Troposphere/mesosphere Coupling by Convective Generation of Gravity Waves: Results from the Kauai MF Radar and Wave Propagation Modeling

We have conducted a three-year study of gravity wave variability, covering 1991, 1992, and 1993, using horizontal wind measurements collected by a partial reflection MF radar located on the island of Kauai, Hawaii (22 N, 160 W). High echo return rates between 70 km and 98 km provided ample time resolution for the study of gravity wave variability in this height range. Horizontal wind measurements were band pass filtered for periods between 20 minutes and 6 hours, establishing a gravity wave band time series. Gravity wave variance and polarization calculations were carried out over various time frames and examined with regard to their response to season and background environment. Among the more interesting observed phenomena were the annual and semiannual oscillations in the gravity wave variances, a significant diurnal variability in the variances, and an extended enhancement of these variances during the latter half of 1992 and the beginning of 1993. The established gravity wave climatology was then examined with regard to possible tropospheric wave sources in the form of deep convective activity using satellite derived cloud-top temperature measurements and monochromatic reverse ray-tracing algorithms. This study suggests a significant link between the springtime variance enhancement observed in the gravity wave variances and the formation of the intertropical convergence zone (ITCZ) in the Pacific. Ray -tracing experiments further linked much of the winter variance enhancement viewed at lower mesospheric altitudes to rainy season convection observed near the Hawaiian Islands. Higher mean winds and increased convective activity suggest that the 1992-1993 non-seasonal variance enhancement is a result of increased wave saturation magnitudes and greater intensity of tropospheric wave generation.