John D. Marwitz

The Structure and Motion of Severe Hailstorms. Part I: Supercell Storms

Abstract A supercell storm was observed in detail near Grover, Colo., on 18 June 1970. The storm was observed by an S-band radar operated alternately in PPI and RHI modes. An instrumented aircraft was flown near cloud base in the updrafts, and packets of chaff were released and tracked by radar within the weak echo region. Examination of the radar data revealed a remarkable similarity between certain RHI photographs of the Grover storm and the Soviet model of a hailstorm. Comparisons among the environmental conditions observed for other supercell storms revealed certain similar characteristics. The descriptive model of supercell storms by Browning was clarified and refined based on these new observations.

Conditions Associated with Large-Drop Regions

Abstract In light of the significant icing hazard large drops pose to general aviation, two conditions have been previously associated with large-drop formation; these being a warm cloud-top temperature and a low droplet concentration. This paper identifies an additional condition associated with the development of large-drop regions. Wind shear is hypothesized as being a necessary but not sufficient condition for the formation of large drops. Wind shear at cloud top may cause turbulence, Kelvin-Helmholtz waves, and thus the inhomogeneous mixing leading to large drops. This hypothesis was tested in 29 cases where the Wyoming King Air aircraft made a climb or descent through the top of stratiform clouds. The presence of a wind shear layer was defined by the magnitude of the wind shear and the value of the bulk Richardson number across the layer. In 23 of the 29 cases, wind shear was associated with large-drop regions. A χ2 statistical test was applied to the data. The null hypothesis, where wind shear and ...

The Structure and Motion of Severe Hailstorms. Part III: Severely Sheared Storms

Abstract A case study of a severe hailstorm which occurred in an extremely sheared environment is presented. The storm occurred near Fort Morgan, Colo., on 15 June 1970, and contained a large, persistent bounded weak echo region (WER). The Fort Morgan storm evolved in a manner and displayed several characteristics similar to another storm previously synthesized by Chisholm which also occurred in an extremely sheared environment. It is proposed that the extreme shear probably acted to erode the turbulent air containing precipitation sized particles from around the updraft core, thus allowing the large, bounded WER to persist in each storm. The erosion or detrainment effect is speculated to have accounted for the evolutionary characteristics of these two storms.

Winter Storms over the San Juan Mountains. Part I: Dynamical Processes

Abstract The Colorado River Basin Pilot Project was conducted over the San Juan Mountains in southwestern Colorado and ran for five winter seasons, terminating in 1974–75. The objective of the project was to demonstrate the feasibility of increasing the amount of snowpack and, therefore, the amount of available runoff. The Bureau of Reclamation, through its contractors, conducted the project. A number of statistical evaluations of the program have been made. This series of papers represents the principal physical evaluation of the seeding potential of San Juan storms. The synthesis of several well-documented San Juan storms indicates that most storms evolve through four distinct stages which are related to thermodynamic stability. The stages in sequence are stable, neutral, unstable and dissipation. During the stable stage, much of the flow below mountain top level is blocked and diverted toward the west. During the neutral stage, the storm is deep; it typically extends throughout much of the troposphere....

Trajectories Within the Weak Echo Regions of Hailstorms

Abstract Three-dimensional tracks of 21 slow-fall chaff packets have been obtained while the packets were rising in the weak echo regions of eight separate Colorado hailstorms. The chaff packets were released at cloud base in the strong smooth updrafts and tracked with a M-33 track radar. In many cases the chaff was released from an instrumented aircraft. From these data it is shown that the inflow air often has its origin near the surface, the inflow air is typically negatively buoyant below cloud base, there exists a significant non-hydrostatic pressure perturbation in most severe storms, and a vertical velocity maximum typically exists within the weak echo region.

The Front Range Blizzard of 1990. Part I: Synoptic and mesoscale structure

Abstract Over 1 m of snow fell in the foothills of southeast Wyoming and northeast Colorado during the storm of 6–7 March 1990. The heavy snowfall combined with strong winds to product blizzard conditions resulting in major highways being closed for several days. The heaviest snow fell in the vicinity of a narrow northerly barrier jet that developed in place along the Front Range of the Colorado Rockies. Strong warm-air advection from the southeast was observed during the development of the barrier jet; there was no cold-air advection from the north. Rapid intensification of the barrier jet took place only after precipitation started and was concurrent with the development of heavy precipitation. A mesoscale front marked the transition from southeasterly to northerly flow. This front remained very close to the rain-snow line and progressed toward the east at 1–2 m s−1 for about 15 h. Melting precipitation appears to be the dominant process contributing to the development of the barrier jet and mesoscale f...

Winter Storms over the San Juan Mountains. Part III: Seeding Potential

Abstract The potential for snowfall augmentation in the San Juan Mountains of southwestern Colorado is considered. We show that the seeding criteria and delivery method used in the Colorado River Basin Pilot Project were not suited to the storm structure and characteristics described in the preceding two papers. New criteria are suggested and compared to the available statistical results. It is suggested that opportunities for precipitation enhancement by seeding occur in the latter part of the storm sequence, are associated with the release of convective instability, and can be identified by the presence of a zone of horizontal convergence upwind of the mountain range.

An Airborne Comparison of Three PMS Probes

Abstract The 1977–78 Sierra Cooperative Pilot Project (SCPP) field season offered an opportunity for comparing several Particle Measuring Systems (PMS) probes. During that winter the University of Wyomings King Air aircraft was instrumented with 1D cloud, 2D cloud and 2D precipitation probes. Data from the three probes were analyzed from several flights in stable oregraphic storms. Various temperature levels (and hence, hydrometeor habits) above and below 0°C were flown in order to compare the data for water drops and for different ice crystal habits. It was found that all three probes agree quite well when sampling water drops of any size; however, when sampling ice crystals the probes agreed only at sizes larger than about 1 mm.

Radar Observations of Transport and Diffusion in Clouds and Precipitation Using TRACIR

Abstract A remote-sensing technique called TRACIR (tracking air with circular-polarization radar) was developed recently for studying air-parcel trajectories in clouds. The technique uses a dual-circular-polarization radar to detect microwave chaff fibers that serve as tracers of the air motion. The radar is able to detect the chaff inside clouds and precipitation by measuring the circular-depolarization ratio, which is much higher for chaff than for hydrometeors. Chaff concentrations are also estimated by the technique, thus permitting turbulent diffusion in clouds to be examined. Demonstrations of TRACIRs capabilities are presented for three cases in which chaff was used to simulate the movement of cloud-seeding nuclei in clouds and precipitation. In two cases involving airborne chaff releases, the gradual drift and diffusion of chaff in a stratiform cloud are contrasted with its abrupt transport and dispersion in a convective cloud. In the third case study, the technique successfully detected a plume ...

A Comparison of AgI and CO2 Seeding Effects in Alberta Cumulus Clouds

Abstract Three convective clouds extending above a stratocumulus layer were identified as being seedable on one day and were then seeded in a random sequence with CO2 pellets, a placebo and droppable AgI flares. The radar and microphysical seeding effects were observed with the Alberta Hail Project S-band radar and with the University of Wyoming Queen Air aircraft. Distinct seeding effects were observed in both seeded clouds by both data systems. The CO2 seeded cloud developed a single curtain of precipitation particles 18 min after seeding which reached the ground 20 min after seeding and ceased precipitating 10 min later. The placebo cloud failed to develop any precipitation-sized particles or radar echo and dissipated after ∼30 min. The AgI seeded cloud developed its first echo 8 min after seeding near the threshold temperature for AgI (−7°C), produced precipitation at the ground 20 min after seeding, and continued to develop a new echo near the −7°C level and precipitate for ∼1 h. A natural echoing st...