Arthur L. Rangno

Ice Particle Concentrations in Clouds

Abstract Measurements and observations have been made on the development of ice in 90 cumulus (cumulus and cumulonimbus) and 72 stratiform (altocumulus, altostratus, nimbostratus, stratocumulus, and stratus) clouds. Ice particle concentrations significantly in excess of those to be expected from ice nucleus measurements (i.e., ice enhancement) were measured in 42 of the cumuliform and 36 of the stratiform clouds. For the complete data set, and for cloud top temperatures (TT) between −6° and −32°C, the maximum concentrations of ice particles (Imax in L−1) in the clouds were essentially independent of TT(r=0.32). However, Imax was strongly dependent on the broadness of the cloud droplet size distribution near cloud top. If the breadth of the droplet size distribution is measured by DT, such that the cumulative concentration of droplets with diameters ≥DT exceeds a prescribed value, then for −32≤TT≤−6°C:where n=8.4 and DO=18.5 μm for the cumuliform clouds and n=6.6 and DO=19.4 μm for the stratiform clouds. W...

Improvement of Microphysical Parameterization through Observational Verification Experiment

Abstract 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 th...

Ice particles in stratiform clouds in the Arctic and possible mechanisms for the production of high ice concentrations

The presence of ice particles in clouds affects precipitation processes, the radiative properties of the clouds, and the derivation of cloud properties from remote sensing measurements. High ice particle concentrations occur often in slightly to moderately supercooled clouds in the Arctic. This paper combines data collected in a common type of ice-producing arctic cloud (stratocumulus) with calculations based on laboratory experiments to elucidate mechanisms that might be responsible for the ice. Ice splinters produced during riming could account for the relatively high concentrations of ice particles in clouds that encompass temperatures between −2.5°C and −8°C. However, it has generally been assumed that ice splinters grow into pristine ice crystal habits, whereas detailed measurements in an arctic stratocumulus cloud showed that only 32% of the ice particles were pristine crystals (needles, sheaths, short columns, and plates) and 10% were broken pieces of needles or sheaths. Thirty-seven percent of the ice particles were not identifiable crystal types, 20% were frozen drops, and 1% were aggregates and graupel. The large numbers of unidentifiable ice particles could have originated from the fragmentation of delicate ice crystals and the shattering of some drops during freezing in free fall. These two mechanisms may also play a role in the production of relatively high ice particle concentrations in moderately supercooled arctic clouds that lie outside of the temperature zone where ice splinter production by riming occurs.

Rapid Development of High Ice Particle Concentrations in Small Polar Maritime Cumuliform Clouds

Abstract Extremely high ice particle concentrations developed rapidly in the ascending tops of maritime cumulus congestus clouds after drizzle drops had already formed below this level by the collision–coalescence mechanism. In one building cloud with a top temperature no colder than −8°C, the ice particle concentrations increased from 0 to >350 L−1 within 9 min. In another cloud with a top temperature no colder than −13°C, the ice particle concentrations increased from ≤1 to ∼1100 L−1 within 12 min. Subsequently, the ice particle concentrations in these clouds decreased, even though the cloud top temperature of one of the clouds continued to decrease to −23.5°C. The mechanism responsible for these prodigious increases in ice particle concentrations is not clear. The concentrations built too fast to be explained by the riming-splintering mechanism as it is presently formulated. It is suggested that high ice particle concentrations might form in localized pockets of high supersaturation with respect to water.

Relationships between cloud droplet effective radius, liquid water content, and droplet concentration for warm clouds in Brazil embedded in biomass smoke

During the Smoke, Clouds, and Radiation-Brazil (SCAR-B) project, the microphysical properties of over 1000 warm, nonprecipitating, clouds were measured from the University of Washington research aircraft. The clouds were partially embedded in the continental-scale, smoky haze that envelops much of Brazil during the biomass-burning season. For the entire data set, the most universal parameterization for the effective cloud droplet radius (r eff ) is as a function of the ratio of cloud liquid water content (LWC) to droplet concentration (essentially the volume mean radius, r v ); this agrees with previous studies under less polluted conditions. Comparisons of SCAR-B data with data from the east coast of the United States and clean oceanic areas show that the r eff -r v relationship is similar in all three cases, suggesting that even the extreme case of clouds impacted by large biomass fires can be treated similarly to more typical clouds. Beyond a certain ambient concentration of accumulation-mode particles (∼3000-4000 cm -3 ), cloud drop number concentrations for cumulus clouds in Brazil were almost constant, so that further increases in the ambient particle concentration did not change r eff , and r eff correlates well with LWC alone. For example, a cumulus cloud, which capped a particularly arge smoke plume with total particle concentrations >150,000 cm -3 , had the same r eff -LWC relationship as other clouds in the region where the ambient particle concentrations were ∼3000 cm -3 . In this study the values of r eff for cumulus clouds in Brazil affected by smoke were between 3 and 8 μm, compared to 9 to 14 μm inferred from satellite measurements of cloud reflectivity at 3.7 μm by Kaufinan and Fraser [1997].

Aerosol measurements in the Arctic relevant to direct and indirect radiative forcing

Airborne measurements in the Arctic in June permit calculation of some of the parameters needed to assess both the direct and indirect radiative forcing by aerosols in the region. Values for the single-scattering albedo of the aerosols suggest that in June the direct effect will produce a net cooling, in contrast to winter arctic hazes. Internal closure calculations comparing aerosol size distribution measurements with directly measured light scattering by aerosols over the first 4 km of the atmosphere show good agreement. Measurements of cloud condensation nucleus (CCN) activation spectra show steeper slopes than previous measurements in the Arctic in winter and early spring. Based on the CCN and collated cloud microphysical measurements, the susceptibility of the clouds encountered in this project to aerosol-induced albedo modification appears quite high.

Production of Ice Particles in Clouds Due to Aircraft Penetrations

Abstract Evidence is presented that the passage of an aircraft through supercooled clouds can produce high concentrations of ice particles (> 1000 L−1 at −8°C in one case). These Aircraft Produced Ice Particles (APIPs) are characterized, initially, both by their high concentration and very uniform size distribution. The ice particles are contained in a cylindrical-like volume of air that is initially oriented along the flight track of the aircraft; the diameter of the cylinder is ∼300 m after 5 min of elapsed time. Possible mechanisms for APIPs are discussed. In view of these findings, care must be taken in the interpretation of data on clouds that have been penetrated by aircraft. It seems likely that some previous observations of abnormally high concentrations of ice particles in clouds were due to APIPs. Also, since APIPs mimic some of the effects produced by deliberate artificial seeding, it could complicate the evaluation of cloud seeding experiments.

Airborne studies of cloud structures over the Arctic Ocean and comparisons with retrievals from ship-based remote sensing measurements

Information on the heights and microphysical structures of two cloud systems derived from a 35 GHz radar, microwave and infrared radiometers, and a lidar aboard a ship in the Arctic Ocean are compared with simultaneous airborne in situ measurements. The cloud systems considered are a single layer of thin altocumulus with virga (June 3, 1998), and a more complex cloud system consisting of several altocumulus-altostratus layers that precipitated into a boundary layer, stratus-stratocumulus system (May 29, 1998). For the first cloud system the cloud top deduced from the 35 GHz radar was close to that measured from the aircraft. The radar detected virga below cloud base even when the virga was composed of very low concentrations of ice crystals; this prevented measurement of the cloud base height with the radar. Because of the sensitivity of the 35 GHz radar to ice crystals, cloud liquid water contents derived from it are confounded by just a few ice crystals. In the case of the second more complex cloud system, embedded cloud liquid water layers into which ice particles fell were not resolved by the radar. Consequently, although five altocumulus layers were intercepted by the aircraft, the radar display depicts a single deep precipitating system. These cases illustrate that liquid water content in mixed-phase clouds cannot be retrieved reliably using the radar-microwave radiometer technique.

A New Look at the Israeli Cloud Seeding Experiments

Abstract Two statistical experiments, carried out in Israel, appeared for a time to have provided a unique demonstration of the ability of cloud seeding to increase rainfall. In this paper the authors examine the possibility that both experiments were compromised by type I statistical errors (i.e., “lucky draws” or false positives). It is concluded that in the first Israeli experiment a type I statistical error produced the appearance of statistically significant effects of artificial seeding on rainfall 1) in the buffer zone and the center target area, 2) in the coastal region of Israel, a few kilometers downwind of the seeding, and 3) in portions of Lebanon, Syria, and Jordan. Analysis of the second Israeli experiment using the original crossover design produced a null result. However, when the two target areas were evaluated separately, naturally heavier rainfall over a wide region on days when the north target area was seeded produced the appearance of increases in rainfall due to seeding in the north...

TRMM Common Microphysics Products: A Tool for Evaluating Spaceborne Precipitation Retrieval Algorithms

Abstract A customized product for analysis of microphysics data collected from aircraft during field campaigns in support of the Tropical Rainfall Measuring Mission (TRMM) program is described. These “common microphysics products” (CMPs) are designed to aid in evaluation of TRMM spaceborne precipitation retrieval algorithms. Information needed for this purpose (e.g., particle size spectra and habit, liquid and ice water content) was derived by using a common processing strategy on the wide variety of microphysical instruments and raw native data formats employed in the field campaigns. The CMPs are organized into an American Standard Code for Information Interchange (ASCII) structure to allow easy access to the data for those less familiar with microphysical data processing and without the tools to accomplish it. Detailed examples of the CMP show its potential and some of its limitations. This approach may be a first step toward developing a generalized microphysics format and an associated community-orie...