Typhoon-generated gravity waves in the lower stratosphere and their influence in the typhoon development

Vertically propagating gravity waves generated by Typhoon Saomai (2006) were simulated using WRF in a moving frame of reference following the typhoon. Waves with large amplitudes appear near the domain center due to strong convection in the eyewall of the typhoon. Convection bands propagating outward from the storm center also generate waves propagating to the stratosphere. Convective forcing is significant in various propagation directions, with maximum power in slowly-moving eastward components due to convection in the eyewall. The typhoon-generated gravity waves (TGWs) in the stratosphere are dominant in the eastward-, northeastward-, and southeastward-propagation directions, since westward waves are mostly filtered by the background wind below z = 25 km. While the typhoon moves northwestward for 78 hours, wave characteristics are different at different times depending on the evolution of the eyewall and spiral bands. Horizontal wavelengths of TGWs are longer in the mature and decaying stages than in the developing stage of the typhoon, with the spectral peak at ~50 km (~20 km) in the mature (developing) stage, and the wave amplitudes are larger in the developing stages.

Effects of TGWs in the typhoon development is investigated by examining changes in the background flow and divergence in the upper troposphere due to TGWs. The momentum flux of TGWs and its vertical divergence/convergence are larger during the developing stage than during the mature and decaying stages of the typhoon. In the developing and mature (decaying) stages of the typhoon, TGWs act to decrease (increase) vertical wind shear, which is known to inhibit a typhoon's intensification. Thus, TGWs can help a typhoon intensify when the typhoon develops and can help a typhoon decay once typhoon intensity begins to decrease. Upper-level divergence increases notably before a typhoon enters both rapidly-developing and mature stages. A significant portion of upper-level divergence is found to be due to TGWs.