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Katrin Lehmann, Holger Siebert, and Raymond A. Shaw

Abstract

The helicopter-borne instrument payload known as the Airborne Cloud Turbulence Observation System (ACTOS) was used to study the entrainment and mixing processes in shallow warm cumulus clouds. The characteristics of the mixing process are determined by the Damköhler number, defined as the ratio of the mixing and a thermodynamic reaction time scale. The definition of the reaction time scale is refined by investigating the relationship between the droplet evaporation time and the phase relaxation time. Following arguments of classical turbulence theory, it is concluded that the description of the mixing process through a single Damköhler number is not sufficient and instead the concept of a transition length scale is introduced. The transition length scale separates the inertial subrange into a range of length scales for which mixing between ambient dry and cloudy air is inhomogeneous, and a range for which the mixing is homogeneous. The new concept is tested on the ACTOS dataset. The effect of entrained subsaturated air on the droplet number size distribution is analyzed using mixing diagrams correlating droplet number concentration and droplet size. The data suggest that homogeneous mixing is more likely to occur in the vicinity of the cloud core, whereas inhomogeneous mixing dominates in more diluted cloud regions. Paluch diagrams are used to support this hypothesis. The observations suggest that homogeneous mixing is favored when the transition length scale exceeds approximately 10 cm. Evidence was found that suggests that under certain conditions mixing can lead to enhanced droplet growth such that the largest droplets are found in the most diluted cloud regions.

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Holger Siebert, Katrin Lehmann, and Raymond A. Shaw

Abstract

The use of a hot-wire anemometer for high-resolution turbulence measurements in a two-phase flow (e.g., atmospheric clouds) is discussed. Experiments in a small wind tunnel (diameter of 0.2 and 2 m in length) with a mean flow velocity in the range between 5 and 16 m s−1 are performed. In the wind tunnel a spray with a liquid water content of 0.5 and 2.5 g m−3 is generated. After applying a simple despiking algorithm, power spectral analysis shows the same results as spectra observed without spray under similar flow conditions. The flattening of the spectrum at higher frequencies due to impacting droplets could be reduced significantly. The time of the signal response of the hot wire to impacting droplets is theoretically estimated and compared with observations. Estimating the fraction of time during which the velocity signal is influenced by droplet spikes, it turns out that the product of liquid water content and mean flow velocity should be minimized. This implies that for turbulence measurements in atmospheric clouds, a slowly flying platform such as a balloon or helicopter is the appropriate instrumental carrier. Examples of hot-wire anemometer measurements with the helicopter-borne Airborne Cloud Turbulence Observation System (ACTOS) are presented.

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Sebastian Schmidt, Katrin Lehmann, and Manfred Wendisch

Abstract

A modified version of the Fast-FSSP (the so-called M-Fast-FSSP) is introduced. It allows minimization of the instrumental broadening of measured cloud drop size distributions caused by laser beam inhomogeneities. This is achieved by applying a new technique based on a postexperiment stepwise reduction of the probe's sampling volume. For monodisperse glass bead samples it is shown that the width of the measured size distribution is considerably reduced when applying this technique, especially for large glass bead diameters. The instrumental broadening may exceed a factor of about 4 for a mean glass bead diameter of 30 μm. The M-Fast-FSSP was applied in two cloud measurement campaigns. For two specific cloud cases, the profile of the width of the measured drop size distribution changes significantly when applying the method.

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Holger Siebert, Katrin Lehmann, and Manfred Wendisch

Abstract

Tethered balloon–borne measurements with a resolution in the order of 10 cm in a cloudy boundary layer are presented. Two examples sampled under different conditions concerning the clouds' stage of life are discussed. The hypothesis tested here is that basic ideas of classical turbulence theory in boundary layer clouds are valid even to the decimeter scale. Power spectral densities S( f ) of air temperature, liquid water content, and wind velocity components show an inertial subrange behavior down to ≈20 cm. The mean energy dissipation rates are ∼10−3 m2 s−3 for both datasets. Estimated Taylor Reynolds numbers (Reλ) are ∼104, which indicates the turbulence is fully developed. The ratios between longitudinal and transversal S( f ) converge to a value close to 4/3, which is predicted by classical turbulence theory for local isotropic conditions. Probability density functions (PDFs) of wind velocity increments Δu are derived. The PDFs show significant deviations from a Gaussian distribution with longer tails typical for an intermittent flow. Local energy dissipation rates ετ are derived from subsequences with a duration of τ = 1 s. With a mean horizontal wind velocity of 8 m s−1, τ corresponds to a spatial scale of 8 m. The PDFs of ετ can be well approximated with a lognormal distribution that agrees with classical theory. Maximum values of ετ ≈ 10−1 m2 s−3 are found in the analyzed clouds. The consequences of this wide range of ετ values for particle–turbulence interaction are discussed.

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Holger Siebert, Harald Franke, Katrin Lehmann, Rolf Maser, Ewe Wei Saw, Dieter Schell, Raymond A. Shaw, and Manfred Wendisch

Helicopter-based measurements provide an opportunity for probing the finescale dynamics and microphysics of clouds simultaneously in space and time. Due to the low true air speed compared with research aircraft, a helicopter allows for measurements with much higher spatial resolution. To circumvent the influence of the helicopter downwash the autonomous measurement payload Airborne Cloud Turbulence Observation System (ACTOS) is carried as an external cargo 140 m below the helicopter. ACTOS allows for collocated measurements of the dynamical and cloud microphysical parameters with a spatial resolution of better than 10 cm.

The interaction between turbulence and cloud microphysical processes is demonstrated using the following two cloud cases from recent helicopter measurements: i) a cumulus cloud with a low degree of turbulence and without strong vertical dynamics, and, in contrast, ii) an actively growing cloud with increased turbulence and stronger updrafts. The turbulence and microphysical measurements suggest that entrainment at the tops of these two clouds occurs by inhomogeneous and homogeneous mixing, respectively.

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