Individual Cloud Particles Research Articles

PHIPS–HALO: the airborne Particle Habit Imaging and Polar Scattering probe – Part 1: Design and operation

Abstract. The number and shape of ice crystals present in mixed-phase and ice clouds influence the radiation properties, precipitation occurrence and lifetime of these clouds. Since clouds play a major role in the climate system, influencing the energy budget by scattering sunlight and absorbing heat radiation from the earth, it is necessary to investigate the optical and microphysical properties of cloud particles particularly in situ. The relationship between the microphysics and the single scattering properties of cloud particles is usually obtained by modelling the optical scattering properties from in situ measurements of ice crystal size distributions. The measured size distribution and the assumed particle shape might be erroneous in case of non-spherical ice particles. There is a demand to obtain both information correspondently and simultaneously for individual cloud particles in their natural environment. For evaluating the average scattering phase function as a function of ice particle habit and crystal complexity, in situ measurements are required. To this end we have developed a novel airborne optical sensor (PHIPS-HALO) to measure the optical properties and the corresponding microphysical parameters of individual cloud particles simultaneously. PHIPS-HALO has been tested in the AIDA cloud simulation chamber and deployed in mountain stations as well as research aircraft (HALO and Polar 6). It is a successive version of the laboratory prototype instrument PHIPS-AIDA. In this paper we present the detailed design of PHIPS-HALO, including the detection mechanism, optical design, mechanical construction and aerodynamic characterization.

The Cloud Particle Spectrometer with Polarization Detection (CPSPD): A next generation open-path cloud probe for distinguishing liquid cloud droplets from ice crystals

The differentiation of small water droplets and ice crystals by in situ measurements, in the size range <50μm, remains a challenge and the lack of such measurements is an obstacle to progress in understanding ice formation in clouds. A new microphysical instrument, the Cloud Particle Spectrometer with Polarization Detection (CPSPD), has been developed that measures light intensity scattered (in forward and backward directions) by individual cloud particles that pass through a focused laser beam and derives their size and thermodynamic phase (liquid or ice) in the optical diameter range from 2 to 50μm. The optical equivalent diameter is derived from the light scattered in the forward direction. The change in polarization state of the incident light, caused by interaction with the cloud particle, is determined from the polarized components of the backscattered light. The CPSPD, along with several other cloud microphysical probes, has been flown on the University of North Dakota Citation aircraft in mixed phase clouds. It has also been deployed and operated at the Zugspitze research station studying mountain clouds. The preliminary results show that liquid cloud droplets can be distinguished from ice crystals and that the ice fraction can be estimated; an important parameter for better understanding of cloud processes, particularly that of glaciation.

IONIZATION IN ATMOSPHERES OF BROWN DWARFS AND EXTRASOLAR PLANETS. III. BREAKDOWN CONDITIONS FOR MINERAL CLOUDS

Electric discharges were detected directly in the cloudy atmospheres of Earth, Jupiter and Saturn, are debatable for Venus, and indirectly inferred for Neptune and Uranus in our solar system. Sprites (and other types of transient luminous events) have been detected only on Earth, and are theoretically predicted for Jupiter, Saturn and Venus. Cloud formation is a common phenomenon in ultra-cool atmospheres such as in Brown Dwarf and extrasolar planetary atmospheres. Cloud particles can be expected to carry considerable charges which may trigger discharge events via small-scale processes between individual cloud particles (intra-cloud discharges) or large-scale processes between clouds (inter-cloud discharges). We investigate electrostatic breakdown characteristics, like critical field strengths and critical charge densities per surface, to demonstrate under which conditions mineral clouds undergo electric discharge events which may trigger or be responsible for sporadic X-ray emission. We apply results from our kinetic dust cloud formation model that is part of the Drift-Phoenix model atmosphere simulations. We present a first investigation of the dependence of the breakdown conditions in Brown Dwarf and giant gas exoplanets on the local gas-phase chemistry, the effective temperature and primordial gas-phase metallicity. Our results suggest that different intra-cloud discharge processes dominate at different heights inside mineral clouds: local coronal (point discharges) and small-scale sparks at the bottom region of the cloud where the gas density is high, and flow discharges and large-scale sparks near, and maybe above, the cloud top. The comparison of the thermal degree of ionisation and the number density of cloud particles allows us to suggest the efficiency with which discharges will occur in planetary atmospheres.

First correlated measurements of the shape and light scattering properties of cloud particles using the new Particle Habit Imaging and Polar Scattering (PHIPS) probe

Abstract. Studying the radiative impact of cirrus clouds requires knowledge of the relationship between their microphysics and the single scattering properties of cloud particles. Usually, this relationship is obtained by modeling the optical scattering properties from in situ measurements of ice crystal size distributions. The measured size distribution and the assumed particle shape might be erroneous in case of non-spherical ice particles. We present here a novel optical sensor (the Particle Habit Imaging and Polar Scattering probe, PHIPS) designed to measure simultaneously the 3-D morphology and the corresponding optical and microphysical parameters of individual cloud particles. Clouds containing particles ranging from a few micrometers to about 800 μm diameter in size can be characterized systematically with an optical resolution power of 2 μm and polar scattering resolution of 1° for forward scattering directions (from 1° to 10°) and 8° for side and backscattering directions (from 18° to 170°). The maximum acquisition rates for scattering phase functions and images are 262 KHz and 10 Hz, respectively. Some preliminary results collected in two ice cloud campaigns conducted in the AIDA cloud simulation chamber are presented. PHIPS showed reliability in operation and produced size distributions and images comparable to those given by other certified cloud particles instruments. A 3-D model of a hexagonal ice plate is constructed and the corresponding scattering phase function is compared to that modeled using the Ray Tracing with Diffraction on Facets (RTDF) program. PHIPS is a highly promising novel airborne optical sensor for studying the radiative impact of cirrus clouds and correlating the particle habit-scattering properties which will serve as a reference for other single, or multi-independent, measurement instruments.

Optical properties of a vertically inhomogeneous mid-latitude mid-level mixed-phase altocumulus in the infrared region

A vertically inhomogeneous mid-latitude mixed-phase altocumulus cloud was observed around 17:26 UTC on Oct. 14, 2001 during the 9th Cloud Layer Experiment (CLEX9). In this study the microphysical and optical properties of this cloud are investigated on the basis of in-situ observed vertical profiles of particle size and habit distributions. Two cloud models, assuming that the cloud properties were vertically homogeneous and inhomogeneous, are adopted to derive the bulk optical and radiative properties of this cloud. The observed microphysical properties are combined with the theoretical solutions to the scattering and absorption properties of individual cloud particles to determine the bulk optical properties at various heights within the cloud layer. The single-scattering properties of spherical liquid water droplets and nonspherical ice crystals are obtained from the Lorenz–Mie theory and an existing database, respectively. The bulk microphysical and optical properties associated with the inhomogeneous model depend strongly on the height above the cloud-base whereas the dependence is smoothed out in the case of the homogeneous model. Furthermore, the transfer of infrared radiation is simulated in conjunction with the two cloud models. It is shown that the brightness temperatures at the top of the atmosphere in the case of the homogeneous model can be 1.5% (3.8 K) higher than their counterpart associated with the inhomogeneous cloud model. This result demonstrates that the effect of the vertical inhomogeneity of a mixed-phase cloud on its radiative properties is not negligible.

An Electrodynamic Levitation System for Studying Individual Cloud Particles under Upper-Tropospheric Conditions

Abstract A laboratory system composed of an electrodynamic levitation cell within an environmental control chamber has been designed and built. The system is ideal for studies of individual particles, such as pure water droplets, aqueous solution droplets, solid salt particles, and ice crystals, under mid- and upper-tropospheric conditions. The experimental system has several features that make it particularly useful for studies of cloud physics. The levitation cell has a cubic geometry with transparent electrodes, thus allowing for full, three-axis positioning of a levitated particle, as well as a large range of viewing angles for optical access and light-scattering measurements. Particles in the approximate diameter range of 10 to 100 μm can be suspended indefinitely with minimal wall influences. The levitation cell is housed within an environmental control chamber capable of operating at temperatures (T), pressures (p), vertical velocities (w), and saturation ratios (with respect to ice, Si) in the ran...

Effects of radiation on clouds

The effects of radiation on the growth of individual cloud particles and on the structure of clouds are reviewed. It is shown that the rate of growth of both water drops and ice crystals may be changed significantly by effects of radiation either near the edges of cloud sheets or below cloud, although the overall effect on particle size depends on the period spent in such regions. Sub-cloud evaporation of ice particles may be reduced by radiative cooling of the crystals giving rise to persistent fall streaks. The role of radiative transfer in the formation of radiation fog is well established. It is shown, however, that the nature of the radiative process is crucial in explaining the observed distribution of liquid water in fogs. The way in which radiation fogs may be dissipated due to changes in the radiation balance are also described. Layer clouds contain regions of radiative flux divergence which give rise to localised heating or cooling. In low-level water clouds and in some higher-level ice clouds this gives rise to destabilisation of the cloud layer leading to the generation of turbulent kinetic energy. This may be enhanced by evaporation of cloud particles into air entrained into the cloud layer. The sensitivity of the cloud structure to the radiative processes may give rise to a pronounced diurnal variation in the structure and to a dependence of the structure on latitude. The turbulent motions generated, in part, by radiative processes feed back on the cloud particle size distribution either by increasing the time spent in the cloud by a small fraction of the particles or by modifying the coupling between those clouds regions in which particles form with those regions where growth occurs. The review brings together theoretical studies and observations. It is concluded that while the effects of radiation on the evolution of stratocumulus cloud sheets are comparatively well understood, some details remain unresolved. However, cirrus clouds are less fully understood owing to the limited observations and the greater coupling between radiative, dynamical and microphysical processes.

On the Measurement of Electric Charge on individual Cloud Particles in Winter Season

On the Measurement of Electric Charge on individual Cloud Particles in Winter Season