Measurements have been made of the fall speeds and masses of a large number of different types of solid precipitation particles. Particular attention is paid to the effects of riming and aggregation on the fall speeds and masses. Empirical expressions are given for the relationships between fall speeds and maximum dimensions and between masses and maximum dimensions for the particles studied. The results are compared with other experimental observations when they exist.The rate of increase in the mass of an ice particle due to collisions with supercooled cloud droplets (riming) and other ice particles (aggregation) as it moves through a cloud is dependent on its mass, dimensions, and fall speed. Also, in a given wind field the trajectory of a particle is determined by its fall speed, and the contribution that it makes to the precipitation rate is proportional to the product of its mass and fall speed. Consequently, as theoretical models of cloud and precipitation processes have become more refined, the need has increased for more detailed measurements of the relationships between the fall speeds, masses, and dimensions of various types of solid precipitation particles.Although several sets of measurements of the fall speeds and masses of solid precipitation particles of various types and sizes have been reported [Zikmunda and Vali, 1972], the available data are still scanty and inadequate for many purposes. Moreover, some of the previous measurements show inconsistencies, and a complete pattern to the results has not emerged. This lack of pattern is not surprising in view of the fact that Magono and Lee [1966] classify snow crystals into 80 different types, and each of these types may exist over a wide range of sizes and with various degrees of riming and aggregation. In this paper we present the results of a new set of measurements of the fall speeds and masses of a wide variety of solid precipitation particles obtained during the winter months of 1971-1972 and 1972-1973 in the Cascade Mountains of Washington. The effects of size, riming, aggregation, and density on the fall speeds and masses of different types of solid precipitation particles are considered. INSTRUMENTATION AND PROCEDURES The instrument used for measuring the fall speeds of solid precipitation particles is shown in Figure 1. The light sources consist of two incandescent lamps (18 W each), transmitted by fiber optics as two parallel beams of light separated by 4.1 cm. These two light beams are received by a similar set of fiber optics on the other side of the instrument, and the intensity of the two signals is recorded with two photomultiplier tubes. Decreases in the intensities of the beams caused by the fall of a precipitation particle through them are detected by the photomultiplier tubes and can be displayed on a storage oscilloscope. The time difference between the changes in intensity of the upper beam and those changes in the lower beam is Light sources Transmitting fiber optics Receiving fiber optics Photomultiplier tubes 155 Dual trace ...

Despite continual increases in numerical model resolution and significant improvements in the forecasting of many meteorological parameters, progress in quantitative precipitation forecasting (QPF) has been slow. This is attributable in part to deficiencies in the bulk microphysical parameterization (BMP) schemes used in mesoscale models to simulate cloud and precipitation processes. These deficiencies have become more apparent as model resolution has increased. To address these problems requires comprehensive data that can be used to isolate errors in QPF due to BMP schemes from those due to other sources. These same data can then be used to evaluate and improve the microphysical processes and hydrometeor fields simulated by BMP schemes. In response to the need for such data, a group of researchers is collaborating on a study titled the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). IMPROVE has included two field campaigns carried out in the Pacific Northwest: an offshore frontal precipitation study off the Washington coast in January–February 2001, and an orographic precipitation study in the Oregon Cascade Mountains in November–December 2001. Twenty-eight intensive observation periods yielded a uniquely comprehensive dataset that includes in situ airborne observations of cloud and precipitation microphysical parameters; remotely sensed reflectivity, dual-Doppler, and polarimetric quantities; upper-air wind, temperature, and humidity data; and a wide variety of surface-based meteorological, precipitation, and microphysical data. These data are being used to test mesoscale model simulations of the observed storm systems and, in particular, to evaluate and improve the BMP schemes used in such models. These studies should lead to improved QPF in operational forecast models.

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From ice particles collected in clouds and at ground stations in the Cascade Mountains during two winter seasons, measurements have been obtained of the lengths and widths of needles and columnar crystals and the thicknesses and diameters of hexagonal plates. Best-fit empirical relationships between these dimensions are presented. The particles have also been examined to obtain detailed information on the properties of aggregates of ice particles. The maximum dimensions of aggregates and the probability of the occurrence of aggregates decrease with decreasing temperature, but both exhibit a local maximum near the dendritic growth region. Below -15øC and for particle concentrations less than 0.1 cm -8, aggregates are unlikely to form; above -5øC and for particle concentrations in excess of 1 cm -8 there is more than a 50% chance of the formation of aggregates. The size spectra of aggregates depend mainly on the sizes of their component crystals. The number of component crystals in aggregates of various sizes is given. ß ß ß ß , ß ß ß ß ß ß Aß ß •ß ß ß ß ß ß <.005 Precipitation rate (inches of water per hour) Fig. 14. Precipitation rates and ground temperatures at which aggregates composed of two or more bullets, side planes, columns, assemblages of plates, and assemblages of sectors were collected at Hyak and Keechelus Dam (triangles) and at Alpental base (circles). Units in parentheses are in millimeters per hour. vestigated or that if such effects exist, they are masked by other variables. Note, however, that aggregates of the same crystal type and size distribution collected on different days may not have the same distributions of mass. Number of crystals in aggregates. From the measurements of the masses of single ice particles and their aggregates given by LocateIll and Hobbs [1974], the number of component ice >1 oto I -I to o = -2t0-1 E ...e -3 to-2 o c• -4 to-3 ac -5 to -4 >(-5) ß ß ß ß ß ß ß ß ß ß ß ß ß ß ßß ßß ß ß © ß ee ß <.005 .005-Di .0i-.02

A mesoscale model simulation of a wide cold-frontal rainband observed in the Pacific Northwest during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-1) field study was used to test the sensitivity of the model-produced precipitation to varied representations of snow particles in a bulk microphysical scheme. Tests of sensitivity to snow habit type, by using empirical relationships for mass and velocity versus diameter, demonstrated the defectiveness of the conventional assumption of snow particles as constant density spheres. More realistic empirical massdiameter relationships result in increased numbers of particles and shift the snow size distribution toward larger particles, leading to increased depositional growth of snow and decreased cloud water production. Use of realistic empirical mass-diameter relationships generally increased precipitation at the surface as the rainband interacted with the orography, with more limited increases occurring offshore. Changes in both the mass-diameter and velocity-diameter relationships significantly redistributed precipitation either windward or leeward when the rainband interacted with the mountain barrier.A method of predicting snow particle habit in a bulk microphysical scheme, and using predicted habit to dynamically determine snow properties in the scheme, was developed and tested. The scheme performed well at predicting the habits present (or not present) in aircraft observations of the rainband. Use of the scheme resulted in little change in the precipitation rate at the ground for the rainband offshore, but significantly increased precipitation when the rainband interacted with the windward slope of the Olympic Mountains. The study demonstrates the promise of the habit prediction approach to treating snow in bulk microphysical schemes.

Particle size spectra collected by the University of Washington's Convair-580 research aircraft at a variety of altitudes and temperatures in winter frontal and orographic precipitation systems during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE) are analyzed in this study. The particle size spectra generally appeared to conform to an exponential size distribution, with well-correlated linear fits between the log of the number concentration and particle diameter. When the particle size spectra were grouped according to the habit composition as determined from airborne imagery, significantly improved correlations between the size spectrum parameters and temperature were obtained. This result could potentially be exploited for specifying the size distribution in a single-moment bulk microphysical scheme, if particle habit is predicted by the scheme. Analyses of "spectral trajectories" suggest that the rime-splintering process was likely responsible for the presence of needle and column habit types and the positive shift in both N 0s and s at temperatures warmer than Ϫ10°C.

On 1–2 February 2001, a strong cyclonic storm system developed over the northeastern Pacific Ocean and moved onto the Washington coast. This storm was one of several that were documented during the first field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). In the 1–2 February case, soundings and wind profiler measurements showed that a wide cold-frontal rainband was coincident with the leading edge of an upper-level cold front in a classical warm occlusion. Ground-based radar observations revealed the presence of subbands within the wide cold-frontal rainband and two layers of precipitation generating cells within this rainband: one at 5–7 km MSL and the other at 9–10 km MSL. The lower layer of generating cells produced fallstreaks that were traced from the cells down to the radar bright band at 2 km MSL. Observations suggest a connection between the subbands and the lower layer of generating cells. A research aircraft, equipped for cloud microphysical measurements, passed through at least two generating cells in the 5–7-km region. These cells were in their formative stage, with elevated liquid water contents and low ice particle concentrations. The microphysical structure of the wide cold-frontal rainband was elucidated by particle imagery from a Cloud Particle Imaging (CPI) probe aboard the research aircraft. These images provide detailed information on crystal habits and degrees of riming throughout the depth of the rainband. The crystal habits are used to deduce the temperature and saturation conditions under which the crystals grew and, along with in situ measurements of particle size spectra, they are used to estimate particle terminal fall velocities, precipitation rates, radar reflectivities, and vertical air motions. The radar reflectivity derived in this way generally compared well with direct measurement. Both the derived and directly measured parameters are used to determine the primary particle growth processes in the wide cold-frontal rainband. Above the melting layer, vapor deposition was the dominant growth process in the rainband; growth of ice particles by riming was small. Significant aggregation of ice particles occurred in the region just above the melting layer. A doubling in the air-relative vertical precipitation mass flux occurred between the region where sheath ice crystals formed (−3° ≤ T ≤ −8°C) and the surface. Substantial amounts of liquid water were found within the melting layer where growth occurred by the accretion of cloud droplets and also by condensation. Growth by the collision and coalescence of raindrops was not significant below the melting layer.

On 13–14 December 2001 a vigorous cyclonic storm passed over the Pacific Northwest, producing heavy orographic precipitation over the Cascade Mountains. This storm was one of several studied during the second field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). A wide variety of in situ and remotely sensed measurements were obtained as this storm passed over the Oregon Cascades. These measurements provided a comprehensive dataset of meteorological state parameters (temperature, pressure, humidity, winds, and vertical air velocity), polarization Doppler radar measurements, and cloud microphysical parameters (cloud liquid water, particle concentrations, size spectra, and imagery). The 13–14 December case was characterized by the passage of a tipped-forward lower-tropospheric front that extended upward to a preceding vigorous upper cold-frontal rainband, which produced clouds up to ∼8–9 km. An important difference between this storm and those studied previously over the Washington Cascades was that the prefrontal low-level airflow over the Oregon Cascades was characterized by strong westerly (as opposed to weak easterly) cross-barrier flow. Consequently, as the upper cold-frontal band passed over the Oregon Cascades there was both strong ice particle production aloft and significant production of liquid water at lower levels in the orographic lifting zone. Airborne in situ measurements, ground-based microwave radiometer measurements, and observations of snow crystals showed the simultaneous presence of high ice crystal concentrations and relatively large values of cloud liquid water aloft, and heavily rimed particles reaching the ground. Analyses indicate that a synergistic interaction occurred between the frontal and orographic precipitation.