Horizontal Divergence of Typhoon-generated Gravity Waves in the Upper Troposphere and Its Contribution to the Intensity of Typhoon Saomai (2006)

Recent study by Kim and Chun (2011) revealed that horizontal divergence of typhoon-generated gravity waves (TGWs) contributes significantly to the total divergence in the upper troposphere. Considering that the horizontal divergence in the upper troposphere is one of the major dynamic factors to determine typhoon intensity, this result implies that TGWs can contribute to the typhoon intensity and that there is a feedback between the induced gravity waves (TGWs) and their sources (convective clouds associated with typhoon). To understand this feedback process, in the present study, we examine the characteristics of horizontal divergence of TGWs (HDTGW) in the upper troposphere and the contribution of HDTGW to the typhoon evolution using the WRF model-simulated results of Typhoon Saomai (2006). The HDTGW in the upper troposphere shows multi-scaled circular-type features that are associated with various scales of convective clouds; relatively short wavelengths and large amplitudes near eyewall and relatively long wavelengths and small amplitudes near spiral rainbands. The 3-D power spectrum of HDTGW in an azimuthal angle-phase speed domain reveals that stronger power exists in the southeastern direction with phase speeds less than 40 m/s. The dominant horizontal wavelengths and period are about 10 km to 50 km and shorter than 3 hours, respectively. The spectral characteristics of HDTGW are generally similar to those of the TGWs reported previously by Kim and Chun. However, when domain-averaged HDTGW is considered its spectral peak exists at 24 hours, because the high-frequency signals having 1 to 3 hours periods, corresponding to the vigorous convections near eyewall, are averaged out and only longer-period components remain. The time series of the domain-averaged HDTGW is well correlated with those of the minimum sea-level pressure, with a maximum correlation coefficient of up to 0.99 at lag 4 hours in the mature stage of typhoon, implying that HDTGW with near-inertia period may contribute best to the evolution of typhoon intensity. In order to understand the origin of the inertia-period GWs, we examined the possibilities of diurnal tide and vertically propagating GWs generated by low-frequency convective clouds associated with typhoon. It is found that near-inertia frequency of convection in the spiral rainbands propagating outward is likely the main source of the near-inertia frequency of HDTGW in the upper troposphere, while the diurnal cycle of convection associated with solar heating is unlikely the main source of the near-inertia period HDTGW, considering the local maximum of precipitation in the present case.