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 4km in height and about 15km 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.