Radar Observation of Rainfall System Modified by Orographic Effects

Abstract

Modification of rainfall system by orographic effects was studied on the basis of the echo structure observed quantitatively by the use of a RHI radar which was set in Owase in the southeastern side of Kii peninsula during the period of June to July in 1977. Four cases of rainfalls were analyzed (cases A, B, C and D). In cases A, B and D continuous rain originated from middle-level clouds occurred over the broad area in Kii peninsula, and in case C convective rain was found out only in the limited area along the sea shore.In case C easterly wind was prevalent at low levels and a precipitating convective cloud system, which had the size of about 4 km in height and about 15 km in width of E to W, was formed around the sea shore. It existed for more than two hours over the nearly same region. This long-lasting system was maintained as the group of small precipitating convective clouds which were formed successively in the same area over the sea and which travelled to the mountain side showing the same evolution. It can be inferred that the horizontal convergence caused by orographic effect in the windward side of mountains and by a downdraft in a previously developed cloud would have played an important role in the successive formation of new convective clouds.In case A easterly wind was prevalent in the lower layer and a low-level cloud system which seemed to be the same type as case C was formed around the sea shore below middlelevel precipitating clouds. Precipitation originated from middle-level clouds increased remarkably in low-level clouds. However, in case B in which westerly wind was prevalent in the lower layer, a low-level cloud system was not detected and precipitation from middlelevel clouds was not observed to increase at low-levels. In case D the "seeding effect" of precipitation particles of middle-level clouds on low-level clouds was suggested.

Improvement of High-Resolution Satellite Rainfall Product for Typhoon Morakot (2009) over Taiwan

“…2c,d) observed by PR over the Kii Peninsula and used for the improvement of orographic rainfall estimates. The orographic precipitation profiles had much lower precipitation top heights than those of the original profiles, which was consistent with the fact that heavy rainfall from shallow clouds in this area has been reported by previous studies (Takeda et al 1976;Takeda and Takase 1980;Sakakibara 1981). Warm-rain processes are enhanced by low-level orographic lifting of maritime air, such that rainfall develops at a low level in the growing convective elements and precipitates while the clouds are still developing.…”

A Case Study of a Heavy Precipitation Event along the Baiu Front in Northern Kyushu, 23 July 1982: Nagasaki Heavy Rainfall

“…A similar seeder-feeder mechanism was discussed by Sakakibara and Takeda (1973) and Takeda et al (1976) for orographic rain associated with typhoons. Badar and Roach (1977) took a numerical approach to investigate the orographic seederfeeder effect over south Wales, assuming an airflow that followed the topography.…”

“…Lavoie (1972) pointed out that the distribution of snow downwind of Lake Erie could be influenced by differential roughness. Takeda and Takase (1980) made an observational study of the modification of rainfall systems by orographic effects, using a RHI radar over the southeastern side of Kii peninsula, Japan, in June and July 1977. In one case they studied, relatively strong (*10 m s-1) low level winds were blowing in the onshore direction.…”

Doppler Radar Observation on the Orographic Modification of a Precipitating Convective Cloud in its Landing

“…These low-top clouds formed locally only around Owase. Further Takeda and Takase (1980) observed precipitating clouds around Owase by a RHI radar. In addition to the modification of cumulonimbus clouds in landing, they showed that when easterly winds were prevailing at low-levels, the precipitating-cloud low top system was maintained for longer than two hours around the coast as a group of small precipitating convective clouds, and that it had a possibility to play an important role in the orographic enhancement of rainfall originated from a travelling cumulonimbus-cloud.…”