The influence of observations propagated by convectively coupled equatorial waves

This paper describes the propagation of the influence of radiosonde observations made during MISMO (Mirai Indian Ocean cruise for the Study of the Madden-Julian oscillation-convection Onset), which was conducted over the Indian Ocean in the boreal autumn of 2006. The impact of these observations was assessed using an experimental reanalysis called 'ALERA' that was produced by the local ensemble transform Kalman filter with the atmospheric global circulation model for the Earth Simulator. The difference of the analysis ensemble mean between the analysis cycles with and without the MISMO observations was used to quantify the influence of these observations on the analysis field, which we call impact signals. Since the impact signals were contaminated by noise, probably due to the model's truncated spectral basis, a significance test was performed using the analysis ensemble spread, and the numerical noise was then successfully eliminated. The results indicated that convectively coupled equatorial waves propagated the impact signals to the central Pacific to the east and to the African continent to the west. In particular, the analysis fields around typhoons Cimaron, Chebi, and Durian over the tropical Western Pacific were significantly modified by the propagation of the impact signals through the Kelvin waves. Here two factors played important roles: (1) the meridional positions of the typhoons; and (2) the duration of the observations. The equatorial Rossby waves also made a significant contribution to the propagation of the impact signals. Such influences through the waves resulted in the reduction of the analysis spread. The shape of the region of reduced spread resembled the Matsuno-Gill pattern with an east-west width of more than 20 000 km. Copyright (C) 2011 Royal Meteorological Society