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- Author or Editor: A. S. Dennis x
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Abstract
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Abstract
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Abstract
The distribution in time of individual lightning flashes recorded visually during 20 storms in New Mexico and on 23-cm radar screens during several storms in the south-central United States has been analyzed. The logarithms of the intervals between flashes within a given storm are normally distributed with a standard deviation σ of approximately one natural-log unit. The available data do not reject the hypothesis that there are no statistically significant variations in σ among storms. The autocorrelation of the intervals between flashes in one storm is very small. The implications of these findings are briefly explored.
Abstract
The distribution in time of individual lightning flashes recorded visually during 20 storms in New Mexico and on 23-cm radar screens during several storms in the south-central United States has been analyzed. The logarithms of the intervals between flashes within a given storm are normally distributed with a standard deviation σ of approximately one natural-log unit. The available data do not reject the hypothesis that there are no statistically significant variations in σ among storms. The autocorrelation of the intervals between flashes in one storm is very small. The implications of these findings are briefly explored.
Abstract
Examination of radar records shows that snow trails frequently occur around the tops of showers. Evidence has been sought that showers are occasionally initiated by the seeding of supercooled cumulus clouds by snow. Certain observed radar patterns suggest strongly that showers were produced in this manner. As a check, plan diagrams with height contours have been synthesized and used to organize vertical sections obtained on different bearings. The constructed plan diagrams show showers occurring in regions of snow trails more frequently than elsewhere.
Abstract
Examination of radar records shows that snow trails frequently occur around the tops of showers. Evidence has been sought that showers are occasionally initiated by the seeding of supercooled cumulus clouds by snow. Certain observed radar patterns suggest strongly that showers were produced in this manner. As a check, plan diagrams with height contours have been synthesized and used to organize vertical sections obtained on different bearings. The constructed plan diagrams show showers occurring in regions of snow trails more frequently than elsewhere.
Abstract
No abstract available.
Abstract
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Abstract
A hailstone model is developed which simulates both wet and dry hailstone growth and partial or complete melting as the hailstone falls to the ground. The simulation is obtained by a consideration of the mass and heat budgets of the hailstone as a function of its size and of the environmental conditions. Competition among hailstone embryos is not treated in this model.
The hailstone model has been run in over 50 versions of a one-dimensional, time-dependent cloud model to derive estimates of maximum hailstone size from a wide variety of convective storms. The model suggests that hailstone diameter at the ground is determined closely by the strongest updraft encountered by the stone and the temperature at that level. In the model, most hailstones spend some time above the maximum updraft and significant growth occurs during descent from that level to the freezing level. A comparison of model predictions to hailstone observations in the Rapid City area during 1968 and 1969 shows fair agreement.
The model has been used to test the concept of hail suppression through artificial glaciation of cloud water and rainwater. It suggests that results would vary with temperature at the level of maximum updraft, but that artificial cloud glaciation would lead in a majority of cases to a reduction in maximum hailstone size at the ground.
Abstract
A hailstone model is developed which simulates both wet and dry hailstone growth and partial or complete melting as the hailstone falls to the ground. The simulation is obtained by a consideration of the mass and heat budgets of the hailstone as a function of its size and of the environmental conditions. Competition among hailstone embryos is not treated in this model.
The hailstone model has been run in over 50 versions of a one-dimensional, time-dependent cloud model to derive estimates of maximum hailstone size from a wide variety of convective storms. The model suggests that hailstone diameter at the ground is determined closely by the strongest updraft encountered by the stone and the temperature at that level. In the model, most hailstones spend some time above the maximum updraft and significant growth occurs during descent from that level to the freezing level. A comparison of model predictions to hailstone observations in the Rapid City area during 1968 and 1969 shows fair agreement.
The model has been used to test the concept of hail suppression through artificial glaciation of cloud water and rainwater. It suggests that results would vary with temperature at the level of maximum updraft, but that artificial cloud glaciation would lead in a majority of cases to a reduction in maximum hailstone size at the ground.
Abstract
Vertical-section radar observations of precipitation in the winter of 1951–1952 have been related to upper-air data. On three of the 22 days, there was very little signal and practically no pattern observed. It is possible that these are cases, such as described recently by Wexler and Austin (1953), in which the snow crystals grow gradually while descending through cloud-free air, at a saturation intermediate between that of ice and water.
On the remaining 19 days, trail patterns were observed. On 13 of them, well-defined snow trails, usually with generating elements visible, were detected. Upper-air analyses showed the air aloft to be stable on these days of good pattern. On six days, the records contained only parts of trails, the whole pattern being less well-defined; the air aloft was unstable on these days. Since the well-defined patterns occur on stable days, instability is not the initiating mechanism; rather, the presence of instability confuses the pattern.
For the 13 stable days, the top of the trail echo was, on the average, near the top of a main middle cloud, the top of this cloud being defined as the height above which radiosonde data show the relative humidity to drop sharply. The average cloud-top height for these 13 days was some 1300 ft above a frontal surface, the average frontal-surface height over Montreal being about 11,000 ft.
The results of this study indicate the presence of a relatively shallow “active layer” of stratiform cloud, straddling a frontal surface, in which snow-generating elements of considerable lifetime exist.
Abstract
Vertical-section radar observations of precipitation in the winter of 1951–1952 have been related to upper-air data. On three of the 22 days, there was very little signal and practically no pattern observed. It is possible that these are cases, such as described recently by Wexler and Austin (1953), in which the snow crystals grow gradually while descending through cloud-free air, at a saturation intermediate between that of ice and water.
On the remaining 19 days, trail patterns were observed. On 13 of them, well-defined snow trails, usually with generating elements visible, were detected. Upper-air analyses showed the air aloft to be stable on these days of good pattern. On six days, the records contained only parts of trails, the whole pattern being less well-defined; the air aloft was unstable on these days. Since the well-defined patterns occur on stable days, instability is not the initiating mechanism; rather, the presence of instability confuses the pattern.
For the 13 stable days, the top of the trail echo was, on the average, near the top of a main middle cloud, the top of this cloud being defined as the height above which radiosonde data show the relative humidity to drop sharply. The average cloud-top height for these 13 days was some 1300 ft above a frontal surface, the average frontal-surface height over Montreal being about 11,000 ft.
The results of this study indicate the presence of a relatively shallow “active layer” of stratiform cloud, straddling a frontal surface, in which snow-generating elements of considerable lifetime exist.
Abstract
Global ozone observations from the Microwave Limb Sounder (MLS) aboard the Upper Atmosphere Research Satellite (UARS) are presented, in both vertically resolved and column abundance formats. The authors review the zonal-mean ozone variations measured over the two and a half years since launch in September 1991. Well-known features such as the annual and semiannual variations are ubiquitous. In the equatorial regions, longerterm changes are believed to be related to the quasi-biennial oscillation (QBO), with a strong semiannual signal above 20 hPa. Ozone values near 50 hPa exhibit an equatorial low from October 1991 to June 1992, after which the low ozone pattern splits into two subtropical lows (possibly in connection with residual circulation changes tied to the QBO) and returns to an equatorial low in September 1993. The ozone hole development at high southern latitudes is apparent in MLS column data integrated down to 100 hPa, with a pattern generally consistent with Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) measurements of total column; the MLS data reinforce current knowledge of this lower-stratospheric phenomenon by providing a height-dependent view of the variations. The region from 30°S to 30°N (an area equal to half the global area) shows very little change in the ozone column from year to year and within each year.
The most striking ozone changes have occurred at northern midlatitudes, with the October 1992 to July 1993 column values significantly lower than during the prior year. The zonal-mean changes manifest themselves as a slower rate of increase during the 1992/93 winter, and there is some evidence for a lower fall minimum. A recovery occurs during late summer of 1993; early 1994 values are significantly larger than during the two previous winters. These results are in general agreement with variations measured by the Nimbus-7 TOMS and Meteor-3 TOMS instruments at midlatitudes. However, the southern midlatitudes exhibit less of a column ozone decrease (relative to the north) in the MLS data (down to 100 hPa) than in the TOMS column results. The timing and latitudinal extent of the northern midlatitude decreases appear to rule out observed CIO enhancements in the Arctic vortex, with related chemical processing and ozone dilution effects, as a unique cause. Local depletion from CIO-related chemical mechanisms alone is also not sufficient, based on MLS CIO data. The puzzling asymmetric nature of the changes probably requires a dynamical component as an explanation. A combination of effects (including chemical destruction via heterogeneous processes and QBO phasing) apparently needs to be invoked. This dataset will place constraints on future modeling studies, which are required to better understand the source of the observed changes.
Finally, residual ozone values extracted from TOMS-minus-MLS column data are briefly presented as a preliminary view into the potential usefulness of such studies, with information on tropospheric ozone as an ultimate goal.
Abstract
Global ozone observations from the Microwave Limb Sounder (MLS) aboard the Upper Atmosphere Research Satellite (UARS) are presented, in both vertically resolved and column abundance formats. The authors review the zonal-mean ozone variations measured over the two and a half years since launch in September 1991. Well-known features such as the annual and semiannual variations are ubiquitous. In the equatorial regions, longerterm changes are believed to be related to the quasi-biennial oscillation (QBO), with a strong semiannual signal above 20 hPa. Ozone values near 50 hPa exhibit an equatorial low from October 1991 to June 1992, after which the low ozone pattern splits into two subtropical lows (possibly in connection with residual circulation changes tied to the QBO) and returns to an equatorial low in September 1993. The ozone hole development at high southern latitudes is apparent in MLS column data integrated down to 100 hPa, with a pattern generally consistent with Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) measurements of total column; the MLS data reinforce current knowledge of this lower-stratospheric phenomenon by providing a height-dependent view of the variations. The region from 30°S to 30°N (an area equal to half the global area) shows very little change in the ozone column from year to year and within each year.
The most striking ozone changes have occurred at northern midlatitudes, with the October 1992 to July 1993 column values significantly lower than during the prior year. The zonal-mean changes manifest themselves as a slower rate of increase during the 1992/93 winter, and there is some evidence for a lower fall minimum. A recovery occurs during late summer of 1993; early 1994 values are significantly larger than during the two previous winters. These results are in general agreement with variations measured by the Nimbus-7 TOMS and Meteor-3 TOMS instruments at midlatitudes. However, the southern midlatitudes exhibit less of a column ozone decrease (relative to the north) in the MLS data (down to 100 hPa) than in the TOMS column results. The timing and latitudinal extent of the northern midlatitude decreases appear to rule out observed CIO enhancements in the Arctic vortex, with related chemical processing and ozone dilution effects, as a unique cause. Local depletion from CIO-related chemical mechanisms alone is also not sufficient, based on MLS CIO data. The puzzling asymmetric nature of the changes probably requires a dynamical component as an explanation. A combination of effects (including chemical destruction via heterogeneous processes and QBO phasing) apparently needs to be invoked. This dataset will place constraints on future modeling studies, which are required to better understand the source of the observed changes.
Finally, residual ozone values extracted from TOMS-minus-MLS column data are briefly presented as a preliminary view into the potential usefulness of such studies, with information on tropospheric ozone as an ultimate goal.
