and we removed a 2 mm bias introduced using International GPS Service orbits by estimating more accurate global orbits. The discrepancies of up to 3-4 mm between the MWR and GPS systems are not caused by rain, waveguide losses, varying waveguide temperature, detector non-linearity or inaccurate estimates of the mean radiating temperature of the atmosphere. However, small differences between mapping functions at low elevations can produce biases comparable with the bias between the two systems. Consequently, we suspect that the biases arise because the mapping functions do not represent the localized atmospheric conditions at Cape Grim. The most accurate GPS estimates are achieved when the G?S analysis contains station separations of more than 2000 km, an elevation cutoff angle of 12 ø is used and the CFA2.2 wet mapping function is used to map the wet delay at any angle to the delay in the zenith.

[1] A model for the vertical variation of microphysical and optical properties of single-layer water clouds is used to design a procedure to obtain cloud droplet concentration from satellite cloud optical thickness/effective radius retrievals. The model allows for smooth vertical variations in microphysical variables including droplet concentration and liquid water content. The procedure is applied to data from the MODIS instrument aboard the EOS-TERRA satellite platform over a region near Cape Grim, Tasmania, over the Southern Ocean. At this unpolluted location, there are seasonally repeated well-described variations in cloud condensation nucleus concentration. The satellite observations show that there is a factor 2.5 difference in retrieved droplet concentration between the summer and winter seasons as measured over the 4 year period (July 2000 to July 2004). Comparison of these results with cloud condensation nuclei concentrations observed at Cape Grim showed good agreement. Furthermore, for a fixed solar zenith angle cos (45°) cloud albedo varied between 0.45 and 0.55 and exhibited a clear correlation with the retrieved droplet concentration (correlation coefficient R = 0.80). Such correlation was absent between albedo and the second retrieval product, namely the cloud depth.

We present an analysis of the evolution of the smoke plume caused by the Black Saturday bushfires, which started on 7 February 2009 in the Australian state of Victoria. Within 3 days this smoke plume was located at altitudes between 15 and 20 km thousands of kilometers away from its source region. Standard explanations for high tropospheric and lower stratospheric absorbing aerosols are either volcanic eruptions or pyroconvection. We performed a detailed analysis of various satellite observations, forward trajectory calculations, and meteorological conditions during the fire episode, yet we could not find evidence of either of these standard mechanisms explaining the observed plume evolution. Pyroconvection observed within the initial smoke plumes remained predominantly below 10 km altitude. Furthermore, there are not active volcanoes in the region. We postulate that the subsequent rise beyond approximately 10 km altitude during the first 3 days after the fires started was caused by absorption of short‐wave solar radiation in the plume. Observations indicate that the plume was highly absorptive and optically very thick. One‐dimensional plume height radiative transfer calculations with realistic assumptions about the optical properties of the smoke show that the plume could rise to 16–18 km after 5 days and up to 20 km after 10 days. The plume rise exhibits a characteristic step‐like time evolution that tracks the variation in diurnal insolation and resembles an escalator. We argue that this is the first time that this mechanism, known as “self‐lifting,” has been observed on a large scale. The key features of this mechanism and its implications are briefly discussed.