Search Results

You are looking at 1 - 10 of 17 items for

  • Author or Editor: Jeffrey L. Stith x
  • Refine by Access: All Content x
Clear All Modify Search
Jeffrey L. Stith

Abstract

The results from an instrumented aircraft penetration of two mamma elements associated with a severe hailstorm are presented. Maximum downdrafts in the clouds were near 2 m s−1. The main downdraft region was warmer than the air outside of the cloud. Aggregate ice particles of average diameter just over 1 mm predominated. The edge of the cloud was not descending. The cloud was smooth and contained only light turbulence. These results are discussed in the context of theories of mamma formation.

Full access
Jeffrey L. Stith

Abstract

Sulfur hexafluoride tracer gas was released during single aircraft passes just above growing convective turrets to study its entrainment into the clouds as they grew through the release altitude. The tracer was sampled in situ from a second research aircraft that carried a real-time sulfur hexafluoride analyzer. The results from three experiments are presented. They were done with clouds ranging in size from a vigorous convective turret to a small cumulus.The observations suggest that during the early stages of entrainment, the tracer remained mostly out of the cloud and was carried alongside the upper cloud regions by the circulation present there. In each experiment, concentrated tracer was first found on the edges of the turrets. Later, the tracer mixed into the central portions of the turrets where it had diluted considerably and mixed through most of the turret. The observations are consistent with the hypothesis that cloud-top entrainment occurs through a vortex-like circulation that brings air from above the cloud into the central region of the cloud. The results are compared to some recent conceptual and numerical models of entrainment.

Full access
Jeffrey L. Stith and Richard L. Benner

Abstract

The airborne applications of two recently developed analyzers for sulfur hexafluoride (SF6) to investigations of cloud top mixing and cloud seeding are described. The analyzers were developed by AeroVironment (AV) and by Washington State University (WSU). Both analyzers were capable of detecting cumulus-scale plume features. The more elaborate flow control mechanism in the AV analyzer was helpful in reducing the effects of altitude on the instrument response, while the faster response and lower baseline noise level of the WSU analyzer were necessary to detect many plume features.

A midcloud injection of SF6 was followed as it mixed through the tops of a small cumulus cloud. The tracer plume was first detected upshear, then mixed through the cloud top region as the cloud top began to collapse.

A plume of AgI cloud seeding agent mixed with SF6 was used to investigate the activation and growth of ice particles in a stratocumulus cloud which was overseeded. The SF6 tracer and ice particle plumes remained colocated during the 45 min sampling period, except for one region of ice particles which had begun to separate from the SF6 26 min after the cloud was treated. The growth of ice was limited by water vapor diffusion into the seeding plume. The measured tracer concentrations were used to estimate the fraction of the seeding nuclei which had activated and grown to detectable sizes. A maximum fraction of 54% was observed 17.5 min after seeding.

Several other applications for SF6 tracer applications are recommended.

Full access
Jeffrey L. Stith and Marcia K. Politovich

Abstract

Sulfur hexafluoride was released at the base of a small nonprecipitating warm cumulus to study cloud mixing and entrainment processes. The tracer gas traveled to the top of the cloud where, during a 2.5 min period, it had mixed to produce a dilute mixture containing 30%, 19% and 51% of air from the original tracer region, an adjacent region of the same cloud, and the environment surrounding the cloud, respectively. The droplet size distributions measured at the top of the cloud represented a mixture of larger droplets that had been growing from the base and smaller, recently activated droplets. The observations suggest that the source region for the small droplets was near cloud top. The large droplet concentration was conserved during the mixing process. These observations are compared with predictions from some recent models for cloud entrainment and droplet evolution.

Full access
Peter V. Hobbs, Jeffrey L. Stith, and Lawrence F. Radke

Abstract

The concentrations of cloud condensation nuclei (CCN) in the plumes from coal-fired electric power plants are generally about 2 to 5 times greater than in the ambient air unaffected by the plumes. However, if the ambient air is very clean, the concentrations of CCN in a coal power plant plume can be up to ∼80 times greater than in the ambient air. The rates of production of CCN due to gas-to-particle (g-to-p) conversion in the plume from one of the plants studied were measured on different occasions to be ∼2 × 1015 and ∼5 × 1013 CCN h−1 per mole of SO2. The maximum current of CCN to be expected in the plume from a coal power plant is ∼1017 CCN s−1. After a travel time of ∼1 h, most of the CCN in power plant plumes have been produced by g-to-p conversion rather than emitted directly from the stack.

The concentrations of ice nuclei in the plumes did not differ significantly from those in the ambient air.

The materials in a plume may be transported rapidly in the vertical if the plume is entrained into a convective cloud. The plume may cause a lowering in the altitude of the cloud base, but any effects that the plume may have on the drop size distribution in a convective cloud are often less than the natural variations. By contrast, in stratiform clouds a plume can produce marked increases in the concentration of small drops (∼10–20 μm diameter) and in the total concentrations of drops

Full access
Timothy J. Lang, Steven A. Rutledge, and Jeffrey L. Stith

Abstract

On a few occasions during the summer and fall of 2002, and again in the fall of 2003, the Colorado State University (CSU)–University of Chicago–Illinois State Water Survey (CHILL) S-band polarimetric Doppler radar observed dumbbell-shaped radar echo patterns in precipitation-free air returns. Dumbbell shaped refers to two distinct and quasi-symmetrical regions of echo surrounding the radar. These were horizontally widespread (thousands of square kilometers) layers, with the highest reflectivity factors (sometimes >20 dBZ) arranged approximately perpendicular to the direction of the mean wind. The echoes coincided with strongly positive differential reflectivity (Z DR) measurements (often >4 dB). Most interestingly, the echoes were elevated near the top of the boundary layer in the 2–3-km-AGL vertical range. Assuming a horizontally uniform layer of scatterers, the observations suggest that targets aloft are quasi prolate in shape and aligned horizontally along the direction of the mean wind. The echoes tended to occur on days when nocturnal inversions persisted into the following day, and solenoidal-like circulations (easterly upslope near the surface, and westerly flow aloft) existed. In some cases, the echoes exhibited diurnal behavior, with dumbbell-shaped echoes only occurring during the day and a more azimuthally uniform echo at night. On occasion, the echoes were coincident with the occurrence of widespread smoke from nearby forest fires. It is suggested that these echoes, which are rare for the CSU–CHILL coverage region, were caused by insects flying in a preferred direction, with the trigger for the migration being either the forest fires or oncoming winter. The local meteorological conditions likely affected the structure of these echoes.

Full access
Nicholas F. Anderson, Cedric A. Grainger, and Jeffrey L. Stith

Abstract

Airborne in situ measurements of updrafts in tropical convective storms were analyzed to determine the similarities and differences between updrafts in a tropical continental and a tropical oceanic region. Two hundred fifteen updraft cores from the Tropical Rainfall Measuring Mission (TRMM) component of the Large Scale Biosphere–Atmosphere (LBA) experiment (tropical continental wet season) and 377 updraft cores from the Kwajalein Experiment (KWAJEX) (tropical oceanic) were analyzed in a similar manner to that of previous studies of tropical updrafts. Average speed, maximum speed, width, and mass flux of the updraft cores from the TRMM-LBA and KWAJEX were generally similar to each other and also were similar to results from previous studies of tropical updrafts.

Full access
Michael W. Huston, Andrew G. Detwiler, Fred J. Kopp, and Jeffrey L. Stith

Abstract

Observations made by three instrumented aircraft, a Doppler radar, and other data sources were used to follow the initiation and development of precipitation in a small cumulus congestus cloud. The cloud was seeded at its base using an airborne silver iodide solution burner. Sulfur hexafluoride tracer gas was released along with the seeding material. Analyzers on two instrumented aircraft detected the tracer gas during subsequent cloud penetrations as it was carried up into the cloud along with the seeding agent. Ice developed initially in the upper regions of the cloud near the −10°C level ∼15 min after the commencement of seeding. This is consistent with primary nucleation by the seeding agent. The cloud developed millimeter-size graupel within the following few minutes. A radar echo approaching 40 dBZ subsequently developed. The echo was observed to descend through the cloud as the cloud dissipated.

One-dimensional, steady-state and two-dimensional, time-dependent bulk water models were used to simulate this cloud. The one-dimensional model produced realistic values for updraft speeds allowing credible estimates of time required for transport from cloud base to upper regions of the cloud. The development of precipitation in the two-dimensional simulation resembled that in the observed cloud. Precipitation developed through riming of snow to graupel. In both the observed and simulated clouds, precipitation development was limited by cloud lifetime. Both clouds collapsed at a time when they were still generating ample supercooled water in their updrafts. Total precipitation on the ground from the seeded cloud simulations was ∼5 times the radar estimated rainfall total of 0.5 mm from the observed seeded cloud. This occurred despite the fact that the simulated cloud went through an accelerated life cycle compared to the observed cloud. A comparison between simulations with a natural ice process and with cloud base release of silver iodide shows that seeding accelerated precipitation formation in the model cloud leading to a fourfold increase in total precipitation for the seeded cases compared to the natural one.

Full access
Jeffrey L. Stith, Julie A. Haggerty, Andrew Heymsfield, and Cedric A. Grainger

Abstract

The distributions of ice particles, precipitation embryos, and supercooled water are examined within updrafts in convective clouds in the Amazon and at Kwajalein, Marshall Islands, based on in situ measurements during two Tropical Rainfall Measuring Mission field campaigns. Composite vertical profiles of liquid water, small particle concentration, and updraft/downdraft magnitudes exhibit similar peak values for the two tropical regions. Updrafts were found to be favored locations for precipitation embryos in the form of liquid or frozen drizzle-sized droplets. Most updrafts glaciated rapidly, removing most of the liquid water between −5° and −17°C. However, occasional encounters with liquid water occurred in much colder updraft regions. The updraft magnitudes where liquid water was observed at cold (e.g., −16° to −19°C) temperatures do not appear to be stronger than updrafts without liquid water at similar temperatures, however. The concentrations of small spherical frozen particles in glaciated regions without liquid water are approximately one-half of the concentrations in regions containing liquid cloud droplets, suggesting that a substantial portion of the cloud droplets may be freezing at relatively warm temperatures. Further evidence for a possible new type of aggregate ice particle, a chain aggregate found at cloud midlevels, is given.

Full access
Jeffrey L. Stith, Don A. Griffith, R. Lynn Rose, John A. Flueck, James R. Miller Jr., and Paul L. Smith

Abstract

A gaseous tracer, sulfur hexafluoride, was used to follow the path of two different AgI cloud seeding aerosols in cumulus clouds. The materials were released at cloud base or midlevels. Plumes sampled at midlevels were found to be relatively narrow and embedded within updrafts or downdrafts; relatively high concentrations of the tracer were observed in some downdrafts. Plumes with diameters comparable to the cloud diameters were found in the upper 20% of the clouds. These observations suggest only limited dispersion of the plumes in the clouds, with greater mixing occurring at cloud top. Similar behavior of the in-cloud plume is observed in results from a two-dimensional, numerical cloud model used to simulate the introduction of seeding materials into convective clouds. Observations of the ice crystal production rates are consistent with the results of recent laboratory findings concerning the properties of the seeding agents. The usefulness of this tracer technique in studying transport, diffusion and ice activation in cumulus clouds is discussed.

Full access