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- Author or Editor: Roelof T. Bruintjes x
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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 precipitation 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 skepticism 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 precipitation 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.
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 precipitation 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 skepticism 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 precipitation 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.
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.
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.
Abstract
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.
Abstract
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.
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 aircraft observations. The CLW regions associated with the waves extended through much deeper layers of the atmosphere and in quantities a factor of 2 larger than those associated with the forced ascent over the ridges. The CLW associated with waves may provide an additional source for precipitation development not previously considered in cloud seeding experiments. In addition, synoptic-scale flow patterns over the area change from one storm system to the next and even during one storm system. Consequently, both the winds and the evolution of clouds over the area are highly space and time dependent
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 aircraft observations. The CLW regions associated with the waves extended through much deeper layers of the atmosphere and in quantities a factor of 2 larger than those associated with the forced ascent over the ridges. The CLW associated with waves may provide an additional source for precipitation development not previously considered in cloud seeding experiments. In addition, synoptic-scale flow patterns over the area change from one storm system to the next and even during one storm system. Consequently, both the winds and the evolution of clouds over the area are highly space and time dependent
Abstract
The purpose of this study is to assess the relative importance of collision and coalescence efficiencies as reported in the literature in different drop size regimes for the development of precipitation via the condensation–coalescence process. The stochastic growth of cloud droplet distributions due to collection processes is studied using a detailed microphysical parcel model. The evolution of rainwater content (L R ) and the radar reflectivity factor (Z) are plotted in order to trace the progress of transfer of cloud water into rainwater and determine the importance of droplet collection in different size ranges.
The results indicate that the van der Waals forces are effective in enhancing droplet collision when the droplets are small and the distributions are narrow. Wake capture is negligible for clouds forming in a continental air mass with low liquid water contents. However, it is effective when coalescence becomes the dominant growth process and rainwater content has reached high values.
When nonunity coalescence efficiencies are used, the drop growth and cloud water to rainwater conversion is reduced compared to the traditional unity coalescence efficiencies used in previous modeling studies. However, the major difference between the results using nonunity and unity coalescence efficiencies is due to the extrapolation of coalescence efficiencies measured in laboratory to size domains outside the domain of the measurements.
Abstract
The purpose of this study is to assess the relative importance of collision and coalescence efficiencies as reported in the literature in different drop size regimes for the development of precipitation via the condensation–coalescence process. The stochastic growth of cloud droplet distributions due to collection processes is studied using a detailed microphysical parcel model. The evolution of rainwater content (L R ) and the radar reflectivity factor (Z) are plotted in order to trace the progress of transfer of cloud water into rainwater and determine the importance of droplet collection in different size ranges.
The results indicate that the van der Waals forces are effective in enhancing droplet collision when the droplets are small and the distributions are narrow. Wake capture is negligible for clouds forming in a continental air mass with low liquid water contents. However, it is effective when coalescence becomes the dominant growth process and rainwater content has reached high values.
When nonunity coalescence efficiencies are used, the drop growth and cloud water to rainwater conversion is reduced compared to the traditional unity coalescence efficiencies used in previous modeling studies. However, the major difference between the results using nonunity and unity coalescence efficiencies is due to the extrapolation of coalescence efficiencies measured in laboratory to size domains outside the domain of the measurements.
Abstract
Measurements within continental convective clouds in the Highveld region of South Africa indicate that the first appreciable nucleation of ice occurs between the −9° to −12°C levels as the cloud top rises through these levels. Moreover, these crystals usually take the form of double plates. Frozen drop centers were observed in 30% of the double crystals, with the diameter of the frozen drop between 10 and 20 μm. The growth of these ice crystals is investigated using ice crystal measurements collected in situ as well as a modeling study.
New information regarding axial dimensions, bulk densities, and riming characteristics of double-plate crystals is presented. Numerical simulations were performed to compare the growth characteristics of ice crystals nucleated at different temperature levels between −6° and −16°C in updrafts of 1 and 3 m s−1. Crystals nucleated at temperatures ≥−12°C are shown to start riming at smaller sizes than previously thought and these crystals are potentially important as graupel embryos in initiating precipitation in growing cumulus congestus clouds.
Abstract
Measurements within continental convective clouds in the Highveld region of South Africa indicate that the first appreciable nucleation of ice occurs between the −9° to −12°C levels as the cloud top rises through these levels. Moreover, these crystals usually take the form of double plates. Frozen drop centers were observed in 30% of the double crystals, with the diameter of the frozen drop between 10 and 20 μm. The growth of these ice crystals is investigated using ice crystal measurements collected in situ as well as a modeling study.
New information regarding axial dimensions, bulk densities, and riming characteristics of double-plate crystals is presented. Numerical simulations were performed to compare the growth characteristics of ice crystals nucleated at different temperature levels between −6° and −16°C in updrafts of 1 and 3 m s−1. Crystals nucleated at temperatures ≥−12°C are shown to start riming at smaller sizes than previously thought and these crystals are potentially important as graupel embryos in initiating precipitation in growing cumulus congestus clouds.
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 measure objectively the skill of icing predictions, a rigorous statistical evaluation was performed in order to compare the different algorithms (details and results are provided in Part II). Both the subjective and objective comparisons are presented here for a particular case study, whereas results from the entire project are found in Part II. By statistically analyzing 2 months worth of data, it appears that further advances in temperature and relative-humidity-based algorithms are unlikely. Explicit cloud liquid water predictions, however, show promising results although still relatively new in operational numerical models.
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 measure objectively the skill of icing predictions, a rigorous statistical evaluation was performed in order to compare the different algorithms (details and results are provided in Part II). Both the subjective and objective comparisons are presented here for a particular case study, whereas results from the entire project are found in Part II. By statistically analyzing 2 months worth of data, it appears that further advances in temperature and relative-humidity-based algorithms are unlikely. Explicit cloud liquid water predictions, however, show promising results although still relatively new in operational numerical models.
Abstract
Polarimetric radar can be used to identify various types of hydrometeors. Ice crystals of the varied growth habits depolarize and backscatter millimeter-wavelength radiation according to crystal aspect ratio, bulk density, and orientation, and the polarization state of the incident radiation. In this paper model calculations of the depolarization caused by various crystal types are extended from previous work, and Ka-band (8.66 mm) radar measurements of linear and elliptical depolarization ratios (LDR and EDR) from various ice hydrometeors are presented. The measurements for regular crystals are related to the models. Drizzle drops, which are quasi-spherical, serve as a reference. Signature discrimination in cloud systems with more than one type of hydrometeor is addressed.
The model calculations illustrate the interplay of the parameters that control depolarization. They predict that in the depolarization signatures, crystals of the various basic planar and columnar habits should generally be most separable, one habit group from another and, to a degree, within each group when they occur in common, mature size distributions. It is verified in this and related papers that measurements of depolarization with a Ka-band dual-polarization radar provide good estimates of hydrometeor identity to separately distinguish drizzle, pristine crystals of various growth habits, graupel, and aggregates in winter storm clouds that have reasonable horizontal homogeneity over short distances (∼10–20 km). Characterization of the mix of two or three hydrometeor types is also possible, once the individual types are identified in some part of the cloud. Quantitative agreement between the measurements and the models, supported by snow crystal samples, was much better for EDR than for LDR; that is, EDR enabled more specific hydrometeor identification. However, LDR provided indications of randomness of crystal orientation and a wider decibel gap differentiating graupel from drizzle.
Abstract
Polarimetric radar can be used to identify various types of hydrometeors. Ice crystals of the varied growth habits depolarize and backscatter millimeter-wavelength radiation according to crystal aspect ratio, bulk density, and orientation, and the polarization state of the incident radiation. In this paper model calculations of the depolarization caused by various crystal types are extended from previous work, and Ka-band (8.66 mm) radar measurements of linear and elliptical depolarization ratios (LDR and EDR) from various ice hydrometeors are presented. The measurements for regular crystals are related to the models. Drizzle drops, which are quasi-spherical, serve as a reference. Signature discrimination in cloud systems with more than one type of hydrometeor is addressed.
The model calculations illustrate the interplay of the parameters that control depolarization. They predict that in the depolarization signatures, crystals of the various basic planar and columnar habits should generally be most separable, one habit group from another and, to a degree, within each group when they occur in common, mature size distributions. It is verified in this and related papers that measurements of depolarization with a Ka-band dual-polarization radar provide good estimates of hydrometeor identity to separately distinguish drizzle, pristine crystals of various growth habits, graupel, and aggregates in winter storm clouds that have reasonable horizontal homogeneity over short distances (∼10–20 km). Characterization of the mix of two or three hydrometeor types is also possible, once the individual types are identified in some part of the cloud. Quantitative agreement between the measurements and the models, supported by snow crystal samples, was much better for EDR than for LDR; that is, EDR enabled more specific hydrometeor identification. However, LDR provided indications of randomness of crystal orientation and a wider decibel gap differentiating graupel from drizzle.
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 compared to the algorithm results.
Because PIREPs are not systematic and are biased toward positive reports, it is difficult to estimate standard verification parameters related to overforecasting (e.g., false alarm ratio). Methods are developed to compensate for these attributes of the PIREPs. The primary verification statistics computed include the probability of detection (POD) of yes and no reports, and the areal and volume extent of the forecast region.
None of the individual algorithms were able to obtain both a higher POD and a smaller area than any other algorithm; increases in POD are associated in all cases with increases in area. The RAP algorithm provides additional information by attempting to identify the physical mechanisms associated with the forecast icing conditions. One component of the RAP algorithm, which is designed to detect and forecast icing in regions of“warm” stratiform clouds, is more efficient at detecting icing than the other components. Cloud liquid water shows promise for development as a predictor of icing conditions, with detection rates of 30% or more in this initial study. AIRMETs were able to detect approximately the same percentage of icing reports as the algorithms, but with somewhat smaller forecast areas and somewhat larger forecast volumes on average. The algorithms are able to provide guidance with characteristics that are similar to the AIRMETs and should be useful in their formulation.
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 compared to the algorithm results.
Because PIREPs are not systematic and are biased toward positive reports, it is difficult to estimate standard verification parameters related to overforecasting (e.g., false alarm ratio). Methods are developed to compensate for these attributes of the PIREPs. The primary verification statistics computed include the probability of detection (POD) of yes and no reports, and the areal and volume extent of the forecast region.
None of the individual algorithms were able to obtain both a higher POD and a smaller area than any other algorithm; increases in POD are associated in all cases with increases in area. The RAP algorithm provides additional information by attempting to identify the physical mechanisms associated with the forecast icing conditions. One component of the RAP algorithm, which is designed to detect and forecast icing in regions of“warm” stratiform clouds, is more efficient at detecting icing than the other components. Cloud liquid water shows promise for development as a predictor of icing conditions, with detection rates of 30% or more in this initial study. AIRMETs were able to detect approximately the same percentage of icing reports as the algorithms, but with somewhat smaller forecast areas and somewhat larger forecast volumes on average. The algorithms are able to provide guidance with characteristics that are similar to the AIRMETs and should be useful in their formulation.
Abstract
No Abstract available.
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No Abstract available.
