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Roger F. Reinking

Abstract

Small but well-organized steam devils were observed forming and propagating over a swimming pool fed by hot springs. The steam devils are described and compared to similar vortices observed in various environs.

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Roger F. Reinking

Abstract

Snow crystals sampled from winter storms of the Sierra Nevada have been examined to determine their riming characteristics in terms of their dimensions and habits of growth. The field data show that accretion can begin on crystals that have grown for only 2–4 min. However, the onset of accretion is delayed over longer growth periods for many crystals, so considerable dispersion occurs in the size at the onset of riming on specific types oaf crystals. This dispersion, which occurs in individual snow showers as well as over the durations of whole storms, and results from interactions of various cloud processes, is very important in describing and modeling snow crystal growth.

The minimum sizes of the observed crystals at the onset of accretion, in terms of major crystal dimensions, ranged from 115 to 320 μm. The minimum widths of different columnar types at the onset were very uniform (30–36 μm). Nevertheless, the basic columnar and planar habits show very systematic differences in minimum dimensions and durations of growth required for riming to begin; the measured sizes and growth times are somewhat less than those predicted by the more rigid of the current theories of accretion.

The systematic differences among the various habits carry through to heavier stages of accretion. In dividually branched planar and radiating crystals and capped columns develop the highest rates of accretion and precipitate the most water in the form of rime. Total precipitation, of course, depends on the concentrations of the various crystals. Assuming that the laboratory evidence for the rime-droplet splintering mechanism can be applied in the field, significant needle and sheath production through such multiplication probably occurred in the Sierran cloud systems.

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Roger F. Reinking

Abstract

Measurements were made of the sizes and concentrations of graupel and snow crystals occurring in seeded and untreated winter storms of the Sierra Nevada. The amounts of rime on individual snow crystals were also determined. The observations show that crystal riming, and formation and precipitation of graupel, are common to all stages of Sierran snowstorms. Graupel occurs simultaneously with all types of snow crystals, but individual crystal types do not consistently occur with or serve as kernels for particles of graupel.Graupel particles do not develop predominantly on kernel snow crystals that grow to relatively large sizes over relatively long periods by deposition and accretion. Graupel often forms instead on a select few small crystals, when it forms utilizing regular crystals as kernels. Graupel frequently develops without kernels, by alternate riming processes. One possible alternative is a mechanism that produces graupel from rime accumulated at localized points on parent snow crystals.

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Joey F. Boatman and Roger F. Reinking

Abstract

It is generally believed that synoptically driven storms, with Offing induced or enhanced by upslope flow over the high plains, produce most of the winter precipitation in eastern Colorado. Two extremely different circulations, the fully developed extratropical cyclone and the shallow arctic anticyclone, bound the range of upslope circulations. Two cases involving shallow upslope circulations were studied in detail for this work.

Aircraft, standard synoptic scale and selected mesoscale data were available for the case studies. The synoptic and (in one case) mesoscale circulations, characteristics of the consequent upslope and overlying midlevel stratiform clouds, and the microphysical processes that generated the precipitation from these events were examined. Dynamically and microphysically, these cases were among the simplest of the varied upslope storm systems. The arctic air masses were about 100 mb in thickness. The troposphere in and above the arctic air was potentially stable in both cases. The upslope clouds resulted from topography induced upward air motions associated with easterly flow. The easterly flow was caused by horizontal pressure gradients within the anticyclones. In one case, mesoscale analyses revealed that the local topography retarded and diverted the approaching arctic air until it became deep enough to flood the entire area.

The observed upslope cloud layers formed within the cold air mass. The limited available moisture was derived from local sources and the arctic air itself. Water contents were generally ∼0.1 g m−3 or less in all cloud layers. Some aircraft icing confirmed the presence of the liquid, and water saturation prevailed in the clouds. Heterogeneous nucleation (primary ice generation) was the most likely source for ice particles in both cases. Ice multiplication could be neither confirmed nor denied. Once nucleated, ice crystals grew predominantly by vapor deposition, to produce some crystals with diameters as large as 2 mm. However, aggregational growth was observed in the storm with the warmest cloud temperatures, and accretional growth was possible in the storm with the greatest water contents. Natural seeding of the upslope clouds by ice particles failing from the midlevel clouds occurred in both cases. Survival of the crystals in descent between the cloud layers was strongly regulated by the atmospheric ice saturation ratio. Crystal growth in the clear-air occurred in one case, whereas substantial sublimation occurred in the other.

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Roger F. Reinking and James E. Lovill

Abstract

The variations of ice nucleus and ozone concentrations were monitored at a high mountain observatory in the Colorado Rockies. Because the observatory is at an altitude of nearly 4 km MSL, increases in ozone at this station are in most instances related to intrusions of stratospheric air into the troposphere. An increase of ice nuclei simultaneously with an increase of ozone in such cases would therefore be indicative of a transport of ice nuclei from the stratosphere to the earth's surface. High ice nucleus concentrations in stratosphere air would substantiate the possibility of an extraterrestrial nucleus source. A negative correlation between the ozone and ice nuclei existed during the time of the case study presented. It is concluded that at least during this period there was not an influx of ice nuclei simultaneously with a downward transport of stratospheric air. Hypotheses for a ground origin of ice nuclei are supported.

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Roger F. Reinking and Brooks E. Martner

Abstract

Questions of delivery, transport, and dispersion of cloud seeding aerosol in a convective feeder cloud are addressed by using radar chaff as a surrogate for aerosol and tracking it with circular-polarization radar. In a case study, a line source of chaff was released by an aircraft at the roots of a growing cloud flanking and feeding into a thunderstorm line. The chaff was tracked as it dispersed in the boundary layer and rose more than 3 km from the cloud base at +14°C to levels cold enough to nucleate ice-forming seeding aerosols. Quantitative measures of the rates of loft and dispersion, and the volume filling and dilution were obtained. The measurements permit examination of the hypotheses and potential efficacy of cloud-base seeding to increase rain and suppress hail. Notably, the problem of delivery, transport, and dispersion of cloud seeding aerosol is much the same as the air quality question of the nature and effect of cloud venting of the boundary layer, and the findings here apply in that context as well.

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Roger F. Reinking, Jack B. Snider, and Janice L. Coen

Abstract

This study illustrates opportunities for much improved orographic quantitative precipitation forecasting, determination of orographic cloud seedability, and flash flood prediction through state-of-the-art remote sensing and numerical modeling of gravity wave clouds. Wintertime field observations with multiple remote sensors, corroborated in this and related papers with a mesoscale–cloud scale numerical simulation, confirm that storm-embedded gravity waves can have a strong and persistent influence on orographic cloud liquid water (CLW) and precipitation. Where parallel mountain ridges dominate the landscape, an upwind ridge can force the wave action, and a downwind ridge can receive the precipitation. The 1995 Arizona Program was conducted in such terrain. In the scenario examined, traveling waves cyclically caused prefrontal cross-barrier winds that produced gravity waves. Significant cloud bands associated with the waves carried substantial moisture to the area. With the passage and waning of the cloud bands, vapor influxes (precipitable water P w) cycled through large changes in magnitude, and prefrontal peaks in P w coincided with the gravity waves in a succession of episodes during a five-day period. Thus, the cyclic trend in P w and the magnitudes of peak P w were simple indicators of wave cloud development. The first two cycles, with minor peak P w, were precursors. Significant wave clouds first appeared during the second episode. During the final two episodes with large vapor influxes, very deep, precipitating wave clouds were coupled with underlying clouds formed in flow up the mountain slopes to create the prefrontal storms. Rain fell on an existing snowpack on the main recipient ridge and, in the end, produced rapid runoff and flash flooding.

The gravity waves persistently condensed CLW that averaged 0.5 mm and reached 1.0 mm in the first of the main storm episodes, and averaged 1.0 mm and reached 2.0 mm and more in the second (column-integrated values). These values equaled or exceeded the larger of those represented in liquid water climate datasets for orographic cloud systems in other locations in the West, where only the upslope and not the wave component had been examined. The effect of shifts between cross-barrier and barrier-parallel flows was reflected in abrupt buildups and declines in wave CLW, but the gravity wave clouds persisted for a total of 22 h during the two storm periods. In the wave updrafts, the condensation rate regularly exceeded the consumption rate by ice, even though ice was usually present. Conversion to ice consumed and precipitated wave CLW. Pulses of available P w and wave CLW on a 2- to 4-h timescale, cyclically followed by partial glaciation, produced the precipitation from the wave clouds. Their seeder effect on the upslope feeder clouds was to enhance the total precipitation from the coupled system. Estimates of the liquid water fluxes in comparison with the precipitation rates suggest precipitation efficiencies in the 11%–33% range from the seeder–feeder couplets. The periods of gravity wave forcing contributed some 80% or more of the total precipitation, and trailing fronts produced the remainder.

Several factors derived from the observed availability of CLW determine the potential for precipitation enhancement by seeding wave clouds; these are enumerated. Given demands for improved water supply, the challenge often presented in mountain watersheds of separating seeding opportunities from potential flash flood situations is examined. The results here show that storms that could threaten flash floods can be readily identified by continuous monitoring with polarization radar and in real-time simulations as those with the altitude of the melting level above the elevation of the highest terrain with existing snowpack.

In the sense that orographically generated gravity waves will significantly influence cloud water and precipitation, geographic transferability of the results is indicated by the existence of wave-generating and precipitation-generating parallel ridges in many places throughout the world. The quantitative effects will, of course, depend on particulars of the locale such as nature of the prevalent forcing, available moisture, and physical stature of the ridges.

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Sergey Y. Matrosov, Roger F. Reinking, and Irina V. Djalalova

Abstract

Single pristine planar ice crystals exhibit some flutter around their preferential horizontal orientation as they fall. This study presents estimates of flutter and analyzes predominant fall attitudes of pristine dendritic crystals observed with a polarization agile Ka-band cloud radar. The observations were made in weakly precipitating winter clouds on slopes of Mt. Washington, New Hampshire. The radar is capable of measuring the linear depolarization ratios in the standard horizontal–vertical polarization basis (HLDR) and the slant 45°–135° polarization basis (SLDR). Both HLDR and SLDR depend on crystal shape. HLDR also exhibits a strong dependence on crystal orientation, while SLDR depends only weakly on orientation. The different sensitivities of SLDR and HLDR to the shape and orientation effects are interpreted to estimate the angular flutter of crystals. A simple analytical expression is derived for the standard deviation of angular flutter as a function of the HLDR to SLDR ratio assuming perfect radar system characteristics. The flutter is also assessed by matching theoretical and observed depolarization patterns as a function of the elevation of the radar’s beam. The matching procedure is generally more robust since it accounts for the actual polarization states and imperfections in the radar hardware. The depolarization approach was used to estimate flutter of falling pristine dendrites that were characterized by Reynolds numbers in a range of approximately 40–100. Using the matching approach, this flutter was found to be about 9° ± 3°, as expressed by the standard deviation of the crystal minor axes from the vertical direction. The analytical expression provides a value of flutter of about 12°, which is at the high end of the estimate obtained by the matching procedure. The difference is explained by the imperfections in the polarization states and radar hardware, so the analytical result serves as an upper bound to the more robust result from matching. The values of flutter estimated from the experimental example are comparable to estimates for planar crystals obtained in laboratory models and by individual crystal sampling.

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Sergey Y. Matrosov, Roger F. Reinking, Robert A. Kropfli, Brooks E. Martner, and B. W. Bartram

Abstract

An approach is suggested to relate measurements of radar depolarization ratios and aspect ratios of predominant hydrometeors in nonprecipitating and weakly precipitating layers of winter clouds. The trends of elevation angle dependencies of depolarization ratios are first used to distinguish between columnar-type and plate-type particles. For the established particle type, values of depolarization ratios observed at certain elevation angles, for which the influence of particle orientation is minimal, are then used to estimate aspect ratios when information on particle effective bulk density is assumed or inferred from other measurements. The use of different polarizations, including circular, slant-45° linear, and two elliptical polarizations, is discussed. These two elliptical polarizations are quasi-circular and quasi-linear slant-45° linear, and both are currently achievable with the National Oceanic and Atmospheric Administration Environmental Technology Laboratory’s Ka-band radar. In comparison with the true circular and slant-45° linear polarizations, the discussed elliptical polarizations provide a stronger signal in the “weak” radar receiver channel; however, it is at the expense of diminished dynamic range of depolarization ratio variations. For depolarization measurements at the radar elevation angles that do not show much sensitivity to particle orientations, the available quasi-circular polarization provides a better depolarization contrast between nonspherical and spherical particles than does the available quasi-linear slant-45°polarization. The use of the proposed approach is illustrated with the experimental data collected during a recent field experiment. It is shown that it allows successful differentiation among pristine planar crystals, rimed planar crystals, long columns, blocky columns, and graupel. When a reasonable assumption about particle bulk density is made, quantitative estimates of particle aspect ratios from radar depolarization data are in good agreement with in situ observations. Uncertainties of particle aspect ratios estimated from depolarization measurements due to 0.1 g cm−3 variations in the assumed bulk density are about 0.1.

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Sergey Y. Matrosov, Robert A. Kropfli, Roger F. Reinking, and Brooks E. Martner

Abstract

Model calculations and measurements of the specific propagation and backscatter differential phase shifts (K DP and δ o, respectively) in rain are discussed for X- (λ ∼ 3 cm) and Ka-band (λ ∼ 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between K DP and rainfall rate R. These relationships, however, are subject to significant variations due to the assumed model of the drop aspect ratio as a function of their size. The backscatter differential phase shift at X band for rain rates of less than about 15 mm h−1 is generally small and should not pose a serious problem when estimating K DP from the total phase difference at range intervals of several kilometers. The main advantage of using X-band wavelengths compared to S-band (λ ∼ 10–11 cm) wavelengths is an increase in K DP by a factor of about 3 for the same rainfall rate. The relative contribution of the backscatter differential phase to the total phase difference at Ka band is significantly larger than at X band. This makes propagation and backscatter phase shift contributions comparable for most practical cases and poses difficulties in estimating rainfall rate from Ka-band measurements of the differential phase.

Experimental studies of rain using X-band differential phase measurements were conducted near Boulder, Colorado, in a stratiform, intermittent rain with a rate averaging about 4–5 mm h−1. The differential phase shift approach proved to be effective for such modest rains, and finer spatial resolutions were possible in comparison to those achieved with similar measurements at longer wavelengths. A K DPR relation derived for the mean drop aspect ratio (R = 20.5K0.80DP) provided a satisfactory agreement between rain accumulations derived from radar measurements of the differential phase and data from several nearby high-resolution surface rain gauges. For two rainfall events, radar estimates based on the assumed mean drop aspect ratio were, on average, quite close to the gauge measurements with about 38% relative standard deviation of radar data from the gauge data.

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