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Arlen W. Huggins

Abstract

Previous studies of the spatial distribution of supercooled liquid water in winter storms over mountainous terrain were performed primarily with instrumented aircraft and to a lesser extent with scans from a stationary microwave radiometer. The present work describes a new technique of mobile radiometer operation that was successfully used during numerous winter storms that occurred over the Wasatch Plateau of central Utah to determine the integrated depth of cloud liquid water relative to horizontal position on the mountain barrier. The technique had the advantage of being able to measure total liquid from the terrain upward, without the usual terrain avoidance problems that research aircraft face in cloudy conditions. The radiometer also collected data during several storms in which a research aircraft could not be operated because of severe turbulence and icing conditions.

Repeated radiometer transects of specific regions of the plateau showed significant variability in liquid water depth over 30–60-min time periods, but also revealed that the profile of orographically generated cloud liquid was consistent, regardless of the absolute quantities. Radiometer liquid depth generally increased across the windward slope of the plateau to a peak near the western edge of the plateau top and then decreased across the relatively flat top of the plateau. These observations were consistent with regions where maximum and minimum vertical velocities were expected, and with depiction of cloud liquid by accretional ice particle growth across the mountain barrier. A comparison of data from the mobile radiometer and a stationary radiometer verified the general decrease in liquid depth from the windward slope to the top of the plateau and also showed that many liquid water regions were transient mesoscale features that moved across the plateau.

Implications of the results, relative to the seeding of orographic clouds, were that seeding aerosols released from valley-based generators could at times be inhibited by stable conditions from reaching appropriate super-cooled liquid water regions and, as found by others, the region of cloud most likely to be encountered by an AgI seeding agent released from the ground was also relatively warm compared to the ice-forming capability of the particular agent used in these experiments. Also, one convective case study that exhibited relatively warm temperatures in the cloud layer indicated that, even in conditions that permit vertical transport to supercooled liquid zones, sufficient time for ice particle growth and fallout from seeded plumes on this plateau may be lacking.

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Arlen W. Huggins and Alfred R. Rodi

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Arlen W. Huggins and Alfred R. Rodi

Abstract

The effect of seeding convective clouds with dry ice was studied using simultaneous aircraft and radar observations. Clouds that were initially ice-free with supercooled liquid water contents of 0.5 g m−3 when the tops reached the −10°C level had similar responses to seeding, although significant natural variability existed. Aircraft particle probes detected sharp increases of small crystals (<100 μm) in 3–6 min followed by > 1 mm aggregates about 10 min after seeding. Observations supported the expectation that riming growth should not be important at these liquid water contents. Initial radar echoes formed in 7 win with distinctive time-height profiles of reflectivity.

Most radar echoes forming downwind of the seeding line were small and relatively weak compared with the natural echoes forming further downwind over the mountains. The impact of the seeding was shown to be observable but relatively small. It was found that unseeded clouds formed radar echoes later, and produced reflectivity time-height profiles that were significantly different from the seeded ones. The difference are considered in part to be due to variability in the initial cloud properties as well as the obvious and well-documented effects of injection of the seeding material early in the cloud lifetime. While the meteorological impact was small, documentation of the evolution of the seeding effect from cloud to ground is a prerequisite to further experimentation.

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Richard J. Matson and Arlen W. Huggins

Abstract

The kinematic behavior of hailstones falling in their natural environment near the surface was studied using stroboscopic photography in a mobile van. The experimental results permitted determination of the shape and dimensions, velocity, and in a few cases the rotation rate, of hailstones failing into the van. Hailstones were sampled in southeast Wyoming, southwest Nebraska and northeast Colorado. About 84% of the hailstones photographed were classified spheroidal, the remainder being roughly conical. Change in orientation of stones is observed in most photographs, though only ∼6% of the hailstones could be assigned a rotation rate. Velocity data were obtained for more than 600 hailstones in the diameter range 5-25 mm. It is shown that the vertical velocity component of hailstones near the surface (air density =9.93×10−4 g cm−3) can be predicted by the expression V T where De0.50 (±m s−1), Deis the equivalent volume diameter of a spherical hailstone in centimeters. Fall-speed and hailstone oblateness are shown to be slightly negatively correlated. Hailstone drag co-efficients, as inferred from the measured vertical velocities, are found to be higher than the values most frequently quoted in the literature. A mean drag coefficient of 0.87 is found over a range of Reynolds numbers from 1032×104 with a tendency for the drag coefficient to decrease with increasing Reynolds number. Implications of the fallspeed and drag coefficient results on hailstone growth and hail instrument calibration are discussed. The time dependence of hailstone size is presented for two storms. A comparison of hailstone size versus arrival time indicates, at least for one of the storms, that the stones may have been size sorted.

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Alexis B. Long and Arlen W. Huggins

Abstract

Some results of the first (1988) Australian Winter Storms Experiment are described. The results shed light on precipitation-enhancement opportunities in winter cyclonic storms interacting with the Great Dividing Range of southeast Australia. The results come from analysis of supercooled liquid water amounts provided by a dual-wavelength microwave radiometer, atmospheric structure from Omegasondes, and precipitation amounts from a large number of tipping-bucket gauges. With these data it is possible to calculate and compare two of the terms in a condensed-phase water budget over a cloud-seeding target area in the Great Dividing Range. The two terms are the horizontal flux of supercooled liquid cloud water entering the budget volume and the vertical precipitation flux at ground level out of the volume. The budget terms have implications for the amount of extra precipitation that may result from seeding. It is found that the amount depends on the frontal or postfrontal stage of activity in the target area and on the wind direction with respect to the mountainous terrain.

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Terry Deshler, David W. Reynolds, and Arlen W. Huggins

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Cloud seeding experiments devoted to physical measurements of the effects of seeding shallow stable winter orographic clouds have been conducted in the central Sierra Nevada of California from 1984 to 1986. Seeding was done by aircraft using either dry ice or silver iodide at temperatures between −6° and −14°C. Aircraft, radar, and surface instruments were used to measure the effects. A trajectory model was used to target seeded precipitation to the ground where the surface instruments were deployed. Results from these experiments are presented in two case studies and a summary analysis of all 36 experiments. Observations from the various measurement platforms conformed with results expected from seeding in 35 percent of the seedlines sampled with a research aircraft, 4 percent of those observed with radar, and 17 percent of these which passed over the surface instrumentation; however, the complete seeding chain was believed to be documented in only 2 of 36 experiments. The failures result from difficult technical and logistical problems, and from the variability of even simple cloud systems, particularly in the spatial and temporal distributions of liquid water and in the natural fluctuations in ice crystal concentrations. Based on the difficulty of these experiments and the magnitude of seeding effects observed, a statistical experiment would be a formidable undertaking.

During the two experiments when seeding effects were detected by all measurement platforms the following effects were observed. A high concentration, 50–100 L−1, of small compact ice crystals formed quickly along the seedline. Although aggregation was seldom observed, riming often began 5–10 min after seeding. The seeded ice crystals dispersed at 1 m s−1 and cloud liquid-water evaporated in regions corresponding to the seedlines. Seeding in a non-echoing region occasionally produced echoes of 3–10 dBZ in portions of the seedlines. At the surface seeding effects arrived 35 to 60 min after seeding, 20–30 km downwind. Snow crystal concentrations increased, snow crystal habits changed to small rimed particles, and precipitation rates increased by 0.1–1.0 mm h −1. The duration of these effects was short, <10 min per seedline. Changes in ice particle development induced by seeding were similar when seeding with either dry ice or silver iodide. This was found to be the case even at temperatures as warm as −6°C using AgI NH4I NH4ClO4 burned in an acetone solution.

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Bernard Campistron, Arlen W. Huggins, and Alexis B. Long

Abstract

This Part III of a multipart paper deals with the analysis of turbulent motion in a winter storm, which occurred over the mountains of southwest Utah. The storm was documented with a long duration single Doppler radar dataset (∼21 h) comprised of volume scan observations acquired at 10-min intervals. Turbulence parameters were determined using a new technique of volume processing of single Doppler radar data.

Physical analysis of turbulence is restricted to three particular storm regions: a prefrontal region far removed from a cold frontal discontinuity, a frontal zone aloft, and a low layer in the post-frontal region where a long lasting (∼6 h) wind-maximum existed. The prefrontal period showed enhancement of turbulent parameters near 2.6 km height, apparently due to disturbed flow caused by an upwind mountain range. Turbulence parameters in this prefrontal region showed good agreement with K-mixing length theory. Within the frontal zone most turbulence parameters reached peak values, but were generally less than orographically induced turbulence values in the prefrontal period.

Turbulence in the low-level postfrontal period experienced periodic oscillations consistent with precipitation and kinematic variables described in Parts I and II, and associated with mesoscale precipitation bands. Acceleration of the valley-parallel wind component was apparent in prefrontal and postfrontal periods and was related to the specific valley configuration through a Venturi effect.

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Bernard Campistron, Alexis B. Long, and Arlen W. Huggins

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In previous work the derivation of turbulence parameters from single-Doppler radar observations was performed with data acquired along a horizontal circle. Here the technique is extended to all the radar data within a horizontal cylindrical slice of finite depth using the same basic assumptions of linearity of the mean wind field and horizontal homogeneity of the turbulence. The method allows the extraction of the six Reynolds stress components, together with their vertical derivatives, and the turbulent fluxes of a scalar quantity deduced from the reflectivity data.

Experimental data were used for the performance evaluation of the methodology. A simple testing procedure was carried out to remove erroneous results. The statistical uncertainty in the measured Reynolds stress terms was found to be about 0.05 m2 s−2, except for the variance of the vertical component, which was poorly retrieved because of an absence of data at high elevation angles. Calculations showed that contamination of the vertical momentum flux measurements by the scatterer fall speed was negligible. An analysis of the response function of the technique to the atmospheric scales tended to show that the diameter of the processing slices corresponded to the largest turbulent scale dimension involved in the measured turbulence quantities.

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Vanda Grubišić, Ramesh K. Vellore, and Arlen W. Huggins

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The skill of a mesoscale model in predicting orographic precipitation during high-impact precipitation events in the Sierra Nevada, and the sensitivity of that skill to the choice of the microphysical parameterization and horizontal resolution, are examined. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) and four bulk microphysical parameterization schemes examined are the Dudhia ice scheme, and the Schultz, GSFC, and Reisner2 mixed-phase schemes. The verification dataset consists of ground precipitation measurements from a selected number of wintertime heavy precipitation events documented during the Sierra Cooperative Pilot Project in the 1980s. At high horizontal resolutions, the predicted spatial precipitation patterns on the upwind Sierra Nevada slopes were found to have filamentary structure, with precipitation amounts over the transverse upwind ridges exceeding severalfold those over the nearby deep river valleys. The verification results show that all four tested bulk microphysical schemes in MM5 produce overprediction of precipitation on both the windward and lee slopes of the Sierra Nevada. The examined accuracy measures indicate that the Reisner2 scheme displays the best overall performance on both sides of the mountain range. The examined statistical skill scores on the other hand reveal that, regardless of the microphysical scheme used, the skill of the MM5 model in predicting the observed spatial distribution of the Sierra Nevada orographic precipitation is fairly low, that this skill is not improved by increasing the horizontal resolution of the model simulations, and that on average the quantitative precipitation forecasting (QPF) skill is better on the windward than on the lee side. Furthermore, a significance test shows that differences in skill scores obtained with the four microphysical schemes are not statistically significant.

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Alexis B. Long, Arlen W. Huggins, and Bernard A. Campistron

Abstract

A winter storm passing across the north–south-orientated Tushar Mountains in southwest Utah is investigated in this multipart paper. This Part I describes the evolving synoptic pattern, mesoscale kinematics, and calculated water release rates (condensation or deposition) in clouds over the western upstope part of the mountains. Horizontal mesoscale kinematic variables come from direct application of Volume Velocity Processing to single C-band Doppler radar data. Water release rates are computed from updrafts derived from the radar data and from the vertical gradient of saturation mixing ratio obtained from soundings.

In Stage I of the storm altostratus was present on the leading side of a long-wave trough. Weak updrafts occurred only at the higher altitudes within the clouds where there was convergence and large-scale synoptically forced lift. Downdrafts as great as −0.6 m s−1 occurred in the lower parts of the cloud where there was divergence. The downdrafts were induced in part by sublimation cooling of solid (ice) precipitation falling from the altostatus. Only virga was observed and the radar echoes did not reach the surface.

Stage II was initially dominated by passage of a short-wave aloft. Drier air associated with the short-wave led to complete evaporation of the altostratus of Stage I. The lower parts of this cloud (≤4.5 km MSL) eventually redeveloped into altocumulus.

Later in Stage II the wind veered more perpendicular to the mountains. Simultaneously, convergence developed in the lower 900–1200 m of the atmosphere, and mesoscale updrafts of 0.1–0.2 in m s−1 were calculated. Maxima in the water release rate were associated with the updrafts.

During Stage III a passing cold front influenced the kinematics and cloud and precipitation. From prior to frontal passage to a few hours afterward the wind beneath the frontal surface veered from southwesterly to northerly. There was strong convergence at low altitudes just upwind of the Tushar Mountains. It was accompanied by strong, deep mesoscale updrafts extending from near the ground up through the frontal surface and by water release maxima.

The storm changed character after the wind at low altitudes had veered to northerly and had become parallel to the Tushar Mountains. Convergence maxima continued to be present beneath the frontal surface but weaker. They preceded by ∼0.5 h maxima in the convergence above the frontal surface. Associated with these paired convergence features were updraft maxima located above the frontal surface. Water release rates were generally lower than earlier in Stage III. The decrease was greatest at low altitudes beneath the frontal surface where the wind had veered to northerly, where there was little uplift by the Tushar Mountains, and where updrafts were weak. Above the frontal surface the decrease in water release rate was not as great inasmuch as lift by the frontal surface was still occurring.

The storm dissipated in Stage IV. The axis of the longwave trough passed through the area, winds at higher altitudes beneath the frontal surface veered more northerly, and there was substantial drying at all altitudes above and below the frontal surface. The winds beneath the frontal surface were divergent, indicative of subsidence, and mesoscale downdrafts were present.

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