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Title:Assessing the dependence of bulk ice properties from probes with anti-shatter tips on environmental conditions
Author(s):Jackson, Robert
Director of Research:McFarquhar, Greg M.
Doctoral Committee Chair(s):McFarquhar, Greg M.
Doctoral Committee Member(s):Rauber, Robert; Stith, Jeff; Lasher-Trapp, Sonia
Department / Program:Atmospheric Sciences
Discipline:Atmospheric Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):optical array probes
aircraft measurements
cirrus clouds
Abstract:Ice clouds have significant impacts on the Earth’s radiative budget. Their radiative impact highly depends on ice cloud microphysical properties. Climate and weather prediction models have to make certain assumptions about how the various processes are represented. Observations of how cloud properties vary with environmental conditions can help evaluate some parameterizations used in models. However, sufficient data are not available to characterize how ice crystal properties vary as a function of environmental conditions. Furthermore, many of these assumptions are derived from historical datasets collected by in situ probes, namely optical array probes that can be contaminated by shattered artifacts generated by large particles shattering on the probe tips and inlets. Therefore this study has two main objectives. Prior estimates of ice crystal size distributions derived from 2D Cloud Probes (2DCs) have been artificially amplified by small ice crystals generated from the shattering of large ice crystals on the probe tips. Although anti-shatter tips and algorithms exist, there is considerable uncertainty in their effectiveness. Therefore, this thesis first examines the differences in ice crystal size distributions, and bulk and optical properties from adjacent 2DCs with standard and anti-shatter tips, and processed with and without anti-shattering algorithms. The measurements were obtained from the National Research Council of Canada Convair-580 during the 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC) and the National Center for Atmospheric Research C-130 during the 2011 Instrumentation Development and Education in Airborne Science 2011 (IDEAS-2011). The 2DC size distributions are compared with those from the Holographic Detector for Clouds (HOLODEC), which has anti-shatter tips and allows for identification of shattering through spatial statistics. The ratio of the number concentration N of particles with maximum dimensions 125 to 500 µm from the 2DC with standard tips to that from the 2DC with modified tips was correlated with median mass diameter and perimeter divided by area, but not with air speed, attack and attitude angles. Anti-shatter tips and algorithms reduced N by up to a factor of 10 for IDEAS-2011 and ISDAC, but neither alone removed all artifacts. For the period with coincident data, N from the HOLODEC and 2DC with modified tips are both around 5 x 10-3 L-1 µm-1, suggesting that anti-shatter tips and algorithms combined remove artifacts from the 2DC for the conditions sampled during IDEAS-2011. To assess the applicability of 2DC data obtained without anti-shatter tips previously used in parameterization schemes for numerical models and remote sensing retrievals, the impacts of artifacts on bulk microphysical and scattering properties were examined by quantifying differences between such properties derived from 2DCs with standard and anti-shatter tips, and with and without the use of shatter detection algorithms using the ISDAC and IDEAS-2011 data. Using either modified tips or algorithms changed quantities dominated by higher order moments such as ice water content, bulk extinction, effective radius, mass weighted terminal velocity, median mass diameter, asymmetry parameter and single scatter albedo at wavenumbers from 5 to 100 cm-1 and wavelengths of 0.5 to 5 µm by less than 20%. This is significantly less than the fractional changes in quantities dominated by lower order moments such as number concentration. The results suggest that model parameterizations and remote sensing techniques based on higher order moments of ice particle size distributions obtained in conditions similar to those sampled during IDEAS-2011 and ISDAC derived from 2DCs are not as biased by shattered remnants compared to those derived from lower order moments. The second main objective of this thesis was to examine the dependence of ice cloud microphysical properties derived from the 2D Stereo and 2D Precipitation Probes on board the Stratton Park Engineering Company (SPEC) Learjet during the Small Ice Particles in Cirrus (SPARTICUS) experiment on temperature, and formation mechanism. An existing approach that represents a size distribution (SD) as a single gamma function using an ellipse of equally realizable solutions in (N0, λ, µ) phase space was modified to automatically identify multiple modes in SDs and characterize each mode by such an ellipse. The dependencies of N0, µ, λ from each mode, total number concentration, bulk extinction, ice water content, and median (mass) diameter as a function of T and formation mechanism were determined. The changes in the observed codependencies between N0, µ, and λ, as well as ice water content, extinction, and median (mass) diameter with environmental conditions indicate that particles grew to larger sizes at higher temperatures during SPARTICUS. No more than 2 modes were observed in SDs during SPARTICUS. The average boundary between the two modes was at 115 μm, similar to past studies not using probes with shatter mitigating tips and artifact removal algorithms. The bimodality of the SD was more frequent with increasing T, consistent with past studies that hypothesized that the bimodality was caused by nucleation occurring in the presence of sedimentation. Furthermore, the differences in the observed codependencies of N0, µ, and λ between the two modes were consistent with past studies attributing these differences to particles in the mode with smaller maximum dimensions D growing primarily by deposition and the particles in the mode with larger D by both aggregation and vapor deposition.
Issue Date:2015-01-16
Rights Information:Copyright 2015 Robert Jackson
Date Available in IDEALS:2015-07-22
Date Deposited:May 2015

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