A modeling study of orographic convection and mountain waves in the landfalling typhoon Nari (2001)
In this study the characteristics of mountain-induced gravity waves (MGWs) and orographic convection in typhoon Nari (2001) and their impacts on the formation of orographic precipitation, as Nari moved across Taiwan's Central Mountain Range (CMR), are investigated by analyzing the cloud-resolving model simulation results. In the distant rain band, convective cells were triggered frequently on the steep windward slope of CMR and advected downstream. Simultaneously MGWs could exist on the lee side with stable environmental stratification. Three phases occurred in the MGWs–convection interaction, including the enhancement of MGWs and convection, breaking, and restoring of MGWs. When the convective cells and MGWs were superposed in (out of) phase, the amplitudes of both MGWs and convective cells were increased (decreased). The vertical wavelength of MGWs was also increased during the in-phase superposition, even inducing the further breaking of MGWs. For the formation of precipitation on the lee side, MGWs–convection interaction contributes on a smaller scale to offset the reduction effect from the larger-scale descending. Moreover, convective cells on the windward slope could contribute a large amount of cloud ice and snow particles aloft to seed hydrometeors over the lee side. More cloud water was produced by the MGWs–convection interaction when their amplitudes were increased during the in-phase superposition. Riming and coalescence processes were responsible not only for the windward-slope precipitation enhancement but also the secondary precipitation maximum over the lee side. Therefore, the MGWs–convection interaction could modify the structure of vertical motion and hydrometeors over the lee side, and then affect orographic precipitation when Nari passed the CMR.



As Nari's eyewall encountered the northern CMR, the terrain generated long-lasting strong updrafts at the upslope, resulting in a doubled rainfall maximum to that in the no-terrain sensitivity experiment. The downdraft branch of the MGW produced a rain shadow on the lee side. Copyright © 2011 Royal Meteorological Society