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John L. Stanford and John S. Davis

The results are presented from a careful, systematic search for reports of stratospheric clouds over the period 1870–1972. For the high-latitude Northern Hemisphere, a total of 156 dates are listed on which mother-of-pearl clouds (MPC) were reported. A small number of other Norwegian sightings exist for which specific dates were not obtained. Several references to general aircraft sightings of clouds above the tropopause are also given.

Five mid- or low-latitude sightings are listed, some of which are perhaps related to rocket firings.

In the Southern Hemisphere, the reported sightings occurred in Antarctica, being either of the MPC type or of more extensive and longer-lasting stratospheric cloud veils. The latter apparently are due to the extremely cold stratospheric temperatures experienced over the Antarctic in winter. Reports of veil-type stratospheric clouds are listed from two Antarctic expeditions, in 1912 and 1950–51. In addition to these, 139 cases of Antarctic MPC sightings were reported, although specific dates were not obtained for all of these cases. Considering the sparsity of Antarctic observers, it appears that stratospheric clouds occur much more frequently in the high-latitude Southern Hemisphere than in the corresponding region of the Northern Hemisphere.

The results of the present investigation are believed to represent the most extensive listing presently available for stratospheric cloud observations.

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John M. Davis and Stephen K. Cox

Abstract

The results of a laboratory experiment are presented that provide additional verification of the methodology adapted for simulation of the radiances reflected from fields of optically thick clouds using the Cloud Field Optical Simulator (CFOS) at Colorado State University. The comparison of these data with their theoretically derived counterparts indicates that the crucial mechanism of cloud-to-cloud radiance field interaction is accurately simulated in the CFOS experiments and adds confidence to the manner in which the optical depth is scaled.

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Maosi Chen, John Davis, and Wei Gao

Abstract

Cloud screening of direct-beam solar radiation is an essential step for in situ calibration and atmospheric properties retrieval. The internal cloud screening module of a Langley analysis program [Langley Analyzer (LA)] used by the U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Program (UVMRP) is used for screening the uncalibrated direct-beam measurements and for deriving Langley offset voltages for calibration of the UV version of the Multifilter Rotating Shadowband Radiometer (UV-MFRSR). The current LA cloud screening module utilizes data from extended clear-sky periods and tends to ignore shorter periods that typify periods of broken cloudiness, and as a result, fewer values are generated for sites with higher frequencies of cloudy days (cloudy sites). A new cloud screening algorithm is presented that calculates the total optical depth (TOD) difference between a target point and pairs of points, and identifies the target as cloudy if the mean TOD difference exceeds a certain threshold. The screening is an iterative process that finishes when no new cloudy points are found. The result at a typical clear/sunny site shows that values from partly cloudy days are consistent with those from cloud-free days, when the new method is employed. The new cloud screening algorithm picks up significantly more values at cloudy sites. The larger decrease of the annual mean value of at cloudy sites than at relatively clear sites suggests the potential for improving calibration accuracy at cloudy sites. The results also show that the new cloud screening method is capable of detecting clear points in short clear windows and in transitional regions.

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John M. Davis and Stephen K. Cox

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Analyses of bidirectional reflectance data are presented with implications regarding the spatial scales appropriate for inferring irradiances from radiances reflected by various surface–atmosphere scenes. Multiple-angle radiance data collected in a nearly simultaneous manner during the 1979 Summer Monsoon Experiment are analyzed using the squared coherency statistic to suggest a method to deduce the minimum spatial scale appropriate for irradiance inferences. Spatial convergence of the irradiances inferred from the component radiances is presented as a function of averaging distance to imply magnitudes of errors that may result from use of“similar scene” bidirectional reflectance models. The reduction in the inference errors with an increasing number of angular viewing positions is also presented. The data are analyzed in search of preferred viewing directions with the result that little improvement is imparted to the inference by viewing the scenes from any specific view direction.

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John M. Davis and Stephen K. Cox

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A set of bi-directional reflectance models is presented for various atmospheric scene types. The models were composited from data collected from an aircraft platform in May-July 1979 during Summer MONEX. The space scale of the composited models is generally from 250 to 1000 km, which corresponds to the scale of interest in climate monitoring and modeling. Composite models for the following scene types are presented: the desert sands of the Saudi Arabian Empty Quarters, the Himalayan mountains, the Arabian Sea with the ever-present fair weather cumulus cloudiness, the semi-arid agricultural land surface of the Indian subcontinent under pre-monsoon conditions, broken middle and low level clouds over ocean, an altostratus cloud deck, and the broken pack-ice fields of Hudson Bay. Nearly all the models display a degree of anisotropy such that serious errors (10–100%) would result in the reflected flux density isotropically inferred from some of the reflected radiances. The features of many of the models are discussed, and all of the models are tabulated in the Appendix. One of the models for altostratus is explicitly compared with theory, and differences between the altostratus and broken cloud models agree with the differences between infinite and finite cloud theory. The models are also compared with models from previous studies. The agreement is generally good (∼100%rms) except in a few cases in which the disagreement may have resulted from natural scene variability or differences between the methods of data collection.

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John D. Tuttle and Chris A. Davis

Abstract

Traveling deep tropospheric disturbances of wavelengths ~1500 km (short waves) have long been known to play an important role in the initiation and maintenance of warm-season convection. To date, relatively few studies have formally documented the climatology of short waves and their relationship to the diurnal heating cycle, the topography, and the diurnal cycle of precipitation. Those that did had to rely on low-resolution global analyses and often could not track short waves across mountain barriers. In this study, 10 yr of the (32 km) NCEP North American Regional Reanalysis (NARR) are used to objectively identify and track short waves in the North American domain. Statistics of short-wave span, duration, phase speed, latitudinal extent, and locations of preferred intensification/decay are presented. Some of the key findings from the climatology include that the lee (windward) of mountain barriers are preferred regions of intensification (decay) and short waves show little evidence of a diurnal cycle and can pass a given point at any time of the day. The second part of the study focuses on the role that short waves play in modulating the diurnal cycle of propagating convection east of the Rocky Mountains. Depending on the timing of short-wave passage, short waves may either significantly enhance the precipitation above the mean or completely disrupt the normal diurnal cycle, causing precipitation to develop at times and locations where it normally does not. While short waves play an important role in modulating the mean precipitation patterns their role is considered to be secondary in nature. The diurnal precipitation signature is prominent even when short waves are not present.

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John D. Tuttle and Chris A. Davis

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During the warm season in the central United States there often exists a corridor of precipitation where a succession of mesoscale convective systems (MCSs) follow similar paths lasting several days. The total cumulative rainfall within a corridor can be substantial while precipitation at nearby regions may be below normal. Understanding the nature of the corridors and the environmental factors important for their formation thus has important implications for quantitative precipitation forecasting and hydrological studies. In this study a U.S. national composite radar dataset and model-analyzed fields are used for the 1998–2002 warm seasons (July–August) to understand the properties of corridors and what environmental factors are important for determining when and where they develop. The analysis is restricted to a relatively narrow longitudinal band in the central United States (95°–100°W), a region where convection often intensifies and becomes highly organized. It is found that ∼68% of MCSs were members of a series and that corridors typically persist for 2–7 days with an extreme case lasting 13 days. Cumulative radar-derived maximum rainfall ranges from 8 to 50 cm, underscoring the fact that corridors can experience excessive rainfall. Combining radar with Rapid Update Cycle model kinematic and thermodynamic fields, 5-yr composites are presented and stratified according to the environmental conditions. While the corridors show the expected association with areas of enhanced CAPE and relatively strong northwesterly/westerly shear, the strongest association is with the northern terminus region of the nocturnal low-level jet (LLJ). Furthermore, the relative intensity of the rainfall is positively correlated with the strength of the LLJ. The LLJ is thought to play a role through enhanced convergence and lifting, moisture transport, and frontogenesis. In the five years analyzed, the large-scale environment varied considerably, but the role of the LLJ in the formation of corridors remained persistent.

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John M. Davis, Stephen K. Cox, and Thomas B. McKee

Abstract

A band absorption model is used in conjunction with a Monte Carlo scattering model to calculate the amount of solar radiation absorbed above, below, within and adjacent to cubic, finite clouds. Horizontally and vertically nonhomogeneous values of absorption within the finite cloud range from 0.16 to 6.4 times the corresponding values in horizontally infinite clouds of the same optical thickness, which were calculated using the same model. Absorption values in the regions adjoining the finite cloud on the solar and anti-solar sides, converge to clear sky values within a distance of two cloud dimensions from the side walls of the cloud.

Absorption below the finite cloud ranges from 1.4 to 4.5 times that below the infinite cloud volume element. Values of absorption above the two cloud types are nearly identical when normalized to the cross-sectional area of the incident beam. If the absorption in the atmospheric column containing the finite cloud is normalized with respect to the horizontal area of the parallel radiation incident on the top plus the side of the cloud, the resulting value is within 3% of the absorption value in the column containing an element of infinite cloud. Thus, infinite cloud total column absorption values may be used to compute areal averages of absorption for a region partially covered by widely separated finite clouds, whose height to width ratio is near unity, if the fractional cloud cover is adjusted in the appropriate manner.

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B. L. Davis, L. R. Johnson, and F. John Moeng

Abstract

The exceptional ice nucleating ability of aersols obtained from combustion of AgI-NH4I-acetone solution is now well known. The high nucleating ability has been determined to come from the existence of a complex compound having a better epitaxial fit with respect to ice than has silver iodide. The compound has a stability region which includes the temperature interval of −20 to +9°C at water saturation and has been observed to be present on AgI aerosol particles produced from standard aircraft seeding generators in a wind tunnel dilution system. Its presence is presumed to occur as a result of incomplete destruction of the NH4 + of the original solution. Although the unit cell of the phase is monoclinic, c-centered, it has a close packed structure nearly identical to silver iodide in the a0-b0 crystallographic plane. In this plane the phase has a misfit with respect to the basal plane of ice of 1.3% at −7°C as compared to 1.5% for silver iodide. The composition of the phase is 3AgI·NH4I·6H2O but with 25% of the silver positions being vacant on the average. Compositions of 2:1 mole ratio AgI:NH4I can also exist in an apparent metastable state. The threshold of ice nucleation for the pure Bn phase was found to be −1°C as contrasted to the recognized −4°C threshold for silver iodide.

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Ronald M. Welch, Stephen K. Cox, and John M. Davis

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No Abstract available.

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