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Roland List, C. Fung, and R. Nissen

Abstract

Previous breakup experiments have been carried out at laboratory pressures (∼100 kPa). However, raindrop interactions mainly take place higher up in the atmosphere, even in the supercooled part of a cloud where drops can be initiated by shedding from hailstones. Thus, 50 kPa, corresponding to a height of ∼5.5 km in the atmosphere at a temperature of ∼−20°C, was selected to bracket the region of interest for rain. Six drop pairs were studied at 50 kPa and laboratory temperature (∼20°C), one of them with reduced surface tension.

The apparatus consists of drop-producing nozzles, acceleration systems, deflectors, a timing and selection control, a pressure regulator, and a photographic unit, mostly set up in a low-pressure chamber. After acceleration to terminal speed, a smaller drop is blown into the path of the larger one while an electronic timing system selects suitable drop pairs that may collide, thereby triggering eight subsequent flashes with a frequency of up to 100 kHz. The results are displayed in terms of a normalized fragment probability per size bin, ready for parameterization in the Part II of this paper.

Five drop pairs were studied in 772 individual events. Overall, 51% resulted in filament breakup, 22% in sheet breakup, 7% in disk breakup, and 20% ended in coalescence. No bag breakups were observed. When compared to the 100-kPa results, the fragment numbers increased at large collision kinetic energies (CKEs) by factors of between 2.64 and 4.37 with pressure decreasing from 100 to 50 kPa, and they remained unchanged at low CKE. Detailed diagrams and tables show the results for the different drop pairs and collision categories. Increasing the sensitivity of the optical measurements from 0.05 to 0.01 cm increased the number of recognized fragments by factors up to 4.4, but only for the two higher-CKE cases. The higher resolution did not increase the fragment numbers detected in the lower-CKE range.

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Roland List, R. Nissen, and C. Fung

Abstract

Fragment size distributions, experimentally obtained for six drop pairs colliding at 50 kPa, are parameterized similarly to the 100-kPa drop pair experiments by Low and List. This information is then introduced into a box model to allow assessment of the spectra evolution and a comparison of the two datasets taken at the two pressures. The differences in breakup patterns include the following: The contributions to mass transfer by breakup and coalescence are very similar at the two pressures, with larger values at lower pressure; the overall mass evolution is not particularly sensitive to pressure; and disk breakup plays an “erratic” role. The situation for the number concentration, however, is totally different and develops gradually. At 50 kPa there is also no three-peak equilibrium developing as for 100 kPa. The times to reach equilibrium are ∼12 h. Note that the box model does not include accretion of cloud droplets—which may well be more important than growth by accretion of fragments.

Application of the new parameterization is not beneficial for low rain rates, but it is strongly recommended for large rain rates (>50 mm h−1).

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I. Gultepe, G. Isaac, D. Hudak, R. Nissen, and J. W. Strapp

Abstract

In this study, observations from aircraft, Doppler radar, and LANDSAT are used to better understand dynamical and microphysical characteristics of low-level Arctic clouds for climate change studies. Observations during the Beaufort and Arctic Storms Experiment were collected over the southern Beaufort Sea and the northern Mackenzie River Basin during 1 September–14 October 1994. Measurements from the cases of 8 September and 24–25 September are analyzed. In situ observations were made by instruments mounted on a Convair-580 research aircraft. Reflectivity and radial winds were obtained from an X-band Doppler radar located near Inuvik. The reflectivity field from LANDSAT observations concurrent with the aircraft and radar observations was also obtained. Dynamical activity, representing vertical air velocity (w a) and turbulent fluxes, is found to be larger in cloud regions. The sizes of coherent structures (e.g., cells) are from 0.1 to 15 km as determined by wavelet analysis and time series of aircraft data. This size is comparable with LANDSAT and Doppler radar–derived cell sizes. Reflectivity in embedded cells for the 8 September case was larger than that of single convective cells for the 24–25 September case. The effective radius for ice crystals (droplets) ranged from 37(7.5) μm to 70(9.5) μm for both cases. Using observations, parameterization of the ice crystal number concentration (N i) is obtained from a heat budget equation. Results showed that N i is a function of w a, radiative cooling, particle size, and supersaturation. The large-scale models may have large uncertainties related to microphysical and dynamical processes (e.g., particle size and vertical air velocity, respectively), which can directly or indirectly influence radiative processes. Overall, the results suggest that the microphysical and dynamical properties of Arctic clouds need to be further explored for climate change studies.

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Greg M. McFarquhar, Roland List, David R. Hudak, Robert P. Nissen, J. S. Dobbie, N. P. Tung, and T. S. Kang

Abstract

During the Joint Tropical Rain Experiment of the Malaysian Meteorological Service and the University of Toronto, pulsating raindrop ensembles, hereafter pulses, were observed in and around Penang Island. Using a Doppler radar on 25 October 1990, a periodic variation of precipitation aloft 30 km from the radar site, with an approximate 8-min period, was established and seemed to be caused by the evolution and motion of horizontal inhomogeneities existing within the same cell. On 30 October 1990, using a new volume scanning strategy with a repetition cycle of 3.5 min, pulsations of the same frequency were observed up to 3 km above the radar and at the ground by a disdrometer. High concentrations of large drops were followed by high concentrations of successively smaller drops at the ground. This provides observational evidence to support the recent argument for using a time-varying release of precipitation-sized particles to model observed pulsating rainfall.

Many cases of nonsteady rain from convective clouds displayed repetition periods of between 8 and 25 min.

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Robert Nissen, Roland List, David Hudak, Greg M. McFarquhar, R. Paul Lawson, N. P. Tung, S. K. Soo, and T. S. Kang

Abstract

For nonconvective, steady light rain with rain rates <5 mm h−1 the mean Doppler velocity of raindrop spectra was found to be constant below the melting band, when the drop-free fall speed was adjusted for pressure. The Doppler radar–weighted raindrop diameters varied from case to case from 1.5 to 2.5 mm while rain rates changed from 1.2 to 2.9 mm h−1. Significant changes of advected velocity moments were observed over periods of 4 min.

These findings were corroborated by three independent systems: a Doppler radar for establishing vertical air speed and mean terminal drop speeds [using extended Velocity Azimuth Display (EVAD) analyses], a Joss–Waldvogel disdrometer at the ground, and a Particle Measuring System (PMS) 2-DP probe flown on an aircraft. These measurements were supported by data from upper-air soundings. The reason why inferred raindrop spectra do not change with height is the negligible interaction rate between raindrops at low rain rates. At low rain rates, numerical box models of drop collisions strongly support this interpretation. It was found that increasing characteristic drop diameters are correlated with increasing rain rates.

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