Effect of isolated elliptical terrain of Jeju Island on localized intense rainfall on the lee side in moist environment

Conference: 
ICMCS-X
Presentation Type: 
Oral
Author(s): 
Keun-Ok Lee (Atmospheric Environmental Research Institute, Pukyong National University)
Dong-In Lee (Department of Environmental Atmospheric Sciences, Pukyong National University)
Hiroshi Uyeda (Hydrospheric Atmospheric Research Center, Nagaoya University)
Abstract: 

During Changma season, orographically enhanced precipitation systems have been frequently observed around Jeju Island with isolated topography (altitude 1.95 km) and elliptic shape. Previous studies reported that warm and moist southwesterly winds are well maintained at low altitudes when a stationary front is located off the northern shore of Jeju Island, and that these winds play an important role in triggering the intensification of a pre-existing rainfall systems in the vicinity of the island. In terms of the orographically-enhanced pre-existing rainfall system, several previous studies have been done in a domain of Jeju Island. However, terrain effect of the island on the sudden occurrence of convective precipitation is unexplored.

To investigate the mechanisms of the occurrence and the evolutionary characteristics of convective system, four convective systems resulting the localized intense rainfall on the lee side (east) of Jeju Island were selected: 1) C1: 0900 1240 LST on 23 June 2011, 2) C2: 1120 1730 LST on 30 July 2011, 3) C3: 1100 1500 LST on 19 July 2012, and 4) C4: 1100 1410 LST on 27 July 2012. The environmental condition and the evolutionary characteristics of C1, C2, C3, and C4 have been investigated using dual S band Doppler radars data and meso scale analyses (MANAL) data. Overall, southwesterly wind prevails in low altitudes below 800 hPa, with surface temperature in range of 20.7 22.1 ! and relative humidity in range of 85 93 %. In the warm and moist environment, C1 was suddenly apparent on the lee side with relatively strong southwesterly (15 m s-1; Fr of 0.8) and high RH (93 %), lasting for 240 min. The height of 30 dBZ for C1 was 6.8 km. With relatively low surface RH (85 %) and weak low-level southwesterly (3 m s-1; Fr of 0.2), C2 was generated, and it lasted for 380 min which is the longest lifetime among the C1 C4. For C2, The height of 30 dBZ was increased to 6.2 km. The C3 was occurred with the high surface RH (93 %) and moderate southwesterly (10 m s-1; Fr of 0.5). As similarly shown in C1 and C2, the height of 30 dBZ was increased to 6.3 km in C3. With similar wind speed of low-level southwesterly (3 m s-1; Fr of 0.2) to C2, but in further moist environment (RH, 93 %), C4 was generated on the lee side recording the maximum height of 30 dBZ among C1 C4 to 8.1 km (height of 0 !, ~4.5 km).

The four convective precipitations C1, C2, C3 and C4 started to occur on the lee side of Jeju Island in the morning, with warm and moist southwesterlies in low altitudes (below 800 hPa). In the environment, the generated C1, C2, C3 and C4 were revealed to have distinct evolutionary characteristics (e.g., lifetime and echo-top height) with the distinct degree of moisture and the intensity of low-level southwesterlies. With relatively strong southwesterlies (Fr, 0.8) and large moist amount (RH, 93 %), C1 lasted for relatively short duration. In contrast, with weak southwesterlies (Fr, 0.2) and the relatively small moist amount (RH, 85 %), C2 lasted for 2 hours longer than C1. With moderate intensity of the low-level southwesterlies (Fr, 0.5) and the large moist amount (RH, 93 %), C3 lasted for relatively short duration similar to be shown in C1. With weak southwesterlies (Fr, 0.2) as similar as C2, but with large moist amount (RH, 93 %), C4 recorded the shortest lifetime, but the maximum echo-top height was shown among C1 C4. The effect of the moist degree and the intensity of low-level southwesterlies on the generation and the evolutionary characteristics of the convective precipitations in the lee side of an isolated elliptical terrain of Jeju Island will be introduced in the conference.