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- Author or Editor: Tamir Reisin x
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Abstract
A hydrodynamic nonhydrostatic anelastic numerical model of an axisymmetric convective cloud is described in which the microphysical processes are treated in detail for different species of hydrometeors: drops. ice crystals, graupel, and snow particles. The size distribution function for each type of particle is divided into 34 spectral bins. In each spectral category two physical moments of the distribution function (number and mass concentrations are independently calculated using the method of moments. The following physical processes are computed: nucleation of drops and ice crystals, freezing of drops, diffusional growth/evaporation of drops and ice particles, collisional coalescence of drops and ice particles, binary breakup of drops, melting of ice particles, and sedimentation. The model describes the different stages of cloud development, the formation of ice, its growth by deposition and riming, the formation of graupel, and the precipitation stage. Analysis of the distribution functions for the different species provides insight into the different microphysical processes active in rain formation in mixed clouds. As an illustration of the capability of the model, the simulation of a mixed-phase continental cloud is presented.
Abstract
A hydrodynamic nonhydrostatic anelastic numerical model of an axisymmetric convective cloud is described in which the microphysical processes are treated in detail for different species of hydrometeors: drops. ice crystals, graupel, and snow particles. The size distribution function for each type of particle is divided into 34 spectral bins. In each spectral category two physical moments of the distribution function (number and mass concentrations are independently calculated using the method of moments. The following physical processes are computed: nucleation of drops and ice crystals, freezing of drops, diffusional growth/evaporation of drops and ice particles, collisional coalescence of drops and ice particles, binary breakup of drops, melting of ice particles, and sedimentation. The model describes the different stages of cloud development, the formation of ice, its growth by deposition and riming, the formation of graupel, and the precipitation stage. Analysis of the distribution functions for the different species provides insight into the different microphysical processes active in rain formation in mixed clouds. As an illustration of the capability of the model, the simulation of a mixed-phase continental cloud is presented.
Abstract
An axisymmetrical nonhydrostatic convective cloud mode with detailed treatment of warm cloud microphysics is presented. Ale microphysical processes considered aere nucleation on cloud condensation nuclei, condensation/evaporation, collisional coalescene/breakup (Low and List kernel), and sedimentation. An accurate multi-moment treatment is implemented in the calculations of the microphysical processes. The results indicate that the collisional breakup process is very important in warm clouds and inhibits the growth of drops to large sizes where spontaneous breakup is significant. This diminishes the importance of the Langmuir chain-reaction mechanism for rain formation. The effect of salt seeding are examined for three different cases: one maritime case and two continental cloud cases. No significant effect followed the injection of up to half a ton of salt particles for the maritime cue, while the effect was very significant for the continental clouds. The sensitivity to various seeding parameters was also investigated, including size of seeding particles, quantity of seeding material, timing and duration of seeding and location of seeding The size of the seeded particles and the timing of seeding were found to be crucial parameters. Premature seeding could have a negative effect. Up to 71% increase in total rainfall was obtained under optimal seeding conditions.
Abstract
An axisymmetrical nonhydrostatic convective cloud mode with detailed treatment of warm cloud microphysics is presented. Ale microphysical processes considered aere nucleation on cloud condensation nuclei, condensation/evaporation, collisional coalescene/breakup (Low and List kernel), and sedimentation. An accurate multi-moment treatment is implemented in the calculations of the microphysical processes. The results indicate that the collisional breakup process is very important in warm clouds and inhibits the growth of drops to large sizes where spontaneous breakup is significant. This diminishes the importance of the Langmuir chain-reaction mechanism for rain formation. The effect of salt seeding are examined for three different cases: one maritime case and two continental cloud cases. No significant effect followed the injection of up to half a ton of salt particles for the maritime cue, while the effect was very significant for the continental clouds. The sensitivity to various seeding parameters was also investigated, including size of seeding particles, quantity of seeding material, timing and duration of seeding and location of seeding The size of the seeded particles and the timing of seeding were found to be crucial parameters. Premature seeding could have a negative effect. Up to 71% increase in total rainfall was obtained under optimal seeding conditions.
Abstract
This paper presents an evaluation of the relative importance of the warm versus cold processes in convective clouds and their relative contribution to the development of rain. For this purpose, an axisymmetrical model of a cold convective cloud with detailed microphysics is used.
Five different types of clouds having characteristics from maritime to extreme continental are simulated. Identical initial conditions are used, leading to the formation of convective clouds of medium depth, with relatively strong updrafts. For these specific conditions, the effects of the different microphysical processes on the production of rain are tested by varying the cloud condensation nuclei (CCN) spectra and the spectra of the nucleated drops. The role of ice crystal concentrations and drop freezing is also reviewed.
The simulations showed that maritime clouds are efficient rain producers. In these clouds, large graupel mass contents develop by the freezing of large drops through their interaction with ice crystals. Rain efficiency decreases with increasing CCN concentration (or with the “continentality” of the clouds). For the same dynamics and liquid water content maritime clouds produce more rain with higher intensifies than continental clouds.
Reducing the ice nuclei concentrations generally produces less rain, especially near the cloud center. In moderate continental clouds, changing the concentration of ice crystals by a few orders of magnitude results in a change in the spatial distribution of the rain but only a small change in the total amount of precipitation.
Self-freezing of drops plays only a minor role in rain production because freezing due to interactions of supercooled drops with ice crystals takes precedent. In the simulated clouds snow is inefficiently produced, especially in maritime ones.
The Bergeron–Findeisen mechanism plays only a minor role in the depletion of supercooled water during the developing and mature stages of the cloud because of the presence of very low ice crystal concentrations as compared to that of the drops. During the dissipation stage of the clouds, however, the Bergeron–Findeisen mechanism helps to accelerate the glaciation.
Abstract
This paper presents an evaluation of the relative importance of the warm versus cold processes in convective clouds and their relative contribution to the development of rain. For this purpose, an axisymmetrical model of a cold convective cloud with detailed microphysics is used.
Five different types of clouds having characteristics from maritime to extreme continental are simulated. Identical initial conditions are used, leading to the formation of convective clouds of medium depth, with relatively strong updrafts. For these specific conditions, the effects of the different microphysical processes on the production of rain are tested by varying the cloud condensation nuclei (CCN) spectra and the spectra of the nucleated drops. The role of ice crystal concentrations and drop freezing is also reviewed.
The simulations showed that maritime clouds are efficient rain producers. In these clouds, large graupel mass contents develop by the freezing of large drops through their interaction with ice crystals. Rain efficiency decreases with increasing CCN concentration (or with the “continentality” of the clouds). For the same dynamics and liquid water content maritime clouds produce more rain with higher intensifies than continental clouds.
Reducing the ice nuclei concentrations generally produces less rain, especially near the cloud center. In moderate continental clouds, changing the concentration of ice crystals by a few orders of magnitude results in a change in the spatial distribution of the rain but only a small change in the total amount of precipitation.
Self-freezing of drops plays only a minor role in rain production because freezing due to interactions of supercooled drops with ice crystals takes precedent. In the simulated clouds snow is inefficiently produced, especially in maritime ones.
The Bergeron–Findeisen mechanism plays only a minor role in the depletion of supercooled water during the developing and mature stages of the cloud because of the presence of very low ice crystal concentrations as compared to that of the drops. During the dissipation stage of the clouds, however, the Bergeron–Findeisen mechanism helps to accelerate the glaciation.
Abstract
Simulations of seeding clouds for rain enhancement with ice nuclei (IN) or hygroscopic particles were conducted using a numerical model of an axisymmetric convective cloud with detailed treatment of both warm and cold microphysical processes. The simulations were performed for three clouds that differed in their cloud condensation nuclei (CCN) concentrations and spectra. Tests were carried out on clouds characterized as maritime (100 CCN cm−3), moderate continental (600 CCN cm−3), and extreme continental (1100 CCN cm−3) using two different initial conditions in which cloud tops reached −20° and −12°C.
The seeding time was found to be a critical parameter for obtaining positive results. The optimal “time window” for IN seeding was found to be very short and to correspond to the time at which the natural ice began to form. Seeding after this time reduced the rain. The optimal concentration of seeding material was about 75–125 L−1. In the maritime clouds rain formation processes were very efficient, and seeding did not produce any significant increase in rain amounts. In the moderate and extreme continental clouds with tops at −20°C, seeding with IN at the optimal time and location increased the precipitation by 9% and 35%, respectively. Ice nuclei seeding of a warmer cloud with a top temperature of −12°C did not change the rainfall when seeding took place in the optimal time window.
Seeding with hygroscopic particles had a dramatic effect on the rainfall. In the moderate and extreme continental clouds increases of 65% and 109% in rain amounts were obtained. In these cases, the optimal time window was longer, and even clouds with tops at −12°C doubled their rain amounts.
Abstract
Simulations of seeding clouds for rain enhancement with ice nuclei (IN) or hygroscopic particles were conducted using a numerical model of an axisymmetric convective cloud with detailed treatment of both warm and cold microphysical processes. The simulations were performed for three clouds that differed in their cloud condensation nuclei (CCN) concentrations and spectra. Tests were carried out on clouds characterized as maritime (100 CCN cm−3), moderate continental (600 CCN cm−3), and extreme continental (1100 CCN cm−3) using two different initial conditions in which cloud tops reached −20° and −12°C.
The seeding time was found to be a critical parameter for obtaining positive results. The optimal “time window” for IN seeding was found to be very short and to correspond to the time at which the natural ice began to form. Seeding after this time reduced the rain. The optimal concentration of seeding material was about 75–125 L−1. In the maritime clouds rain formation processes were very efficient, and seeding did not produce any significant increase in rain amounts. In the moderate and extreme continental clouds with tops at −20°C, seeding with IN at the optimal time and location increased the precipitation by 9% and 35%, respectively. Ice nuclei seeding of a warmer cloud with a top temperature of −12°C did not change the rainfall when seeding took place in the optimal time window.
Seeding with hygroscopic particles had a dramatic effect on the rainfall. In the moderate and extreme continental clouds increases of 65% and 109% in rain amounts were obtained. In these cases, the optimal time window was longer, and even clouds with tops at −12°C doubled their rain amounts.
Abstract
A mesoscale model RAMS (the Regional Atmospheric Modeling System) was used to investigate the effectiveness of the broadcast static seeding method for dispersing particles into clouds, as it is used in Israel. The model was run using three nested grids, with 500 m × 500 m horizontal resolution in the finest grid. In this paper, the particles were assumed to be inert; namely, only the wind field controlled the dispersal of the tracer particles, and no interaction with cloud or precipitation particles was considered. Although the resolution of the model is good for mesoscale studies, it could not resolve individual plumes. The results, therefore, present average values of the concentrations at each level. The simulations showed that seeding particles reach altitudes at which they could become effective as ice nuclei. These cases were primarily the ones in which the updrafts developed over the seeding lines when the seeding plane was just passing underneath. In these cases only, seeding at about 1-km level (∼4°C) with 500 g h−1 of inert material (simulating AgI particles) resulted in about 1 × 103–2 × 103 L−1 being lifted to the −10°C level. Based on previous laboratory studies of the seeding agent used in Israel, out of these total concentrations, only 1–2 L−1 could form ice at −10°C. The simulations also suggest that in most other cases the horizontal advection diluted the particles in the air and only very low concentrations (<10−3 L−1, active at −10°C) reached the −10°C level. Most other released particles were transported horizontally with the winds and were later on forced down by downdrafts. Although these simulations await some experimental verification, they suggest that the broadcast seeding method used in Israel is not so effective for widespread rain enhancement operations.
Abstract
A mesoscale model RAMS (the Regional Atmospheric Modeling System) was used to investigate the effectiveness of the broadcast static seeding method for dispersing particles into clouds, as it is used in Israel. The model was run using three nested grids, with 500 m × 500 m horizontal resolution in the finest grid. In this paper, the particles were assumed to be inert; namely, only the wind field controlled the dispersal of the tracer particles, and no interaction with cloud or precipitation particles was considered. Although the resolution of the model is good for mesoscale studies, it could not resolve individual plumes. The results, therefore, present average values of the concentrations at each level. The simulations showed that seeding particles reach altitudes at which they could become effective as ice nuclei. These cases were primarily the ones in which the updrafts developed over the seeding lines when the seeding plane was just passing underneath. In these cases only, seeding at about 1-km level (∼4°C) with 500 g h−1 of inert material (simulating AgI particles) resulted in about 1 × 103–2 × 103 L−1 being lifted to the −10°C level. Based on previous laboratory studies of the seeding agent used in Israel, out of these total concentrations, only 1–2 L−1 could form ice at −10°C. The simulations also suggest that in most other cases the horizontal advection diluted the particles in the air and only very low concentrations (<10−3 L−1, active at −10°C) reached the −10°C level. Most other released particles were transported horizontally with the winds and were later on forced down by downdrafts. Although these simulations await some experimental verification, they suggest that the broadcast seeding method used in Israel is not so effective for widespread rain enhancement operations.
Abstract
Numerical calculations using a cloud model with detailed microphysics are conducted to investigate the possible effects of hygroscopic flare seeding on the changes in the spectra of hydrometeors and the resulting radar-derived properties, such as storm rain mass, rain flux, and rainfall amount. The results indicate that, in continental clouds, seeding can significantly change the distribution functions of the precipitation particles, the radar reflectivity–rainfall (Z–R) relationship, and the radar-derived properties. Therefore, different Z–R relationships derived respectively from unseeded and seeded clouds should be used to estimate properly the effects of seeding with hygroscopic flares. The results also show that the effects of hygroscopic seeding on maritime clouds are small and there is little difference in the Z–R relationship and the precipitation properties between the seeded and the unseeded cases.
Abstract
Numerical calculations using a cloud model with detailed microphysics are conducted to investigate the possible effects of hygroscopic flare seeding on the changes in the spectra of hydrometeors and the resulting radar-derived properties, such as storm rain mass, rain flux, and rainfall amount. The results indicate that, in continental clouds, seeding can significantly change the distribution functions of the precipitation particles, the radar reflectivity–rainfall (Z–R) relationship, and the radar-derived properties. Therefore, different Z–R relationships derived respectively from unseeded and seeded clouds should be used to estimate properly the effects of seeding with hygroscopic flares. The results also show that the effects of hygroscopic seeding on maritime clouds are small and there is little difference in the Z–R relationship and the precipitation properties between the seeded and the unseeded cases.
Abstract
Numerical experiments were conducted to evaluate the role of hygroscopic flare seeding on enhancement of precipitation in convective clouds. The spectra of seeding particles were based on measurements of the particles produced by hygroscopic flares used in field experiments in South Africa. The seeding effects were investigated by comparing the development of precipitation particles and rain production between the seeded and unseeded cases for clouds with different cloud condensation nuclei (CCN) concentrations and spectra.
The South African hypothesis that the introduction of larger and more efficient artificial CCN below cloud base at the early stage of cloud development would influence the initial condensation process in the cloud, resulting in a broader droplet spectrum and in acceleration of the precipitation growth by coalescence, was tested. The results show that the largest seeding particles broaden the cloud droplet distribution near cloud base, leading to an earlier formation of raindrops, graupel particles, and, therefore, stronger radar echoes at a lower altitude. The results also show that the large artificial CCN prevent some of the natural CCN from becoming activated. It was found that seeding with the full particle spectrum from the flares could increase rainfall amount in continental clouds having CCN concentrations of more than about 500 cm−3 (active at 1% supersaturation). Seeding more maritime clouds resulted in reducing the integrated rain amount, although in some cases rain formation was accelerated. The physical mechanisms responsible for these results were explored by investigating the relative importance of different segments of the size spectrum of the seeding particles to precipitation development. It was found that, out of the full spectrum, the most effective particles were those with radii larger than 1 μm, especially those larger than 10 μm; the particles smaller than 1 μm always had a negative effect on the rain development.
The sensitivity of seeding effects to seeding time, seeding height, and seeding amounts also was tested. The biggest precipitation enhancement was obtained when seeding was conducted a few minutes after cloud initiation and above cloud base. The radar reflectivity at that time period was lower than 0 dBZ. Rain enhancement also increased with the increase in the concentration of the large seeding particles in the spectrum (at least for the amounts tested here).
Abstract
Numerical experiments were conducted to evaluate the role of hygroscopic flare seeding on enhancement of precipitation in convective clouds. The spectra of seeding particles were based on measurements of the particles produced by hygroscopic flares used in field experiments in South Africa. The seeding effects were investigated by comparing the development of precipitation particles and rain production between the seeded and unseeded cases for clouds with different cloud condensation nuclei (CCN) concentrations and spectra.
The South African hypothesis that the introduction of larger and more efficient artificial CCN below cloud base at the early stage of cloud development would influence the initial condensation process in the cloud, resulting in a broader droplet spectrum and in acceleration of the precipitation growth by coalescence, was tested. The results show that the largest seeding particles broaden the cloud droplet distribution near cloud base, leading to an earlier formation of raindrops, graupel particles, and, therefore, stronger radar echoes at a lower altitude. The results also show that the large artificial CCN prevent some of the natural CCN from becoming activated. It was found that seeding with the full particle spectrum from the flares could increase rainfall amount in continental clouds having CCN concentrations of more than about 500 cm−3 (active at 1% supersaturation). Seeding more maritime clouds resulted in reducing the integrated rain amount, although in some cases rain formation was accelerated. The physical mechanisms responsible for these results were explored by investigating the relative importance of different segments of the size spectrum of the seeding particles to precipitation development. It was found that, out of the full spectrum, the most effective particles were those with radii larger than 1 μm, especially those larger than 10 μm; the particles smaller than 1 μm always had a negative effect on the rain development.
The sensitivity of seeding effects to seeding time, seeding height, and seeding amounts also was tested. The biggest precipitation enhancement was obtained when seeding was conducted a few minutes after cloud initiation and above cloud base. The radar reflectivity at that time period was lower than 0 dBZ. Rain enhancement also increased with the increase in the concentration of the large seeding particles in the spectrum (at least for the amounts tested here).
