Observational Evidence of High Ice Concentration in a Shallow Convective Cloud Embedded in Stratiform Cloud over North China

The formation of high ice concentrations via secondary ice production processes in clouds is important in determining precipitation and atmospheric radiation transmission. During the past several decades, many researchers (e.g., Mossop et al., 1972; Heymsfield and Hjemfelt, 1984; Hobbs and Rangno, 1985; Huang et al., 2011) have reported high ice number concentrations; they are usually several orders of magnitude larger than the concentrations of ice nuclei and occur in warmer regions of lower-tropospheric convective clouds. Generally, high ice concentrations in convective clouds can be attributed to the splinter ejection during the riming of ice crystals from -3^°C to -8^°C due to large water drops as well as the development of ice generated by intense updrafts across a large range of temperatures, which is a process first proposed by Hallett and Mossop (hereafter referred to as the HM process). Several aircraft and modeling studies further supported that the appearance of a high ice number concentration in convective clouds, which span the temperature range of -3^°C to -8^°C (Clark et al., 2005; Huang et al., 2011), can be attributed to the HM process. During the Ice in Clouds Experiment-Tropical (ICE-T), (Heymsfield and Willis, 2014) also found that secondary ice particles are primarily present in regions where liquid water content (LWC) is relatively low and the updraft is weak in tropical maritime cumulus. Although different to that in an isolated convective cloud, the ice formed in stratus clouds is expected to be lower in concentration compared with that in convective clouds and would be quickly precipitated due to rapid growth (Meyers et al., 1992). High ice concentrations are also usually observed in slightly supercooled stratus clouds (cloud top temperature greater than -15^°C), where the updraft is relatively small and very few ice particles would be expected to form. For example, (Rangno and Hobbs, 2001) demonstrated that high ice number concentrations can be observed in arctic stratiform clouds within slightly or moderately supercooled stratus clouds with a cloud top temperature greater than -15^°C, and they speculated that there are many mechanisms responsible for this phenomenon. (Rangno, 2008) further showed evidence of ice formation induced by the freezing of drops during free fall in a shallow, supercooled marine stratiform cloud. (Yang et al., 2014) and (Crawford et al., 2012) presented observations that supported the HM mechanism in stratiform and convective clouds in North China and England, respectively. Moreover, experiments in the laboratory confirm that there are several mechanisms, different from those mentioned above, that can account for the production of high ice number concentrations within relatively higher temperature ranges. Those mechanisms include the fracturing of fragile ice particles when coalescing with each other (Hobbs and Farber, 1972), and the evaporation of large cloud drops at -5^°C (Knight, 2012). Additionally, mineral dust and organic compounds have been identified as efficient ice nuclei at relatively higher temperatures (>15^°C), which may significantly increase ice number concentrations when entering a supercooled cloud (Fukuta, 1966). However, the relative importance of these mechanisms remains uncertain (DeMott et al., 2010).

The formation of high ice concentrations in clouds is still poorly quantified or understood, thus usually resulting in underestimates of ice number concentrations when using ice nucleation schemes in cloud resolving models (Klein et al., 2009). Therefore, understanding the formation of high ice concentrations in different cloud types is very important, not only for water balance and climate, but also for numerical simulations of cloud microphysical processes and cloud seeding. In this paper, we present in situ observations of stratiform cloud behind a cold front in North China on 9 September 2015. The main purpose of the study was to observe the microphysics in terms of the vertical distributions of ice particles, cloud droplets, and particle size spectra within a slightly supercooled stratiform cloud. The conditions in which secondary ice particles formed are also demonstrated. Moreover, we hope that the in situ observational data collected during this study will contribute to improving microphysical schemes used in numerical models.

During the last five years (2010-15), several in situ airborne observations were conducted in the eastern part of Inner Mongolia by the Key Laboratory of Cloud-Precipitation Physics and Severe Storms (LACS), Institute of Atmospheric Physics (IAP), in association with Tongliao Meteorological Bureau. These observations were conducted to build datasets and investigate the microphysical properties of stratiform cloud precipitation and aerosol-cloud-precipitation interactions in North China. The field study area was mainly located in the surrounding areas of Tongliao city. A set of airborne and ground-based instruments, including a particle measuring system and a C-band radar, were used to measure the aerosol, cloud condensation nuclei and detailed microphysical properties of precipitation systems that passed through the area. During the field observations, the airborne instruments provided important microphysical quantities and two-dimensional (2D) images of ice particles. For example, a passive cavity aerosol spectrometer probe (PCASP, 0.1-3 μm), for obtaining aerosol particles, and a cloud-aerosol spectrometer (CAS, 0.6-50 μm), for sampling large aerosol particles and cloud droplets, were employed. Meanwhile, a cloud image probe (CIP, 25-1550 μm) and precipitation image probe (PIP, 100-6200 μm) were used to provide 2D images, and concentrations and size spectra of ice particles. An AIMMS-20 instrument was used to provide the flight track and basic state parameters, such as temperature, pressure and wind speed. These airborne instruments were mounted on a Y-12 twin-engine aircraft with a cruising speed of around 50 m s^-1. Meanwhile, a C-band research radar, located in Tongliao city, performed continuous plan position indicator (PPI) scans to capture the variations of the cloud system, and to provide the cloud conditions where secondary ice particles were observed.

The synoptic situation in this study was acquired using China Meteorological Administration reanalysis synoptic maps. Meanwhile, radar reflectivities between different elevation scans were interpolated on the cross sections of the radar data along the flight track to compare the data obtained by radar with those obtained by aircraft, because RHI (range height indicator) scans were not performed during our study. Details of the aircraft data processing were provided in (Yang et al., 2014). Recently, many researchers (e.g., Korolev et al., 2013, Jackson and McFarquhar, 2014; Jackson et al., 2014) have found that ice shattering on the tips or inlet of probes can result in overestimates of small ice particles (diameter <500 μm). This miscounting can be reduced by using an inter-arrival time algorithm (ITA) or specially designed tips, although not all of the shattering particles can be removed. (Baumgardner and Korolev, 1997) found the depth of field of a 2D probe for smaller particles (diameter <150 μm) was not well defined. Moreover, 2D probes are unable to directly distinguish ice-phase particles from liquid ones, because small particles (diameter <100 μm) are hard to classify. Based on RICE (Rosemount icing detector), HVPS (high-volume precipitation sampler), FSSP (forward scattering spectrometer probe) and CPI (cloud particle imager) data, (Cober et al., 2001) and (McFarquhar et al., 2007) found that the shape of cloud size distributions measured in liquid-phase cloud has a strong peak between 5 μm and 20 μm, while counterparts within ice-phase and mixed-phase cloud are usually broader due to the existence of ice particles. However, this criterion was not used in our study because of the different resolutions of FSSP and CAS; quantitative studies regarding the differences between these two instruments are still required.

As mentioned above, the microphysical properties and structure of clouds measured by the 2D probes can be largely distorted if the ice shattering and DOF (depth of field) issues are not avoided. Thus, two methods were used to reduce the effects of particle shattering on ice concentrations. The first method rejected particles using the particle inter-arrival times method proposed by (Field et al., 2006). This method is based on the theory that the inter-arrival time between two neighboring, natural particles is much longer than that between two artificial ones. (Field et al., 2006) proposed 1× 10^-5 s as the threshold to filter out shattered particles for a CIP probe. However, this threshold seemed too short to eliminate most of the shattered particles, since the inter-arrival time depends highly on the cruising speed of t he aircraft. Therefore, this study used a threshold of 2× 10^-5 s, owing to the lower speed of the Y-12 aircraft compared with the DC-9. Comparisons indicated that ice number concentrations were usually reduced by a factor of one to five when using the ITA algorithm, and the number of artificial particles detected increased as the size distribution was broadened. The second method estimated the ice number concentrations based on the theory that the ice growth rate is larger than that of water droplets due to the Bergeron-Findeisen process in mixed-phase cloud. Therefore, only particles with diameters larger than a minimum cutoff size were taken into account when determining the ice number concentrations. For example, (Rangno and Hobbs, 2001), (Heymsfield et al., 2002) and (Heymsfield and Willis, 2014) proposed 100 μm as the cutoff size (hereafter referred to as N₁₀₀). In this paper, in order to maintain consistency with previous studies, N₁₀₀ is also used to present the ice number concentrations. Additionally, the number concentrations of particles with diameter >500 μm were calculated to minimize the effects of ice shattering and sample volume issues on ice concentrations (Plummer et al., 2015). Moreover, cloud droplet concentrations, including large droplets with diameter >24 μm, and size spectra, were determined using the date measured by the CAS probe. LWC was derived from the size spectra of cloud droplets (CAS), but particles with diameter <1 μm were removed to minimize the influence of large aerosols.

The synoptic situation over our study area on 9 September 2015 was primarily influenced by a cold front associated with an upper-level trough. This mesoscale precipitation system passed through North China from west to east, with a surface frontal boundary roughly aligned northeast to southwest. The 0000 UTC surface synoptic map indicated a surface low pressure center located in Northeast China and Southeast Russia, with a cold front extending southwestwards from the low pressure center towards central China (Fig. 1a). The upper-level synoptic pattern (illustrated by Fig. 1b) indicated an upper level trough with a low pressure center located at approximately 500 km west to the surface low pressure center. Infrared imagery from the Japan Meteorological Agency METSAT satellite at 0200 UTC (Fig. 2) showed that the frontal clouds, approximately parallel with the surface cold front, mainly covered the areas northeast of Tongliao city. The basic structure of the cloud field further suggested the upper-level front was a white cloud line crossing the study area. Meanwhile, a wide-range light precipitation event was generated by the unstable region behind the cold front from 0130 to 0300 UTC, with the maximum rainfall intensity reaching 1 mm h^-1 (rain gauge data, not shown). The radar composite equivalent reflectivity obtained by PPI scans during flight sampling from 0210 to 0300 UTC demonstrated a large area of stratiform clouds, with reflectivities <30 dBZ in most regions. Several small convective areas (maximum reflectivities >30 dBZ) were mainly located northwest and west of the radar site. Meanwhile, the radar data also showed that the precipitation was nearly parallel to the surface cold front, extending from the northeast to southwest. During this precipitation event, two penetrations were performed to investigate the ice particles within discrete regions of the cloud. From 0210 to 0240 UTC, the aircraft departed from Tongliao city and flew northeastwards, nearly 55^° radial from the radar ground site, to penetrate the convective area of the cloud system (Fig. 3a), which was located 20 km northeast of the radar site. Then, the aircraft turned southwestwards to observe the surrounding stratiform cloud (as illustrated in Fig. 3b) from 0240 to 0300 UTC. To obtain the horizontal distributions of the microphysical properties at different levels, several horizontal flight legs, each extending 20-30 km, were also performed at altitudes ranging from 3.5 km to 5 km and temperatures ranging from -1^°C to -7^°C.

Figure 1.  Synoptic situation during the present study: (a) surface pressure and location of the cold front; (b) 500 hPa geopotential height valid at 0000 UTC 9 September 2015. Source: China Meteorological Administration.

Figure 2.  Flight tracks of the Y-12 aircraft (in red) and the radar reflectivities measured by the C-band radar located at Tongliao city on 9 September 2015. Locations identified by markers include Tongliao (red) and Kezuozhongqi (yellow). (a) Radar data obtained at 0220 UTC; flight track from 0200 to 0230 UTC. (b) Radar data at 0240 UTC; flight track from 0230 to 0300 UTC.

Figure 3.  JMA (Japan Meteorological Agency) METSAT infrared image at 0200 UTC 9 September 2015. The black square and concentric circles indicate the study area and Tongliao city, respectively.

Variations of LWC, cloud droplet concentrations, N₁₀₀, N₅₀₀ and the 2D images obtained during the first cloud penetration, are shown in Fig. 4. The radar cross section along the flight track at 0220 UTC is shown in Fig. 5. It is shown that, in the lower and higher parts of the supercooled cloud, where the ambient temperature varied from -1^°C to -2.6^°C and from -4^°C to -7^°C, the N₁₀₀ and N₅₀₀ measured by CIP were generally less than 50 L^-1 and 5 L^-1, respectively. The high ice number concentrations region, identified by N₁₀₀, exceeded 100 L^-1 and was mainly obtained during 0220 to 0225 UTC, with ambient temperatures ranging from -2.6^°C to -3^°C. In this region, N₁₀₀ and N₅₀₀ apparently increased by a factor of five to ten compared with those in other regions of the penetration, and their maximum values exceeded 300 L^-1 and 30 L^-1, respectively. Meanwhile, cloud droplet concentrations measured simultaneously by the CAS probe during the first penetration confirmed that this region was a mixed-phase cloud, due to the appearance of a large number of cloud droplets. As shown in Fig. 4a, throughout the majority of the first penetration, cloud droplet concentrations were greater than 20 cm^-3, and the maximum values of 80-120 cm^-3 appeared in the high ice number concentration area. The LWC derived by the CAS probe was usually lower than 0.01 g m^-3 in the lower and higher parts of the cloud, except for the high ice number concentration area. The LWC in the high ice number concentration area exceeded 0.015 g m^-3, and the maximum value was approximately 0.025 g m^-3. These results suggested that larger cloud droplets may have existed in this region.

Figure 4.  (a) Total cloud droplet number concentrations (units: cm^-3; black solid line) and mass concentrations (LWC; units: g m^-3; gray dashed line) derived by the CAS probe, 1-50 μm. (b) Number concentration of cloud droplets with diameter >24 μm (units: cm^-3; black solid line) obtained by the CAS probe and ambient temperatures (units: ^°C; gray dashed line). (c) Ice number concentration (units: L^-1) with diameter >100 μm (black solid line) and >500 μm (gray dashed line) derived from CIP.

The HM mechanism, as investigated in the laboratory and in situ observations of (Mossop, 1985), shows a dependence on the coexistence of drops with diameter <12 μm and >25 μm, with a lesser effect of small drops. Therefore, we also present the number concentrations of large modes of the droplets obtained during the first penetration (Fig. 4b). The variations of large cloud droplet (diameter >25 μm) concentrations along the flight track were consistent with those of N₁₀₀ and N₅₀₀, and the maximum value of 0.4 cm^-3 was comparable with the value of N₁₀₀ in the same region. Moreover, the radar reflectivity cross section observed at 0220 UTC demonstrated that the altitudes of cloud top varied from 5-7 km, and cloud top temperatures ranged from -7^°C to -18^°C along the flight track. The radar reflectivity also indicated that the high ice number concentration region was located in the supercooled part of a shallow convective area, which was approximately 20 km in width and roughly 70-100 km northeast of the radar site. Meanwhile, the maximum reflectivity reached 40 dBZ at 2-3 km in altitude, where the ambient temperature was higher than 0^°C (below the melting layer, Fig. 5). According to the radar reflectivities obtained by the C-band radar from 0210 to 0230 UTC, this convective cloud was in a steady state during the first penetration.

Figure 5.  Radar reflectivity (units:dBZ) cross section derived from different evaluation angles of PPI scans, 55^° radial from the radar ground site, at 0220 UTC. The black line indicates the flight track during 0210 to 0230 UTC 9 September 2015.

Figure 6.  Typical ice particle types obtained in different regions of the supercooled cloud during the first penetration between 0210 and 0240 UTC 9 September 2015. The left panel labels a-g represent the points which were indicated by the same characters in Fig. 4.

Figure 6 illustrates the typical ice particle types obtained by CIP at several points during the first cloud penetration (a to g points as indicated in Fig. 4). The columnar and needle-like ice crystals, typically approximately 300-500 μm in length, were predominant in the high ice concentration area, as indicated by the 2D images. The ice particle types in the high ice concentration region were similar to those reported in previous studies (Rangno and Hobbs, 2001; Crawford et al., 2012) in middle level stratus clouds in South England and Arctic stratus clouds. Meanwhile, aggregates, and some moderately and lightly rimed ice particles (or graupels) with diameters from 500-1000 μm, were also obtained in the same region. It should be noted that columnar and needle-like ice crystals were measured in the southwest of the main convective area (Figs. 6a, b and c), where the ambient temperatures ranged from -0.5^°C to -2.25^°C. However, their maximum lengths of 500-1000 μm were usually longer than their counterparts at Figs. 6d and e. The needles occurred in the southwest part of the main convective cloud, most likely due to the divergence of air at the cloud top. Then, these needles fell from higher altitudes and kept growing via deposition in the surrounding stratiform cloud. Note that many spherical particles with diameter >100 μm, which may be freezing drops, also appeared in some regions (as indicated by the 2D images at a, b and c points illustrated in Figs. 6a-d). In the higher parts of the cloud, from -3.5^°C to -7.5^°C, the ice particle types were much more uniform compared with t hose mentioned above. Most of the ice particle types appeared to be heavily and lightly rimed stellar ice crystals (Figs. 6f and g), which we would expect to be produced at temperatures of around -15^°C, while some spherical particles were also obtained in this region.

Vertical profiles of ice particle number concentrations from CIP obtained during the first penetration for flight legs in different temperature regions are shown in Fig. 7. The plot, which shows the variations of the 5th, 25th, 50th and 95th percentiles of N₁₀₀ and N₅₀₀, indicates that N₁₀₀ and N₅₀₀ were highly variable at -3.2^°C, where the HM process may have been active, as compared with other parts of the cloud penetration. The median value of N₁₀₀ (30 L^-1) during the first penetration was comparable with previous in situ observations. For example, (Crosier et al., 2011) showed that the median value of irregular ice particles, measured within a weakly convective cloud embedded in a supercooled mid-level stratus cloud in England, was approximately 5 L^-1. (Crawford et al., 2012) demonstrated that ice number concentrations, obtained in a mature convective cloud, was approximately 30 L^-1 and the maximum value exceeded 100 L^-1.

Figure 7.  Median and 5th, 25th, 75th and 95th percentiles for (a) N₁₀₀ (units: L^-1) and (b) N₅₀₀, (units: L^-1) for flight legs at different temperatures, from the sampling flight from 0210 to 0240 UTC 9 September 2015. Whiskers extend to the 5th and 95th percentiles, and boxes encompass the 25th to 75th percentiles and 50th percentiles (median value).

Figure 8.  Ten-second average size spectra N(D) (units: L^-1 μm^-1) obtained during 0218-0228 UTC 9 September 2015. Data were acquired from the CAS (1-50 μm), CIP and PIP instruments, as indicated by the legend.

Figure 9.  Concentrations of ice particles measured by CIP during the first penetration, N₅₀₀ and N₁₀₀ as a function of (a) large cloud droplets (diameter >24 μm) obtained by the CAS probe, and (b) the LWC derived by the CAS probe.

Combined particle size spectra (10 s averaged) measured simultaneously by CAS, CIP and PIP at different points are shown in Fig. 8. The size spectra of large particles were flatter within the high ice number concentration region compared with those in the surrounding area in lower and higher parts of the cloud. Figure 8 also shows that the ratio of size spectra between Fig. 8c and e and between Figs. 8a and b were maximized for diameters from 500 μm to 1000 μm. Additionally, the cloud droplet spectra in the high ice concentration region (or in the higher part of the shallow convective region, Fig. 8) showed that the larger cloud drops (with diameters from 10 μm to 50 μm) increased by a factor of one to five compared with those in other parts of the penetrations.

In this study, N₁₀₀ and N₅₀₀ were related to both LWC (Fig. 9) and the large cloud droplets (diameter >24 μm) derived by the CAS probe. This finding was similar to that of (Heymsfield and Willis, 2014), who also presented a relationship between ice particles with diameter >125 μm and large cloud droplets measured in tropical maritime convective clouds during the African Monsoon Multidisciplinary Analyses (NAMMA). However, there are still some discrepancies. For example, although the cloud droplet concentrations were comparable, the LWCs (usually less than 0.015 g m^-3) and number concentrations of the large cloud droplets derived by the CAS probe were much smaller than those presented in (Heymsfield and Willis, 2014) within the high ice number concentration zone in a marine convective cloud. These results implied that the large cloud droplet number concentrations were smaller than those measured in NAMMA. In fact, the LWC derived by the CAS probe in the high ice number concentration zone in this study was significantly smaller than that reported by most previous studies, especially those measured in cumulus clouds with intense updraft. For example, (Crawford et al., 2012) demonstrated that the maximum value of LWC in aged wintertime cumulus clouds over the United Kingdom usually exceeded 0.5 g m^-3. (Mossop, 1978) found that a large droplet concentration of 10 cm^-3 is necessary for an efficient operation of the HM process in cumulus. (Rangno and Hobbs, 2001) also found that the maximum value of LWC in an Arctic stratocumulus cloud exceeded 0.2 g m^-3.

On the other hand, large cloud droplets and high values of LWC are not always captured in some clouds where ice multiplication apparently appeared. Observations conducted recently also presented concurrences of low LWC and high ice number concentration in weakly convective cloud and maritime chimney cloud. For instance, based on data gathered in England, (Crosier et al., 2011) demonstrated that LWC and large cloud droplets (diameter >10 μm), within the temperature range -4^°C to -5^°C, were usually less than 0.04 g m^-3 and 0.1 cm^-3, respectively, while the ice number concentrations were usually greater than 30 L^-1 and the types of ice particles were predominated by pristine needles and columns. Additionally, based on data obtained in chimney cloud during the ICE-T project, (Heymsfield and Willis, 2014) concluded that secondary ice particles were observed primarily in regions of low LWC and weak vertical velocity, while LWC in the regions where secondary ice crystals were observed were dominantly below 0.1 g m^-3, with a median value of only 0.03 g m^-3. These data are comparable with those obtained in the present study in the same temperature range. We speculate that the updraft in this case was relatively weaker compared to that in typical convective cloud. Therefore, the majority of droplets may have been removed quickly by the Bergeron-Findeisen process after high ice concentrations were produced via ice multiplication, which resulted in the coexistence of large droplets and ice crystals for a few minutes only. Thus, it is hard to capture large droplets and high ice concentrations simultaneously.

Figure 10.  As in Fig. 4, but for the second penetration.

During 0240-0300 UTC, to obtain the vertical microphysical properties and compare them with those in the first penetration, a quasi-Lagrange (fast descent in 10 minutes) sampling was performed within the cloud north of the first penetration. Unfortunately, 3 minutes of data (from 0243 to 0246 UTC) with temperatures ranging from -4^°C to -3^°C were not uploaded to the data collection system due to hardware problems. Radar data in the same region were also unavailable because its wave band was blocked by buildings (Fig. 3b). Therefore, it was hard to estimate whether this area was a convective or a stratiform cloud and whether the high ice number concentrations appeared in this region. Despite this inherent shortcoming, the rest of the data showed (Fig. 10) that concentrations of N₁₀₀, N₅₀₀, LWC and cloud droplets were much smaller than their counterparts obtained during the first penetration. For example, N₁₀₀ and N₅₀₀ were usually less than 8 cm^-3 and 3 cm^-3, respectively, while the LWC and cloud droplet concentrations derived from the CAS probe (1-50 μm) were usually smaller than 0.01 g m^-3 and 60 cm^-3, respectively. The maximum values mainly occurred in the higher and lower parts of the cloud, with temperature ranging from -5^°C to -6^°C and -0.3^°C to -1^°C, respectively. The concentrations of large cloud droplets decreased by a factor of three to five compared with those in the high ice concentration area in the first penetration. Meanwhile, the 2D images obtained by CIP showed large discrepancies compared with those in the first penetration (Fig. 11). Sector plates and rim ice particles, with diameter >1000 μm, occurred in the higher part of the cloud, where ambient temperatures were lower than -5^°C. The heavily rimed ice particles dominated in the lower part of the cloud, where the ambient temperatures were higher than -3^°C. The occurrence of rimed ice particles could be attributed to the riming growth of ice particles during free fall. The pristine ice crystals, such as column and needle crystals, which may relate to secondary ice production, were very rare throughout the second penetration (even just below the missing data area), and freezing drops were also absent. It was possible that the updraft during the second penetration was relatively small because the cloud was in its dispersion stage. Therefore, the majority of the supercooled cloud droplets and ice particles were removed by evaporation/sublimation, leading to only very low concentrations of large cloud droplets and ice particles, which were insufficient for the activation of the HM process. Overall, it was unlikely that secondary ice production was active during the second penetration, and the ice formed in this region may have been primarily from ice nucleation.

Figure 11.  2D imageries obtained during the second penetration. The left panel labels a-d represent the points which were indicated by the same characters in Fig. 10

On 9 September 2015, in situ observations of cloud microphysical properties in a slightly supercooled stratiform cloud behind a cold front were made using aircraft instruments along with radar and satellite data. The measurement errors induced by ice shattering on the tips of probes and airflow distortion were reduced using an ITA. The results showed that the high ice concentrations region was in the supercooled region of a shallow convective cloud, with cloud top temperature ranging from about -12^°C to -15^°C, embedded in a stratiform cloud. In this region, the maximum values of N₁₀₀ and N₅₀₀ exceeded 250 L^-1 and 25 L^-1, respectively. The ice particle types in the high ice number concentration zone were predominated by columnar and needle-like crystals, together with some graupels and heavily rimed ice particles. Meanwhile, the maximum value of large supercooled water droplet (diameter >24 μm) concentrations was 0.4 cm^-3, and was generated by the intense updraft of the shallow convective cloud in this region. In the other parts of the cloud, their counterparts decreased by a factor of five to ten, and N₁₀₀ and N₅₀₀ were usually less than 50 L^-1 and 5 L^-1, respectively. The 2D images obtained during the first penetration also demonstrated that the column and needle-like ice crystals falling from a weak convective area could keep growing in the surrounding stratiform cloud via deposition. Throughout the first penetration, fragile ice particles, such as dendrites, were very rare, while freezing drops mainly occurred in the warmer (above -2^°C) parts of the surrounding stratiform cloud. These results suggested that the fracturing of fragile ice particles and the freezing of drops during free fall may play insignificant roles in ice multiplication. The microphysical properties in terms of the drops and ice particles just below the high ice number concentration zone, were not obtained in this study. Therefore, it is difficult to speculate whether freezing drops existed in relatively warmer regions of the convective cloud, which could have been carried upward into the HM zone to eventually become graupels with the ability to produce secondary ice.

Figure 12.  The ice concentrations at different ambient temperatures, as predicted by the Fletcher scheme (red line) and the DeMott scheme using different large aerosol (diameter >0.5 μm) concentrations as inputs (black lines). The red circle and red rectangle stand for the maximum values of N₁₀₀ and N₅₀₀ obtained during the first penetration, while the blue circle and blue rectangle are the N₁₀₀ and N₅₀₀ measured during the second penetration. In order to compare the ice concentrations predicted by the different schemes, the cloud top temperature was assumed to be -15^°C.

Additionally, the aerosol particle number concentrations (0.1-3 μm) measured by PCASP just below the stratiform cloud base were in the order of 100 cm^-3, while the large aerosol particle (diameter >0.5 μm) concentrations were approximately 2 cm^-3. These were highly related to the IN (Ice Nuclei) concentration as proposed by (DeMott et al., 2010): \begin{equation} n_{{\rm IN},T_{\rm K}}=a(273.16-T_{\rm K})^bn_{{\rm aer},0.5}^{c(273.16-T_{\rm K})+d} , (1)\end{equation}

where a,b,c and d are constants, T _K stands for the temperature in Kelvin degree, and n _aer,0.5 is the number concentration of large aerosol particles (diameter >0.5 μm). n _{IN,T K} stands for ice nuclei concentration at temperature T _K. Compared with schemes proposed in previous studies, the variability in the IN number concentrations at a specific temperature is reduced from 10³ L^-1 to less than a factor of 10 by using this new scheme. If we assume that all of the large aerosols were transported upward to the cloud top (-15^°C) by updraft without any wet partitioning induced by rainout or nucleation, the maximum values of N₁₀₀ and N₅₀₀ are two orders and one order of magnitude larger, respectively, than those of the IN number concentration predicted by the DeMott scheme (Fig. 12), and about four and three orders of magnitude larger than those predicted by the Fletcher scheme, when the cloud top temperature is -15^°C, respectively. Although there was some uncertainty in determining the cloud top temperature based on radar reflectively, to match the same order of N₁₀₀, the temperature should decrease to approximately -30^°C, which means the cloud top altitude should be approximately 9 km. It should be noted that the N₅₀₀ affirmatively underestimated the ice number concentrations, at the very least by a factor of two to three in the high ice number concentration area in this case. Thus, it was unlikely that the high number concentrations of ice particles were produced by ice nucleation alone. Meanwhile, the radar reflectivity cross section obtained at 0220 UTC also ruled out any possibility of ice seeding from generating cells above, which may increase the ice concentrations in low-level clouds via the "seeder-feeder" process (Plummer et al., 2015). Therefore, ice formed in generating cells also seemed very unlikely.

The data were not collected at approximately 3 minutes. The second penetration in the surrounding stratiform cloud provided evidence of regions where the HM process was not active because the columnar and needle-like pristine ice crystals were not measured in this area (even just below the missing data area). The N₁₀₀ and N₅₀₀ obtained during the second penetration were one to two orders of magnitudes smaller than those obtained during the first penetration, while the ice particle types were predominated by lightly and heavily rimed ice particles without any particular shape. Meanwhile, the number concentrations of large cloud droplets and freezing drops were also smaller than those obtained during the first penetration, which may have been insufficient for the ice multiplication process.

Previous laboratory studies suggest that the riming splintering mechanism does occur in a temperature range centered at about -5^°C. The presumption has been that new ice particles could be produced during riming, and can be identified by the occurrence of large amounts of needle/column ice crystals, since the growth of those particle types is favored in the temperature range -3^°C to -8^°C. In this case, the cloud microphysical properties obtained during the first penetration indicated that the ice number concentrations of N₁₀₀ and N₅₀₀ were related to large cloud droplets (diameter >24 μm) and the LWC produced by the relatively intense updraft in the shallow convective cloud; meanwhile, the ice particle types in the high ice number concentration zone were predominated by column and needle crystals. Heavily rimed ice particles or graupels falling from the upper-level cloud become the initial rimers. After the formation, the splinters transformed into columns or needles in the region where ambient temperature ranged from -3^°C to -8^°C, via the HM process. Therefore, the appearance of the high ice number concentration zone containing pristine ice, such as columns and needles, was likely related to the HM process. On the other hand, many processes other than HM mechanism cannot be totally ruled out. For instance, some in situ observations and numerical simulations (e.g., Scott and Hobbs, 1977; Lawson et al., 2015) have indicated that the drop-freezing secondary ice production mechanism is found at -8^°C to -11^°C in strong tropical convective cores. Also, a laboratory study (Knight, 2012) suggested the possibility of ice multiplication during sublimation growth of ice crystals around -5^°C, even without any riming. It should be emphasized that perhaps more than one mechanism can be responsible for the occurrence of high ice number concentrations. However, it was hard to obtain direct evidence of the mechanisms mentioned above, because the resolutions of the airborne probes used in this study were too low to discern very small freezing drops and ice crystals. Thus, it seems to be very difficult to confirm which mechanism is predominant in determining secondary ice production. Meanwhile, the limited observations of the present study allow only a very minimal investigation of the different ice multiplication processes in cloud. Overall, the potential effects of different ice multiplication processes on high ice concentrations still require further investigation.