Abstract.We have investigated the formation of cloud droplets under pyro-convective conditions using a cloud parcel model with detailed spectral microphysics and with the κ-Köhler model approach for efficient and realistic description of the cloud condensation nucleus (CCN) activity of aerosol particles. Assuming a typical biomass burning aerosol size distribution (accumulation mode centred at 120 nm), we have calculated initial cloud droplet number concentrations (N CD ) for a wide range of updraft velocities (w=0.25-20 m s −1 ) and aerosol particle number concentrations (N CN =200-10 5 cm −3 ) at the cloud base. Depending on the ratio between updraft velocity and particle number concentration (w/N CN ), we found three distinctly different regimes of CCN activation and cloud droplet formation:(1) An aerosol-limited regime that is characterized by high w/N CN ratios (>≈10 −3 m s −1 cm 3 ), high maximum values of water vapour supersaturation (S max >≈0.5%), and high activated fractions of aerosol particles (N CD /N CN >≈90%). In this regime N CD is directly proportional to N CN and practically independent of w.(2) An updraft-limited regime that is characterized by low w/N CN ratios (<≈10 −4 m s −1 cm 3 ), low maximum values of water vapour supersaturation (S max <≈0.2%), and low activated fractions of aerosol particles (N CD /N CN <≈20%). In this regime N CD is directly proportional to w and practically independent of N CN .Correspondence to: H. Su (hsu@mpch-mainz.mpg.de) (3) An aerosol-and updraft-sensitive regime (transitional regime), which is characterized by parameter values in between the two other regimes and covers most of the conditions relevant for pyro-convection. In this regime N CD depends non-linearly on both N CN and w.In sensitivity studies we have tested the influence of aerosol particle size distribution and hygroscopicity on N CD . Within the range of effective hygroscopicity parameters that is characteristic for continental atmospheric aerosols (κ≈0.05-0.6), we found that N CD depends rather weakly on the actual value of κ. A compensation of changes in κ and S max leads to an effective buffering of N CD . Only for aerosols with very low hygroscopicity (κ<0.05) and also in the updraft-limited regime for aerosols with higher than average hygroscopicity (κ>0.3) did the relative sensitivities ∂lnN CD /∂lnκ≈ ( N CD /N CD )/( κ/κ) exceed values of ∼0.2, indicating that a 50% difference in κ would change N CD by more than 10%.The influence of changing size distribution parameters was stronger than that of particle hygroscopicity. Nevertheless, similar regimes of CCN activation were observed in simulations with varying types of size distributions (polluted and pristine continental and marine aerosols with different proportions of nucleation, Aitken, accumulation, and coarse mode particles). In general, the different regimes can be discriminated with regard to the relative sensitivities of N CD against w and N CN (∂lnN CD /∂lnw and ∂lnN CD /∂lnN CN ). We propose to separate the different regimes by rel...

Abstract. We have investigated the formation of cloud droplets under (pyro-)convective conditions using a cloud parcel model with detailed spectral microphysics and with the κ-Köhler model approach for efficient and realistic description of the cloud condensation nucleus (CCN) activity of aerosol particles. Assuming a typical biomass burning aerosol size distribution (accumulation mode centred at 120 nm), we have calculated initial cloud droplet number concentrations (NCD) for a wide range of updraft velocities (w=0.5–20 m s−1) and aerosol particle number concentrations (NCN=103–105 cm−3) at the cloud base. Depending on the ratio between updraft velocity and particle number concentration (w/NCN), we found three distinctly different regimes of CCN activation and cloud droplet formation: 1. An aerosol-limited regime that is characterized by high w/NCN ratios (>≈10−3 m s−1 cm3), high maximum values of water vapour supersaturation (Smax>≈0.5%), and high activated fractions of aerosol particles (NCD/NCN>≈90%). In this regime NCD is directly proportional to NCN and practically independent of w. 2. An updraft-limited regime that is characterized by low w/NCN ratios (

The Midlatitude Cirrus experiment (ML-CIRRUS) deployed the High Altitude and Long Range Research Aircraft (HALO) to obtain new insights into nucleation, life cycle, and climate impact of natural cirrus and aircraft-induced contrail cirrus. Direct observations of cirrus properties and their variability are still incomplete, currently limiting our understanding of the clouds’ impact on climate. Also, dynamical effects on clouds and feedbacks are not adequately represented in today’s weather prediction models. Here, we present the rationale, objectives, and selected scientific highlights of ML-CIRRUS using the G-550 aircraft of the German atmospheric science community. The first combined in situ–remote sensing cloud mission with HALO united state-of-the-art cloud probes, a lidar and novel ice residual, aerosol, trace gas, and radiation instrumentation. The aircraft observations were accompanied by remote sensing from satellite and ground and by numerical simulations. In spring 2014, HALO performed 16 flights above Europe with a focus on anthropogenic contrail cirrus and midlatitude cirrus induced by frontal systems including warm conveyor belts and other dynamical regimes (jet streams, mountain waves, and convection). Highlights from ML-CIRRUS include 1) new observations of microphysical and radiative cirrus properties and their variability in meteorological regimes typical for midlatitudes, 2) insights into occurrence of in situ–formed and lifted liquid-origin cirrus, 3) validation of cloud forecasts and satellite products, 4) assessment of contrail predictability, and 5) direct observations of contrail cirrus and their distinction from natural cirrus. Hence, ML-CIRRUS provides a comprehensive dataset on cirrus in the densely populated European midlatitudes with the scope to enhance our understanding of cirrus clouds and their role for climate and weather