Abstract
A modified digital filter in favor of mesoscale gravity waves was developed and applied to a real case study of rainfall assimilation using the four‐dimensional variation data assimilation (4D‐Var) approach. The results show that the rainfall assimilation experiment with the modified digital filter produced further improvements in quantitative precipitation forecasts compared with the rainfall assimilation experiment with a regular digital filter. Spectral analysis confirms that the mesoscale gravity waves were intensified not only within the rainfall assimilation window during which the modified digital filter was applied but also beyond the assimilation window. The gravity wave–induced vertical motions along the direction of wave propagation were also intensified, resulting in a more realistic time evolution of the precipitation pattern. It is also found that the assimilation of 6 h accumulated rainfall outperformed the assimilation of hourly rainfall within the same 6 h window.
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