"An attempt is made to examine the credibility and explore the implications of the reports of convective clouds to extreme heights which have been indicated by radar in recent years. The extreme cases have not been verified by independent observations and it is shown that inherent limitations in radar observations of convective cloud heights are such as to raise serious doubts in regard to the accuracy of many of the extreme echo heights which have been reported"--Abstract. / Typescript. / "Scientific report to the U.S. Navy Weather Research Facility, Norfolk, Virginia, under contract N189-50775A." / "15 April 1962." / Includes bibliographical references (leaves 32-34).

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A ground-based near-infrared spectroradiometer was built and used to measure relative spectral reflectance from cumulus congestus and cumulonimbus clouds during the 1985 and 1986 Arizona summer monsoon seasons. Thermodynamic phase was inferred from spectral features in the regions between 1.55-1.75μm and 2.1-2.3μm where there are distinct differences between absorption in liquid water and ice and absorption by water vapor is very weak. Although liquid water and ice are nearly transparent in the visible, they absorb weakly in the near-infrared and that absorption is amplified by multiple scattering in clouds. Reflectance measurements are simple to make, requiring neither high spectral resolution nor absolute detector response. Three distinct aspects of differences between absorption in liquid water and ice were used to infer phase: (a) Ratio of the signal at 1.65 μm to that at 2.2 μm; (b) Wavelength of peak signal in the 1.65 μm water vapor transmission window; (c) Half-bandwidth of the 2.1-2.3 μm feature. Representative spectra are presented and analyzed on the basis of the predicted behavior of liquid water and ice cloud absorption. The results are consistent with young cumuli rapidly glaciating as they reach cooler levels, well before evidence of anvil formation or fibrous structure, contrary to the notion that phase can be inferred from visible cloud features.

Thesis (Ph.D.) - University of St Andrews, March 2007.

Thesis (M.S.)--University of Wisconsin, 1967. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaf 39).

We present the development of the adjoint of a physically based cirrus formation parameterization that computes the sensitivity of formed crystal number concentration to numerous model variables (e.g., updraft velocity, soluble aerosol geometric mean diameter and number concentration, insoluble aerosol geometric mean diameter and number concentration, and ice deposition coefficient). The adjoint is demonstrated in the CESM Community Atmosphere Model Version 5.1, where sensitivity information is computed and used to quantify which variables are most responsible for modeled variability in formed crystal number concentration. The sensitivity of formed crystal number concentration to updraft velocity is positive and largest over the tropics where regions of deep convection are collocated with large sulfate number concentrations. Sensitivity to sulfate number concentration is largest over the tropics where updraft cooling is sufficient and sulfate number concentration is low, pointing to a sulfate limited regime. Outside of the tropics, crystal production is dominated by heterogeneous freezing; unexpectedly, sensitivities to insoluble aerosol number concentration for accumulation and coarse mode dust, black carbon, and organic carbon are negative in sign here. This is a result of infrequent, anomalously high updraft velocity events causing shifts in the dominant modes of freezing which act to bias sensitivity information when annually averaged. Updraft velocity is responsible for ~95% of the variability in formed crystal number concentration in the high latitudes of the Northern Hemisphere. In the tropics, sulfate number concentration controls variability in formed crystal number concentration since crystal production here is sulfate limited. Insoluble aerosol species play a secondary role in influencing the variability of crystal concentrations; coarse mode dust is the largest contributor to crystal number variability at nearly 60%, although the spatial extent of this influence is small and concentrated over highly localized dust events. When globally averaged, nearly 90% of the variability in crystal number concentration can be described by only updraft velocity, sulfate number, temperature, and coarse mode dust number concentration. Although these results depend on parameter assumptions, the robustness of the underlying physics of the cirrus formation parameterization used throughout this work suggests that this approach can be a powerful method for efficiently identifying the origin of microphysical dependencies within large scale atmospheric simulations.

Airborne lidar measurements of Arctic polar stratospheric clouds (PSCs) in January 1984 and January 1986 are reported. The locales and altitudes of the clouds coincided in both years with very cold ambient temperatures. During the 1984 experiment, PSCs were observed on three flights north of Thule, Greenland; peak backscatter occurred near 20 km (at temperatures below 193 K). A single PSC formation was seen between Iceland and Scotland during the 1986 experiment, with beak backscatter occurring near 22 km (at temperatures from 188-191 K). A sequence of observations in this same area by the SAM II satellite sensor depicts the history of cloud development and dissipation. Enhancements in aerosol backscattering in excess of a factor of 100 were measured during the 1984 experiment at latitudes near the Pole where 50-mb temperatures approached the frost point. Depolarization in the backscattered signal was estimated as 30-40%, similar to that measured in cirrus clouds. Farther south, with 50-mb temperatures several degrees warmer, backscatter enhancement factors ranged from 20-30, and little or no depolarization was observed. Results similar to the latter were found during the 1986 experiment--enhancement factors near 50 (at the 30-mb level, with temperatures 3-5 K above the frost point), and little depolarization. The contrast in observations suggested the existence of distinct cloud growth regimes delineated by temperatures, as proposed in recent articles addressing Antarctic ozone depletion. A theoretical model was developed which interposes a stage of nitric acid trihydrate deposition between the two stages of cloud formation and growth assumed in earlier models (aerosol droplet precursors and ice particles). The calculated temperature dependence of backscatter and extinction agreed well with experimentally observed values, except for small systematic errors at the 30-mb level which may be due to poor characterization of the temperature field there. A companion theoretical study of PSC formation at 70 mb in the Antarctic showed that about 80% and 30% of the nitric acid and water vapor supplies, respectively, may be sequestered in relatively large (4-μm radius) cloud particles at a temperature near 189 K. Such large particles would fall at a rate of about 2 km wk⁻¹, suggesting that PSCs may act as a sink for these stratospheric trace gases.

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