298 Journal of the Meteorological Society of Japan Vol. 47, No. 4 Precipitation Bands of Typhoon Vera in 1959 (Part I)* By Staff Members** Division of Meteorology, Geophysical Institute, Tokyo University, Tokyo (Manuscript received 22 March 1969) Abstract A case study of the rainfall associated with Typhoon Vera (the Ise Bay Typhoon) in 1959 is made. From an inspection of hourly precipitation charts, one gets an imprestion that the rainfall amount with the typhoon is solely dependent upon the orographic elect. However, the analysis of time changes of rainfall intensity reveals the existence of well-organized zones of alternating heavy and weak rainfall intensity surrounding the storm center. The surface pressure field also contains similar banded structure. A comparison with the radar pictures shows the correspondence of these zones to groups of radar rain bands. the rainfall amount is the forced lifting of warm 1. Introduction and moist air around the storm along the upslope Some of the heaviest rainfalls in Japan occur of a mountain range or the existing frontal in connection with typhoons. The total rainfall surface rather than the convective rain inherent during the passage of a typhoon frequently to the typhoon (Arakawa, 1939; Takahashi, 1952; amounts to 400-500 mm. There are a number Masuda and Kasahara, 1956; Dunn and Miller, of factors contributing to the torrential rain 1960; and many others). associated with a typhoon. Probably the observed rainfall over Japan Consider an idealized typhoon which is at sea. Islands during the passage of a typhoon has dual Although we do not have direct rainfall measure- characteristics of both convective and continuous ments over the sea, the average rainfall rates in rains. Then we may raise questions like the various sectors within the typhoon may be calculated by the low-level inflow and the moisture content of the air. Hughes (1952) made such a calculation. The calculated rainfall rates were such that if a storm center had passed directly over a station while moving in a straight line, there would have been a total rainfall of about 280 mm in 48 hours. It is also known that the rainfall in a typhoon is highly convective and concentrated within a series of spiral bands as seen by radar pictures (Maynard, 1945; Wexler, 1947; and many others). However, the observed features of the typhoon rainfall over land are much more complex. The rainfall rates are often much larger and their horizontal distribution is dependent upon many factors. It has been widely recognized that one of the most important factors which determine * Division of Meteorology , Contribution No. 174. ** M. Hamuro , late Y. Kawata, S. Matsuda, T. Matsuno, N. Nakamura, T. Pak, T. Takeda and M. Yanai. Fig. 1. Track of Typhoon Vera. August 1969 Staff Members, Tokyo University 299 Fig. 2. Hourly precipitation amounts from 1600 JST to 2100 JST 26 September 1959. The orography is indicated by light and heavy shadings. 300 Journal of the Meteorological Society of Japan Vol. 47, No. 4 following. Can we detect the banded convective the rainfall over land is mostly controlled by nature of the rainfall from the observed precipita- orographic effects. However, when one examines tion data? What is the relation between the the time change of rainfall amount at various original convective rain of the typhoon and the stations, there are marked variations of rainfall rain caused by forced lifting of air which appears intensity with time, which are commonly observed to dominate in the distribution of rainfall amount? at separate stations. In this paper we present a case study of the Fig. 3 shows time changes of hourly rainfall rainfall during the passage of Typhoon Vera in amounts at two rain gauge stations, Ookawa and 1959. Typhoon Vera, or the Ise Bay Typhoon, which occurred in late September 1959 was one of the most intense typhoons in Japanese weather records. The path of the typhoon is shown in Fig. 1. At its maximum intensity on the 23 rd September, the minimum sea-level pressure was 894 mb and the maximum wind speed was more than 70 m sec-l. The typhoon landed near Shionomisaki weather station at the southern- most tip of the Kii Peninsula on the evening of the 26th September. The recorded minimum surface pressure was 929.5 mb at the station. After the landing, the typhoon took a course heading towards the NNE. By the passage of the typhoon, unusually large-scale storm surges were caused in Ise Bay. The highest meteorological tide at Nagoya harbor reached 3.45 m. The storm killed 4,800 people in Nagoya and its vicinity. A detailed documentation of this typhoon was published by Japan Meteorological Agency (1961). 2. Hourly rainfall amounts Based upon the data from the rain gauge stations mostly belonging to Japan Meteorological Agency, hourly precipitation charts were construct- ed. Fig. 2 shows the distribution of rainfall amount in units of mm during the period from 1600 JST (Japan Standard Time) to 2100 JST 26 September. The positions of the storm center at 1800, 1900, 2000 and 2100 JST are marked in the figures. The orographic feature is also indicated in the figures by light and dark shadings. When we look at the hourly precipitation charts, we note a high correlation between the rainfall amount and the orography. The maxima of the hourly rainfall amount are generally located at upslopes of mountain ranges. It is difficult to detect banded structure of the rainfall from the analysis of the hourly precipitation charts. 3. Analysis of the time change of rainfall intensity Fig. 3. Time changes of hourly precipitation amounts at Ookawa and Shizuoka. Black From the hourly rainfall distribution presented circles correspond to positive R and white in Section 2, one might get an impression that circles to negative OR (see text). August 1969 Staff Members, Tokyo University 301 Fig. 4. Horizontal distributions of oR from 1600 JST to 2100 .1ST 26 September 1959. 302 Journal of the Meteorological Society of Japan Vol. 47, No. 4 Fig. 5. Major zones of positive R relative to the storm center (from 1300 JST to 2400 JST 26 September 1959). August 1969 Staff Members, Tokyo University 303 Shizuoka. These two stations are only 20 km to the storm center studied by Senn and Hiser apart (see Fig. 8). Ookawa is situated in a (1959). valley and its elevation is 200 m from the mean sea level, while Shizuoka is in a plain and its 4. Band structure of surface pressure elevation is 13.5 m. We note two marked features Many investigators have shown that minor in this figure. Firstly, the hourly rainfall amounts pressure dips are associated with the passage of at Ookawa are much larger than those at Shizuoka, typhoon rain bands (Wexler, 1947; Ligda, 1955; showing the high correlation between the rainfall Ushijima, 1958; Tatehira, 1961, 1962; and several amount and the station's elevation. Secondly, others). Most of these studies are concerned there is a parallel time change of the hourly rain- with small-scale fluctuations of the barograph fall amounts at the two stations. Four peaks of trace in comparison with the passage of individual intense rainfall at about 13 JST, 17 JST, 20 JST rain bands above the station. It is generally and 23 JST are commonly observed at the two recognized that there is a rapid fall of the stations. pressure before the arrival of a rain band and As a simple measure of the time change of the then it is followed by a slow recovery of the hourly rainfall data, we define barograph trace. The pressure dip is usually of the order of 1 mb. Similar features have been observed in upper levels by flights penetrating rain where h(t) is the hourly rainfall amount ending bands (Simpson, 1954; Simpson and Starrett, at time t. Then positive values of oR should 1955). correspond to the peaks in the time series of The zones of positive R analyzed in Section hourly rainfall data. On the contrary, negative 3 have much larger horizontal scale as compared values correspond to the minima in the time with individual bands observed by radar. Each series. In Fig. 3, signs of OR are shown below zone may correspond to a group of several by black ( R>0) and white ( R<0) circles. bands rather than a single band. To reveal the The four peaks mentioned before are well traced fluctuations of the surface pressure with time and by this method. space scales comparable to those of OR, we read We expand the computation of OR for all the the barograms of all regular stations at every 15 rain gauge stations. The result is presented in minutes and computed the average hourly pressure Fig. 4. It is rather surprising that positive and p(t), where t is the ending time. Then we negative signs of R are systematically distributed calculated in several zones and that they rarely mix together except for a few stations. We further note that the zones of positive or negative R are propaga- The analysis of Op eliminates the sea-level reduc- ting with the movement of the storm center. tion problem which usually complicates the meso- To see the latter point more clearly, we mark scale pressure analysis. centers of the zones of positive R at every 1 Fig. 6 shows the horizontal distribution of 4 P hour and plot the position of the zones relative in units of mb during the same period as for the to the moving storm center (.Fig. 5). We recognize analysis of OR. We notice a general correspond- four major zones of positive R around the ence between the zones of negative P and those center of the typhoon.