## 3 Results
### Comparing \(\omega\) with Retrieval Result from a Disdrometer  
To preliminarily validate the reliability of radar-derived vertical air velocities, indirect retrieval results from a ground-based optical disdrometer were used for comparison.  
The disdrometer can simultaneously detect the equivalent diameter (D), concentration (N), and MV of raindrops near the ground (1.4 m above the surface) with a 1 min temporal resolution. With the knowledge of raindrop terminal speed (V\({}_{\rm T}\)), the \(\omega\) can be simply inferred by subtracting V\({}_{\rm T}\) from MV. For liquid raindrops, many previous studies have indicated that a relationship between V\({}_{\rm T}\) and D can be established using an exponential function [36; 37]:  
\[\rm V_{T}=(9.65-10.3\times\exp(-0.6\times D))/f \tag{7}\]  
where f is a correction factor related to the atmospheric density. Figure 10 shows the frequency distribution of D and MV of the observed raindrops, collected from 15 July to 30 August 2015, under weak precipitating conditions (\(10\leq Z\leq 25\) dBZ). A fitting equation with a correction factor of 0.7338 can represent the V\({}_{\rm T}-D\) relationship of the raindrop on the surface of the TP. In the mapping, the measured MV is marked by a color block, and the deviation between MV and V\({}_{\rm T}\) is induced by \(\omega\). From the above, the \(\omega\) derived from the disdrometer can be used for comparison with the radar-derived results.  
Figure 11 shows the time series of retrievals from two instruments for case two, where the radar retrievals are selected from the first detectable ranging gate (150 m above the surface). The comparison illustrates that the trends of the two retrievals are basically similar, with the appearances of the updrafts or downdrafts mostly coinciding. However, the radar may have a larger fluctuation due to its high temporal resolution, and the value of the two series include certain deviations, with a mean bias of nearly 0.5 m\(\cdot\)s\({}^{-1}\).  
There are several reasons for the retrieval deviations between the two instruments. First, the cloud radar minimum detectable height is 150 m, but the disdrometer collects data only near the ground (1.4 m height), which means that the radar retrievals are located higher than that of the disdrometer's (although they are deployed co-located). Thus, two different vertical air flows are retrieved, and naturally the retrieved values will be different, as the air flow in convective cells can be changed rapidly on small scales. Second, the retrieval method for the disdrometer has some uncertainties which will affect the comparison. The most crucial uncertainty is the assumed relationship between the falling speed of particles (V\({}_{\rm T}\)) and their diameters (D). The \"V\({}_{\rm T}-D\)\" relationship will be relatively  
Figure 10: Frequency distribution of diameter (D) and MV of the observed raindrops collected from 15 July to 30 August 2015 under weak precipitating conditions. The blue solid line represents a fitting equation for the V\({}_{\rm T}-D\) relationship.  
more reliable for moderate raindrops (liquid), yet for ice hydrometeors, the \"V\({}_{\rm T}-{\rm D}\)\" relationship is complicated and less reliable. On the TP, environmental temperatures are relatively low, which will, therefore, complicate retrievals in ice precipitation. Third, the disdrometer is two-dimensional and cannot distinguish multiple droplets when they pass through the sensor in parallel, which will also affect the retrieval results.