Roelof T. Bruintjes

A Review of Cloud Seeding Experiments to Enhance Precipitation and Some New Prospects

Water is one of the most basic commodities on earth sustaining human life. In many regions of the world, traditional sources and supplies of ground water, rivers and reservoirs, are either inadequate or under threat from ever-increasing demands on water from changes in land use and growing populations. This has prompted scientists and engineers to explore the possibility of augmenting water supplies by means of cloud seeding. This paper provides an overview of the current scientific status of weather modification activities to enhance pre- cipitation for both glaciogenic and hygroscopic seeding experiments. It is important to emphasize that although funding for scientific studies has decreased substantially during the past decade, operational programs have actually increased. During the last 10 years there has been a thorough scrutiny of past experiments involving experiments using glaciogenic seeding. Although there still exist indications that seeding can increase precipitation, a number of recent studies have questioned many of the positive results, weakening the scientific credibility. As a result, considerable skep- ticism exists as to whether these methods provides a cost-effective means for increasing precipitation for water resources. Recent results from hygroscopic seeding experiments provided for some renewed optimism in the field of precipita- tion enhancement. Although promising results have been obtained to date, some fundamental questions remain that need to be answered in order to provide a sound scientific basis for this technology.

Calculations Pertaining to Hygroscopic Seeding with Flares

Some possible effects of hygroscopic seeding with flares are explored by calculating how such seeding would modify the initial size distribution of cloud droplets and the subsequent evolution of that size distribution by coalescence. To be representative of recent experiments in South Africa, the calculations emphasize the effects of hygroscopic particles that can be produced by flares, instead of the larger particles used in most past hygroscopic-seeding experiments. Parcel calculations representing simultaneous condensation and coalescence suggest that the formation of rain through the warm-rain process can be accelerated significantly by the addition of such hygroscopic particles. Some observations of the effects of hygroscopic material near cloud base support at least the early stages of the calculations. The results suggest that the positive effects being obtained in the South African experiment may occur through such acceleration of the warm-rain process. Possible cloud-seeding applications and climate implications are discussed.

Interactions between Topographic Airflow and Cloud/Precipitation Development during the Passage of a Winter Storm in Arizona

Abstract A case study showing comparisons between observations and numerical simulations of the passage of a winter storm over complex terrain is presented. The interactions between the mesoscale and cloud environments and the microphysical and dynamical processes are addressed using both observations and numerical simulations. A three-dimensional, time-dependent nested grid model was used to conduct numerical simulations of the three-dimensional airflow and cloud evolution over the Mogollon Rim and adjacent terrain in Arizona. The modeling results indicated that the flow patterns and cloud liquid water (CLW) were closely linked to the topography. To a large extent, gravity waves excited by the flow over the mountains determine the distribution of clouds and precipitation. The waves extend through deep layers of the atmosphere with substantial updrafts and downdrafts, at times exceeding 5 m s−1. The simulated vertical velocities and horizontal wavelengths of about 20 km were in good agreement with the air...

Cloud activating properties of aerosol observed during CELTIC

Measurements of aerosol size distribution, chemical composition, and cloud condensation nuclei (CCN) concentration were performed during the Chemical Emission, Loss, Transformation, and Interactions with Canopies (CELTIC) field program at Duke Forest in North Carolina. A kinetic model of the cloud activation of ambient aerosol in the chamber of the CCN instrument was used to perform an aerosol–CCN closure study. This study advances prior investigations by employing a novel fitting algorithm that was used to integrate scanning mobility particle sizer (SMPS) measurements of aerosol number size distribution and aerosol mass spectrometer (AMS) measurements of the mass size distribution for sulfate, nitrate, ammonium, and organics into a single, coherent description of the ambient aerosol in the size range critical to aerosol activation (around 100-nm diameter). Three lognormal aerosol size modes, each with a unique internally mixed composition, were used as input into the kinetic model. For the two smaller size modes, which control CCN number concentration, organic aerosol mass fractions for the defined cases were between 58% and 77%. This study is also unique in that the water vapor accommodation coefficient was estimated based on comparing the initial timing for CCN activation in the instrument chamber with the activation predicted by the kinetic model. The kinetic model overestimated measured CCN concentrations, especially under polluted conditions. Prior studies have attributed a positive model bias to an incomplete understanding of the aerosol composition, especially the role of organics in the activation process. This study shows that including measured organic mass fractions with an assumed organic aerosol speciation profile (pinic acid, fulvic acid, and levoglucosan) and an assumed organic aerosol solubility of 0.02 kg kg 1 still resulted in a significant model positive bias for polluted case study periods. The slope and y intercept for the CCN predicted versus CCN observed regression was found to be 1.9 and 180 cm 3 , respectively. The overprediction generally does not exceed uncertainty limits but is indicative that a bias exists in the measurements or application of model. From this study, uncertainties in the particle number and mass size distributions as the cause for the model bias can be ruled out. The authors are also confident that the model is including the effects of growth kinetics on predicted activated number. However, one cannot rule out uncertainties associated with poorly characterized CCN measurement biases, uncertainties in assumed organic solubility, and uncertainties in aerosol mixing state. Sensitivity simulations suggest that assuming either an insoluble organic fraction or external aerosol mixing were both sufficient to reconcile the model bias.

Smoke aerosol from biomass burning in Mexico: Hygroscopic smoke optical model

The May 1998 transport of smoke from fires in Mexico and Central America into the United States is examined. We combine data from ground-based Interagency Monitoring of Protected Visual Environments aerosol chemical sampling sites with in situ airborne and Sun photometer measurements to develop a consistent picture of the transported smoke-impacted aerosol optical and chemical properties. The aerosol observed in Mexico and the southern United States is found to have a higher sulfate mass fraction, higher single-scattering albedo, and larger accumulation mode radius than biomass burning aerosols observed by similar instrumentation in South America and Africa. We postulate that the smoke-impacted aerosol in the 1998 event was more hygroscopic than that observed in the other locations, because of the higher mass fractions of sulfate, and show that a simple model of corresponding changes in aerosol water content yields agreement with the observed variations in refractive index and radii. We further show that the single-scattering albedo cannot be fully explained by hygroscopic growth alone. Modifications to the model invoking variations in aerosol light-absorbing carbon content, which are consistent with differences in observed composition among the various smoke-impacted aerosols, bring the predictions of single-scattering albedo into alignment with our observations. The model demonstrates that the particle size, single-scattering albedo, and real refractive index of smoke-impacted aerosols are not independent but vary in tandem with variations in particle hygroscopicity and with variations in black carbon content. This relationship is an important consideration in the assessment of the effects of biomass burning aerosols, particularly those subject to long-range transport, on radiative forcing and climate.

Intercomparison of In-Flight Icing Algorithms. Part II: Statistical Verification Results

Abstract Recent research to improve forecasts of in-flight icing conditions has involved the development of algorithms to apply to the output of numerical weather prediction models. The abilities of several of these algorithms to predict icing conditions, as verified by pilot reports (PIREPs), are compared for two numerical weather prediction models (Eta and the Mesoscale Analysis and Prediction System) for the Winter Icing and Storms Program 1994 (WISP94) time period (25 January–25 March 1994). Algorithms included in the comparison were developed by the National Aviation Weather Advisory Unit [NAWAU, now the Aviation Weather Center (AWC)], the National Center for Atmospheric Research’s Research Applications Program (RAP), and the U.S. Air Force. Operational icing forecasts (AIRMETs) issued by NAWAU for the same time period are evaluated to provide a standard of comparison. The capabilities of the Eta Model’s explicit cloud liquid water estimates for identifying icing regions are also evaluated and compar...

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...

Haze layer characterization and associated meteorological controls along the eastern coastal region of southern Africa

[1] Episodes of regionally extensive haze were observed over southern African during the dry season intensive of the Southern African Regional Science Initiative (SAFARI 2000). Several case studies of southern African haze layers were examined and characterized in terms of physical structure as they exited off of the eastern coastal region of southern Africa. In situ observations of aerosols and trace gases and their physical and chemical characteristics were collected on board South African Weather Service Aerocommander research aircraft. Haze structure, based on these measurements, is examined as it varies with synoptic type. Despite strong differences in the observed ENSO regime between SAFARI 2000 and that observed during the Southern African Fire-Atmosphere Research Initiative (SAFARI-92) and their respective aerosol accumulation mechanisms (col Rrgions/weak anticyclones versus strong anticyclones), a surprising degree of consistency in the observed vertical structure of the lower troposphere was found in southern Africa. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; KEYWORDS: haze layers, synoptic circulations, ENSO

The Dispersion of Tracer Plumes in Mountainous Regions in Central Arizona: Comparisons between Observations and Modeling Results

Abstract A three-dimensional, time-dependent, nested-grid model is used to calculate the targeting of tracer or Seeding material over complex terrain in northern Arizona. Good agreement with measurements of SF6 tracer is reported in three case studies. Released in upwind valleys, the tracer movement and dispersion are strongly influenced by both valley flow and gravity waves excited by the mountains, as well as by changes in the synoptic flow, which can change substantially even during a single storm. The interaction between the airflow and the topography seem to be the dominant factor determining the dispersion and transport of tracer material.

Intercomparison of In-Flight Icing Algorithms. Part I: WISP94 Real-Time Icing Prediction and Evaluation Program

Abstract The purpose of the Federal Aviation Administration’s Icing Forecasting Improvement Program is to conduct research on icing conditions both in flight and on the ground. This paper describes a portion of the in-flight aircraft icing prediction effort through a comprehensive icing prediction and evaluation project conducted by the Research Applications Program at the National Center for Atmospheric Research. During this project, in- flight icing potential was forecast using algorithms developed by RAP, the National Weather Service’s National Aviation Weather Advisory Unit, and the Air Force Global Weather Center in conjunction with numerical model data from the Eta, MAPS, and MM5 models. Furthermore, explicit predictions of cloud liquid water were available from the Eta and MM5 models and were also used to forecast icing potential. To compare subjectively the different algorithms, predicted icing regions and observed pilot reports were viewed simultaneously on an interactive, real-time display. To m...