Session6: Water-related Disaster Risk Reduction
Title: A DIAGNOSTIC STUDY OF ATMOSPHERIC BLOCKING USING ANALOGY BETWEEN JET STREAM AND OPEN CHANNEL FLOW
Yoshikazu Kitano, Tomohito J. Yamada, Yasuo Hattori
Atmospheric blockings due to persistent meandering of jet streams often cause meteorological extremes in mid-latitudes, such as long-term rainfalls, heat waves, draughts, and cold spells. Previous studies proposed various diagnosis methods of blockings, which are based on the detection of anomalous spatial distribution of pressure systems (Lejenäs and Okland, 1983; Tibaldi and Molteni, 1990; Pelly and Hoskins, 2003). This study introduces a new dynamical detection method based on energy conservation in the jet stream. This idea was first presented by Riffler (2005). In this method, we use sum of kinetic and potential energy flux in the jet stream, called “specific energy”, which is found to be analogous to flows studied in the open channel hydraulics. The latitudinal width of the jet stream has similarities of the depth of an open channel flow. Moreover, Rossby (1950) showed that a dynamically possible state characterized by blocking may exist which are compatible with the hydraulic jump in an open channel flow. This analogy suggests that specific energy controls a jet width expansion, which is observed near a blocking. To apply this theory to reanalysis, we derived a new specific energy equation considering changes of jet latitude and meridional profile of eastward wind speed. Utilizing this equation, Kitano and Yamada (2017) revealed minimum specific energy to form meandering jet stream (i.e. atmospheric blocking). In this extended abstract, we analyze a Pacific blocking episode using reanalysis and found energetic signals over Japan and its conservation and transport before occasion of atmospheric blocking.