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This study examines two strategies for improving the analysis of an hourly update three-dimensional variational data assimilation (3DVAR) system and the subsequent quantitative precipitation forecast (QPF). The first strategy is to assimilate synoptic and radar observations in different steps. This strategy aims to extract both large-scale and convective-scale information from observations typically representing different scales. The second strategy is to add a divergence constraint to the momentum variables in the 3DVAR system. This technique aims at improving the dynamic balance and suppressing noise introduced during the assimilation process. A detailed analysis on how the new techniques impact convective-scale QPF was conducted using a severe storm case over Colorado and Kansas during 8 and 9 August 2008. First, it is demonstrated that, without the new strategies, the QPF initialized with an hourly update analysis performs worse than its 3-hourly counterpart. The implementation of the two-step assimilation and divergence constraint in the hourly update system results in improved QPF throughout most of the 12-h forecast period. The diagnoses of the analysis fields show that the two-step assimilation is able to preserve key convective-scale as well as large-scale structures that are consistent with the development of the real weather system. The divergence constraint is effective in improving the balance between the momentum control variables in the analysis, which leads to less spurious convection and improved QPF scores. The improvements of the new techniques were further verified by eight convective cases in 2014 and shown to be statistically significant.

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A rainstorm occurred over the central part of Japan (*36*N/140*E) within Typhoon 8124 (Gay) for 22-23 October 1981 and it was studied mainly using Doppler radar data. The main purposes of the present study are to clarify the structure of the rainstorm and to know whether it was a typhoon spiral band or another type of precipitation system. The typhoon was under transformation into an extratropical cyclone in the southeastern portion of a large-scale trough. Satellite and radar data show that this rainstorm occurred on the southeastern edge of a wide cloud band to the north of the typhoon center. The most outstanding feature of this rainstorm found by Doppler radar was the existence of a slant axis of strong wind from lower levels on the southeastern side to upper levels on the northwestern side. This means the existence of a mesoscale slant updraft. Below the axis of the slantwise updraft, convective-scale vertical motion was embeded in the mesoscale updraft. Above the axis, convective-scale vertical motion was generally small. The middle-level air intruded into the northwestern portion of the rainstorm in the southern part. It is suggested that the intruded air was cooled by evaporation of precipitation particles and formed a mesoscale downdraft. Although a pronounced surface convergence line was associated with the rainstorm, its effect on the rainstorm was subsidiary except in the southern part. The structure of the rainstorm was partially similar to those of a typhoon spiral band, eyewall clouds and a squall line in the middle latitudes. However, this structure is considered as a characteristic feature of a rainstorm which occurs to the north of a typhoon in extratropical transition. The result and interpretation of a mesoscale budget of condensed water in the form of precipitation particles are also shown to study the relative importance of production and transport of precipitation particles in the rainstorm.

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