Abstract
The Japan Meteorological Agency Regional Atmospheric Transport Model (JMA-RATM, previously called the Mesoscale Tracer Transport Model) that is used operationally for the Volcanic Ash Fall Forecast has been revised. Major improvements of the JMA-RATM are as follows: (i) For the initial condition of the eruption column model, the time-series variation of eruption cloud echo height data observed by weather radars is used instead of visual camera observation. (ii) For the input meteorological field, the grid point values of the Local Forecast Model (LFM, 2 km grid spacing and 60 vertical layers) are available instead of the Mesoscale Model (MSM, 5 km spacing and 50 layers); both models originate from the JMA Nonhydrostatic Model (JMA-NHM). (iii) In the atmospheric transport model calculations, Suzuki's resistance law is extended with the Cunningham slip correction, and rainout (in-cloud scavenging) and washout (below-cloud scavenging) processes by snow and graupel are incorporated in addition to rain. The target of the model predictions is tephra fall, which includes both ash fall quantity and lapilli fall area. Comparative calculations with the JMA-RATM were conducted for the lapilli fall event during the eruptions of Shinmoe-dake volcano on 26-27 January, 14 February, 13 March and 18 April 2011. The use of the time-series data of eruption cloud echoes and the LFM grid point values was effective for predictions of both ash fall quantity and lapilli fall area. The Cunningham slip correction had a marginal effect on the ash fall prediction. In-cloud and below-cloud scavenging processes had a large influence on the ash fall prediction; therefore, the scavenging rate will need to be calibrated against ash fall observation data in rain or snow. The values for the density and form of tephra, based on observation data, also had a large impact on the lapilli fall prediction; however, the occurrence of undetected error requires future research on the effect of wind in the eruption column model.
