Vertical Evolution of Microphysical Properties during Snow Events in Middle Latitudes of China Observed by a C-band Vertically Pointing Radar

This study applied the C-band vertically pointing radar with frequency-modulation continuous-wave technology to obtain the continuous observation data of four shallow 25 and two deep snow events during the winter of 2015–2016 in the midlatitudes of 26 China. Generating cells (GCs) were found near the echo tops in every event. The ice 27 particle number concentration ( N ), ice water content (IWC), and median mass 28 diameter ( D m ) retrieved from radar Doppler spectra were used to analyze the 29 microphysical properties in the snow clouds. The clouds were divided into upper GC 30 and lower stratiform (St) regions according to their vertical structure. The fall streaks 31 (FSs) associated with GCs were embedded in the St regions. In the GC regions, the N 32 values in shallow events were smaller compared with deep events, while D m and IWC 33 were larger. In the St regions, N decreased compared with the GC regions, while the 34 D m and IWC increased, implying the existence of aggregation and deposition growth. 35 The growth of particle size and mass mainly occurred in the St regions. The increases 36 of N were usually observed near −5°C accompanied by bimodal Doppler spectra, 37 which might be caused by ice multiplication. The average ratios of the median N , D m , 38 and IWC inside GCs to those outside GCs are 2, 1.3, and 2.5 respectively for shallow 39 events, with 1.7, 1.2, and 2.3 respectively for deep events. These values were 40 basically the same as those for the FSs, implying the importance of GCs to the 41 enhanced ice growth subsequently found in FSs. The larger values of N , D m , and IWC 42 inside GCs could be related to the upward air motions inside GCs. The first Z e –IWC 43 relationship suitable for snow clouds in the midlatitudes of China was also


Introduction
The generating cells (GCs) and associated fall streaks (FSs) were observed and 8 to be the main particle type in the GC regions of shallow events, since the Z-Vt 155 relationship established for these regions was very close to the Z-Vt relationship for 156 hexagonal plates. For the same reason, C20 considered that the GC regions of deep 157 events were mainly composed of bullet rosettes, and the St regions in the two types of 158 snow events were mainly composed of aggregates. The deduced particle types in GC 159 and St regions are consistent with those observed by Plummer et al. (2014) and 160 Plummer et al. (2015), respectively. Based on the main ice particle types identified in 161 C20, this study will retrieve and analyze the microphysical parameters (N, Dm, and 162 IWC) in the snow clouds.
163 Table 1: The information of the six snow events.  shows the ERA5 data as the red asterisks. As shown in Fig. 1, the ERA5 data were 183 close to the air sounding profiles, and the air sounding data points were much more 184 than the ERA5 data, so that the variation of atmospheric temperature and humidity 185 reflected by air sounding data was more detailed. Therefore, the following sections 186 will analyze the microphysical processes by using air sounding data. where Δv is the interval between two adjacent spectral lines, with the value of 204 0.0895 m s −1 , and NFFT is the number of spectral lines, with the value of 512. 205 Since the calculation of Ze is closely related to particle size (D), it is necessary to 206 convert Zehi(Vt) into the equivalent reflectivity of a single spectral line with D being 207 the independent variable (Zehi(D)). Note that particle size in this study means the 208 maximum dimension of the particle. The equivalent reflectivity of the ith spectral line where C = 10 6 λ 4 /π 5 /|K| 2 , λ is wavelength with the unit of cm, Di is the particle 11 maximum dimension of the ith spectral line with the unit of cm, Nhi(Di) is the particle 213 number density of the ith spectral line with the unit of m −3 cm −1 , and σ(Di) is the 214 backscattering cross-section of a single particle with the unit of cm 2 . The 215 dimensionless quantity K is the Clausius-Mossotti factor of liquid water that can be 216 calculated by the complex refraction index of liquid water (m), K = (m 2 − 1)/(m 2 + 2).
where m(Di) is the mass of a single particle with the size of Di. The sum of 224 IWChi(Di) of each spectral line in a range gate is the IWC at the corresponding height.

225
According to Eq. (3), the m-D relationship is indispensable for calculating the IWC, 226 which is described in Section 3.1.

227
The calculation expression of Dm at a certain height (Delanoë et al. 2007) is shown 228 as Eq. (4): In conclusion, the key problems of retrieving N, IWC, and Dm are finding a 231 reasonable Vt-D and m-D relationships, and calculating the backscattering 232 cross-section of ice particles (σ). given shape, size, and mass. These relationships usually have the following form: where D is the maximum dimension of the particle; the constants a and b were 239 measured from a variety of particle habits (e.g., Locatelli  Heymsfield and Westbrook 2010) was selected to obtain the Vt-D relationship.

245
The fall speeds of solid particles are related to their shape, volume density, and air 246 density. In general, the drag on a falling particle can be expressed by a dimensionless 247 drag coefficient Cd: where ρair is the air density, Fd is the drag force, and A is the projected area. In the Best number X = CdRe 2 is introduced. When the Fd in Eq. (6) is equal to the 253 weight of the particle mg, where g is gravity, the following results can be obtained: The area ratio of the particle Ar is introduced, defined as the ratio of the particle's shallow events, the T within the snow clouds was from −20°C to 0°C (Table 1).

279
Columns and bullets are rarely seen in this temperature range (Bailey and Hallett 280 2009); therefore, the aggregates of plates were regarded as the dominant particle type 281 in the St regions of the shallow events. Table 2 shows the parameters corresponding to 282 different types of ice particles (Mitchell 1996). According to Mitchell (1996), the 283 parameters are different for different ranges of particle size. To select the appropriate 284 parameters, the ranges of particle size were estimated roughly by the retrieved Vt (C20)   is used for aggregates.

16
In the Rayleigh-Gans theory, the σ of an arbitrarily shaped particle illuminated by a 312 plane wave propagating in the direction s is given by (e.g., van de Hulst 1957; ( ) ( ) where k is the wavenumber calculating by 1/λ, and Ki is the Clausius-Mossotti 316 factor of solid ice. The D and A(s) here can be considered the same as D and A in 3.1.

317
Note that exp(i2ks) can be replaced by cos(2ks) + i sin(2ks).  The changes in N, IWC, and Dm must be related to environmental T and humidity.

405
The detected T and RH can be converted into actual vapor pressure (e) and 406 supersaturation with respect to ice (Si) by the WMO formulations, as shown in C20.

407
The profiles of T, e, and Si are shown in Figs. 3d, 3e, and 3f respectively. According  The same period of 1230-1330 UTC during the 20 January 2016 event was 541 selected to represent the shallow events, as described in C20. The Ze, Vr, and SW are 542 exhibited by the time-height contours in Fig. 6, indicating that the main distribution 543 characteristics of spectral parameters were similar to those in the deep event. Figure   544 6a shows the detailed structure of the GCs and FSs, and upward and downward air 545 motions coexisted near the echo top (Fig. 6b), where the SW was apparently larger 28 than that of the underlying cloud (Fig. 6c). Below about 3.5 km, the Vr was more According to the profiles of T, e, and Si in Fig. 7, the corresponding T in the GC 580 region ranged from −21°C to −15°C, and e values were between 100 and 200 hPa.

581
The Si was greater than 30% and reached a maximum of 40% near the echo top.

582
Compared with the GC region of the deep event, Si was similar, but the T increased, The N values increased around 1.7 km and 0.8 km. According to the T profile of St 628 region in Fig. 7d, the corresponding T near the above two heights were between −3°C 629 and −8°C, which might be associated with Hallett-Mossop process (Mossop 1976).

630
The production of secondary ice particles also increased the IWC at the same heights.

631
The bimodal Doppler spectra near 1.7 km shown in Fig. 8d as an example, also 632 indicated the possible existence of small secondary ice crystals, but note that the      It can be seen from Fig. 10  It is assumed that the CVPR-FMCW used in this study has almost no attenuation in 820 snowfall detection, and its power spectral density distribution can accurately reflect 821 the size distribution of snow particles. However, the retrieval errors of microphysical 822 parameters are inevitable. The errors are caused by: 1) the hypothesis of particle types; 823 2) calculation of σ; 3) Vt-related errors. 824 We supposed that the GC regions of shallow events were dominated by hexagonal aggregates. We also assumed that there was only one particle type in a single radar 828 sampling volume. The particle types affect the selection of m-D and A-D 829 relationships, thus affecting the calculated m and A. Then the m and A affect the 830 calculation of IWC, σ and Vt-D relationship. The particle types adopted in this study 831 have been tested in C20, which is more in line with the given snow events, so that the 832 error caused by hypothesis of particle types can be reduced here.

833
In the case of the present radar band and particle types, the RGA underestimates the  (1) Similar to C20, the clouds were divided into upper GC and lower St regions.

859
For the GC regions, hexagonal plates were regarded as the dominant particle