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- Author or Editor: J. M. MITCHELL JR. x
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Abstract
Power spectrum techniques are applied to series of daily, weekly, and monthly average temperature and precipitation values, recorded since 1870 at the Woodstock Climatological Benchmark Station in Maryland, in order to gain a reasonable interpretation of the extent and frequency distribution of periodic variations in these data. Analysis procedures are outlined, and the results presented, interpreted, and collated with the results of earlier literature in some detail.
Apparent short-period variations are found whose periods lie near 3 days, between 5 and 7 days, and between about 15 and 25 days; various of them however, are absent from some portions of the data series and also differ somewhat in character with the season of the year.
Significant long-period variations are more prevalent in the temperature series than in the precipitation series. Spectral peaks in temperature, of periods near 2 years and greater than 50 years, both achieve high levels of statistical significance. The 11-year sunspot cycle, and to some extent its second harmonic as well, is suggested in the temperature data. The double (22-year) sunspot cycle and the longer Brückner cycle, however, are almost totally absent. The basis of Abbot's statistical long-range prediction scheme, which utilizes numerous higher harmonics of the double sunspot cycle, is tested against the Woodstock data, and is found in this caw to lack measurable skill above chance.
Abstract
Power spectrum techniques are applied to series of daily, weekly, and monthly average temperature and precipitation values, recorded since 1870 at the Woodstock Climatological Benchmark Station in Maryland, in order to gain a reasonable interpretation of the extent and frequency distribution of periodic variations in these data. Analysis procedures are outlined, and the results presented, interpreted, and collated with the results of earlier literature in some detail.
Apparent short-period variations are found whose periods lie near 3 days, between 5 and 7 days, and between about 15 and 25 days; various of them however, are absent from some portions of the data series and also differ somewhat in character with the season of the year.
Significant long-period variations are more prevalent in the temperature series than in the precipitation series. Spectral peaks in temperature, of periods near 2 years and greater than 50 years, both achieve high levels of statistical significance. The 11-year sunspot cycle, and to some extent its second harmonic as well, is suggested in the temperature data. The double (22-year) sunspot cycle and the longer Brückner cycle, however, are almost totally absent. The basis of Abbot's statistical long-range prediction scheme, which utilizes numerous higher harmonics of the double sunspot cycle, is tested against the Woodstock data, and is found in this caw to lack measurable skill above chance.
Abstract
Further evidence is presented for the presence of a persistent periodicity somewhat in excess of 2 years duration. Its existence can be shown in the surface temperature at widely separated stations along two meridians from Norway to South Africa and from Canada to Cape Horn. Time series analysis by means of a narrow band-pass filter indicates phase relations among the stations, the most important of these being that the intertropical regions appear to be mutually in phase and that the higher-latitude stations are out of phase with the tropical stations. In the Northern Hemisphere, at extratropical stations, the amplitudes of the pulse are largest when the pulse extremes coincide with the winter months.
Abstract
Further evidence is presented for the presence of a persistent periodicity somewhat in excess of 2 years duration. Its existence can be shown in the surface temperature at widely separated stations along two meridians from Norway to South Africa and from Canada to Cape Horn. Time series analysis by means of a narrow band-pass filter indicates phase relations among the stations, the most important of these being that the intertropical regions appear to be mutually in phase and that the higher-latitude stations are out of phase with the tropical stations. In the Northern Hemisphere, at extratropical stations, the amplitudes of the pulse are largest when the pulse extremes coincide with the winter months.
Abstract
Tornadic vortex signatures (TVSs) of 52 tornadoes were identified and analyzed, then characterized as either descending or nondescending. This characterization refers to a known tendency of radar-observed tornadic vortices, namely, that of their initial detection aloft and then of their subsequent descent leading to tornadogenesis. Only 52% of the sampled TVSs descended according to this archetypal model. The remaining 48% were detected first near the ground and grew upward or appeared nearly simultaneously over a several kilometer depth; these represent primary modes of tornado development that have been explained theoretically. The descending–nondescending TVSs were stratified according to attributes of the tornado and TVS. Significantly, tornadoes within quasi-linear convective systems tended to be associated with nondescending TVSs, identification of which provided a mean tornado lead time of 5 min.
Two case studies are presented for illustrative purposes. On 1 July 1997 in southern Minnesota, nondescending TVSs and associated tornadogenesis were revealed in the leading edge of a squall line, with a squall line–supercell merger, and later during that day, with the cyclonic bookend vortex of a bow echo. On 22 June 1995 in southern Colorado, a low-topped supercell storm produced a tornado that was associated with a descending TVS.
Abstract
Tornadic vortex signatures (TVSs) of 52 tornadoes were identified and analyzed, then characterized as either descending or nondescending. This characterization refers to a known tendency of radar-observed tornadic vortices, namely, that of their initial detection aloft and then of their subsequent descent leading to tornadogenesis. Only 52% of the sampled TVSs descended according to this archetypal model. The remaining 48% were detected first near the ground and grew upward or appeared nearly simultaneously over a several kilometer depth; these represent primary modes of tornado development that have been explained theoretically. The descending–nondescending TVSs were stratified according to attributes of the tornado and TVS. Significantly, tornadoes within quasi-linear convective systems tended to be associated with nondescending TVSs, identification of which provided a mean tornado lead time of 5 min.
Two case studies are presented for illustrative purposes. On 1 July 1997 in southern Minnesota, nondescending TVSs and associated tornadogenesis were revealed in the leading edge of a squall line, with a squall line–supercell merger, and later during that day, with the cyclonic bookend vortex of a bow echo. On 22 June 1995 in southern Colorado, a low-topped supercell storm produced a tornado that was associated with a descending TVS.
