Mountain Waves and Orographic Convection in Typhoon Nari (2001)

In this study the characteristics of orographic convection and mountain-induced gravity waves (MGWs) in Typhoon Nari (2001) are investigated including their impacts on the formation of orographic precipitation by analyzing the cloud-resolving model simulation results. As Nari moved across Taiwan's Central Mountain Range (CMR), convective cells were triggered frequently on the steep windward slope of CMR and advected downstream in the distant rainband. Simultaneously MGWs could exist on the lee side with stable environmental stratification. In the MGWs-convection interaction three phases were found, including the enhancement of MGWs and convection, breaking, and restoring of MGWs. The amplitudes of both MGWs and convective cells were increased (decreased) during the superposition of convective cells and MGWs in (out of) phase. During the in-phase superposition the vertical wavelength of MGWs was also increased, even inducing the further breaking of MGWs.

For the formation of precipitation, MGWs-convection interaction has a contribution in smaller scale to offset the reduction effect from the larger-scale descending on the lee side. Convective cells on the windward slope could further contribute large amount of cloud ice and snow particles aloft to seed hydrometeors over the lee side. The MGWs-convection interaction produced more cloud water when their amplitudes were increased during the in-phase superposition. The riming and coalescence processes might be responsible for not only 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 consequently affected orographic precipitation during the passage of Nari.

The differences of mountain effects over different regions of a landfall typhoon, i.e. the distant rain band versus eyewall were also examined. As Nari' eyewall encountered the northern CMR, the terrain generated long-lasting strong updrafts at the upslope, resulting in 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.

The study aims to provide insights of orographic convection and MGWs and their interaction, and impacts on orographic precipitation during typhoon passage from a dynamical–microphysical perspective quantitatively.