The design, laboratory calibrations, and flight tests of a new optical imaging instrument, the two-dimensional stereo (2D-S) probe, are presented. Two orthogonal laser beams cross in the middle of the sample volume. Custom, high-speed, 128-photodiode linear arrays and electronics produce shadowgraph images with true 10-μm pixel resolution at aircraft speeds up to 250 m s−1. An overlap region is defined by the two laser beams, improving the sample volume boundaries and sizing of small (<∼100 μm) particles, compared to conventional optical array probes. The stereo views of particles in the overlap region can also improve determination of three-dimensional properties of some particles. Data collected by three research aircraft are examined and discussed. The 2D-S sees fine details of ice crystals and small water drops coexisting in mixed-phase cloud. Measurements in warm cumuli collected by the NCAR C-130 during the Rain in Cumulus over the Ocean (RICO) project provide a test bed to compare the 2D-S with 2D cloud (2D-C) and 260X probes. The 2D-S sees thousands of cloud drops <∼150 μm when the 2D-C and 260X probes see few or none. The data suggest that particle images and size distributions ranging from 25 to ∼150 μm and collected at airspeeds >100 m s−1 by the 2D-C and 260X probes are probably (erroneously) generated from out-of-focus particles. Development of the 2D-S is in its infancy, and much work needs to be done to quantify its performance and generate software to analyze data.

Abstract. In situ measurements of ice crystal concentrations and sizes made with aircraft instrumentation over the past two decades have often indicated the presence of numerous relatively small (< 50 µm diameter) crystals in cirrus clouds. Further, these measurements frequently indicate that small crystals account for a large fraction of the extinction in cirrus clouds. The fact that the instruments used to make these measurements, such as the Forward Scattering Spectrometer Probe (FSSP) and the Cloud Aerosol Spectrometer (CAS), ingest ice crystals into the sample volume through inlets has led to suspicion that the indications of numerous small-crystals could be artifacts of large-crystal shattering on the instrument inlets. We present new aircraft measurements in anvil cirrus sampled during the Tropical Composition, Cloud, and Climate Coupling (TC4) campaign with the 2-Dimensional Stereo (2D-S) probe, which detects particles as small as 10 µm. The 2D-S has detector "arms" instead of an inlet tube. Since the 2D-S probe surfaces are much further from the sample volume than is the case for the instruments with inlets, it is expected that 2D-S will be less susceptible to shattering artifacts. In addition, particle interarrival times are used to identify and remove shattering artifacts that occur even with the 2D-S probe. The number of shattering artifacts identified by the 2D-S interarrival time analysis ranges from a negligible contribution to an order of magnitude or more enhancement in apparent ice concentration over the natural ice concentration, depending on the abundance of large crystals and the natural small-crystal concentration. The 2D-S measurements in tropical anvil cirrus suggest that natural small-crystal concentrations are typically one to two orders of magnitude lower than those inferred from CAS. The strong correlation between the CAS/2D-S raCorrespondence to: E. J. Jensen (eric.j.jensen@nasa.gov) tio of small-crystal concentrations and large-crystal concentration suggests that the discrepancy is likely caused by shattering of large crystals on the CAS inlet. We argue that past measurements with CAS in cirrus with large crystals present may contain errors due to crystal shattering, and past conclusions derived from these measurements may need to be revisited. Further, we present correlations between CAS spurious concentration and 2D-S large-crystal mass from spatially uniform anvil cirrus sampling periods as an approximate guide for estimating quantitative impact of large-crystal shattering on CAS concentrations in previous datasets. We use radiative transfer calculations to demonstrate that in the maritime anvil cirrus sampled during TC4, small crystals indicated by 2D-S contribute relatively little cloud extinction, radiative forcing, or radiative heating in the anvils, regardless of anvil age or vertical location in the clouds. While 2D-S ice concentrations in fresh anvil cirrus may often exceed 1 cm −3 , and are observed to exceed 10 cm −3 in turrets, they are typically 0.1 cm −3 and rarely ex...

[1] The size, shape and concentration of ice particles in tropical anvil cirrus and in situ cirrus clouds have a significant impact on cloud radiative forcing, and hence on global climate change. Data collected in tropical anvil and cirrus clouds with a 2D-S probe, an optical imaging probe with improved response characteristics and the ability to remove shattered artifacts, are analyzed and discussed. The data were collected with NASA DC-8 and WB-57F research aircraft near Costa , ice water contents exceeding 2 g m −3 and total particle concentrations exceeding 10 cmat 12.2 km. Ice water contents in the anvils declined outward from the center of convection, decreasing to <0.1 g m −3 in aged anvil cirrus. The data show that microphysical and radiative properties of both tropical anvils and cirrus are most strongly influenced by ice particles in the size range from about 100 to 400 mm. This is contrary to several previous investigations that have suggested that ice particles less than about 50 mm control radiative properties in anvils and cirrus. The 2D-S particle area and mass size distributions, plus information on particle shape, are input into an optical properties routine that computes cloud extinction, asymmetry parameter and single scattering albedo. These optical properties are then input into two-stream radiative code to compute radiative heating profiles within the various cloud types. The results produce short-and long-wave heating/cooling vertical profiles in these tropical clouds. A simple parameterization based on 2D-S measurements is derived from the particle mass size distribution that yields an area size distribution. The parameterized area size distribution can then be used in large-scale numerical simulations that include radiative transfer packages.

Abstract. Subvisible cirrus (SVC) clouds are often observed within the tropical tropopause layer (TTL). Some studies suggest that SVC has a significant impact on the earth radiation budget. The Costa Rica Aura Validation Experiment (CR-AVE) sponsored by the National Aeronautics and Space Administration (NASA) took place near San Jose, Costa Rica from 14 January–15 February 2006. The NASA WB-57F sampled SVC in the TTL from −75°C to −90°C with an improved set of cloud particle probes. The first digital images of ice particles in the TTL are compared with replicator images of ice particles collected in 1973 by a WB-57F in the TTL. The newer measurements reveal larger particles, on the order of 100 μm compared with <50 μm from the earlier measurements, and also different particle shapes. The 1973 particles were mainly columnar and trigonal, whereas the newer measurements are quasi-spherical and hexagonal plates. The WB-57F also measured very high water vapor contents with some instruments, up to 4 ppmv, and aerosols with mixed organics and sulfates. It is unknown whether these ambient conditions were present in the 1973 studies, and whether such conditions have an influence on particle shape and the development of the large particles. A companion paper (Jensen et al., 2008) presents crystal growth calculations that suggest that the high water vapor measurements are required to grow ice particles to the observed sizes of 100 μm and larger.

In early February 2001 (during the austral summer), over 900 000 digital images of ice crystals were recorded at the South Pole using two ground-based cloud particle imagers (CPIs). Of these, 721 572 crystals Ͼ50 m were classified into crystal habits. When sorted by number, 30% of the crystals were rosette shaped (mixed-habit rosettes, platelike polycrystals, and rosette shapes with side planes), 45% were diamond dust (columns, thick plates, and plates), and 25% were irregular. When sorted by area, rosette shapes comprised 50%, diamond dust 30% and irregular 20%. By mass, the percentages were 57% rosette shapes, 23% diamond dust, and 20% irregular. Particle size distributions as a function of maximum dimension and equivalent radius are compared with previous studies. Particles are generally found to be slightly larger than previous austral wintertime studies. In 2002, a polar nephelometer (PN) that measures scattering phase function was incorporated with one of the CPIs. Correlated measurements between the two instruments showed that 22°and 46°peaks in the phase function were present when diamond dust was recorded by the CPI, but not when rosette shapes were present. Visual observations confirmed the presence of 22°and 46°a tmospheric halos in some, but not all, of the diamond dust events. No visual halos were observed when rosette shapes were precipitating. Average PN phase functions are presented for diamond dust and rosette shapes. The diamond dust and rosette-shaped ice crystals appear to be very similar in shape to those observed by CPIs in cirrus clouds. Cloud conditions at the South Pole that were associated with various crystal types are discussed, as are some effects of blowing snow.