Precipitation production is investigated for 9 intense thunderstorms that developed over the Lowv... more Precipitation production is investigated for 9 intense thunderstorms that developed over the Lowveld in South Africa. A C-band radar is used to observe the 3-dimensional reflectivity pattern. Using an empirical relation between reflectivity factor and precipitation content and integrating over the storm volume provides an estimate of the total precipitation content aloft. Likewise, an area integration of the instantaneous rain rate at cloud base yields an estimate of the rate of total outflow. At their maturing stage, the storms had precipitation contents of 0.2 to 5.0 Tg and rainfall rates of about 0.3 to 2.0 Gg s-1. The total accumulation of rain at the ground ranged from 1 to 10 Tg. The characteristic storm updraft, defined as the ratio of the area-averaged rainfall rate to the volume-averaged precipitation content, was about 5 ms-1 for all storms. The time evolution of integral storm parameters is also presented and related to the overall storm development. The precipitation production values observed in the Lowveld storms compares well with previous estimates reported for large thunderstorms observed in Alberta and New England. RÉSUMÉ On étudie la précipitation produite par 9 orages intenses qui se sont développés sur le Lowveld de l'Afrique du sud. La configuration de la réflectivité tri-dimensionnelle est observée à l'aide d'un radar de bande C. On arrive à une estimation du contenu total de précipitation en altitude en utilisant une relation empirique entre le facteur de réflectivité et la précipitation et en intégrant ces valeurs pour le volume entier de l'orage. De même, une integration de surface de l'intensité des pluies instantanée à la base du nuage nous donne une estimation de l'intensité de sortie totale. Les orages avaient un contenu de précipitation de 0,2 à 0,5 Tg et une intensité de 0,3 à2,0 Gg s-1 à leur stade de maturité. La hauteur totale de pluie au sol était de 1 à 10 Tg. Le courant caractéristique ascendant d'un orage, défini comme le rapport entre l'étendue moyennée de l'intensité des pluies et le volume moyenne du contenu de précipitation, était d'environ de 5 ms-1 pour tous les orages. On présente aussi l'évolution dans le temps des paramètres intégraux des orages et leur association au développement général des orages. La production de précipitation observée dans ces orages se compare positivement aux estimations antérieures provenant d'intenses orages observés en Alberta et en Nouvelle-Angleterre.
HAILCAST is a numerical model developed specifically to predict the size of the largest hail reac... more HAILCAST is a numerical model developed specifically to predict the size of the largest hail reaching the ground. It consists of a steady-state cloud model combined with a time-dependent hailstone growth model. The regional version of the Canadian Global Environmental Multiscale (GEM) model is used to provide prognostic model soundings that are used as input data for HAILCAST. A map of forecasted maximum hail size is thereby obtained. Because hail is typically accompanied by rain, it would be advantageous if the GEM–HAILCAST system were to predict the occurrence of hail only in those regions where the GEM model was predicting precipitation. Hence, the utility of applying a forecast rainfall mask from the GEM model to restrict hail forecasts to those areas where rainfall is forecast during a 12-h window centered on 0000 UTC was tested. The accumulated precipitation filter is objective and integrates both the thermodynamic and dynamic output from the GEM model over many time steps. To test the utility of applying the GEM forecast precipitation mask, the masking technique was applied to HAILCAST-predicted maximum hail size maps for the three Canadian prairie provinces between 1 June and 31 August 2000. Several case studies will be presented to illustrate the usefulness of adding the precipitation mask. Verification statistics confirm that applying the rainfall mask tends to slightly reduce the false alarm ratio while still identifying the majority of hail events within a special study area over southern Alberta. The performance of the precipitation masking technique was not as effective on severe hail days, especially when attempting to identify both the occurrence and location of severe hail swaths.
ABSTRACT The frequency and duration of hail day episodes were analyzed based on 29 summers of hai... more ABSTRACT The frequency and duration of hail day episodes were analyzed based on 29 summers of hail data from central Alberta, Canada and 19 summers of data from the Highveld, South Africa. A hail episode consists of one or more consecutive days with at least one hail report. A severe hail episode is deþned as an episode that has at least one day of maximum hail size ≥ 33 mm for Alberta, (≥ 31 mm for the Highveld). Severe and non‐severe hail episodes are found to occur, on average, 6.6 and 11.2 times in Alberta, respectively, with mean durations of 4.2 and 2.1 days. In the Highveld, severe and non‐severe hail episodes occur, on average, 2.8 and 21.9 times per six‐month summer and have mean durations of 4.9 and 2.5 days.
A morphogenetic model has been developed to predict the ice accretion on an ice sensor resulting ... more A morphogenetic model has been developed to predict the ice accretion on an ice sensor resulting from water spray. In the numerical model, the spray is divided into an ensemble of fluid elements and their motion and freezing are considered simultaneously. The model predicts both the ice accretion on the vertical cylindrical surface of the sensor and the accretion below the sensor, including icicle formation. The model also allows the simulation and investigation of the stochastic variability of the accretion shape, something that cannot be achieved with conventional continuous, deterministic models. The model predictions of ice accretion shape and mass have been analyzed as function of water spray mass flux and heat transfer conditions. The prediction of the overall ice structure agrees with a simple heat-balance model and our experimental data. y
Precipitation production is investigated for 9 intense thunderstorms that developed over the Lowv... more Precipitation production is investigated for 9 intense thunderstorms that developed over the Lowveld in South Africa. A C-band radar is used to observe the 3-dimensional reflectivity pattern. Using an empirical relation between reflectivity factor and precipitation content and integrating over the storm volume provides an estimate of the total precipitation content aloft. Likewise, an area integration of the instantaneous rain rate at cloud base yields an estimate of the rate of total outflow. At their maturing stage, the storms had precipitation contents of 0.2 to 5.0 Tg and rainfall rates of about 0.3 to 2.0 Gg s-1. The total accumulation of rain at the ground ranged from 1 to 10 Tg. The characteristic storm updraft, defined as the ratio of the area-averaged rainfall rate to the volume-averaged precipitation content, was about 5 ms-1 for all storms. The time evolution of integral storm parameters is also presented and related to the overall storm development. The precipitation production values observed in the Lowveld storms compares well with previous estimates reported for large thunderstorms observed in Alberta and New England. RÉSUMÉ On étudie la précipitation produite par 9 orages intenses qui se sont développés sur le Lowveld de l'Afrique du sud. La configuration de la réflectivité tri-dimensionnelle est observée à l'aide d'un radar de bande C. On arrive à une estimation du contenu total de précipitation en altitude en utilisant une relation empirique entre le facteur de réflectivité et la précipitation et en intégrant ces valeurs pour le volume entier de l'orage. De même, une integration de surface de l'intensité des pluies instantanée à la base du nuage nous donne une estimation de l'intensité de sortie totale. Les orages avaient un contenu de précipitation de 0,2 à 0,5 Tg et une intensité de 0,3 à2,0 Gg s-1 à leur stade de maturité. La hauteur totale de pluie au sol était de 1 à 10 Tg. Le courant caractéristique ascendant d'un orage, défini comme le rapport entre l'étendue moyennée de l'intensité des pluies et le volume moyenne du contenu de précipitation, était d'environ de 5 ms-1 pour tous les orages. On présente aussi l'évolution dans le temps des paramètres intégraux des orages et leur association au développement général des orages. La production de précipitation observée dans ces orages se compare positivement aux estimations antérieures provenant d'intenses orages observés en Alberta et en Nouvelle-Angleterre.
HAILCAST is a numerical model developed specifically to predict the size of the largest hail reac... more HAILCAST is a numerical model developed specifically to predict the size of the largest hail reaching the ground. It consists of a steady-state cloud model combined with a time-dependent hailstone growth model. The regional version of the Canadian Global Environmental Multiscale (GEM) model is used to provide prognostic model soundings that are used as input data for HAILCAST. A map of forecasted maximum hail size is thereby obtained. Because hail is typically accompanied by rain, it would be advantageous if the GEM–HAILCAST system were to predict the occurrence of hail only in those regions where the GEM model was predicting precipitation. Hence, the utility of applying a forecast rainfall mask from the GEM model to restrict hail forecasts to those areas where rainfall is forecast during a 12-h window centered on 0000 UTC was tested. The accumulated precipitation filter is objective and integrates both the thermodynamic and dynamic output from the GEM model over many time steps. To test the utility of applying the GEM forecast precipitation mask, the masking technique was applied to HAILCAST-predicted maximum hail size maps for the three Canadian prairie provinces between 1 June and 31 August 2000. Several case studies will be presented to illustrate the usefulness of adding the precipitation mask. Verification statistics confirm that applying the rainfall mask tends to slightly reduce the false alarm ratio while still identifying the majority of hail events within a special study area over southern Alberta. The performance of the precipitation masking technique was not as effective on severe hail days, especially when attempting to identify both the occurrence and location of severe hail swaths.
ABSTRACT The frequency and duration of hail day episodes were analyzed based on 29 summers of hai... more ABSTRACT The frequency and duration of hail day episodes were analyzed based on 29 summers of hail data from central Alberta, Canada and 19 summers of data from the Highveld, South Africa. A hail episode consists of one or more consecutive days with at least one hail report. A severe hail episode is deþned as an episode that has at least one day of maximum hail size ≥ 33 mm for Alberta, (≥ 31 mm for the Highveld). Severe and non‐severe hail episodes are found to occur, on average, 6.6 and 11.2 times in Alberta, respectively, with mean durations of 4.2 and 2.1 days. In the Highveld, severe and non‐severe hail episodes occur, on average, 2.8 and 21.9 times per six‐month summer and have mean durations of 4.9 and 2.5 days.
A morphogenetic model has been developed to predict the ice accretion on an ice sensor resulting ... more A morphogenetic model has been developed to predict the ice accretion on an ice sensor resulting from water spray. In the numerical model, the spray is divided into an ensemble of fluid elements and their motion and freezing are considered simultaneously. The model predicts both the ice accretion on the vertical cylindrical surface of the sensor and the accretion below the sensor, including icicle formation. The model also allows the simulation and investigation of the stochastic variability of the accretion shape, something that cannot be achieved with conventional continuous, deterministic models. The model predictions of ice accretion shape and mass have been analyzed as function of water spray mass flux and heat transfer conditions. The prediction of the overall ice structure agrees with a simple heat-balance model and our experimental data. y
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