Mesoscale data assimilation for improving quantitative precipitation forecasts

NCEP multi-sensor hourly rainfall data and ground-based GPS zenith total delay (ZTD) were used for data assimilation and evaluation of quantitative precipitation forecasts (QPFs) through three case studies. Improvements in QPFs were obtained through direct assimilation of these rainfall observations and ZTD data using 4-dimensional variational assimilation (4D-Var). Inclusion of the observed no-rain information was shown to be beneficial to QPFs. Although the assimilation of ZTD observations does not produce a rainfall distribution as close to the observations as does the assimilation of rainfall within the assimilation window, the improvement in the QPFs beyond the window from the ZTD experiment is comparable to that from the rainfall experiment.; Assimilation of ZTD and rainfall observations modifies the thermodynamic structures of the atmosphere, favoring development of precipitation in the observed rainy areas. The horizontal and vertical wind velocities are also adjusted consistent with the precipitation process. Sensitivity studies indicated that the adjustments in the moisture and temperature fields resulting from precipitation assimilation played a more important role than those o£ other state variables for improving QPFs. Spectral analysis indicates that rainfall assimilation adjusts the model variables on smaller scales (25 to 50 km) while the ZTD assimilation adjusts the model variables mainly on larger scales (>50 km).; A modified digital filter for intensifying mesoscale gravity wave signatures is developed and applied to a real case study of rainfall assimilation. 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 are intensified not only within the rainfall assimilation window during which the modified digital filter is applied, but also beyond the assimilation window. The gravity-wave-induced vertical motions along the direction of wave propagation are 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 outperforms the assimilation of hourly rainfall within the same 6-h window.