Evidence for liquid-phase cirrus cloud formation from volcanic aerosols: climatic implications.

Supercooled droplets in cirrus uncinus cell heads between -40 degrees and -50 degrees C are identified from Project FIRE [First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment] polarization lidar measurements. Although short-lived, complexes of these small liquid cells seem to have contributed importantly to the formation of the cirrus. Freezing-point depression effects in solution droplets, apparently resulting from relatively large cloud condensation nuclei of volcanic origin, can be used to explain this rare phenomenon. An unrecognized volcano-cirrus cloud climate feedback mechanism is implied by these findings.

Highly supercooled cirrus cloud water: confirmation and climatic implications.

Liquid cloud droplets supercooled to temperatures approaching -40 degrees C have been detected at the base of a cirrostratus cloud through a combination of ground-based, polarization laser radar (lidar) and in situ aircraft measurements, Solar and thermal infrared radiative budget calculations based on these observatoins indicate that significant changes in the atmospheric heating distribution and the surface radiative budget may be attributed to liquid layers in cirrus clouds.

Formation of low-temperature cirrus from H2SO4/H2O aerosol droplets.

We present experimental results obtained with a differential scanning calorimeter (DSC) that indicate the small ice particles in low-temperature cirrus clouds are not completely solid but rather coated with an unfrozen H2SO4/H2O overlayer. Our results provide a new look on the formation, development, and microphysical properties of low-temperature cirrus clouds.

Possible halo depictions in the prehistoric rock art of Utah.

In western American rock art the concentric circle symbol, which is widely regarded as a sun symbol, is ubiquitous. We provide evidence from Archaic and Fremont Indian rock art sites in northwestern Utah that at least one depiction was motivated by an observation of a complex halo display. Cirrus cloud optical displays are linked in both folklore and meteorology to precipitation-producing weather situations, which, in combination with an abundance of weather-related rock art symbolism, indicate that such images reflected the ceremonial concerns of the indigenous cultures for ensuring adequate precipitation. As has been shown to be the case with rock art rainbows, conventionalization of the halo image may have resulted in simple patterns that lacked recognizable details of atmospheric optical phenomena. However, in one case in which an Archaic-style petroglyph (probably 1500 yr or more old) satisfactorily reproduced a complicated halo display that contained parhelia and tangent arcs, sufficient geometricinformation is rendered to indicate a solar elevation angle of ~ 40° at the time of observation.

Optical backscattering from near-spherical water, ice, and mixed phase drops.

An experimental assessment of the scattering behavior of freely falling artificial raindrops and mechanically suspended drops in the ice and mixed phase has been undertaken with a device which simultaneously measures the parallel and cross polarized components of backscattered linearly polarized laser light (6328 A). Among the findings are that linear depolarization ratios (delta) are generally <0.01 for raindrops up to nearly 6-mm diam, near 0.5 for regularly shaped frozen drops, and between 0.35 and 1.0 for more irregular ice particles. Anomalous scattering behavior has been observed during the liquid to solid drop phase transition (delta > 1.0) and in the relatively great amounts of parallel polarized energy returned from raindrops >~4 mm. Backscattered signal variations produced during drop melting reveal that delta values tend to remain near the initial ice value until most of the ice has changed phase. The details of the variations aid in the determination of the dominant scattering mechanisms responsible for the b ckscatter from large, near-spherical particles. The results are shown to have some bearing on measurements of atmospheric hydrometeors obtained by lidar.

Backscattering cross sections for hydrometeors: measurements at 6328 A.

Reported are estimates of backscattering cross sections and other scattering parameters determined experimentally at 6328 A for major atmospheric hydrometeor types.

Infrared (10.6-mum) scattering and extinction in laboratory water and ice clouds.

Measurements of the angular scattering and extinction of IR (10.6-mum) laser radiation in laboratory water and ice clouds are reported and compared to theoretical predictions for spheres and visible (0.633-mum) light scattering data. Randomly oriented cloud particles with dimensions ranging from several times smaller to larger than the incident wavelength generated phase functions span the Rayleigh and Mie scattering domains and illustrate the effects caused by strong internal energy absorption. Dual-wavelength extinction measurements reveal information on the growth and dissipation of laboratory water clouds and the effects of cloud seeding. The remote sensing significance of the findings is discussed.

Corona-producing cirrus cloud properties derived from polarization lidar and photographic analyses.

Polarization lidar data are used to demonstrate that clouds composed of hexagonal ice crystals can generate multiple-ringed colored coronas. Although relatively uncommon in our mid-latitude cirrus sample (derived from Project FIRE extended time observations), the coronas are associated with unusual cloud conditions that appear to be effective in generating the displays. Invariably, the cirrus cloud tops are located at or slightly above elevated tropopauses (12.7-km MSL average height) at temperatures between -60 degrees and -70 degrees C. The cloud top region also generates relatively strong laser backscattering and unusually high 0.5-0.7 linear depolarization ratios. Color photograph analysis of corona ring angles indicates crystals with mean diameters of from 12 to 30 microm. The cirrus cloud types were mainly subvisual to thin (i.e., bluish-colored) cirrostratus, but also included fibrous cirrus. Estimated cloud optical thicknesses at the 0.694-microm laser wavelength ranged from 0.001 to 0.2, where the upper limit reflects the effects of multiple scattering and/or unfavorable changes in particle characteristics in deep cirrus clouds.

Rainbows in the Indian rock art of desert western America.

For thousands of years the image of the rainbow was pecked and painted by native Americans onto the rocks of the Great Basin and the Southwest. This long-lived tradition, which transcended major developments in lifestyles and cultures, underscores the important symbolic significance of the rainbow to the inhabitants of this arid region. The rainbow rock art depictions were usually associated with humanlike ceremonial figures, snakes, clouds, rain, and lightning bolts, suggesting that the rainbow symbol was employed as part of an elaborate sacred tradition. Although such ceremonial usage of the rainbow image tends to lead to abstraction and symbolic representation, there are examples, including a properly colorized rainbow painting from central Utah (approximately a thousand years old), that indicate observationally based rainbow reproductions of relatively great antiquity.

Parry arc: a polarization lidar, ray-tracing, and aircraft case study.

Using simple ray-tracing simulations, the cause of the rare Parry arc has been linked historically to horizontally oriented columns that display the peculiar ability to fall with a pair of prism faces closely parallel to the ground. Although we understand the aerodynamic forces that orient the long-column axis in the horizontal plane, which gives rise to the relatively common tangent arcs of the 22 degrees halo, the mechanism leading to the Parry crystal orientation has never been resolved adequately. On 16 November 1998, at the University of Utah Facility for Atmospheric Remote Sensing, we studied a cirrus cloud producing a classic upper Parry arc using polarization lidar and an aircraft with a new high-resolution ice crystal imaging probe. Scanning lidar data, which reveal extremely high linear depolarization ratios delta a few degrees off the zenith direction, are simulated with ray-tracing theory to determine the ice crystal properties that reproduce this previously unknown behavior. It is found that a limited range of thick-plate crystal axis (length-to-diameter) ratios from approximately 0.75 to 0.93 generates a maximum delta approximately 2.0-5.0 for vertically polarized 0.532-microm light when the lidar is tilted 1 degrees -2 degrees off the zenith. Halo simulations based on these crystal properties also generate a Parry arc. However, although such particles are abundant in the in situ data in the height interval indicated by the lidar, one still has to invoke an aerodynamic stabilization force to produce properly oriented particles. Although we speculate on a possible mechanism, further research is needed into this new explanation for the Parry arc.

Lidar crossover function and misalignment effects.

The lidar crossover function f(R) in the lidar equation accounts for the incomplete overlap of the laser-pulse volume and the receiver field of view at short ranges and so is pertinent to near-surface returns from aerosols and precipitation. Using a Gaussian-intensity transverse profile for the laser pulse we present a comparatively simple formulation for f(R) and provide some numerical results for realistic lidar geometries, including the effects of optical axes misalignment. It is shown that for lidar systems constrained to narrow beam-widths (approximately 1 mrad) for polarization or other observations, accurate alignment is of great importance to lidar operations.

Light and color in the open air: introduction to the feature issue.

The topical meeting on light and color in the open air was held 9-12 February 1997 in Santa Fe, New Mexico. The series of papers that follows represents the fruition of this meeting, revealing the range of current scientific explorations into the play of light and color in nature.

Backscatter laser depolarization studies of simulated stratospheric aerosols: crystallized sulfuric acid droplets.

The optical depolarizing properties of simulated stratospheric aerosols were studied in laboratory laser (0.633 microm) backscattering experiments for application to polarization lidar observations. Clouds composed of sulfuric acid solution droplets, some treated with ammonia gas, were observed during evaporation. The results indicate that the formation of minute ammonium sulfate particles from the evaporation of acid droplets produces linear depolarization ratios of delta approximately 0.02, but delta approximately 0.10-0.15 are generated from acid droplet crystallization effects associated with recycled aerosols and the introduction of ammonia gas into the chamber. It is concluded that partially crystallized sulfuric acid droplets are a likely candidate for explaining the lidar delta approximately 0.10 values that have been observed in the lower stratosphere in the absence of the relatively strong backscattering from homogeneous sulfuric acid droplet (delta approximately 0) or ice crystal (delta approximately 0.5) clouds.

Simulated polarization diversity lidar returns from water and precipitating mixed phase clouds.

The dependence of polarization lidar returns on basic microphysical and thermodynamic variables is assessed by using a cloud model to simulate the growth of water and mixed (water and ice) phase clouds. Cloud contents that evolve with height in updrafts are converted, by using Mie theory, into cloud droplet single and double backscattering and attenuation coefficients. The lidar equation includes forward multiple scattering attenuation corrections based on diffraction theory for droplets and ice crystals, whose relative scattering contributions are treated empirically. Lidar depolarization is computed from droplet and crystal single scattering and an analytical treatment of droplet double scattering. Water cloud results reveal the expected increases in linear depolarization ratios (delta) with increasing lidar field of view and distance to cloud but also show that depolarization is a function of cloud liquid water content, which depends primarily on temperature. Ice crystals modulate mixed phase cloud liquid water contents through water vapor competition effects, thereby affecting multiple scattering delta values as functions of updraft velocity, temperature, and crystal size and concentration. Although the minimum delta at cloud base increases with increasing ice content, the peak measurable delta in the cloud decreases. Comparison with field data demonstrate that this modeling approach is a valuable supplement to cloud measurements.

Effects of ice-crystal structure on halo formation: cirrus cloud experimental and ray-tracing modeling studies.

During the 1986 Project FIRE (First International Satellite Cloud Climatology Project Regional Experiment) field campaign, four 22° halo-producing cirrus clouds were studied jointly from a groundbased polarization lidar and an instrumented aircraft. The lidar data show the vertical cloud structure and the relative position of the aircraft, which collected a total of 84 slides by impaction, preserving the ice crystals for later microscopic examination. Although many particles were too fragile to survive impaction intact, a large fraction of the identifiable crystals were columns and radial bullet rosettes, with both displaying internal cavitations, and radial plate-column combinations. Particles that were solid or displayed only a slight amount of internal structure were relatively rare, which shows that the usual model postulated by halo theorists, i.e., the randomly oriented, solid hexagonal crystal, is inappropriate for typical cirrus clouds. With the aid of new ray-tracing simulations for hexagonal hollow ended column and bullet-rosette models, we evaluate the effects of more realistic ice-crystal structures on halo formation and lidar depolarization and consider why the common halo is not more common in cirrus clouds.

Volcanic Bishop's ring: evidence for a sulfuric acid tetrahydrate particle aureole.

Following the massive 1883 Krakatoa volcanic eruption, a new atmospheric optical phenomeon was identified by Rev. S. E. Bishop. This inconspicuous one-ringed corona, or aureole, was immediately linked to the global spread of volcanic debris injected into the stratosphere, but little refinement in the mechanisms responsible for Bishop's ring has since been made. On the basis of our combined studies of sulfuric acid droplet-freezing theory and polarization (0.694-µm) lidar measurements of Bishop's ring aerosols from the June 1991 Mt. Pinatubo eruption that show average linear depolarization ratios of ;~0.05, it appears that this solar diffraction phenomenon is caused by accumulations of nonspherical sulfuric acid tetrahydrate (SAT) particles. The diffraction-theory aureole-derived SAT particle radius of ~0.8 µm is consistent with the freezing of the large mode of volcanic acid droplets created by coagulation, which, according to theory, is necessary for concentrating a sufficient insoluble mass to promote het rogeneous drop freezing at temperatures below approximately -65 °C.

Can cirrus clouds produce glories?

A vague glory display was photographed over central Utah from an airplane beginning its descent through a cirrus cloud layer with an estimated cloud top temperature of -45 and -55 degrees C. Photographic analysis reveals a single reddish-brown ring of 2.5-3.0 degrees radius around the antisolar point, although a second ring appeared visually to have been present over the brief observation period. Mie and approximate nonspherical theory scattering simulations predict a population of particles with modal diameters between 9 and 15 mum. Although it is concluded that multiple-ringed glories can be accounted for only through the backscattering of light from particles that are strictly spherical in shape, the poor glory colorization in this case could imply the presence of slightly aspherical ice particles. The location of this display over mountainous terrain suggests that it was generated by an orographic wave cloud, which we speculate produced numerous frozen cloud droplets that only gradually took on crystalline characteristics during growth.

Corona-producing ice clouds: a case study of a cold mid-latitude cirrus layer.

A high (14.0-km), cold (-71.0 degrees C) cirrus cloud was studied by ground-based polarization lidar and millimeter radar and aircraft probes on the night of 19 April 1994 from the Cloud and Radiation Testbed site in northern Oklahoma. A rare cirrus cloud lunar corona was generated by this 1-2-km-deep cloud, thus providing an opportunity to measure the composition in situ, which had previously been assumed only on the basis of lidar depolarization data and simple diffraction theory for spheres. In this case, corona ring analysis indicated an effective particle diameter of ~22 mum. A variety of in situ data corroborates the approximate ice-particle size derived from the passive retrieval method, especially near the cloud top, where impacted cloud samples show simple solid crystals. The homogeneous freezing of sulfuric acid droplets of stratospheric origin is assumed to be the dominant ice-particle nucleation mode acting in corona-producing cirrus clouds. It is speculated that this process results in a previously unrecognized mode of acid-contaminated ice-particle growth and that such small-particle cold cirrus clouds are potentially a radiatively distinct type of cloud.