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  • View in gallery

    Schematic of the observational region and facilities (the colors indicate the altitudes of terrain).

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    Synoptic weather background on 18 Apr 2009: (a) geopotential height (gpm) and wind fields at 500 hPa and 0800 BT, (b) FY-2C infrared cloud image at 1900 BT. The black square indicates the study area.

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    Superimposed image of radar reflectivity (shading) at 1900 BT with flight tracks recorded on 18 Apr 2009 (green line represents 3817 path, red line represents 3830 path, blue line represents 3625 path).

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    Synoptic weather background at 0800 BT on 1 May 2009: (a) geopotential height (gpm) and wind fields of 500 hPa, The black square indicates the study area. (b) FY-2C visible cloud image at 0900 BT. The red square indicates the study area, the pink line indicates the land boundary of China, and the blue line represents the Yellow River.

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    Superimposed image of radar reflectivity (shading) at 0930 BT with flight tracks on 1 May 2009 (green line represents 3817 path, red line represents 3830 path).

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    Typical ice crystal images recorded by the three aircraft on 18 Apr 2009 (ice crystal habits at 5.1 and 4.8 km were recorded by 2DC aboard 3625, those at 4.2 km were recorded by CIP aboard 3817, those at 3.6 km were recorded by CIP aboard 3830).

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    Typical ice crystal images recorded by the three aircraft on 1 May 2009: (a) images recorded by 2DC aboard 3625, (b) images recorded by CIP aboard 3817, (c) images recorded by CIP aboard 3830.

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    The penetration data recorded by aircraft 3625 on 18 Apr 2009: (a) cross-section of radar reflectivity for flight path, ice crystal images recorded with 2DC (panel top) and 2DP (panel bottom), and T (black line); (b) 2DC instantaneous spectrum; (c) 2DP instantaneous spectrum; and (d) LWC.

  • View in gallery

    The penetration data recorded by aircraft 3817 on 18 Apr 2009: (a) cross section of radar reflectivity for flight path, ice crystal images recorded through CIP (panel top)and PIP (panel bottom), and T (black line); (b) CIP instantaneous spectrum; and (c) PIP instantaneous spectrum.

  • View in gallery

    The cloud penetration data recorded by aircraft 3830 on 18 Apr 2009: (a) cross section of radar reflectivity for flight path and ice crystal habits recorded through CIP, where black solid line represents flight track, red dashed line represents the 0°C layer; (b) CIP instantaneous spectrum; and (c) PIP instantaneous spectrum, (d) temperature (black) and LWC (blue).

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    As in Fig. 9, but on 1 May 2009 and with (d) Hotwire-LWC liquid water content.

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    As in Fig. 11, but by aircraft 3830.

  • View in gallery

    (a) The flight tracks of three aircraft 3625 (red line), 3817 (black line), and 3830 (green line) superimposed with the cross section of radar reflectivity on 1 May 2009; (b) RH (green line, %), LWC (black line, g m−3), and temperature (blue line, °C) recorded from aircraft 3625; (c) as in (b), but from aircraft 3630; (d) as in (b), but from 3630 aircraft; (e) comparison of particle mean volume diameter; (f), average spectrum between 0935 and 0940 BT in convection region; and (g) as in (f), but between 0930 and 0935 BT in stratiform region.

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Ice Crystal Habits and Growth Processes in Stratiform Clouds with Embedded Convection Examined through Aircraft Observation in Northern China

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  • 1 Key Laboratory for Cloud Physics, Chinese Academy of Meteorological Sciences, Beijing, and Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, Nanjing, and Anhui Weather Modification Office, Hefei, China
  • 2 Key Laboratory for Cloud Physics, Chinese Academy of Meteorological Sciences, Beijing, China
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Abstract

Ice crystal habits and growth processes in two cases of stratiform clouds with embedded convection are investigated using data observed simultaneously from three aircraft on 18 April 2009 and 1 May 2009 as part of the Beijing Cloud Experiment (BCE). The results show that the majority of ice crystal habits found in the two cases at temperatures between 0° and −16°C included platelike, needle column, capped column, dendrite, and irregular. A mixture of several ice crystal habits was identified in all of the clouds studied. However, the ice crystals recorded in the embedded convection regions contained more dendrites and possessed heavier riming degrees, and the ice crystals identified in the stratiform clouds contained more hexagonal plate crystals. Both riming and aggregation processes played central roles in the broadening of particle size distributions (PSDs), and these processes were more active in embedded convection regions than in stratiform regions. However, riming was more prevalent in the 18 April case than aggregation, though aggregates were evident. In contrast, the 1 May case had a more dominant aggregation processes, but also riming. With the decrease in height, PSDs broadened in both embedded convection regions and stratiform regions, but the broadening rates between 4.8 km (T ≈ −11.6°C) and 4.2 km (T ≈ −8°C) were larger than those between 4.2 km (T ≈ −8°C) and 3.6 km (T ≈ −5°C). In addition, the broadening rates of PSDs in the embedded convection regions were larger than those in the stratiform clouds, as the aggregation and riming processes of ice particles in embedded convection regions were active. High supercooled water content is critical to enhancing riming and aggregation processes in embedded convection regions.

Corresponding author address: Xueliang Guo, Chinese Academy of Meteorological Sciences, No.46, Zhongguancun Street, Haidian District, Beijing 100081, China. E-mail: guoxl@mail.iap.ac.cn

Abstract

Ice crystal habits and growth processes in two cases of stratiform clouds with embedded convection are investigated using data observed simultaneously from three aircraft on 18 April 2009 and 1 May 2009 as part of the Beijing Cloud Experiment (BCE). The results show that the majority of ice crystal habits found in the two cases at temperatures between 0° and −16°C included platelike, needle column, capped column, dendrite, and irregular. A mixture of several ice crystal habits was identified in all of the clouds studied. However, the ice crystals recorded in the embedded convection regions contained more dendrites and possessed heavier riming degrees, and the ice crystals identified in the stratiform clouds contained more hexagonal plate crystals. Both riming and aggregation processes played central roles in the broadening of particle size distributions (PSDs), and these processes were more active in embedded convection regions than in stratiform regions. However, riming was more prevalent in the 18 April case than aggregation, though aggregates were evident. In contrast, the 1 May case had a more dominant aggregation processes, but also riming. With the decrease in height, PSDs broadened in both embedded convection regions and stratiform regions, but the broadening rates between 4.8 km (T ≈ −11.6°C) and 4.2 km (T ≈ −8°C) were larger than those between 4.2 km (T ≈ −8°C) and 3.6 km (T ≈ −5°C). In addition, the broadening rates of PSDs in the embedded convection regions were larger than those in the stratiform clouds, as the aggregation and riming processes of ice particles in embedded convection regions were active. High supercooled water content is critical to enhancing riming and aggregation processes in embedded convection regions.

Corresponding author address: Xueliang Guo, Chinese Academy of Meteorological Sciences, No.46, Zhongguancun Street, Haidian District, Beijing 100081, China. E-mail: guoxl@mail.iap.ac.cn

1. Introduction

Stratiform clouds with embedded convection constitute an important precipitation system that typically has a long lifetime and that may bring either continuous or intermittent precipitation to a large region (Hobbs and Locatelli 1978; Matejka et al. 1980; Hobbs et al. 1980; Herzegh and Hobbs 1981; Evans et al. 2005). Because of the presence of high ice crystal concentrations and supercooled water content in embedded convection regions (Evans et al. 2005; Hobbs and Rangno 1990; Matejka et al. 1980), these systems can improve the precipitation efficiency of stratiform clouds by up to 20%–35% (Herzegh and Hobbs 1980; Houze et al. 1981; Rutledge and Hobbs 1983). Although ice crystal habit is central to understanding the generation and growth processes of precipitation particles in these clouds, it is difficult to obtain accurate characteristics of crystals and their continuous growth processes.

At present, the airborne observation is an important method used to obtain information on ice crystal habits and growth mechanisms in natural clouds. In deep stratiform clouds, the formation and growth processes of ice particles are complex. Such processes include riming (Ono 1969), the freezing of large drizzle drops (Korolev et al. 2004), and the aggregation of ice crystals (Takahashi and Fukuta 1988). However, the growth processes of ice crystals in stratiform clouds with embedded convection are more complex.

A wide variety of particle habits were observed in both convection and stratiform regions of clouds at temperatures from 0°C to ~−20°C (Stith et al. 2002). McFarquhar and Black (2004) analyzed data recorded by Particle Measuring Systems (PMS) two-dimensional cloud (2DC) probe placed in tropical cyclones and found that the composition (i.e., graupel or snow), number, and size of ice particles can vary substantially in convection and stratiform regions. In stratiform regions, small ice crystals, columns, and medium-sized graupel were observed, while in convection regions, medium to large graupel, aggregates, and raindrops were identified. Stark et al. (2013) collected solid precipitation habits at ground during two winter storms in coastal, extratropical cyclones and showed that convective seeder cells resulted in relatively cold (<−15°C) ice crystal characteristics (side planes, bullets, and dendrites). Needlelike crystals were prevalent during the pre-band period when the maximum vertical motion occurred in the −5° to −10°C layer. Moderately rimed dendritic crystals were observed at snowband maturity.

Ice crystal habits heavily depend on the cloud temperatures at which the ice crystals form. Hogan et al. (2003) identified pristine planar crystals in aircraft observations of a multilayered altocumulus cloud with a cloud-top temperature (CTT) of −15°C and found that at a CTT of −24°C, crystals formed as complex polycrystals. Carey et al. (2008) sampled several altocumulus cloud layers at temperatures between −12° and −26°C and found that pristine planar crystals developed close to the top of the cloud. Westbrook and Heymsfield (2011) summarized previous studies and concluded that single pristine crystals are common in thin, mixed-phase clouds and that the critical temperature for polycrystal particles lies in the range of approximately −20° to −26°C, depending on the characteristics of the individual cloud.

The findings of laboratory (Fukuta and Takahashi 1999; Heymsfield et al. 2010; Magono and Iwabuchi 1972; Pruppacher and Klett 1997) and field (Evans et al. 2005; Heymsfield et al. 2002; Stith et al. 2002; Woods et al. 2008) studies indicate that ice crystals formed between 0° and −4°C are predominantly plates, while those formed between −4° and −8°C are predominantly needle columns. Bailey and Hallett (2009) produced a new ice crystal habit diagram based on field and laboratory datasets while retaining descriptions of ice crystal habits in temperatures above −18°C from older diagrams. The authors proposed that the ice crystal form changes from plate (from 0° to −4°C) to column (from −4° to −8°C) and then to platelike (plate or dendrite, from −8° to −22°C).

A wide variety of particle growth processes have also been observed in convection and stratiform regions of clouds. Analyzing PMS probe data in midlatitude cyclones, Herzegh and Hobbs (1980) found that riming growth was dominant in embedded convection regions where updrafts reached 60 cm s−1, while deposition and aggregation growth dominated at lower layers where updraft velocities were less than 15 cm s−1. Analyzing 2DC and two-dimensional precipitation (2DP) probe data from winter storms on the coast of Newfoundland, Canada, Lawson et al. (1993, 1998) showed that ice crystals grow rapidly through aggregation processes in embedded convection regions, and similar ice crystal growth characteristics were found in Arctic clouds during the summer (Lawson and Zuidema 2009). Studying 2DC and cloud imaging particle (CIP) probe data from a strong cyclonic storm over the northeastern Pacific Ocean, Evans et al. (2005) showed that above the melting layer, vapor deposition is the dominant growth process in the rainband. Ice particle growth via riming was found to be negligible, while significant ice particle aggregation occurred in the region just above the melting layer. Therefore, dominant ice crystal growth processes found in different cloud types were not consistent with previous studies.

The Beijing Cloud Experiment (BCE), a component of the National Science and Technology Pillar Program (2006–11) Key Project, was conducted from April to May 2009 in the Zhangjiakou area, which is located in the upstream area of Beijing in northern China. Throughout the experiment, three aircraft observed aerosol and cloud features simultaneously at different cloud levels, generating valuable data on aerosol–cloud interactions in this area (Lu and Guo 2012). Northern China is located in the middle latitude and it has a complex precipitating cloud system including both convective clouds (e.g., Fu and Guo 2012) and stratiform clouds with embedded convection. The cloud system in this region is also heavily affected by aerosols and urbanization processes owing to rapid industrialization and population growth (Guo et al. 2006, 2014).

Ice crystal habits and growth processes in two cases of stratiform clouds with embedded convection were investigated on 18 April 2009 and 1 May 2009 through simultaneous observation from three aircraft during the BCE. The aircraft tracks were layered with radar echoes to reveal ice crystal distribution and growth processes in different cloud regions (convection and stratiform) and at different temperatures and cloud levels. The broadening rate of vertical particle size distribution (PSD) in different cloud regions was quantitatively calculated using data collected through simultaneous observations from three aircraft at different levels of clouds.

A brief description of the instruments and field experiment are provided in section 2. Section 3 introduces the synoptic weather conditions and the properties of the cloud system observed. The ice crystal habits observed from the three aircraft are listed in section 4, and the distribution and growth processes of ice crystal habits are presented in section 5. Section 6 provides the calculations and discusses the broadening rate of PSD. A conclusion and discussion are provided in section 7.

2. Instruments and field experiment

Throughout the BCE field observations, simultaneous measurements were conducted through ground-based meteorological stations, radar, and aircraft observation. A schematic diagram of the study area and the observational facilities used for the BCE are shown in Fig. 1. The aircraft employed in this experiment included the Cheyenne III-A from the Shijiazhuang Weather Modification Office (number 3625), the Datong Y-12 from the Shanxi Weather Modification Office (number 3817), and the Beijing Y-12 from the Beijing Weather Modification Office (number 3830). The three aircraft departed from airports in Zhangjiakou, Shijiazhuang, and Taiyuan, respectively, and simultaneously collected data at different cloud levels within the study area.

Fig. 1.
Fig. 1.

Schematic of the observational region and facilities (the colors indicate the altitudes of terrain).

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

During the experiment, two cold-frontal systems passed through the study area on 18 April 2009 and 1 May 2009, respectively, and the dominant cloud type over the study area was stratiform clouds with embedded convection. The three aircraft simultaneously observed aerosol, cloud condensation nuclei (CCN), and cloud microphysics processes at different cloud levels and collected valuable data on cloud microphysical properties in the area. Table 1 provides a list of the instruments used aboard the three research aircraft. All instruments were calibrated by Droplet Measuring Technologies (DMT) in the United States before the experiment was conducted (Lu and Guo 2012).

Table 1.

Instruments and measurements collected aboard three BCE aircraft.

Table 1.

3. Synoptic weather conditions and properties of the cloud system observed

On 18 April 2009, a weak cold-frontal system passed through the study area and produced stratiform clouds with embedded convection regions over the study area. The total rainfall recorded at the Zhangjiakou meteorological station was 6.7 mm. The synoptic weather conditions at 0800 BT (Beijing time) on 18 April 2009 are shown in Fig. 2, and the black box shown in the image denotes the study area. Figure 2 shows that on 18 April 2009, a weak trough at 500 hPa approached the study area, and a high pressure center in the East China Sea blocked the movement of the weak trough to the southeastern direction. The wind at 500 hPa was dominated by southwesterly winds. Figure 2b shows infrared cloud images from satellite FY-2C at 1900 BT on 18 April 2009. The figure shows that the cloud band was dispersed throughout the northeast–southwestern region, and the main cloud band was located in the northern area of the Zhangjiakou region. Clouds with nonuniform top developed over the study area.

Fig. 2.
Fig. 2.

Synoptic weather background on 18 Apr 2009: (a) geopotential height (gpm) and wind fields at 500 hPa and 0800 BT, (b) FY-2C infrared cloud image at 1900 BT. The black square indicates the study area.

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

The radar composite reflectivity shown in Fig. 3 demonstrates that the strong echo band was located along the front boundary of the cloud system, which was produced through low-level airflow convergence. A stratiform cloud with a weak echo formed behind the convection region. The entire cloud band was positioned in the northeast–southwestern area and moved from northwest to southeast. Some obvious strong echo regions were embedded in the cloud band, and the maximum radar reflectivity of the embedded convection region was approximately 50 dBZ, which was approximately 10–20 dBZ higher than reflectivities recorded in surrounding stratiform clouds. Both aircraft 3817 and 3830 recorded observations from the front edge of the cloud system, and the flight route of aircraft 3625 followed a north–south loop through the cloud band.

Fig. 3.
Fig. 3.

Superimposed image of radar reflectivity (shading) at 1900 BT with flight tracks recorded on 18 Apr 2009 (green line represents 3817 path, red line represents 3830 path, blue line represents 3625 path).

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

On 1 May 2009, a stronger frontal system passed through the study area and generated 6–10 mm of total rainfall in the study area. The synoptic weather conditions at 0800 BT on 1 May 2009 are shown in Fig. 4. The figure shows that on 1 May 2009, the 500-hPa level over the study area was controlled by a trough (Fig. 4a). Comparing these conditions with those of 18 April 2009, the trough was more obvious and was not blocked by the high pressure center located in the East China Sea. Winds at 500 hPa were dominated by southwesterly winds. Figure 4b provides a visible cloud image taken from satellite FY-2C at 0900 BT on 1 May 2009 and shows that the main cloud system, moving in a northwest to southeast direction, had already moved out of the study area. The cloud observed from the aircraft was mainly located in the postfrontal region.

Fig. 4.
Fig. 4.

Synoptic weather background at 0800 BT on 1 May 2009: (a) geopotential height (gpm) and wind fields of 500 hPa, The black square indicates the study area. (b) FY-2C visible cloud image at 0900 BT. The red square indicates the study area, the pink line indicates the land boundary of China, and the blue line represents the Yellow River.

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

Figure 5 provides a superimposed image of radar reflectivity at 0930 BT with flight tracks on 1 May 2009. The figure shows that the entire cloud band assumed a northeast–southwest formation and moved from northwest to southeast. Similar to the case on 18 April 2009, some obvious strong echo regions were embedded in the cloud band, and the highest radar reflectivity in the embedded convection region was approximately 40 dBZ, which is approximately 5–10 dBZ higher than the reflectivities in the surrounding stratiform cloud. The flight route of aircraft 3625 was not recorded because the GPS system in the plane malfunctioned during the experiment.

Fig. 5.
Fig. 5.

Superimposed image of radar reflectivity (shading) at 0930 BT with flight tracks on 1 May 2009 (green line represents 3817 path, red line represents 3830 path).

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

4. Ice crystal habits observed from three aircraft

Ice crystal habits were observed using the DMT Cloud Imaging Probe (CIP) and PMS 2DC, and larger ice particles were observed via the DMT Precipitation Imaging Probe (PIP) and PMS 2DP. CIP and PIP are basically modified 2DC and 2DP, respectively (Field et al. 2006). A detailed description of the probe features can be found in Table 1.

Typical ice crystal habits identified on 18 April 2009 are shown in Fig. 6. The ice crystal images were classified into the stratiform clouds and embedded convection regions according to radar reflectivity and LWC in order to identify differences of ice crystal habits between two regions.

Fig. 6.
Fig. 6.

Typical ice crystal images recorded by the three aircraft on 18 Apr 2009 (ice crystal habits at 5.1 and 4.8 km were recorded by 2DC aboard 3625, those at 4.2 km were recorded by CIP aboard 3817, those at 3.6 km were recorded by CIP aboard 3830).

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

The platelike ice habits in this study included both plate (Bailey and Hallett 2009; Kajikawa and Heymsfield 1989; Woods et al. 2005) and small spheroid (Connolly et al. 2005). The identification of riming and aggregation levels of ice crystals may roughly depend on their size and density. The ice particles with large size and high density are generally identified as heavily rimed ice crystals, and those with large size and low density (with clear structure) are classified as aggregate.

Ice crystal habits identified by three aircraft between −2° and −9.5°C were predominantly platelike, needle column, dendrite, and irregular. But needles and plates could be identified at all temperatures and the only actual difference is that of size. It can be seen from Fig. 6 that most of the ice crystals identified in the stratiform clouds were platelike and needle column and some dendritic crystals could be also identified. However, in the embedded convection regions, in addition to platelike and needle-column crystals, more dendritic crystals existed between −6° and −9.5°C and more aggregate existed between −4° and −5°C. This phenomenon suggests that the aggregation process in the embedded convection regions was active. In addition, it also can be seen from Fig. 6 that the ice crystal images recorded in embedded convection regions had higher density than those recorded in the stratiform regions, suggesting that riming were also evident and larger ice particles such as graupel might occur.

Ice crystal habits observed in this case are generally consistent with previous laboratory research (Fukuta and Takahashi 1999; Heymsfield et al. 2010; Magono and Iwabuchi 1972; Pruppacher and Klett 1997) and field observations (Evans et al. 2005; Heymsfield et al. 2002; Stith et al. 2002; Woods et al. 2008) as well as with the new ice crystal habit diagram developed by Bailey and Hallett (2009).

Another case of stratiform clouds with embedded convection was identified in the study area on 1 May 2009 (Fig. 4). Figure 7 shows the typical ice crystal habits recorded by three aircraft on 1 May 2009. The temperature ranges identified in the cloud from aircraft 3625, 3817, and 3830 were from −11° to –16°, from −8° to –11.6°, and from 0° to –11°C, respectively.

Fig. 7.
Fig. 7.

Typical ice crystal images recorded by the three aircraft on 1 May 2009: (a) images recorded by 2DC aboard 3625, (b) images recorded by CIP aboard 3817, (c) images recorded by CIP aboard 3830.

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

The ice crystal images recorded by aircraft 3625 at temperatures between −11° and −16°C in Fig. 7a indicate that the ice crystal habits were predominantly needle column, platelike, capped column, dendrite, and irregular. The ice crystal images recorded by aircraft 3817 at temperatures between −8° and −11.6°C shown in Fig. 7b demonstrate that ice habits in these conditions were predominantly platelike, needle column, capped column, and dendrite. Ice crystal images recorded by aircraft 3830 at temperatures between 0° and −11°C indicate that the majority of ice crystals were platelike, dendrite, and irregular. Heavily rimmed ice crystals were common at these layers.

Differences of ice crystal habits recorded between the stratiform clouds and embedded convection regions were basically similar to those recorded on 18 April 2009. However, there were still some differences between two cases. The 18 April case had more heavily rimed ice particles and 1 May case had more aggregate. This will be discussed in detail in the following sections. The ice crystal habits recorded in the embedded convection regions contained more dendritic crystals and possessed a heavier riming degree. In addition, the ice crystals recorded in the stratiform clouds contained more hexagonal plate crystals (shown in Figs. 7a,b).

In summary, the ice crystal habits sampled in the two cases with stratiform clouds containing embedded convection were complex; the ice crystal habits between 0° and −16°C were predominantly platelike, needle column, capped column, dendrite, and irregular. This indicates that although temperature may have an effect on crystal habit, it may not be the primary factor.

It can be concluded from this comparison of ice crystal habits in different clouds that a mixture of several ice crystal habits can always be identified. However, the ice crystal habits recorded in the embedded convection regions contained more dendritic crystals and heavier riming degree, and the ice crystals recorded in the stratiform clouds contained more hexagonal plate crystals. Previous studies suggest that the characteristics of ice crystal habits in different stratiform clouds with embedded convection are different because of different cloud conditions (e.g., temperatures, cloud layers, cloud tops, LWC, etc.) (McFarquhar and Black 2004; Evans et al. 2005; Stark et al. 2013). The deep clouds with high cloud top and low temperature could produce more complex ice crystal habits such as bullet, dendrite, and aggregate.

5. Distribution and growth processes of ice crystal habits

a. The 18 April 2009 case

Table 2 lists observation data recorded from the aircraft on 18 April 2009. The three aircraft flew at different cloud levels, and the maximum LWC recorded from aircraft 3817 and 3830 both exceeded 1 g m−3, suggesting high LWC in the clouds. To investigate the distribution and growth processes of ice crystal habits in the clouds, a detailed description of the procedure through which the three aircraft penetrated the clouds is discussed in this section.

Table 2.

Observed results from aircraft taken on 18 Apr 2009.

Table 2.

A cross section of radar reflectivity in the flight path and relevant observed data recorded from aircraft 3625 on 18 April 2009 are shown in Fig. 8. Aircraft 3625 primarily flew horizontally at 4.8 km (T ≈ −6°C) in the early stage and then at 5.1 km (T ≈ −9.5°C) in the later stage, penetrating several embedded convection regions. Pristine ice crystal habits identified from the aircraft were primarily platelike, needle column, and dendrite between −6° and −9.5°C. The cloud particle spectra derived from 2DC and 2DP shown in Fig. 8b and Fig. 8c indicate that high concentrations of large ice particles were present in the embedded convection region. The formation and growth of these large particles are difficult to document, but the properties of the ice habits sampled from aircraft 3625 demonstrate that both riming and aggregation processes were obvious in the embedded convection regions. Figure 8d shows that LWC was not uniformly distributed in the clouds and high LWC corresponded well with embedded convection regions. The ice crystal riming degree was heavy in the embedded convection region with high LWC, such as that between 1715 and 1728 BT shown in Fig. 8a, suggesting that riming growth was an important growth process in the embedded convection regions. At the same time, it can be seen from the 2DP images show in Fig. 8a (such as 1727 BT) that the apparent aggregates appeared between −6° and −9.5°C, suggesting that aggregation growth was also an important growth process within the embedded convection region. But it can be seen from the ice particle images shown in Fig. 8a that riming was more prevalent in this case than aggregation, though aggregates were evident. In contrast, the 1 May case discussed later had a more dominant aggregation processes, but also riming.

Fig. 8.
Fig. 8.

The penetration data recorded by aircraft 3625 on 18 Apr 2009: (a) cross-section of radar reflectivity for flight path, ice crystal images recorded with 2DC (panel top) and 2DP (panel bottom), and T (black line); (b) 2DC instantaneous spectrum; (c) 2DP instantaneous spectrum; and (d) LWC.

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

Aircraft 3817 penetrated the clouds at lower layers in −4° to approximately −5°C conditions on 18 April 2009. As the hotwire-LWC equipment used on aircraft 3817 was in poor condition, no LWC data are available. Figure 9 shows the radar reflectivity cross section in the flight path and relevant spectra of cloud particles sampled through CIP and PIP. Figure 9a shows that aircraft 3817 penetrated both weak and strong embedded convection regions as well as the stratiform region. The pristine ice crystals from −4° to approximately −5°C were predominantly needle column; however, a wide variety of ice crystal habits, such as heavily rimed dendrite and ice aggregate, were also sampled at these layers. From 1743 to 1745 BT, a weak embedded convection region with a thickness of less than 2 km and a cloud top at less than 5 km were sampled. The corresponding CTT was warmer than −8°C and the 0°–8°C layers are suitable for plate and needle-column crystal formation (Takahashi et al. 1991, 1986). In addition, most of the cloud particles were single crystals similar to those recorded by the PIP probe at 1744 BT, suggesting that the ice crystal aggregation process was weak and that the growth process should be dominated by deposition growth.

Fig. 9.
Fig. 9.

The penetration data recorded by aircraft 3817 on 18 Apr 2009: (a) cross section of radar reflectivity for flight path, ice crystal images recorded through CIP (panel top)and PIP (panel bottom), and T (black line); (b) CIP instantaneous spectrum; and (c) PIP instantaneous spectrum.

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

At 1748 BT, when the CTT was colder than −13°C, aircraft 3817 sampled lightly rimed dendritic crystals in the cloud. Laboratory studies (Takahashi et al. 1991, 1986) and field (Bailey and Hallett 2009; Heymsfield and Kajikawa 1987) have shown that the optimal temperature for dendritic crystal formation is approximately −15°C. However, at 4.2 km, the temperature recorded from aircraft 3817 was approximately −5°C. Therefore, lightly rimed dendritic crystals likely fell from higher, colder cloud layers.

From 1748 to 1750 BT, the heavily rimed and aggregated crystals (such as the images recorded via CIP probing at 1749 and 1750 BT) were sampled in the strong embedded convection region, suggesting that both riming and aggregation processes were important in the strong embedded convection region.

Since the maximum reflectivity of this cloud system reached about 50 dBZ and with the cloud base temperature of 12°C, cloud-top temperature of −28°C (estimated from radar data in Fig. 9), and LWC of 1 g m−3, it is possible for occurrence of graupel in the lower regions of the embedded convection. Graupel formation has been found in several other studies with embedded convection.

Figure 10 shows the penetration data recorded by aircraft 3830 on 18 April 2009. From 1800 to 1805 BT, ice crystal habits recorded at a layer of 3.6 km (T ≈ −2°C) was dominated by rimed and aggregated plates and needle columns in the embedded convection region (Fig. 10a). Previous laboratorial studies (Pruppacher and Klett 1997; Bailey and Hallett 2009) have shown that temperatures at this layer are suitable for plate formation. Thus, the aggregated plates recorded at this layer may have formed locally because of the longevity of the stratiform region. In addition, Fig. 10a shows that crystals identified at 1803 BT included some pristine columns. This is an interesting finding given that most previous studies (Bailey and Hallett 2009; Crosier et al. 2011) show that this crystal habit typically forms at temperatures lower than −2°C. Ice formation at this temperature is very rare. A more plausible explanation could be a secondary ice formation event driven by Hallett–Mossop processes.

Fig. 10.
Fig. 10.

The cloud penetration data recorded by aircraft 3830 on 18 Apr 2009: (a) cross section of radar reflectivity for flight path and ice crystal habits recorded through CIP, where black solid line represents flight track, red dashed line represents the 0°C layer; (b) CIP instantaneous spectrum; and (c) PIP instantaneous spectrum, (d) temperature (black) and LWC (blue).

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

Both aggregation and riming processes became active when the aircraft passed from the shallow stratiform region (1801 BT) to the relatively deep embedded convection region (1804 BT) with an increase in supercooled cloud water (Fig. 10d). The corresponding spectra recorded by the CIP and PIP probes also broadened rapidly (Figs. 10b,c). From 1805 to 1811 BT, the aircraft flew downward and passed through the melting layer as it descended. A small number of melting ice crystals were recorded by the CIP probe while the aircraft passed through the melting layer. With the melting of ice particles, the cloud particle spectra gradually narrowed because of changes in particle diameters in the transition from snow to raindrop form. From 1811 to 1820 BT, the aircraft flew at 2.7 km (T ≈ 5°C), and particles at this layer were dominated by cloud and rain drops. From 1813 to 1815 BT, PSD broadened again owing likely to larger particles aloft melting into larger raindrops with the increase of cloud top. Upon traveling from 1815 to 1817 BT, the aircraft passed through a region of strong radar echo (>30 dBZ). However, at this moment, the CIP and PIP probes malfunctioned, and thus, the PSDs recorded in this region were incorrect, and the particle images recorded at 1816 BT were dominated by larger raindrops.

b. The 1 May 2009 case

Table 3 lists observations from the three aircraft recorded on 1 May 2009. The three aircraft flew at different cloud levels, and the maximum LWC identified in this case were lower than those recorded on 18 April 2009, suggesting that the ice crystal riming process was relatively weaker than the process recorded on 18 April 2009. Therefore, pristine ice crystal habits recorded on 1 May 2009 were much clearer. The flight route of aircraft 3625 was not recorded because the GPS system malfunctioned during the experiment. Therefore, only the cloud penetration data recorded by aircraft 3817 and 3830 will be discussed in this section.

Table 3.

Observed results from aircraft taken on 1 May 2009.

Table 3.

Figure 11 shows the cloud penetration data recorded by aircraft 3817 for 1 May 2009. It indicates that aircraft 3817 penetrated both stratiform and embedded convection regions with temperatures ranging from −7° to −11.6°C throughout the flight. The CIP images show that the ice crystal habits sampled in the stratiform cloud region were predominantly plate and needle column (Fig. 11a, 0925 BT, 1017 BT), and those observed in the embedded convection regions were predominantly dendrites (Fig. 11a, 0937 BT, 1026 BT). The ice crystal images sampled by CIP and PIP show that both riming and aggregation processes existed in the embedded convection regions, but riming was not as strong a growth process as aggregation.

Fig. 11.
Fig. 11.

As in Fig. 9, but on 1 May 2009 and with (d) Hotwire-LWC liquid water content.

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

The particle images recorded by PIP probe (Fig. 11a) show that aggregates were very common in the embedded convection region over a height range of 4.2 km (T ≈ −8°C) to 4.9 km (T ≈ −12°C), suggesting that aggregation was also a primary growth process in the embedded convection regions. At the same time, the regions of stronger reflectivity (>20 dBZ) contained larger aggregates. For example, the aggregates recorded at 0937, 0942, and 1005 BT were identifiably larger than those recorded at 0926, 0948, and 1017 BT, indicating that the aggregation process was more active in the embedded convection region than in the stratiform regions.

Riming growth at this layer also showed a nonuniform distribution. The riming degrees of the ice crystals identified at 0937 and 1000 BT were clearly heavier than those recorded at 0934 BT. The LWC detected at 0937 BT were higher than 0.1 g m−3, but the LWC at 0934 BT were lower than 0.02 g m−3, suggesting that the nonuniform riming growth distributions found in the clouds were caused by variations of LWC in the stratiform and embedded convection clouds. These results echo those of the 18 April 2009 case. After 0957 BT was reached, the Hotwire-LWC malfunctioned, so there are no available data on cloud LWC. However, the particle images show that riming growth was still nonuniform, which is consistent with the results of Stark et al. (2013), who found that the riming degree of ice crystals is influenced by cloud updrafts in extratropical cyclones. The authors also noted that heavy riming in high updraft regions was caused by high supercooled LWC in these areas.

In embedded convection regions (such as at 0938 BT), both riming and aggregation processes were active (shown in Fig. 11a), and the PSDs recorded by PIP probe broadened in the embedded convection regions (Fig. 11c). Although both riming and aggregation processes contributed to the PSD broadening, the particle images recorded by PIP probes show that the aggregation process was more critical to the broadening of PSDs at this layer. Therefore, comparing with 18 April case, the 1 May case had a more dominant aggregation processes but also riming.

The ice crystal habits at this layer were also strongly affected by CTT, which varied from −8°C at the lowest cloud top of approximately 4.5 km (such as 0925 BT, 1017 BT) to approximately −27.3°C at the highest cloud top of approximately 7 km (such as 0935 BT, 1008 BT). In addition to plate, needle-column, and dendritic crystals, capped-column crystals also appeared as the CTT was colder than −18°C (Fig. 11a, 0934 BT). The initial columns likely formed over a temperature range of −18° to −25°C, and then capped columns formed as these initial columns fell through the planar-crystal growth region where temperatures ranged from −12° to −18°C (Heymsfield et al. 2002).

The particle images recorded by PIP probe also show that the large aggregates typically consisted of dendritic crystals (Fig. 11a). Higher cloud tops (>6 km) with lower temperatures (−20°–27°C) are suitable for generating dendritic and capped-column crystals. Dendritic aggregate more easily than other crystal habits and can easily capture supercooled cloud droplets (Hashino and Tripoli 2011; Hobbs et al. 1974). In addition, Fig. 11b shows that the embedded convection regions with higher cloud tops (such as 0933 BT, 1009 BT) often contained more ice crystals than other regions, and high concentrations of ice crystals can facilitate the aggregation process (Cooper and Lawson 1984; Dye et al. 1976; Holroyd and Jiusto 1971). Because the embedded convection regions with higher cloud tops contained more ice crystals, especially dendritic crystals, PSD broadening should be apparent and precipitation efficiencies should also be higher. Hobbs et al. (1980) found that precipitation efficiency in wide, cold-frontal rainbands with generating cells reaches levels of at least 80%, while levels in warm-sector and narrow, cold-frontal rainbands reach only 40%–50% and 30%–50%, respectively.

Figure 12 shows the cloud penetration data recorded by aircraft 3830 on 1 May 2009. Although the crystal habits recorded by CIP probe varied greatly as the flight altitude of aircraft 3830 changed considerably, ice crystals in this region were dominated by heavily rimed dendritic and irregular particles. More dendritic crystals were found in embedded convection regions (such as 0938 BT, 0958 BT), which is consistent with the observed results of aircraft 3817.

Fig. 12.
Fig. 12.

As in Fig. 11, but by aircraft 3830.

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

The particle images recorded via CIP probing show that most of the ice crystals were heavily rimed, indicating that riming is an important growth process for ice crystals in clouds in the range of 2.3 km (T ≈ 0°C) to 3.6 km (T ≈ −5°C). Compared to the results observed from aircraft 3817, the riming process was more active in the lower cloud layer because of the high supercooled LWC in this layer. Figure 12d, which shows the LWC distribution detected from aircraft 3830, demonstrates that the maximum LWC in the clouds was more than 1 g m−3, and most of regions observed from aircraft 3830 exhibited higher LWC values than the upper layer observed from aircraft 3817. The particle images recorded by PIP probe (Fig. 12a) suggest that the aggregation was an important growth process in clouds in the range of 2.3 km (T ≈ 0°C) to 4.9 km (T ≈ −12°C) and in stronger embedded convection regions especially.

In summary, the distribution and growth processes of ice crystal habits in two cases of stratiform clouds with embedded convection show that the distribution of ice crystal habits varied greatly between stratiform cloud regions and embedded convection regions. Higher concentrations of large ice particles, such as dendritic crystals, and higher supercooled water content were found in the embedded convection regions. Both riming and aggregation processes were more active in the embedded convection regions than in the stratiform clouds.

6. The broadening rate of PSDs

To investigate how the PSDs changed with vertical distance, the three aircraft flew horizontally at different cloud levels while maintaining constant longitude and latitude locations (beginning at 0930 BT, 40.807°N, 114.982°E, and ending at 0940 BT, 40.984°N, 115.292°E). The flight levels of aircraft 3625, 3817, and 3830 were 4.8 km (T ≈ −11.6°C), 4.2 km (T ≈ −8°C), and 3.6 km (T ≈ −5°C), respectively. Although GPS data were lost, the flight route of aircraft 3625 was recovered for the first 10 min from the pilot’s flight log.

Figure 13a shows the flight routes of the three aircraft superimposed with radar echo distributions from 0930 to 0940 BT. Figure 13a shows that, from 0935 to 0940 BT, the three aircraft simultaneously passed through an embedded, strong echo region. In this region, the LWC detected from the three aircraft reached its peak value (Figs. 13b,c,d), and the LWC detected from aircraft 3830 was higher than 1 g m−3.

Fig. 13.
Fig. 13.

(a) The flight tracks of three aircraft 3625 (red line), 3817 (black line), and 3830 (green line) superimposed with the cross section of radar reflectivity on 1 May 2009; (b) RH (green line, %), LWC (black line, g m−3), and temperature (blue line, °C) recorded from aircraft 3625; (c) as in (b), but from aircraft 3630; (d) as in (b), but from 3630 aircraft; (e) comparison of particle mean volume diameter; (f), average spectrum between 0935 and 0940 BT in convection region; and (g) as in (f), but between 0930 and 0935 BT in stratiform region.

Citation: Journal of the Atmospheric Sciences 72, 5; 10.1175/JAS-D-14-0194.1

The averaged spectra between 0935 and 0940 BT at convection areas are shown in Fig. 13f. In the embedded convection region, the Marshall–Palmer (MP) fit of the PSDs shows that the PSDs broadened from 4.8 km (T ≈ −11.6°C) to 3.6 km (T ≈ −5°C) but most noticeably from 4.8 km (T ≈ −11.6°C) to 4.2 km (T ≈ −8°C). Figure 13e provides a comparison of particle mean volume diameters. Figure 13e shows that the particle mean volume diameters at 4.8 km (T ≈ −11.6°C), 4.2 km (T ≈ −8°C), and 3.6 km (T ≈ −5°C) were 1.2 mm (Dmv4.8 = 1.2 mm), 3 mm (Dmv4.2 = 3 mm), and 3.2 mm (Dmv3.6 = 3.2 mm), respectively. The PSD broadening rate from 4.8 km (T ≈ −11.6°C) to 4.2 km (T ≈ −8°C) can be calculated as follows:
e1
From 4.2 km (T ≈ −8°C) to 3.6 km (T ≈ −5°C), the PSD broadening rate is
e2
The three aircraft passed through stratiform clouds from 0930 to 0935 BT (Fig. 13a), and the averaged spectra between 0930 and 0935 BT at stratiform region are shown in Fig. 13g. In the stratiform region, PSDs broadened from 4.8 km (T ≈ −11.6°C) to 3.6 km (T ≈ −5°C), but most noticeably from 4.8 km (T ≈ −11.6°C) to 4.2 km (T ≈ −8°C). Figure 13e shows that the particle mean volume diameters at 4.8 km (T ≈ −11.6°C), 4.2 km (T ≈ −8°C), and 3.6 km (T ≈ −5°C) were 1.3 mm (Dmv4.8 = 1.3 mm), 2.8 mm (Dmv4.2 = 2.8 mm), and 2.9 mm (Dmv3.6 = 2.9 mm), respectively. The PSD broadening rate from 4.8 km (T ≈ −11.6°C) to 4.2 km (T ≈ −8°C) can be calculated as follows:
e3
From 4.2 km (T ≈ −8°C) to 3.6 km (T ≈ −5°C), the PSD broadening rate is
e4

The PSD broadening processes in the convection and stratiform regions demonstrate that with the decrease of height, the PSDs broadened in both regions. However, the broadening rates between 4.8 km (T ≈ −11.6°C) and 4.2 km (T ≈ −8°C) were larger than those between 4.2 km (T ≈ −8°C) and 3.6 km (T ≈ −5°C). This phenomenon may be because the temperature range between 4.8 km (T ≈ −11.6°C) and 4.2 km (T ≈ −8°C) was more suitable for dendritic crystal formation. Furthermore, once aggregates are formed, riming is more effective as well (Bailey and Hallett 2009; Hashino and Tripoli 2011).

The PSD broadening rate in the convection region was larger than that in the stratiform region. At the same time, Figs. 13f and 13g show that concentrations of ice particles of sizes less than 1.2 mm (convection region) and 1 mm (stratiform region) decreased with the decrease in height owing to collision and collection forces on larger ice particles. Heymsfield et al. (2002) showed from TRMM field campaigns that in deep tropical cirrus and stratiform precipitating cloud, PSDs broaden from the cloud top toward the cloud base. The broadening rate was found to be 1–3 mm km−1, and concentrations of particles less than 1 mm in size decreased with decreasing height. Our case showed similar characteristics and results on the vertical evolution of PSDs. Particle images recorded by PIP probe show that the aggregation process was the main cause of the broadening phenomenon (Fig. 13a).

The PSD broadening rate in the embedded convection region was found to be higher in our study. Lawson and Zuidema (2009) analyzed 2DP data from stratiform clouds with embedded convection regions at high latitudes and found that in embedded convection regions (CTT ≈ −27°C), the maximum diameter of ice particles at 4 km (T ≈ −13°C) was 15 mm, and at 2.2 km (T ≈ 0°C), the value was approximately 30 mm. The PSD broadening rate was also approximately 6.25 mm km−1. While the broadening rate was higher in that study, their results were similar to the results of this study, suggesting that PSD broadening rates are higher in embedded convection regions.

PSD broadening rates were higher in the embedded convection than in the stratiform clouds. The reason is too complicated to clearly clarify by the observations of this study. Studies have shown that riming does enhance aggregation (Hallgren and Hosler 1960), but when high concentrations of dendritic crystals exist, aggregation occurs rapidly (Hashino and Tripoli 2011; Hobbs et al. 1974), creating larger particles favored for riming growth. The observations in this paper do not consistently support one over the other. Some of the images show large aggregates with little riming while others show heavily rimed particles. The importance of one process over another probably varies, but both are active and lead to PSD broadening. Therefore, in the higher LWC regions of embedded convection, both particle growth processes are important and feedback on one another.

7. Conclusions and discussion

In this study, we investigated ice crystal habits, distribution, and growth processes in two cases of stratiform clouds with embedded convection on 18 April 2009 and 1 May 2009 in northern China. Three aircraft were employed to observe clouds simultaneously at different levels of the clouds while maintaining a constant longitude and latitude position. This flight pattern facilitated an investigation of PSD change with height in the clouds. This combination of airborne observations and radar echo distributions allows us to conduct an accurate evaluation of ice particle distributions and main growth processes in stratiform clouds with embedded convection.

The 18 April 2009 case observed prefrontal clouds, and the convection embedded in stratiform clouds had a stronger radar echo with a maximum reflectivity of approximately 50 dBZ, which was stronger than reflectivities in the surrounding stratiform clouds of approximately 10–20 dBZ. The 1 May 2009 case observed postfrontal clouds with a maximum reflectivity of approximately 40 dBZ in the embedded convection region, which was stronger than reflectivities in the surrounding stratiform clouds of approximately 5–10 dBZ. Two cases studied here comprised typical stratiform clouds with embedded convection.

The ice crystal habits sampled in the two cases of stratiform clouds with embedded convection were complex, and the ice crystal habits present between 0° and −16°C were predominantly platelike, needle column, capped column, dendrite, and irregular. Aggregates and heavily rimed ice crystals were common in the clouds.

The ice crystal habits recorded from the aircraft were strongly affected by CTT and cloud type. First, the plate and needle-column crystals were predominant in clouds with CTT warmer than −8°C, and dendritic and capped-column crystals were found only in CTT conditions colder than −13° and −18°C, respectively. Second, comparisons of ice crystal habits in different clouds show that a mixture of several ice crystal habits can always be found. However, ice crystal habits identified in embedded convection regions contained more dendrites and possessed a heavier riming degree, and the ice crystals recorded in stratiform clouds contained more hexagonal plate crystals.

Our case showed similar results to those of Heymsfield et al. (2002), who identified ice crystal habits recorded with a cloud particle imager (CPI) probe in deep tropical cirrus and stratiform precipitating clouds, and concluded that small, pristine particles are observable in clouds formed by more transient convective clouds, with CTT controlling the habits. Most of larger particles were aggregates and rimed ice particles that were sampled near deep convective clouds with sustained convection.

The dominant growth process is different for different clouds. In shallow stratiform clouds, the growth process is dominated by deposition growth, but in deep stratiform clouds or embedded convection, both riming and aggregation are important. The two cases in this study show that riming was more prevalent in 18 April case than aggregation, though aggregates were evident. In contrast, the 1 May case had a more dominant aggregation processes but also riming. These results are generally in agreement with previous studies conducted in other regions, such as Washington, United States (Herzegh and Hobbs 1980); Newfoundland, Canada (Lawson et al. 1998); Kwajalein, United States (Heymsfield et al. 2002); and the Arctic (Lawson and Zuidema 2009; Lawson et al. 1998, 1993).

In addition, with changes of height in deep stratiform clouds or embedded convection, ice crystal growth processes were different. Both aggregation and riming processes constituted main growth processes between 0° and −11.6°C, but riming processes were more active in lower cloud layers because of high supercooled LWC in the clouds. Hou et al. (2010) analyzed 2DC and 2DP data of a stratiform cloud with embedded convection regions in northeastern China and found that ice particles primarily form and grow at layers between 4.0 and 5.5 km (from −1° to –8°C), with depositional growth dominating this process in addition to aggregation. In contrast, our study shows that riming was also an important growth process in embedded clouds owing to high supercooled LWC in the clouds.

The PSD broadening processes in convection and stratiform regions show that with a decrease in height, PSD broadened in both regions, but the broadening rates between 4.8 km (T ≈ −11.6°C) and 4.2 km (T ≈ −8°C) were larger than those between 4.2 km (T ≈ −8°C) and 3.6 km (T ≈ −5°C). In addition, the PSD broadening rates in embedded convection regions were larger than those in stratiform clouds, as the aggregation and riming processes of ice particles in embedded convection regions were active. The existence and extent of supercooled liquid water is critical to enhancing riming and aggregation processes in embedded convection region.

It should be noted that the quality of the microphysical data due to the relatively low resolution of airborne instruments and the limited data collections in this study lead to many difficulties and uncertainties for analyzing classification of particle habits and their growth processes. In many cases, we cannot separate plates from irregular shapes unless the images appear to be larger size and identify riming and aggregation processes that come first. On the one hand, riming comes first and then enhances aggregation. On the other hand, when high concentrations of dendritic crystals exist, aggregation occurs rapidly, creating larger particles favored for riming growth. The observations in this paper do not consistently support one over the other. Some of the images show large aggregates with little riming, and others show heavily rimed particles. The importance of one process over another probably varies, but both are active and lead to PSD broadening. Therefore, in the higher LWC regions of embedded convection, both particle growth processes are important and feedback on one another.

The ice particle shattering problems induced by the older-technology cloud probes (Field et al. 2006; Korolev et al. 2013) used in this study are not considered in this paper, which was likely taking place during observations in these clouds and influencing the data quality. All these uncertainties need to be further clarified in future studies.

Acknowledgments

The authors are grateful for critical and constructive comments from three anonymous reviewers and editor, which greatly improved the quality of this paper. This research was supported by the Key Project of National Science and Technology Pillar Program (Grant 2006BAC12B03), the Research and Development Special Fund for Public Welfare Industry (Meteorology) (Grants GYHY200806001 and GYHY201306040), the Third Tibetan Plateau Scientific Experiment: Observations for Boundary Layer and Troposphere (GYHY201406001), the Key Project of Basic Scientific Research and Operation fund of Chinese Academy of Meteorological Sciences (Grant 2013Z009), and the Open Funding from Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration (KDW1402).

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