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 and their sources. To understand this feedback process, we examine the characteristics of horizontal divergence of TGWs (HDTGW) in the upper troposphere and the contribution of the HDTGW to the typhoon intensity 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. Power spectrum of HDTGW shows dominant horizontal wavelengths and period of 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, domain-averaged HDTGW has its spectral peak at 24 hours and is well correlated with the minimum sea-level pressure of the simulated typhoon, with a maximum correlation coefficient of 0.99 at lag 4 hours in a mature stage of the typhoon. The analyses of the domain-averaged DHTGWs reveal that high-frequency signals of 1 to 3 hours periods, corresponding to the TGWs generated by vigorous convections near eyewall, are averaged out, and that the dominant spectral peak at 24 hours is associated with the inertia-gravity waves (IGW) generated by relatively weak and low-frequency convective clouds in the spiral rainbands, without clear association with the thermal tides and diurnal trend of precipitation.