A high concentration episode occurred at the beginning of 2009 pollen season in southern Kanto of Japan

Normally Japanese spring pollen season (mainly cedar and cypress pollen) starts in early or mid February, and the number of pollen increases gradually with temperature rising. However, in 2009 season, at the beginning of pollen season (during 14th to 16th in February of 2009), a high concentration episode (the number of pollen observed in 3 days exceeded more than 1,000 grains/m3) occurred in southern Kanto area of Japan. It was less than 10 days since the starting date of the pollen season at the area (06/Feb/2009), and the ratio of pollen observed during these 3 days was 23 to 28% of the total number of cedar pollen in 2009 season. From the end of January to the first half of February, temperature was 2.0 degrees (in Celsius) higher than usual, and the flowering ratio of cedar was estimated 15 percent at 10/Feb/2009 and it became 20 to 25% at 15/Feb/2009 in southern Kanto area. Additionally pollen forecast model (with MM5 and Lagrangian dispersion model) showed (1) high concentration pollen was provided from southwest cedar forests, (2) the pollen was carried by westerly wind to Kanto area in upper layer, (3) at night a local convergence line with sea breeze and land breeze developed, and (4) dense pollens sunk along local convergence line. Compared with observed pollen, counted by the observation network system named ‘Hanako-san', the model estimated the distribution and concentration of pollen at the episode correctly.