Obtaining best estimates for the microphysical and radiative properties of tropical ice clouds from TWP-ICE in situ microphysical observations.
Résumé
Best estimates of the bulk microphysical and radiative properties (ice water content, visible extinction, effective radius, and total concentration) are derived for three case studies of tropical ice clouds sampled during the Tropical Warm Pool International Cloud Experiment (TWP-ICE). Two case studies are aged cirrus clouds produced by deep convection (the 27/01 and 29/01 cases), and the third (02/02) is a fresh anvil produced by deep convective activity over the Tiwi Islands. Using crystal images obtained by a Cloud Particle Imager (CPI), it is observed that small ice particles (with maximum dimension D < 50 to 100 μm) were predominantly quasi-spherical, with the degree of non-sphericity increasing rapidly in the 50 < D < 100 μm range. For D > 100 μm, the aged cirrus clouds were predominantly characterized by bullet rosettes and aggregates of bullet rosettes, plates, and columns. In contrast, the fresh anvil had more frequent occurrences of plates, columns, aggregates of plates, and occasionally capped columns. The impact of shattering of large ice crystals on probe tips and the overall quality of the TWP-ICE in-situ microphysical measurements are assessed. It is suggested that shattering has a relatively small impact on the CPI and CDP TWP-ICE data, and a large impact on the CAS data, as already documented by others. It is also shown that the CPI size distributions must be multiplied by a factor 4 to match those of the CIP for maximum dimension larger than 100 μm (taken as a reference). A technique (named BEAD, Best Estimate of Area and Density) to minimize errors associated with the density (ρ) -D and projected area (A) - D assumptions in bulk microphysics calculation is introduced and applied to the TWP-ICE data. The method makes direct use of the frequency of occurrence of each particle habit as classified from the CPI data and prescribed ρ- D and A - D relationships from the literature. This approach produces IWC estimates that are virtually unbiased compared to bulk measures obtained from a Counterflow Spectrometer and Impactor (CSI) probe. In contrast, the use of ρ- D and A - D relationships for single habits does produce large biases compared to the CSI observations: from −50% for bullet rosettes to +70-80% for aggregates. The so-called WLAP technique, which also makes use of individual CPI images, is found to produce positively biased IWCs (by 40% or so), which is not the case of the BEAD technique, but a standard deviation of the errors similar to the BEAD technique. The impact of the large variability of the size distributions measured by different probe combinations on the resulting bulk microphysical properties is characterized. The mean fractional differences with respect to the CSI measurements are small for the CPI+CIP, CPI, and CDP+CIP combinations (2.2, −0.8, and −1.1 %, respectively), with standard deviations of the fractional differences ranging from 7 to 9 %. This result provides an independent validation of the CPI scaling factor. The fractional differences produced between the CPI+CIP, CPI, and CDP+CIP combinations for extinction, effective radius, and total concentration are 33%, 10-20%, and 90%, respectively, with relatively small standard deviations of 5 to 8%. The fractional difference on total concentration varies greatly over the concentration range though, with values being larger than a factor 2 for total concentrations smaller than 40 L−1, but reducing to 10-20% for concentrations larger than 100 L−1. Therefore, caution should be exercised when using total concentrations smaller than 60-80 L−1 as references for radar-lidar retrieval evaluation.
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