Harold D. Orville

Bulk Parameterization of the Snow Field in a Cloud Model

Abstract A two-dimensional, time-dependent cloud model has been used to simulate a moderate intensity thunderstorm for the High Plains region. Six forms of water substance (water vapor, cloud water, cloud ice, rain, snow and hail, i.e., graupel) are simulated. The model utilizes the “bulk water” microphysical parameterization technique to represent the precipitation fields which are all assumed to follow exponential size distribution functions. Autoconversion concepts are used to parameterize the collision-coalescence and collision-aggregation processes. Accretion processes involving the various forms of liquid and solid hydrometeors are simulated in this model. The transformation of cloud ice to snow through autoconversion (aggregation) and Bergeron process and subsequent accretional growth or aggregation to form hail are simulated. Hail is also produced by various contact mechanisms and via probabilistic freezing of raindrops. Evaporation (sublimation) is considered for all precipitation particles outsi...

Numerical Simulation of the Life History of a Hailstorm

Abstract A two-dimensional, time-dependent cloud model has been used to simulate the evolution of hail cells and hailstorms. The model has been under development for several years tested on a few days of real data, and gives reasonable predictions of the convective characteristics and essential precipitation processes for both severe and nonsevere convection days. Hydrodynamic equations for. deep convection are integrated over a 20 km square grid with 200 m spacing between grid points. Cloud formation and precipitation processes employing bulk water techniques are simulated in the model. Autoconversion and accretion are used to transform cloud water to rain. Precipitating ice (hail) is formed by the freezing of rain and through an approximation to the Bergeron-Findeisen process to transform cloud liquid to precipitating ice. Accretion of cloud water by rain, and accretion of cloud water, cloud ice and rain by hail are modeled. Wet and dry growth of hail and the shedding of rain from hail are simulated. Cl...

Numerical Simulation of Ice-Phase Convective Cloud Seeding

Abstract A two-dimensional time-dependent cloud model which covers a region 19.2 km × 19.2 km in the x and z directions with 200 m grid intervals, has been used to simulate silver iodide (AgI) seeding effects on strong convective clouds. The model is a set of conservation equations for momentum, energy and mass (air and water contents). One extra conservation equation is applied to trace the seeding agent which advects and diffuses along the flow field and interacts with the supercooled cloud fields. Contact and deposition nucleation are simulated. Only inertial impact and Brownian collection are considered as possible mechanisms for contact nucleation. Most of the AgI particles work as deposition or sorption nuclei in this study. Three different soundings are tested. Most of the effort is used in testing sounding H1, from Miles City, Montana, 29 July 1975. Seeding at a different place (see H1/P1), at a different time (case H1/T1), and with different amounts of AgI (cases H1/M1 and H1/M2) are simulated. T...

Effects of Mesoscale Convergence on Cloud Convection

Abstract A two-dimensional time-dependent cloud model was used in this research to investigate the effects of mesoscale convergence on cloud convection. A Klemp and Wilhelmson type boundary condition was tested which allows inflow and outflow through the lateral boundary and also lets gravity waves pass out through the boundaries with minimal reflection. A relatively stable and easy method of superimposing a mesoscale convergence field was also introduced in this study. The main idea of this superposition is to decompose the velocity into mesoscale and cloud-scale velocities. The cloud-scale velocity is governed by the cloud convection, while the mesoscale velocity is governed by the mesoscale variable. Two types of atmospheric soundings were run in this model. The first type is an unstable sounding and the second type is conditionally unstable with a low-level inversion. The results show that convergence weakens the temperature inversion and leads to strong convection in one case. Fewer, broader and more...

Weather Modification: Finding Common Ground

Research and operational approaches to weather modification expressed in the National Research Councils 2003 report on “Critical Issues in Weather Modification Research” and in the Weather Modification Associations response to that report form the basis for this discussion. There is agreement that advances in the past few decades over a broad front of understanding physical processes and in technology have not been comprehensively applied to weather modification. Such advances need to be capitalized upon in the form of a concerted and sustained national effort to carry out basic and applied research in weather modification. The need for credible scientific evidence and the pressure for action should be resolved. Differences in the perception of current knowledge, the utility of numerical models, and the specific needs of research and operations in weather modification must be addressed. The increasing demand for water and the cost to society inflicted by severe weather require that the intellectual, tec...

A Review of Cloud Modeling in Weather Modification

The numerical modeling of clouds has a history almost as long as the modern-day concepts of the seeding of clouds. The various models and theoretical developments emerging from the model results are reviewed in this paper. Significant advances have been made in understanding cloud-seeding effects, which bodes well for the continued application of this form of weather modification in the future.

Observational and Numerical Study of a Microburst Line-Producing Storm

Abstract An analysis is performed on a microburst line-producing cloud that occurred near Denver, Colorado on 13 July 1982. The cloud line developed in an environment conducive to the production of low-reflectivity microbursts. Doppler radar analysis revealed strong convergence above cloud base into the region of downdraft 3.5 to 4.5 km above ground. Aircraft measurements detected light rain with graupel aloft in microburst downdrafts. A two-dimensional cloud model simulation captured many of the observed features of the cloud line structure and wind fields. In particular, both the development of multiple microbursts and the convergence aloft were well simulated. The formation of graupel/hail was important to the precipitation process in the model. The loading of rain and graupel and the cooling effect of rain evaporation and graupel melting were all important in microburst production—the graupel in the formative stages of the downdraft, and the rain in the further intensification of the downdraft and enh...

Some Surprising Results from Simulated Seeding of Stratiform-Type Clouds

Abstract Studies have been conducted to determine the cloud seeding potential of stratiform type clouds using a two-dimensional, time-dependent cloud model. An atmospheric sounding from Villanubla, Spain, in February 1980, was used to initialize the model. The model is designed to allow mesoscale convergence in the lower levels and divergence in the upper levels, which results in a stratiform-type cloud in this Spanish situation. The seeding of clouds using either dry ice or silver iodide has been tested and rather surprising results are indicated. The silver iodide seeding simulations produce strong dynamic responses in the model clouds, even with small amounts of supercooled liquid available and a few natural ice crystals per liter in the cloud. These effects occur in a nearly moist adiabatic layer as well as in a convectively unstable layer. The effects appear to be due to the heat released as the liquid freezes and the cloudy environment switches from liquid saturation to ice saturation. Cloud vertica...

The North Dakota Thunderstorm Project: A Cooperative Study of High Plains Thunderstorms

Abstract The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification. Of primary interest were tracer experiment that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF6), carbon monoxide, ozone, radar chaff, and silver iodide. Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunder...

Effects of Cloud Seeding, Latent Heat of Fusion, and Condensate Loading on Cloud Dynamics and Precipitation Evolution: A Numerical Study

Abstract This study attempts to isolate the dynamic and microphysical effects of seeding. A two-dimensional, time-dependent cloud model has been used to simulate silver iodide (AgI) seeding of convective clouds. Two major dynamic effects (latent heat of fusion and condensate loading) are separated through a sequence of differential processes to examine their individual effect. A High Plains sounding is used for the tests. The effects of condensate loading and latent heat of fusion are due to natural processes as well as to cloud seeding. Separate discussions and comparisons are made of both processes. Condensate loading has the greater influence on cloud development. A method of differencing the results from different cases is used to illustrate the overall seeding effects and to isolate those portions of the latent heat of fusion and loading effects which are due solely to ice-phase cloud seeding. The results indicate significant fusion and loading effects due to seeding, but at 10 min or so after the se...