Study on effects of an isolated elliptical terrain (Jeju Island) on rainfall enhancement in a moist environment

Keun-Ok Lee (李根玉)

Jeju Island, southern part of Korea has frequently suffered from flooding and landslides due to orographically-intensified rainfall systems during the rainy season. The island is an isolated terrain with an elliptical-shaped mountain extending east-to-west (width 35 km, length 78 km, height 1.95 km). During the rainy season (June to mid-July) when a stationary front is located off the northern shore of the island, a moist environment and ambient southwesterly winds are well predominant around the island. In last 10 years (2003-2012), when the southwesterly wind prevails in low altitudes during June and July, the maximum rainfall amount of 3,805 mm was recorded locally on the northern slope of the mountain showing the relatively large rainfall (> 700 mm) on the northern and eastern side of the island, while rainfall less than 700 mm was accumulated on the western and southern sides of the island; the large regional-rainfall distinction was apparent on a mountainous region with a small horizontal scale. This study aims to reveal the effects of an isolated elliptical-shaped terrain on the flow modification and the related enhancement of rainfall system, especially on the northern side of the terrain, in a moist environment by both of data analyses and numerical simulations.

Two intense rainfall systems occurred on 30 June 2006 and 6 July 2007 were selected to identify the orographic effect of an elliptically-shaped terrain on rainfall enhancement. Both of the selected rainfall systems moved eastward and passed over the northern side of the island, showing the system enhancement on the northern side of the island. Four-dimensional structures of reflectivity and wind of convective cell within both systems were analyzed using S-band dual-Doppler radar dataset and a non-hydrostatic numerical model, Cloud Resolving Storm Simulator (CReSS). Comparison of the enhancement mechanisms of two systems reveals that first, moist environment and the southwesterlies at low altitudes are the favourable condition for the system enhancement, and secondly the isolated elliptical-shaped terrain of Jeju Island induces a horizontal rerouting of relatively-weak low-level southwesterly winds, generating a local updraft region on the northwestern lateral side of the island; in result, rainfall enhances on the lateral side of the island. Sensitivity experiments reveals that (a) 30.6 % of rainfall amount is induced by the steep topography of Jeju Island, and (b) concentrated moisture in low altitudes works for increasing rainfall; a reduction in the low-level RH of 2 % results in a 20.8 % reduction in rainfall amount.

Comparison of the two systems also shows some differences on the system evolution. In order to clarify the key parameter to induce the differences, a series of idealized experiments using CReSS were conducted. One key parameter is the arrival location of the eastward-moving system on the island. When the system travels off the northwestern shore of the island, further intensified system arrives on the island. As the distance between the system and terrain decreased, the southwesterlies blowing parallel to the coastline accelerates in the space between the system and terrain; thus updraft is locally generated. Subsequently, abundant vapour over sea surface is supplied to interior of the convective cell; thus warm rain process is mainly important to the system enhancement on the northwestern part of the island. Another key parameter is revealed to be intensity of low-level southwesterlies. Fairly weak southwesterly wind (Fr = 0.2) at low altitudes allows enhancement of the system on the lee side of the island by generating the convergence of a relatively weak go-around northwesterly from the northern island and a relatively strong moist southwesterly from the southern island, thus producing a relatively long-lived rainfall system is resulted. With the strong southwesterly wind over the southeastern slope, the moist air is supplied to interior of the system to above the melting layer; thus both warm and cold rain processes are important in enhancing the convective cell on the eastern side of the island. As the low-level southwesterly winds strengthen at low altitudes (Fr - 0.2), a dry descending air mass intensified on the northeastern downwind side, which actively and rapidly dissipates rainfall system, resulting in a relatively short-lived rainfall system.

In this study, the effect of an elliptically-shaped terrain (Jeju Island) on the enhancement of rainfall system in a moist environment is revealed. The results imply us two meteorological elements for improving rainfall forecast over the mountainous region in a moist environment; first, predominant moist southwesterly wind has potential to bring out intense rainfall on the lateral (northern) side of the island, besides secondly, the fairly-weak southwesterly wind has potential to induce intense rainfall, also on the lee (eastern) side of the island. Furthermore, in the identical environment, if the eastward-moving pre-existing rainfall system passes off the northwestern shore of the terrain, the system has potential to bring further intense rainfall on the lateral (northern) side of the terrain. These results suggest us key meteorological elements for rainfall enhancement which are applicable to a terrain with further complicated topographic feature in a moist environment so that it can help to improve the rainfall forecast over the mountainous region.