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Deep Convective Clouds over the Northern Pacific and Their Relationship with Oceanic Cyclones


doi: 10.1007/s00376-014-4056-9

  • Based on combined CloudSat/CALIPSO detections, the seasonal occurrence of deep convective clouds (DCCs) over the midlatitude North Pacific (NP) and cyclonic activity in winter were compared. In winter, DCCs are more frequent over the central NP, from approximately 30°N to 45°N, than over other regions. The high frequencies are roughly equal to those occurring in this region in summer. Most of these DCCs have cloud tops above a 12 km altitude, and the highest top is approximately 15 km. These wintertime marine DCCs commonly occur during surface circulation conditions of low pressure, high temperature, strong meridional wind, and high relative humidity. Further, the maximum probability of DCCs, according to the high correlation coefficient, was found in the region 10°-20° east and 5°-10° south of the center of the cyclones. The potential relationship between DCCs and cyclones regarding their relative locations and circulation conditions was also identified by a case study. Deep clouds were generated in the warm conveyor belt by strong updrafts from baroclinic flows. The updrafts intensified when latent heat was released during the adjustment of the cyclone circulation current. This indicates that the dynamics of cyclones are the primary energy source for DCCs over the NP in winter.
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Manuscript received: 29 April 2014
Manuscript revised: 07 October 2014
通讯作者: 陈斌, bchen63@163.com
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Deep Convective Clouds over the Northern Pacific and Their Relationship with Oceanic Cyclones

  • 1. University of Science and Technology of China, Hefei 230026
  • 2. Anhui Provincial Academy of Environmental Sciences, Hefei 230022
  • 3. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081

Abstract: Based on combined CloudSat/CALIPSO detections, the seasonal occurrence of deep convective clouds (DCCs) over the midlatitude North Pacific (NP) and cyclonic activity in winter were compared. In winter, DCCs are more frequent over the central NP, from approximately 30°N to 45°N, than over other regions. The high frequencies are roughly equal to those occurring in this region in summer. Most of these DCCs have cloud tops above a 12 km altitude, and the highest top is approximately 15 km. These wintertime marine DCCs commonly occur during surface circulation conditions of low pressure, high temperature, strong meridional wind, and high relative humidity. Further, the maximum probability of DCCs, according to the high correlation coefficient, was found in the region 10°-20° east and 5°-10° south of the center of the cyclones. The potential relationship between DCCs and cyclones regarding their relative locations and circulation conditions was also identified by a case study. Deep clouds were generated in the warm conveyor belt by strong updrafts from baroclinic flows. The updrafts intensified when latent heat was released during the adjustment of the cyclone circulation current. This indicates that the dynamics of cyclones are the primary energy source for DCCs over the NP in winter.

1. Introduction
  • Deep convective clouds (DCCs) are common in low latitudes during summer (Mapes and Houze, 1993; Liu et al., 1995; Hall and Vonder Haar, 1999; Gettelman et al., 2002). With the development of satellite technology, particularly for the remote sensing of clouds and precipitation, great progress has been made in the study of DCCs (Alcala and Dessler, 2002; Luo et al., 2008; Luo et al., 2011). DCCs can extend from near the surface to a high level above the tropopause. Because of their large depths and high tops, DCCs play an important role in the Earth's climate system via their influence on precipitation and radiation (Stephens, 2005; Stephens et al., 2008; Feng et al., 2011; Li et al., 2013).

    Approximately 1.3% of tropical convective systems reach 14 km, and 0.1% of these systems penetrate the tropical tropopause. Overshooting convection is more frequent over land than over the ocean, especially over Central Africa, Indonesia and South America (Liu and Zipser, 2005). Because the activities of deep convection are generally attributed to the large-scale seasonal migration of the intertropical convergence zone (ITCZ) and the Indian and East Asian, South American and African monsoons, the distribution of deep convection is usually consistent with these general circulation systems (Liu et al., 2007, 2012). Most extreme deep convection events occur in the western Pacific (December-March) and in the Indian monsoon regions (July-September) (Jiang et al., 2004). The peak depth and strength of deep convection is associated with the strong interactions of tropical clouds, precipitation and radiation in summer (Lin et al., 2006; Li et al., 2013). The characteristic features of tropical deep convection have been studied in recent years due to the rapid development of remote sensing technology (Liu et al., 1995; Dessler, 2002; Yuan and Li, 2010). These deep convection events and their associated precipitation play an important role in regional weather variations and in the cross-tropopause transport of chemical tracer gases (Poulida et al., 1996; Sherwood and Dessler, 2000).

    However, whether the deep convection events in the extratropics are similar to those that occur in tropical regions has yet to be determined. Generally, the convective activities that occur in the extratropics are weaker than those that occur in the tropics, especially in winter (Liu et al., 2012). However, there are exceptions in some areas. The most notable exception is in the North Pacific (NP), where storm tracks have a distinct path during boreal winter (Chang and Fu, 2002; Chang et al., 2002). Every year, dozens of strong cyclones are generated in the west portion of the ocean and migrate eastward from November to the next April (Gulev et al., 2001; Rodionov et al., 2007). These cyclones usually form in low latitudes and then develop rapidly. Their central pressure at sea level falls at a rate of 24 hPa d-1 (Sanders and Gyakum, 1980). Because of the vigorous development of these synoptic-scale systems, strong jet streams and vertical motion are also common (Holland et al., 1987). When there is sufficient accumulation of moisture to achieve saturation, deep clouds are generated in the uplifted flows (Govekar et al., 2011). The optical thickness of these convective deep clouds is usually large, which is expected to affect the radiation flux significantly. Moreover, an extended vertical transfer that is sustained in deep convection can also play a role in the cross-tropopause exchange of atmospheric constituents (Fischer et al., 2003). Our understanding of deep convection and embedded deep clouds over the wintertime ocean has remained incomplete due to the sparse coverage of ground-based and ship-based observations. This dilemma has recently been addressed as satellite remote sensing now routinely collects data on global clouds; thus, DCCs over the ocean can be characterized (Luo et al., 2008). We used CloudSat/CALIPSO data in this study to focus on DCCs over the NP and to investigate their relationship with cyclonic activity. This study aims to reveal the characteristics of deep convection in the extratropics during the cold season; thus, it may advance understanding of the effects on convection of synoptic-scale oceanic cyclones.

    Figure 1.  (a, b) Seasonal occurrences of a double tropopause, and (c, d) average tropopause height and standard deviation (units: km): (a, c) winter (December-February); (b, d) summer (June-August).

2. Data
  • Cloud structure data from CloudSat/CALIPSO were used in this study to illustrate the vertical development of convection systems. The CloudSat satellite orbits the Earth approximately 14.5 times per day at a 750 km orbit. The cloud profiling radar (CPR) on CloudSat operates at a frequency of 94 GHz, points nominally towards the nadir and emits a pulse of 3.3 microseconds. The backscattered signals are over-sampled and averaged to produce a nominal footprint of 1.4 km across-track and 2.5 km along-track and a vertical sampling of 250 m (Mace et al., 2007). The CloudSat data were also combined with measurements from the CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP; hereafter referred to as the Lidar) to produce an authoritative mask of hydrometeor coverage. The CPR and the Lidar are designed to fly in close synchronization. The obvious synergy of these combined observational capabilities is significant. Considering the ability of the CPR to probe optically thick large-particle layers and the ability of the Lidar to sense optically thin layers and tenuous cloud tops, the two instruments have the potential to provide a complete depiction of cloud generation (Mace et al., 2009; Naud and Chen, 2010). The 2B-CLDCLASS-LIDAR data used in this study identify cloud types by primarily combining information available from the CloudSat and CALIPSO satellites. It classifies clouds by using vertical and horizontal cloud properties, the presence or absence of precipitation from CloudSat and CALIPSO, cloud temperature from ECMWF (European Centre for Medium-Range Weather Forecasts), and upward radiance from MODIS (Moderate-Resolution Imaging Spectroradiometer) measurements (Wang et al., 2013). The CPR (CloudSat) and Lidar (CALIPSO) provide vertical cloud profiles and horizontal extents of clouds, which provide important information on differentiate cloud types.

    The ERA-Interim archive from 2007 to 2010, which was produced by ECMWF, was also used in this study, including variables such as potential vorticity (PV), pressure, temperature, wind speed, and relative humidity. The PV was used to define tropopause height, and the other variables were used to reveal the atmospheric conditions both in the statistical analysis and in a case study. ERA-Interim is a reanalysis of the global atmosphere that covers the data-rich period since 1979 (originally, ERA-Interim started in 1989, but a 10-year extension to 1979-88 was produced in 2011) and continues in real time (Dee et al., 2011). The dataset from ERA-Interim is widely used in climate and weather analyses. It includes not only the principle atmospheric fields, such as sea level pressure, temperature, wind and relative humility, but also indirect variables, such as PV and convective available potential energy (CAPE). The data used here have a spatial resolution of 1.5°× 1.5° and 60 vertical levels, with a top level of 0.1 hPa.

3.Statistical analysis method
  • The deep convection in the tropical and subtropical regions during summer usually extends from the low levels to above the tropopause. It has been determined that 1.3% of convection occurs over a 14 km depth, and 0.1% occurs over a 17 km depth (Liu and Zipser, 2005). One of the highest cloud tops observed, which occurred over northern Australia, formed at a height of more than 19 km (Alcala and Dessler, 2002). To categorize deep convection, (Alcala and Dessler, 2002) referred to tops exceeding 10 km as "deep convection" and tops exceeding 14 km as "overshooting convection". This index is not appropriate for the deep convection that occurs over the NP in winter because the troposphere is thinner than it is in the tropics. In terms of PV, which is a quantity that is conserved under adiabatic and frictionless conditions, the tropopause is frequently defined by 2 PVU (potential vorticity unites) (Hoskins et al., 1985; Gouget et al., 2000), where 1 PVU = 10-6 K m2 kg-1 s-1. This "dynamical tropopause definition" ideally represents the concept of the tropopause as a quasi-impermeable discontinuous surface. Obviously, the tropopause height is exactly the height of the 2 PVU isosurface when the tropopause is not folded. Tropopause folds can be defined as regions where vertical soundings reveal multiple crossings of the tropopause. When the tropopause folds, the 2 PVU isosurface is vertically intersected at least twice, i.e., multiple tropopauses exist (Sprenger et al., 2003). Figures 1a and b show the tropopause fold frequency distributions over the NP for winter (December-January-February; DJF) and summer (June-July-August; JJA). The fold frequency at a certain location represents the percentage of instances (daily ECMWF fields have been analyzed) in which a fold is identified at this location during the particular season. During both seasons, tropopause folds are most frequent in the subtropics between 20°N and 40°N, where the maximum occurrences are approximately 0.12 during DJF and approximately 0.04 during JJA. We defined the tropopause height of the fold as the average height of its upper and lower boundary to reduce the influence of the double tropopause when identifying DCCs.

    Figures 1c and d show the average tropopause height and its standard deviation for DJF and JJA. The highest tropopause occurs over the West Pacific during DJF. The tropopause descends sharply with increasing latitude. North of 40°N, the height drops to less than 10 km. The tropopause is higher during JJA than DJF, except over the West Pacific, and has a smaller standard deviation. The average tropopause height in the midlatitudes is approximately 11 km. Because of the large variation in the tropopause between winter and summer, it is not appropriate to use a constant height to define DCCs, such as that used by (Alcala and Dessler, 2002) for the tropics.

4.Statistical analysis results
  • Figure 2.  Occurrence of deep convective clouds in (a) winter (December-February) and (b) summer (June-August).

    Figure 3.  Occurrence of cloud top heights in winter (December-February): (a) latitudinal occurrence averaged from 30°N to 60°N; (b) longitudinal occurrence averaged from 120°E to 120°W.

    Figure 4.  Probability density of the anomalies during DCCs for (a) surface pressure (units: hPa), (b) 2-m temperature (units: K), (c) 10-m velocity of zonal wind (units: m s-1), (d) 10-m velocity of meridional wind (units: m s-1), (e) 2-m relative humidity (units: %), and (f) convective available potential energy (CAPE) (units: J kg-1).

  • In this study, we collected the altitude data of cloud layer tops and bases in summer and winter from 2007 to 2010. We defined "deep clouds" as the cloud layers that have a base below 3 km and a top above the in-situ tropopause (we defined the tropopause height of the fold as the average height of its upper and lower boundary). The DCC data were gridded into 1.5°× 1.5° for DJF and JJA, as shown in Fig. 2. Note that the average number of observations by the CPR/Lidar in each grid was greater than 5000, which is large enough to avoid the spurious bias introduced by this approach. Figure 2 shows a much greater difference in the DCC occurrences between winter and summer. During JJA, high frequencies of DCCs occur over a large region from southeastern Asia to the East Pacific. Particularly, the DCCs occur most frequently in eastern Asia and the western Pacific, which are regions dominated by Asian summer monsoons (Wang et al., 2001). These distributions are generally consistent with previous studies based on the TRMM PR (Tropical Rainfall Measuring Mission Precipitation Radar) (Alcala and Dessler, 2002; Liu et al., 2012). The definition of deep convection works well in the midlatitudes, where CloudSat/CALIPSO fills the TRMM PR detection gap north of 38°N (Kummerow et al., 1998). During DJF, the occurrence of DCCs over land greatly decreases, especially in East Asia and the West Pacific. However, the DJF occurrence remains approximately equal to the JJA occurrence over the ocean north of 30°N. The comparison between DJF and JJA suggests the importance of DCCs over the NP in winter because the DCC activities do not weaken from summer to winter, as seen in East Asia and the West Pacific. Note that the high occurrence region roughly coincides with a section of the prevailing storm tracks in boreal winter (Zhang et al., 2004). In general, most cyclones are not strong in their early stages when they are over the western portion of the NP; they generally exhibit a mature stage and peak depth around the dateline after continuously deepening during their eastward migration (Gyakum et al., 1989). The jet stream flow around cyclones and their associated updrafts also peak when the cyclones are deepest (Ding, 2005; Yi et al., 2012). These circulation conditions provide a favorable dynamic force for deep convection, resulting in larger occurrences of DCCs in central and eastern portions of the NP. Because of the particular features of DCCs in DJF and their potential relationship to marine cyclones, we subsequently only focus on the DCCs in DJF.

  • To identify the vertical extension of these DCCs, cloud top height (CTH) information for DJF were selected and averaged over the designated area. Figure 3a shows the variation of CTHs over a range of longitudes. Most cloud tops occur at a height of 9-14 km. The highest cloud tops, which occur west of the dateline, are as high as 15 km. Figure 3b shows the CTH variation with latitude. From low latitudes to high latitudes, the highest cloud tops descend gradually, but the occurrences have different variations. The tops of DCCs occur most frequently at a height of approximately 12 km north of 30°N.

    The occurrences of DCCs from CloudSat/CALIPSO appear to be highest in the ITCZ; this finding validates the major hypothesis of previous studies, which emphasizes that intense convective activities occur in this area (not shown). However, it is important to note that DCCs distinctly decrease between 15°N and the Tropic of Cancer, where the large-scale descent of the Hadley circulation dominates. In the midlatitudes, from 30°N to 45°N, the cloud tops are slightly lower than those in low latitudes, but the occurrence is much higher and nearly equal to that in summer. The DCCs in winter occur much more frequently over the ocean and over the belt where the concentrated DCCs in winter are very close to the agglomeration of cyclones (Chang et al., 2002; Wang et al., 2007; Zhang et al., 2012). In general, the DCCs in the wintertime midlatitude ocean appear different from those that occur in the summer ITCZ. It has been established that the thermodynamic force is very weak during winter, but it is very strong in the summer convective regions where deep convective activities occur. Thus, we hypothesize that this deep, marine convection in winter is associated with an essential mechanism that differs from convection that is thermally driven.

  • To clarify the potential relationship of DCCs with atmospheric circulation conditions and cyclones that frequently occur over the NP in winter, we calculated the probability distribution of the anomalies for several fundamental quantities of the atmospheric circulation and the original CAPE when DCCs occur. The surface pressure, temperature and relative humidity at 2 m, the zonal and meridional wind velocity at 10 m, and the CAPE data were obtained from the ECMWF 6-hourly dataset. As shown in Fig. 4, there is obviously lower surface pressure, higher 2 m temperatures, stronger 10 m meridional winds, and a higher 2 m relative humidity when DCCs occur. At times, the zonal wind greatly decelerates, but this situation is not dominant; the wind speed deviation is ambiguous. Approximately 45% of the samples have a CAPE value near 0, and the probability density decreases sharply with the increase in CAPE. These features indicate the circulation conditions near DCCs. The surface below DCCs is characterized by warm and moist air, low pressure and meridional acceleration. CAPE is often used as an indicative index for estimating the potential convection development (Ye et al., 2006), but a large CAPE value does not necessarily mean deep convection will occur. A large number of DCCs occur with small CAPE values in the NP in winter. Although partially contributed by the coarse resolution of the ERA-interim, the small value posed problems for an explanation in terms of pure thermal initiation, which plays an important role in tropical convection; however, the large number of DCCs appears to have a close connection to the atmospheric circulation. This finding motivated us to determine the relationship between DCCs and cyclones, which represent the most prominent type of weather system over the NP in winter.

    Figure 5.  (a) Tracks and (b) occurrences of cyclones in winter.

    Figure 6.  Schematic diagram of the (a) calculation methods and (b) distribution of the correlation coefficients.

    To clarify the potential linkage between DCCs and cyclones, we identified and traced all cyclones that occurred in the NP from 2007 to 2010. All of these cyclones exhibited central pressures less than 975 hPa for at least 24 hours. Figure 5 shows the statistical results of these cyclones. Most of them originated in the western part of the NP at low latitudes, then migrated toward the northeast and dissipated near the high latitudes or the western coast of the American continent (Fig. 6a). The highest occurrence was around the dateline, near 45°N (Fig. 6b), and appeared close to the region with intense DCCs. To quantify the linkage in the relative locations between cyclones and DCCs, we calculated the correlation coefficients between the frequencies of the cyclones and the DCCs. The coefficients were based on the frequency of the central low pressure in a fixed region and the frequency of the DCCs within a moving window using the same region size (Fig. 6a). The offset longitude and offset latitude between the designated fixed and moving regions were called LON and LAT, respectively, and were assigned as the coordinate indices of the calculated correlation index. The calculation was performed for a LON range from -10 to 30 and a LAT range from -5 to 15 to produce a correlation index distribution (Fig. 6b). The regions with larger correlation index values have a higher probability of DCC formation. As shown in Fig. 6b, the maximum index occurs 10°-20° east of the cyclone center and 5°-10° south of the cyclone center. Figure 6b depicts the pattern in the geographic relationship between the cyclones and DCCs; the southeastern portion of a cyclone is the most convective and tends to have higher occurrences of deep clouds. Thus, the strongest convection is not located near the center of the cyclone system but is most likely to occur in the warm conveyor belt along the outer edge of a cold front, which is in agreement with the classic cyclone model.

5. Case study
  • Figure 7.  Two satellite passes of the cyclone: (a, b) surface pressure (contours, units: gpm) and cloud top temperature (shading, units: K); (c, d) vertical cross sections of radar reflectivity factors along the tracks (units: dBZ). The solid gray line in (a, b) represents the orbit of CloudSat; the gray dashed line in (c) represents the tropopause.

    Figure 8.  Divergence (units: 10-5 s-1) on the isobaric surface (shading, 850 hPa; contours, 200 hPa: (a) 0000 UTC 8 February 2009; (b) 0600 UTC 8 February 2009. The solid gray line in (a, b) represents the orbit of CloudSat according to Figs. 7a and b.

    The analysis of the DCCs and the circulation environment indicates the typical characteristic features and inherent processes of their relationship. However, a typical case study is necessary to obtain more detailed cyclone characteristics. Among the thousands of cloud profiles in the study area, we chose the strong cyclones with deep clouds that were detected in the Northwest Pacific on 8 February 2009. On that day, CloudSat passed over the Northwest Pacific twice at 0058 UTC and 0237 UTC, as shown in Fig. 7. The first satellite track passed over the eastern part of the cyclone. The radar reflectivity of the CPR along this track clearly indicated a deep cloud located between 40°N and 45°N. The cloud extended from a low level to above 10 km in the vertical direction, and most of its top was above the tropopause. The region with the DCCs had cold cloud top temperatures (based on MODIS), and was located in a larger cold belt in the spiral arm of the cyclone. Approximately 2 hours later, CloudSat passed over the western part of the cyclone again, when it was at the tail of the cold belt of the spiral arm. Along the second track, there were no deep clouds as detected in the first track, except for a mid-level cloud in the northwestern part of the cyclone. This case study offered a direct understanding of the deep clouds in the NP in winter. The location of DCCs along the southeastern leading edge of the cyclone provides convincing evidence for the conclusions of the statistical results in the previous section.

    This typical case provides detailed characteristics of the DCCs and the associated cyclone. Therefore, our study focused on the analysis of the surrounding atmospheric circulation environment. As shown in Fig. 8, the low-level (850 hPa) converging flow and the high-level (200 hPa) diverging flow in the southeastern part of the cyclone migrated from west to east. At 0000 UTC, the high-level diverging flow lagged behind the low-level converging flow. Then, it moved northeast and strengthened. At 0600 UTC, the diverging flow region extended from 30°N to north of 50°N, almost covering the entire low-level converging flow region. It is reasonable to deduce that the lower convergence and the upper divergence were concurrent during the migration process, and the time and location of the concurrence was consistent with the DCCs. The strong vertical motion generated during the concurrence reveals that the large-scale baroclinicity played an important role in the development of the convection.

    Figure 9.  Relative humidity (shading, units: %), temperature (contours, units: K) and stream vectors (arrows, units: m s-1) on the isobaric surface of 850 hPa: (a) 0000 UTC 8 February 2009; (b) 0600 UTC 8 February 2009.

    Figure 10.  (a) Regional averaged (40°-45°N, 180°-175°W) relative humidity (shading, units: %), temperature (contours, units: K) and vertical velocity vectors (units: Pa s-1) in the time-pressure cross section. (b) The contours of the vertical velocity (units: Pa s-1) in the same cross section.

    Some studies have emphasized the importance of the moisture content of the low-level troposphere in the development of deep convection (Derbyshire et al., 2004; Kuang and Bretherton, 2006). (Derbyshire et al., 2004), for instance, documented the sensitivity of convective development in a wide range of environmental relative humidity levels. Their results showed that a wet environment is favorable for deep convection because the entrainment of wetter environmental air leads to weaker evaporative cooling and positive buoyancy. As shown in Fig. 9, the distributions of relative humidity, temperature and wind vectors were consistent with the divergence in Fig. 8. The leading edge of the cyclone, especially the southwestern part, most notably had the strongest poleward flow, with a maximum velocity exceeding 50 m s-1. This warm air advection transported abundant moisture into the cyclonic system. At the same time, the negative temperature gradient during the conveyance caused the relative humidity to approach saturation rapidly. Note that the wettest air was found in the warm and moist transfer belt, where updrafts prevail. Abundant water vapor, warm air advection and updrafts create the most favorable environmental conditions for enhancing the depth of convection.

    The temperature, velocity and humidity were averaged over the most convective region (40°-45°N, 180°-175°W) to reveal precisely how the characteristic features varied with time (Fig. 10). Initially, a temperature anomaly occurred. Because the air temperature above 300 hPa dropped sharply to less than 220 K on 7 February, weak sinking motion began in the high-level air. However, the air temperature below 300 hPa was warming at this time. This temperature structure increased the atmospheric instability. After a slow and gradual increase over dozens of hours, the warming of the low-level air became significant in the daytime hours of 7 February, and the lifting started at the mid-level. As the lifting strengthened, an updraft developed from the sea surface to above 200 hPa. This indicates that there was an explosive development of deep convection. The updraft in this convection system peaked at 1 Pa s-1 at nightfall on 7 February. The relative humidity increased prior to the strongest lift; however, this increase was not very significant and appeared only in the mid-level for a brief time. When the strongest vertical motion was triggered, the humidity peaked and moistened the air over a wide vertical range. The time series of the basic meteorological variables describes the regional variation of the convection. The deep convection strengthened following enhanced instability and increased moisture. The time-pressure cross section shows that the strong updraft motion started at 0000 UTC 7 February and peaked 24 hours later (Fig. 10b). The updraft extended from the surface to above 200 hPa. The geographical and temporal correlations between the updraft and DCCs also indicate the close relationship of these factors.

6. Conclusion
  • Characteristic features of deep clouds that extend from the atmospheric boundary layer to the tropopause were described using a climatological analysis and a typical case study based on cloud information from the combined observations of the CloudSat CPR and the CALIPSO Lidar. The results demonstrated that particular DCC features occur over the NP in winter. Unlike the occurrence of summer DCCs, which are more frequent in East Asia and the West Pacific, winter DCCs occur mainly over the NP. From 30°N to 45°N, winter DCCs are most frequent, approximately equal to the number in summer. Most of the clouds have a top above 12 km in altitude. The highest cloud tops, which occur to the west of the dateline, are as high as 15 km.

    The statistics of the circulation conditions showed that there is obviously low surface pressure, high 2-m temperatures, strong 10-m meridional winds, and high 2-m relative humidity when DCCs occur; however, the zonal wind and CAPE deviations are ambiguous. Further, the correlation between the locations of the DCCs and the cyclone center is strong between 10° and 20° east, and 5° and 10° south, of the center of the cyclone. Thus, the outer southeastern area of the cyclone is most beneficial for the occurrence of DCCs.

    The case study revealed detailed characteristics of a cyclone in which DCCs were embedded. In the spiral arm of the cyclone, a belt with very cold cloud tops existed. A deep convective cloud was detected by the CPR when it crossed this belt. The deep convective cloud located 17° east and 6° south of the center of the cyclone, where the warm conveyor belt was located. Warm and moist low-level air was lifted when the high-level diverging flow migrated along with the low-level converging flow. Latent heat released from the condensation of cloud particles strengthened the updraft. This rapid strengthening of the cyclone resulted in deep convective motion that stretched from the low levels to above the tropopause. As indicated by satellite observations, thick clouds were generated.

    This study characterizes DCCs that occur outside of the tropics in winter. From a basic dynamic perspective, both the climatological statistics and the case study confirm that the development of DCCs over the midlatitude ocean in winter is closely related to synoptic-scale cyclones. The deep convection along the prevailing winter storm tracks is important due to its potential effects in the atmosphere.

    Acknowledgements. The authors thank the reviewers for their comments and recommendations, the NASA CloudSat project for providing the CloudSat/CALIPSO dataset, and the ECMWF Interim Reanalysis project for providing the global atmospheric data. This work was funded by the National Natural Science Foundation of China (Grant Nos. 41105031, 41230419, 91337213 and 41205126), the China pecial Fund for Meteorological Research in the Public

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