9:02 AM - 9:42 AM
[O05-01] Mechanisms and predictability of quasi-stationary band-shaped precipitation systems, named “senjo-kousuitai” in Japanese
★Invited Papers
Keywords:senjo-kousuitai, heavy rainfall, numerical prediction
In Japan, heavy rainfall (HR) events producing 3-hour precipitation amounts exceeding 200 mm are often observed. Such events are mainly brought about by quasi-stationary band-shaped precipitation systems, named “senjo-kousuitai (SENJO)” in Japanese. SENJO is defined as a band-shaped HR area with a length of 50–300 km and a width of 20-50 km, produced by successively formed and developed convective cells, lining up to organize multi-cell clusters, and passing or stagnating at almost the same place for a few hours. The formation processes of SENJO are categorized mainly into two types; the broken line type in which convective cells simultaneously form on a quasi-stationary local front by the inflow of warm and humid air, and the back building type in which new convective cells successively forming on the upstream side of low-level winds linearly organize with pre-existing cells.
In the 1980s, weather radar data covering Japan were digitized and combined to operate nationwide precipitation nowcasts. In the 1990s, the distribution of one-hour precipitation amounts (R-A) corrected by rain gauges was produced. Using this data, it became clear that most of HR events are brought about by SENJO. The term "SENJO" was used around 2000 by researchers at the Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA), and it was defined in a textbook published in 2007. Subsequently, it was first used in a press release from MRI for the 2012 Niigata-Fukushima HR event, and it was picked up by many news after the 2014 Hiroshima HR event and was nominated for the 2017 Epidemic Word of the Year award, which gains public recognition.
The occurrence characteristics of SENJO have been investigated based on the aspect ratio of the distribution of areas with precipitation of 50 mm or more in 3-hour precipitation amounts, using R-A of 130 mm as a threshold. The share of SENJO in HR events is about 50%, or about two-thirds when excluding direct precipitation associated with tropical cyclones (TCs) within the distance of 500 km from TC centers (DTC). In southern Japan, including Kyushu Island, most of HR events belong to SENJO, except DTC, which is especially pronounced around the Baiu front during the rainy season. In eastern Japan, SENJO occurs most frequently in September, mostly due to indirect precipitation associated with TCs (500-1500 km from TC centers).
For the 2014 Hiroshima HR event, the reproducibility and predictability of SENJO are examined. The model with a horizontal resolution of 5 km is not sufficient to predict the SENJO, and the maximum 3-hour precipitation amounts up to 04 JST (= UTC + 9hs) on 20 August was about 32 mm compared to R-A of 232 mm. The 2-km model was able to predict the maximum rainfall of 227 mm, which is almost the same as R-A. This successfully prediction is because the initial conditions were optimal; however, as initial conditions were updated, predictions deteriorated considerably. Using initial conditions at one hour later (19 JST), the 2-km model predicted about half the maximum precipitation amount. At more recent initial times, predicted precipitation amounts became even lower. The slight differences in initial conditions have a significant impact on predicted results, which indicates the difficulty in deciding whether to issue a warning based on one predicted result.
Most of SENJO events during the rainy season occur 100-300 km south of the Baiu front, which makes them rather difficult to predict. The area is known as the Baiu frontal zone, and the following favorable occurrence conditions of SENJO are conducive, despite the absence of forcing such as converging winds associated with the front; large inflow of low-level water vapor causing HR, inflow of low-level warm and humid air providing favorable conditions for the formation and development of cumulonimbus clouds, moist conditions up to mid-level to prevent evaporation of clouds in cumulonimbus clouds, and presence of moderate differences in wind speed and direction between mid- and low-levels for cumulonimbus clouds to organize.
In the 1980s, weather radar data covering Japan were digitized and combined to operate nationwide precipitation nowcasts. In the 1990s, the distribution of one-hour precipitation amounts (R-A) corrected by rain gauges was produced. Using this data, it became clear that most of HR events are brought about by SENJO. The term "SENJO" was used around 2000 by researchers at the Meteorological Research Institute (MRI) of the Japan Meteorological Agency (JMA), and it was defined in a textbook published in 2007. Subsequently, it was first used in a press release from MRI for the 2012 Niigata-Fukushima HR event, and it was picked up by many news after the 2014 Hiroshima HR event and was nominated for the 2017 Epidemic Word of the Year award, which gains public recognition.
The occurrence characteristics of SENJO have been investigated based on the aspect ratio of the distribution of areas with precipitation of 50 mm or more in 3-hour precipitation amounts, using R-A of 130 mm as a threshold. The share of SENJO in HR events is about 50%, or about two-thirds when excluding direct precipitation associated with tropical cyclones (TCs) within the distance of 500 km from TC centers (DTC). In southern Japan, including Kyushu Island, most of HR events belong to SENJO, except DTC, which is especially pronounced around the Baiu front during the rainy season. In eastern Japan, SENJO occurs most frequently in September, mostly due to indirect precipitation associated with TCs (500-1500 km from TC centers).
For the 2014 Hiroshima HR event, the reproducibility and predictability of SENJO are examined. The model with a horizontal resolution of 5 km is not sufficient to predict the SENJO, and the maximum 3-hour precipitation amounts up to 04 JST (= UTC + 9hs) on 20 August was about 32 mm compared to R-A of 232 mm. The 2-km model was able to predict the maximum rainfall of 227 mm, which is almost the same as R-A. This successfully prediction is because the initial conditions were optimal; however, as initial conditions were updated, predictions deteriorated considerably. Using initial conditions at one hour later (19 JST), the 2-km model predicted about half the maximum precipitation amount. At more recent initial times, predicted precipitation amounts became even lower. The slight differences in initial conditions have a significant impact on predicted results, which indicates the difficulty in deciding whether to issue a warning based on one predicted result.
Most of SENJO events during the rainy season occur 100-300 km south of the Baiu front, which makes them rather difficult to predict. The area is known as the Baiu frontal zone, and the following favorable occurrence conditions of SENJO are conducive, despite the absence of forcing such as converging winds associated with the front; large inflow of low-level water vapor causing HR, inflow of low-level warm and humid air providing favorable conditions for the formation and development of cumulonimbus clouds, moist conditions up to mid-level to prevent evaporation of clouds in cumulonimbus clouds, and presence of moderate differences in wind speed and direction between mid- and low-levels for cumulonimbus clouds to organize.