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Schmeissner T.,Leibniz Institute for Tropospheric Research | Shaw R.A.,Michigan Technological University | Ditas J.,Max Planck Institute for Chemistry | Stratmann F.,Leibniz Institute for Tropospheric Research | And 2 more authors.
Journal of the Atmospheric Sciences | Year: 2015

Helicopter-borne observations of the impact of turbulent mixing and cloud microphysical properties in shallow trade wind cumuli are presented. The measurements were collected during the Cloud, Aerosol, Radiation and Turbulence in the Trade Wind Regime over Barbados (CARRIBA) project. Basic meteorological parameters (3D wind vector, air temperature, and relative humidity), cloud condensation nuclei concentrations, and cloud microphysical parameters (droplet number, size distribution, and liquid water content) are measured by the Airborne Cloud Turbulence Observation System (ACTOS), which is fixed by a 160-m-long rope underneath a helicopter flying with a true airspeed of approximately 20 m s-1. Clouds at different evolutionary stages were sampled. A total of 300 clouds are classified into actively growing, decelerated, and dissolving clouds. The mixing process of these cloud categories is investigated by correlating the cloud droplet number concentration and cubed droplet mean volume diameter. A significant tendency to more inhomogeneous mixing with increasing cloud lifetime is observed. Furthermore, the mixing process and its effects on droplet number concentration, droplet size, and cloud liquid water content are statistically evaluated. It is found that, in dissolving clouds, liquid water content and droplet number concentration are decreased by about 50% compared to actively growing clouds. Conversely, the droplet size remains almost constant, which can be attributed to the existence of a humid shell around the cloud that prevents cloud droplets from rapid evaporation after entrainment of premoistened air. Moreover, signs of secondary activation are found, which results in a more difficult interpretation of observed mixing diagrams. © 2015 American Meteorological Society. Source

Wehner B.,Leibniz Institute for Tropospheric Research | Werner F.,Leipzig Institute for Meteorology | Werner F.,Joint Center for Earth Systems Technology | Ditas F.,Leibniz Institute for Tropospheric Research | And 4 more authors.
Atmospheric Chemistry and Physics | Year: 2015

During the CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over Barbados) campaign, the interaction between aerosol particles and cloud microphysical properties was investigated in detail, which also includes the influence of clouds on the aerosol formation. During two intensive campaigns in 2010 and 2011, helicopter-borne measurement flights were performed to investigate the thermodynamic, turbulent, microphysical, and radiative properties of trade-wind cumuli over Barbados. During these flights, 91 cases with increased aerosol particle number concentrations near clouds were detected. The majority of these cases are also correlated with enhanced irradiance in the ultraviolet (UV) spectral wavelength range. This enhancement reaches values up to a factor of 3.3 greater compared to background values. Thus, cloud boundaries provide a perfect environment for the production of precursor gases for new particle formation. Another feature of cloud edges is an increased turbulence, which may also enhance nucleation and particle growth. The observed events have a mean length of 100 m, corresponding to a lifetime of less than 300 s. This implies that particles with diameters of at least 7 nm grew several nanometers per minute, which corresponds to the upper end of values in the literature (Kulmala et al., 2004). Such high values cannot be explained by sulfuric acid alone; thus extremely low volatility organic compounds (ELVOCs) are probably involved here. © 2015 Author(s). Source

Merk D.,Leibniz Institute for Tropospheric Research | Deneke H.,Leibniz Institute for Tropospheric Research | Pospichal B.,Leipzig Institute for Meteorology | Seifert P.,Leibniz Institute for Tropospheric Research
Atmospheric Chemistry and Physics | Year: 2016

Cloud properties from both ground-based as well as from geostationary passive satellite observations have been used previously for diagnosing aerosol-cloud interactions. In this investigation, a 2-year data set together with four selected case studies are analyzed with the aim of evaluating the consistency and limitations of current ground-based and satellite-retrieved cloud property data sets. The typically applied adiabatic cloud profile is modified using a sub-adiabatic factor to account for entrainment within the cloud. Based on the adiabatic factor obtained from the combination of ground-based cloud radar, ceilometer and microwave radiometer, we demonstrate that neither the assumption of a completely adiabatic cloud nor the assumption of a constant sub-adiabatic factor is fulfilled (mean adiabatic factor 0.63±0.22). As cloud adiabaticity is required to estimate the cloud droplet number concentration but is not available from passive satellite observations, an independent method to estimate the adiabatic factor, and thus the influence of mixing, would be highly desirable for global-scale analyses. Considering the radiative effect of a cloud described by the sub-adiabatic model, we focus on cloud optical depth and its sensitivities. Ground-based estimates are here compared vs. cloud optical depth retrieved from the Meteosat SEVIRI satellite instrument resulting in a bias of -4 and a root mean square difference of 16. While a synergistic approach based on the combination of ceilometer, cloud radar and microwave radiometer enables an estimate of the cloud droplet concentration, it is highly sensitive to radar calibration and to assumptions about the moments of the droplet size distribution. Similarly, satellite-based estimates of cloud droplet concentration are uncertain. We conclude that neither the ground-based nor satellite-based cloud retrievals applied here allow a robust estimate of cloud droplet concentration, which complicates its use for the study of aerosol-cloud interactions. © Author(s) 2016. Source

Dipu S.,Indian Institute of Tropical Meteorology | Dipu S.,Leipzig Institute for Meteorology | Pandithurai G.,Indian Institute of Tropical Meteorology | Panicker A.S.,Indian Institute of Tropical Meteorology | And 3 more authors.
Advances in Meteorology | Year: 2014

Ground-based network of cloud measurements is presently limited and there exists uncertainty in the cloud microphysical parameters derived from ground-based measurements. Bias in the i-skyradiometer derived cloud optical depth (τ c) and droplet effective radius (R eff) and the importance of these parameters in the parameterization of clouds in climate models have made us intend to develop a possible method for improving these parameters. A new combination method, which uses zenith sky transmittance and surface radiation measurements, has been proposed in the present study to improve the retrievals. The i-skyradiometer derived parameters τ c and R eff have been provided as a first guess to a radiative transfer model (SBDART) and a new retrieval algorithm has been implemented to obtain the best combination of τ c and R eff having minimum bias (-0.09 and -2.5) between the simulated global and diffuse fluxes at the surface with the collocated surface radiation measurements. The new retrieval method has improved τ c and R eff values compared to those derived using the transmittance only method and are in good agreement with the MODIS satellite retrievals. The study therefore suggests a possible improvement of the i-skyradiometer derived cloud parameters using observed radiation fluxes and a radiative transfer model. © 2014 S. Dipu et al. Source

Henrich F.,Johannes Gutenberg University Mainz | Henrich F.,Leipzig Institute for Meteorology | Siebert H.,Leibniz Institute for Tropospheric Research | Jakel E.,Johannes Gutenberg University Mainz | And 4 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2010

First data from collocated, helicopter-based measurements of boundary layer cloud microphysical properties (effective droplet radius Reff, droplet number concentration N) and spectral radiative quantities (cloud optical thickness τ, cloud top albedo ρ, reflectivity ℛ) are presented. The in situ measurements of the microphysical cloud properties were collected by the Airborne Cloud Turbulence Observation System (ACTOS) attached to a helicopter by a 145 m long rope. Cloud spectral reflectivity was derived from radiances measured by grating spectrometers combined with downward looking optical inlets installed underneath the helicopter. Correlations between cloud microphysics and reflected radiation are presented for two cloud cases with different optical and geometrical thicknesses. On the basis of common retrieval techniques, τ and Reff are derived using a radiative transfer model. The results of the retrieval are compared to the collocated in situ measurements and data from the Moderate-Resolution Imaging Spectroradiometer (MODIS). Within the limitations of the relatively small data set, the feasibility of closely collocated microphysics and radiation data and their benefits were demonstrated. Copyright 2010 by the American Geophysical Union. Source

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