**5. Conclusions**

This research demonstrated that rain microstructure varies significantly between weather types in both stratiform and convective rain. Easterly circulations had the highest drop concentration and the smallest drop size while westerly circulations were associated with large drops and low drop concentration. A plausible explanation for these differences is the high humidity content in westerly circulations and abundant cloud condensation nuclei concentration in easterly circulation. These finding offer potential new applications for radar-based quantitative precipitation estimates. Z–R parameters vary substantially with synoptic weather patterns effectively summarized by regional wind direction classes. This variation in Z–R parameters with wind direction approximates their station-to-station spatial variability for stratiform, but not for convective rain. We therefore conclude that building separate Z–R retrieval equation for regional wind direction classes should improve radar-based QPE, especially for stratiform rain events. This approach should be feasible for operational level forecasts, especially since daily large-scale weather types can be predicted with high accuracy several days in advance.

**Author Contributions:** Conceptualization, W.G. and A.M.; methodology and formal analysis, W.G.; writing—original draft preparation, W.G.; supervision, A.M.; writing—review and editing, W.G., J.B., N.E., A.H. and A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** J.B. was partly funded by project RTI2018-098693-B-C32 (AEI/FEDER).

**Acknowledgments:** We thank the Deutscher Wetterdienst (German Meteorological Service-DWD) for providing the disdrometer data, the classification of weather types, and the cloud observation data. We appreciate the valuable comments provided by the anonymous reviewers. The first author thanks the Deutscher Akademischer Austauschdienst (DAAD) for financial support.

**Conflicts of Interest:** The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **Appendix A**

**Table A1.** Summary of events selected for the fitting of Gamma distribution and the two methods of R–Z parameters extraction (see Section 2.4).


**Figure A1.** Raindrop concentration per millimeter and cubic meter for a selection of ten events; one event for each combination of wind direction and rain type. The points represent the event average one-minute concentrations for each diameter range colored by the relevant wind directions. The colored lines represent the fitted gamma distribution for these points.

**Figure A2.** *Cont*.

**Figure A2.** The probability density of the fitted gamma parameters in stratiform and convective rain colored by wind directions. The vertical lines represent the mean values of the three parameters based on a selection of 2449 events (see Section 2.4).

**Figure A3.** Density plot of reflectivity (dBZ) and rain intensity (R) for convective and stratiform rain. This plot includes the 9914 h of rain within the selected 2449 events (see Section 2.4).

**Figure A4.** The fitted Z–R lines for one event (start: 2014-09-20 08:48:30, duration: 54 min, rain type: stratiform, wind direction: SW) using both the conventional method (black line) and the alternative method (red line).
