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H. R. Pruppacher and R. Rasmussen

Abstract

An experimental study of the effect of ventilation on the rate of evaporation of millimeter sized water drops failing at terminal velocity in air has been carried out in a wind tunnel where drops were suspended freely in the tunnel air stream. It was found that for drops in the size range 1150 µm≤a 0≤2500 µm, the mean ventilation coefficient f̄vh could be expressed as f=(0.78±0.02)+(0.308±0.010)X, where X=N &frac13 Sc,v N½ Re. Previously, we showed that this relation holds for drops in the size range 60 µm≤a 0≤400 µm. Taken together, our present and previous data suggest that with reasonable accuracy f̄=0.78+0.308X, for 1.4≤X≤51.4 (60 µm≤a 0≤2500 µm). For 0≤X≤1.4 (0≤a≤60 µm), one may use our previous result f=1.00+0.108 X 2. To illustrate how the present data may be applied, we computed the distance which is required for a water drop to travel from cloud base through a NACA Standard Atmosphere of various relative humidities, in order to reach the earth's surface with a given size.

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R. Rasmussen and H. R. Pruppacher

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A study has been made on the melting behavior of frozen drops suspended freely at terminal velocity in the UCLA Cloud Tunnel. The relative humidity of the air ranged between 25 and 95%. The warming rates of the tunnel air stream ranged from 2 to 5°C min−1, which for the studied ice particles of radius between 200 and 500 μm, corresponded to warming rates of 0.8 to 5.2°C per 100 m of fall. The rate of melting of the frozen drops and their fall behavior during melting were continually monitored by motion picture. From these observations the dimensions of the ice core inside the melting drop was determined as a function of time, and from the latter the total melting time was found. The present wind tunnel observations were compared with the theoretical predictions of Mason (1956). Considerable disagreement with Mason's theory was found. This disagreement was attributed to the pronounced asymmetric melting and the internal circulation in the melt water, both of which were disregarded in Mason's theory.

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R. M. Rasmussen and A. J. Heymsfield

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A simple parameterization is presented which allows calculation of surface averaged radial impact velocities for droplets colliding with spheres as a function of the Reynolds and Stokes numbers. These impact velocities are averaged over the collector particle surface, assuming that the incoming droplets are uniformly distributed in space. The results extend the experimental graupel density measurements of Pflaum and Pruppacher to a wide range of graupel masses, sizes, and fallspeeds.

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K. L. Rasmussen and R. A. Houze Jr.

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Satellite radar and radiometer data indicate that subtropical South America has some of the deepest and most extreme convective storms on Earth. This study uses the full 15-yr TRMM Precipitation Radar dataset in conjunction with high-resolution simulations from the Weather Research and Forecasting Model to better understand the physical factors that control the climatology of high-impact weather in subtropical South America. The occurrence of intense storms with an extreme horizontal dimension is generally associated with lee cyclogenesis and a strengthening South American low-level jet (SALLJ) in the La Plata basin. The orography of the Andes is critical, and model sensitivity calculations removing and/or reducing various topographic features indicate the orographic control on the initiation of convection and its upscale growth into mesoscale convective systems (MCSs). Reduced Andes experiments show more widespread convective initiation, weaker average storm intensity, and more rapid propagation of the MCS to the east (reminiscent of the MCS life cycle downstream of lower mountains such as the Rockies). With reduced Andes, lee cyclogenesis and SALLJ winds are weaker, while they are stronger in increased Andes runs. The presence of the Sierras de Córdoba (secondary mountain range east of the Andes in Argentina) focuses convective initiation and results in more intense storms in experiments with higher Andes. Average CAPE and CIN values for each terrain modification simulation show that reduced Andes runs had lower CIN and CAPE, while increased Andes runs had both stronger CAPE and CIN. From this research, a conceptual model for convective storm environments leading to convective initiation has been developed for subtropical South America.

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R. M. Rasmussen, V. Levizzani, and H. R. Pruppacher

Abstract

An experimental and theoretical study has been performed on the melting of spherical ice particles between 3 and 20 mm in diameter. For the experimental study the UCLA Cloud Tunnel was employed in determining the melting rate, the mode of melting, the shedding rate, and the hydrodynamic behavior of the melting ice particles. Our experimental results demonstrate that the melting mode of ice particles can be grouped into distinct categories depending on the Reynolds number. For these categories, comparison was made to various theoretical expressions reported in literature and to our own formulations. These comparisons show that experiment and the appropriate theory agree within experimental error.

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R. M. Rasmussen, V. Levizzani, and H. R. Pruppacher

Abstract

The internal and external heat transfer of a melting spherical ice particles less than 500 μm radius has been investigated theoretically. The effect of an internal circulation and eccentric location of the ice core was modeled. These two effects combined to reduce total melting times by ∼10%. However, this still left a 10–15% difference between theoretical and experimental melting times which could not be explained by experimental error. The external heat transfer was subsequently investigated, and it is postulated that: 1) surface irregularities and nonsphericity, 2) rear eddy shedding, and 3) nonsteady motions, are able to increase the external ventilation coefficient by a factor of two, and thus account for the observed discrepancy in melting times.

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V. N. Bringi, R. M. Rasmussen, and J. Vivekanandan

Abstract

This paper presents an analysis of coordinated measurements taken by the NCAR CP-2 radar and the Wyoming King Air aircraft flown by NCAR during the May Polarization Experiment, which was held new Boulder, Colorado. A key feature of this paper is the rigorous computation Of ZH, Z DR and LDR using an electromagnetic model that is coupled to a detailed one-dimensional microphysical model of melting graupel. The graupel melting model was initialized with aircraft-measured graupel spectrum and sounding data. Two case studies during MAYPOLE '83 were considered where model-derived vertical profiles of ZH and Z DR were computed and compared to radar measurements, resulting in excellent agreement. The RHI profiles of ZH, Z DR and LDR through the core of an isolated convective storm revealed new and interesting microphysical data which are consistent with model computations. The radar measurements and model results of this paper show that a significant breakthrough has been made in the radar remote sensing of storm microphysics.

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Julie M. Thériault, Nicolas R. Leroux, and Roy Rasmussen

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Accurate snowfall measurement is challenging because it depends on the precipitation gauge used, meteorological conditions, and the precipitation microphysics. Upstream of weighing gauges, the flow field is disturbed by the gauge and any shielding used usually creates an updraft, which deflects solid precipitation from falling in the gauge resulting in significant undercatch. Wind shields are often used with weighing gauges to reduce this updraft and transfer functions are required to adjust the snowfall measurements to consider gauge undercatch. Using these functions reduce the bias in precipitation measurement but not the Root Mean Square Error (RMSE) (Kochendorfer et al. 2017a, b). The analysis performed in this study shows that the hotplate precipitation gauge bias after wind correction is near zero and similar to wind corrected weighing gauges but improves on the RMSE or scatter of the collection efficiency of weighing gauges for a given wind speed. To do this, the accuracy of the hotplate was compared to standard unshielded and shielded weighing gauges collected during the WMO SPICE program. The RMSE of the hotplate measurements is lower than weighing gauges (with or without an Alter shield) for wind speeds up to 5 m s-1; the wind speed limit at which sufficient data were available. This study shows that the hotplate precipitation measurement has a low bias and RMSE due to its aerodynamic shape, making its performance mostly independent of the type of solid precipitation.

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R. A. Houze Jr., K. L. Rasmussen, S. Medina, S. R. Brodzik, and U. Romatschke

Abstract

No abstract available.

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Jeffrey K. Lew, Derek C. Montague, Hans R. Pruppacher, and Roy M. Rasmussen

Abstract

The effects of porosity on the accretional growth characteristics of ice crystal aggregates (snowflakes) are investigated by riming circular disks of ice in a cloud tunnel. Twelve disk models were used, sized 5 to 6 mm and 10 to 11 mm in diameter, with various hole sizes and numbers, resulting in porosities ranging between 15% and 50%. The onset for riming occurred much earlier for porous dish than for similarly sized nonporous disks as the tunnel airflow speed was increased. For porosities in excess of 15%, the rime growth rates were found to be relatively insensitive to the extent of porosity. However, these rates were an order of magnitude greater than those for nonporous disks of the same size and rimed at the same flow velocity. The appearance of the rime was similar to that observed in the atmosphere for similar conditions. A large stellar model was rimed using the same techniques, and its riming rate was found to be in fair agreement with previous experiments.

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