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- Author or Editor: Roland A. Madden x
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Abstract
Diurnal and semidiurnal variations in the budget of atmospheric angular momentum are evident in a simulation by the NCAR Community Climate Model (CCM2). These variations depicted with 20-min time resolution (each time step) are used as guides to study similar variations determined from 6-hourly NCEP/NCAR Reanalysis data. A semidiurnal variation in relative angular momentum and in angular momentum related to solid body rotation of the atmosphere is found in the reanalysis. Although there is evidence that both frictional and gravity-wave drag torques play roles, the effects of semidiurnal variations in mountain torque, resulting from the migrating semidiurnal pressure wave, are most thoroughly documented.
Abstract
Diurnal and semidiurnal variations in the budget of atmospheric angular momentum are evident in a simulation by the NCAR Community Climate Model (CCM2). These variations depicted with 20-min time resolution (each time step) are used as guides to study similar variations determined from 6-hourly NCEP/NCAR Reanalysis data. A semidiurnal variation in relative angular momentum and in angular momentum related to solid body rotation of the atmosphere is found in the reanalysis. Although there is evidence that both frictional and gravity-wave drag torques play roles, the effects of semidiurnal variations in mountain torque, resulting from the migrating semidiurnal pressure wave, are most thoroughly documented.
Abstract
Two patterns dominate changes of monthly mean temperature and pressure-height in the stratosphere. In the one, the middle latitudes vary oppositely to low and high latitudes, and in the other the changes at higher latitudes are out of phase with those at lower latitudes.
A shorter trend consisting of opposite changes at middle and high latitudes is superposed on the above variations which a cross-spectrum analysis shows has a preferred time scale of one to three weeks. The contrast between middle and high latitudes thus undergoes a series of corresponding fluctuations and we show that these are associated with amplitude changes in waves 1 and 2 in that the meridional contrast decreases when the amplitude of one or both waves is large, and vice versa.
Abstract
Two patterns dominate changes of monthly mean temperature and pressure-height in the stratosphere. In the one, the middle latitudes vary oppositely to low and high latitudes, and in the other the changes at higher latitudes are out of phase with those at lower latitudes.
A shorter trend consisting of opposite changes at middle and high latitudes is superposed on the above variations which a cross-spectrum analysis shows has a preferred time scale of one to three weeks. The contrast between middle and high latitudes thus undergoes a series of corresponding fluctuations and we show that these are associated with amplitude changes in waves 1 and 2 in that the meridional contrast decreases when the amplitude of one or both waves is large, and vice versa.
Abstract
Upper tropospheric and lower stratospheric wind data spanning 31 years from 1964 to 1994 were analyzed at rawinsonde stations in the central/western Pacific. Traditional spectral and cross-spectral analysis led to the conclusion that there is a significant signal with periods between 3 and 4.5 days, which the authors link with the dominant antisymmetric waves predicted by theory to have these periods, mixed Rossby–gravity waves, and equatorial Rossby waves. Then the authors applied the seasonally varying spectral analysis method developed by Madden to study the average seasonal variation of these waves. The seasonally varying analysis suggested that there are significant twice-yearly maxima in equatorial wave activity throughout the upper troposphere and lower stratosphere, with peaks occurring in late winter–spring and in late summer–fall. The twice-yearly signal was most prominent at the 70-hPa and 100-hPa levels. Similar and consistent results were also shown by an autoregressive cyclic spectral analysis. The cyclic spectral analysis suggested that the frequency characteristics of the υ-wind wave power are different during the two maxima at some stations. In addition, the seasonally varying squared coherence between the u and υ winds and the associated phase implied that there is horizontal momentum flux associated with these waves and that the sign of the flux is different during the two maxima. The differences in wave characteristics during the maxima periods may be related to different wave modes, seasonal variation of the basic zonal state, or possibly to different equatorial wave forcing mechanisms (i.e., convective versus lateral excitations).
Abstract
Upper tropospheric and lower stratospheric wind data spanning 31 years from 1964 to 1994 were analyzed at rawinsonde stations in the central/western Pacific. Traditional spectral and cross-spectral analysis led to the conclusion that there is a significant signal with periods between 3 and 4.5 days, which the authors link with the dominant antisymmetric waves predicted by theory to have these periods, mixed Rossby–gravity waves, and equatorial Rossby waves. Then the authors applied the seasonally varying spectral analysis method developed by Madden to study the average seasonal variation of these waves. The seasonally varying analysis suggested that there are significant twice-yearly maxima in equatorial wave activity throughout the upper troposphere and lower stratosphere, with peaks occurring in late winter–spring and in late summer–fall. The twice-yearly signal was most prominent at the 70-hPa and 100-hPa levels. Similar and consistent results were also shown by an autoregressive cyclic spectral analysis. The cyclic spectral analysis suggested that the frequency characteristics of the υ-wind wave power are different during the two maxima at some stations. In addition, the seasonally varying squared coherence between the u and υ winds and the associated phase implied that there is horizontal momentum flux associated with these waves and that the sign of the flux is different during the two maxima. The differences in wave characteristics during the maxima periods may be related to different wave modes, seasonal variation of the basic zonal state, or possibly to different equatorial wave forcing mechanisms (i.e., convective versus lateral excitations).
