Kinematic and Microphysical Structures of Multi-Cellular Storm Developing over the Zoshigaya area of Tokyo, Japan

A heavy rainfall event accompanied by multi-cellular storm was observed by two X-band polarimetric radars, as operated by the National Research Institute for Earth Science and Disaster Prevention (NIED), in Zoshigaya area of Tokyo, Japan on 5 August 2008. It lasted for more than 2 hours and produced approximately 120 mm accumulated rainfall. To investigate the evolution of precipitation cells and show how they are related to low level wind, dual-Doppler radar analysis and three-dimensional structure of liquid water content (LWC) were analyzed. The heavy precipitation system was consisted with a total of twenty precipitation cells. Each precipitation cell has a precipitation core. The convective cells are grouped into three types according to the lifetime of the precipitation core: ordinary type, long lasting type, and intermediate type, respectively. The number of the ordinary type cells is 18 and the average lifetime of their precipitation core, rainfall amount, and horizontal scale are 16 minutes, 15 mm, and 5 km, respectively. The long lasting type cell has a precipitation core, which has the lifetime of 100 minutes. The long-lasting cell has a horizontal scale of 10 km, and produced 110 mm rainfall amount. The intermediate type has a lifetime of 40 minutes precipitation core and its horizontal scale and total rainfall amount are 5 km and 27 mm, respectively.

The ordinary cells are developed with updraft by low level convergence. The updraft usually exists for about 10 minutes, and it is turned to downdraft due to the fall of precipitation particles in precipitation core. Without the supporting updraft, the precipitation core falls to the ground 5–25 minutes after the first appearance of the precipitation core. This type cells show one descending precipitation core. On the other hand, the long-lived cell is characterized by several substituting precipitation cores during mature stage. In the long-lived cell, 5 precipitation cores are alternately produced and descended to the ground during its lifetime. This replacement of precipitation cores is associated with the periodic updraft surge originated from the southeasterly inflow in the lower level. This southeasterly inflow is considered to have been the main supply of warm and moist air to the precipitation cell. The outflow from the adjacent short-lived cells, especially located in southern part of long-lived cell, is another source to be updraft and to supply a moist air for development of new precipitation core. Intermediate-lived precipitation cell is maintained for some time after descending precipitation core by balance of outflows produced from long-lived cell and short-lived cell, though it does not develop the new precipitation core.

Dual-polarization radars provides useful parameter such as differential reflectivity Z_DR, differential propagation phase K_DP, and copular correlation coefficient ρ_HV as well as horizontal polarization Z_H to be able to study about microphysical process in precipitation cell more quantitatively. The two governing parameters of a normalized gamma DSD (drop size distribution), N_w (normalized number concentration) and D₀ (drop median diameter), are retrieved from the polarization measurements based on the “constrained-gamma” method. Large D₀ more than 2 mm is estimated in ordinary type cells and long-lived cell of developing stage. The temporal and vertical variation of the DSD parameters within the lifetimes of each precipitation cells will be presented.