Investigation of Kelvin Waves in the Stratosphere Using FORMOSAT-3/COSMIC Temperature Data

Abstract

Temperature data from Global Positioning System based Radio Occultation (GPS RO) soundings of the Formosa Satellite mission 3/Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC or F-3/C) micro satellites has been investigated in detail to study the Kelvin wave properties. The high temporal and spatial resolution satellite data from August 2006 to August 2009 have enabled the investigation of Kelvin wave activity on each day. The dominant waves of wave numbers 1 and 2 (W1 and W2) have been investigated in detail at three altitudes—19, 25 and 30 km, and it is found that the amplitude of W1 is greater than that of W2 during 60% of the time. A statistical study of the amplitudes of W1 and W2 is also presented and it is found that the dominant amplitudes are 0.5 to 1.0 K for both waves. At lower altitudes (19 km), the amplitudes of W1 are larger and the distribution is also broader. The amplitudes of both waves in the stratosphere are higher during the easterlies of the quasi-biennial oscillation (QBO) and are maximum when the zonal wind changes from easterlies to westerlies. In the lower altitudes near the tropopause they vary in consonance with the outgoing long wave radiation, a proxy of deep convection. Deduction of the Kelvin wave periods and phase velocities has been possible with better accuracy with the use of the F-3/C data. The average periods of W1 for all years are 15 ± 3, 13 ± 4, and 10 ± 3 days at altitudes 19, 25, and 30 km, respectively and the average periods of W2 for all years are 10 ± 2, 7 ± 2, and 6 ± 2 days, respectively. These standard deviations are geophysical and are due to the variation in the periods of the individual Kelvin wave events and identification of the period for a single Kelvin wave event is correct to within ± one day. We found that the Kelvin waves of both the zonal wave numbers are slow in the lower altitudes and fast in the higher altitudes. Also, the periods decrease gradually with height. This is the most important result of the present study.