J. Meteor. Res.  2017, Vol. 31 Issue (5): 890-905   PDF    
http://dx.doi.org/10.1007/s13351-017-7038-x
The Chinese Meteorological Society
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Article Information

WANG, Rui, and Yunfei FU, 2017.
Structural Characteristics of Atmospheric Temperature and Humidity inside Clouds of Convective and Stratiform Precipitation in the Rainy Season over East Asia. 2017.
J. Meteor. Res., 31(5): 890-905
http://dx.doi.org/10.1007/s13351-017-7038-x

Article History

Received March 20, 2017
in final form June 15, 2017
Structural Characteristics of Atmospheric Temperature and Humidity inside Clouds of Convective and Stratiform Precipitation in the Rainy Season over East Asia
Rui WANG1, Yunfei FU1,2,3     
1. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026;
2. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081;
3. Key Laboratory of Atmospheric Sciences and Satellite Remote Sensing of Anhui Province, Anhui Institute of Meteorological Sciences, Hefei 230031
ABSTRACT: In this study, a merged dataset constructed from Tropical Rainfall Measuring Mission precipitation radar rain products and Integrated Global Radiosonde Archive data is used to investigate the thermal structural characteristics of convective and stratiform precipitation in the rainy season (May–August) of 1998–2012 over East Asia. The results show that the storm tops for convective precipitation are higher than those for stratiform precipitation, because of the more unstable atmospheric motions for convective precipitation. Moreover, the storm tops are higher at 1200 UTC than at 0000 UTC over land regions for both convective and stratiform precipitation, and vice versa for ocean region. Additionally, temperature anomaly patterns inside convective and stratiform precipitating clouds show a negative anomaly of about 0–2 K, which results in cooling effects in the lower troposphere. This cooling is more obvious at 1200 UTC for stratiform precipitation. The positive anomaly that appears in the middle troposphere is more than 2 K, with the strongest warming at 300 hPa. Relative humidity anomaly patterns show a positive anomaly in the middle troposphere (700–500 hPa) prior to the occurrence of the two types of precipitation, and the increase in moisture is evident for stratiform precipitation.
Key words: vertical structure of precipitation     atmospheric stability     temperature anomaly     relative humidity anomaly    
1 Introduction

Traditionally, rain gauge observations or ground-based weather radar detections are used to obtain rain rates or three-dimensional (3D) rain echoes of precipitating clouds. Meanwhile, advanced satellite-borne precipitation radar, such as the earlier Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and the more recent Global Precipitation Measurement Dual-frequency Precipitation Radar (DPR), can homogeneously measure precipitating clouds on a larger scale, providing the rain rate and 3D precipitation structures of precipitating clouds associated with their spatiotemporal distribution for different precipitation types, such as convective and stratiform precipitation (Liu and Fu, 2001; Fu and Liu, 2003; Fu et al., 2013). The vertical structure of precipitation obtained from PR or DPR is very useful in retrieving the latent heat released through cloud precipitation processes (Li et al., 2013). Currently, satellite-derived observational products are used in many studies (e.g., Zhou and Wang, 2006; Houze et al., 2007; Li et al., 2012; Qin and Fu, 2016).

On the other hand, the vertical atmospheric temperature and humidity measured by the Integrated Global Radiosonde Archive (IGRA) offer substantially higher vertical resolution than reanalysis, as well as much longer records than GPS radio occultation, and has thus supplied an essential dataset in analyzing and understanding the vertical structures of temperature and humidity at the global scale (Durre et al., 2006). For example, Alexeev et al. (2012) evaluated the Arctic temperature trend around 1990 using IGRA. They found that the temperature changed from negative to positive in the lower stratosphere (200–70 hPa). Based on IGRA, Feng et al. (2012) analyzed the global trend in the thickness of the tropopause layer during 1965–2004. The results revealed that the tropopause layer thickened during this period, with a positive trend of 0.16 ± 0.12 km (10 yr) –1.

Combining radiosonde sounding and TRMM products, Folkins et al. (2008) found that deep convection cooled the atmosphere in the lower troposphere of equatorial regions over a horizontal distance of about 1000 km. Using TRMM 3B42 products and IGRA, Mitovski and Flokins (2014) indicated the regional similarities and differences in the interaction between high rainfall events and the background atmosphere from 1998 to 2010. Xian and Fu (2015) revealed that the frequency of tropopause-penetrating convection decreased exponentially with height in the tropics, based on TRMM PR data together with the data from Constellation Observing System for Meteorology, Ionosphere, and Climate.

Nevertheless, knowledge about the vertical structure of atmospheric temperature and humidity inside precipitating clouds, such as convective and stratiform precipitation, remains insufficient. This is the motivation behind the present study, in which we focus on the characteristics of atmospheric temperature and humidity inside precipitating clouds over the East Asian summer monsoon (EASM) region, where the characteristics of precipitation are strongly affected by the local topography. According to previous studies (Zhou et al., 2008; Bao et al., 2011; Zhu et al., 2011; Xu, 2013), four regions are selected: the Sichuan basin (SCB; 28.0°–32.0°N, 102.0°– 110.0°E), Southeast China (SEC; 23.0°–26.0°N, 112.5°– 118.5°E), middle-eastern China (MEC; 28.0°–32.0°N, 112.5°–123.0°E), and Northwest Pacific (NWP; 25.0°–29.0°N, 127.5°–142.5°E), as shown inFig. 1.

Figure 1 Topography of the East Asian summer monsoon region (shaded) and the IGRA stations (red solid circles) in four regions of East Asia (SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific).

Following this introduction, the data and methodology are described in Section 2. The water vapor transport, vertical structure of convective and stratiform precipitation, atmospheric stability, temperature, and relative humidity anomaly patterns inside convective and stratiform precipitating clouds are explored in Section 3. Meanwhile, the interactions between precipitation and these atmospheric parameters are also discussed in this section. Section 4 summarizes and draws conclusions from the study.

2 Data and methodology

The TRMM PR 2A25 version 7 products issued by the Goddard Space Flight Center (GSFC) are used to investigate the vertical structure of convective and stratiform precipitation over the EASM region from 1998 to 2012. These products provide precipitation information, including location, scanning time, precipitation type, and 3D radar reflectivity from the surface to 20 km, with a horizontal resolution of 4.3 km and a vertical resolution of 0.25 km (Kummerow et al., 1998). Owing to the orbit boost of TRMM after August 2001, the detection threshold of PR is 17 dBZ in radar reflectivity and 0.4 mm h–1 in rain rate (Schumacher and Houze, 2003). Consequently, in this study, only PR 2A25 pixels with radar reflectivity larger than 17 dBZ and rain rate greater than 0.4 mm h–1 in the near-surface layer are included.

To obtain the structures of atmospheric stability associated with temperature and humidity for convective and stratiform precipitation, the IGRA data archived at the National Climatic Data Center (NCDC) are used. These data provide the pressure, temperature, dew-point temperature, geopotential height, and other meteorological parameters at standard levels. The data are subject to rigorous quality control and are measured twice daily (0000 and 1200 UTC) (Durre et al., 2006). Considering the period of overlap between TRMM PR and IGRA, IGRA data from 1998 to 2012 are selected. Moreover, missing data at each individual IGRA station should be less than 20% (Guo and Ding, 2009).

To analyze the water vapor transport in the rainy season from 1998 to 2012, reanalysis data obtained from NCEP at 0000 and 1200 UTC are selected, including horizontal wind and specific humidity with a horizontal resolution of 2.5° at 850 hPa.

To investigate the vertical structures of temperature and humidity for convective and stratiform precipitation, we merge the PR 2A25 products with the IGRA dataset from 1998 to 2012. The location and sounding time of IGRA observations are fixed, whereas the geographical location and scanning time of rain pixels measured by PR vary with TRMM PR orbit positions. Therefore, it is necessary to match both datasets in time and space. As suggested by Xia and Fu (2016), it is practicable to match the PR 2A25 and IGRA data at each IGRA station within its 0.25° grid before and after 2 h of IGRA observations. Ultimately, a new dataset with quasi spatiotemporal synchronization is obtained for our study. Note that the atmospheric profiles from IGRA represent the mean state of the atmosphere, rather than the instantaneous state, for temperature and dew-point temperature during the period of observation; whereas the precipitation profiles from TRMM PR represent the instantaneous state of precipitation structure.

Table 1 Numbers of IGRA and TRMM PR 2A25 samples for convective and stratiform precipitation in the rainy season during 1998–2012 in the four regions of East Asia
0000 UTC 1200 UTC
Convective Stratiform Convective Stratiform
IGRA PR 2A25 IGRA PR 2A25 IGRA PR 2A25 IGRA PR 2A25
SCB 67 981 161 3827 108 852 135 2246
SEC 162 1856 190 3912 176 2694 250 5826
MEC 266 3555 510 12820 302 3351 510 10866
NWP 165 1395 173 4048 140 912 168 3865
Note: SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific

In this paper, convective (stratiform) precipitation is defined where convective (stratiform) pixels comprise more than 30% (70%) of the total pixels in each PR sample. Table 1 shows the number of IGRA samples classified as under convective or stratiform precipitation and the associated precipitating pixels detected by PR. The samples are statistically sufficient in each selected region. Clearly, the numbers of samples for stratiform precipitation at 0000 and 1200 UTC are more than those for convective precipitation, which is consistent with previous studies (Fu et al., 2007, 2008).

3 Results 3.1 Water vapor transport

It is well-known that the variation in precipitation is influenced by general circulation over the EASM region (Ding et al., 2008). Therefore, before studying the structure of precipitation, we examine the water vapor transport at 850 hPa in the rainy season. As shown in Fig. 2, water vapor comes from different directions and has different influences in the four regions. The SCB is dominated by southeasterly water vapor transport. Meanwhile, there are two different brances of water vapor trasnport in SEC and MEC, one is southerly from the East China Sea, and the other is northerly from the area of Bohai or Huanghai. Water vapor from the western Pacific converges over the NWP. These results are consistent with previous studies (Fu et al., 2005; Zhou and Yu, 2005). Water vapor transport drives warm and moist air from the ocean to the land regions (SCB, SEC, and MEC) in the lower troposphere, which provides sufficient moisture for precipitation over the EASM region. In addition, the moisture flux divergence field at 850 hPa over the EASM region is shown by the shading in Fig. 2. A negative (positive) moisture flux divergence indicates water vapor convergence (divergence) (Juneng and Tangang, 2005), and water vapor convergence is conducive to the formation of precipitation (Liu et al., 2005). Significant water vapor convergence exists in SCB, SEC, and MEC at 1200 UTC, whereas water vapor divergence exists in NWP. Moreover, the convergence intensity of the water vapor at 1200 UTC is stronger than that at 0000 UTC in SCB and SEC, which could enhance the formation of precipitation in these two regions at 1200 UTC.

Figure 2 Mean 850-hPa moisture flux field (vector; g s–1 hPa–1 cm–1) superimposed on the moisture flux divergence field (shaded; g s–1 hPa–1 cm–2) at (a) 0000 UTC and (b) 1200 UTC over the EASM region in the rainy season during 1998–2012.
3.2 Precipitation cases

To preliminarily check the results of the merged dataset and demonstrate the thermal and vertical structures of precipitation inside precipitating clouds in SCB, SEC, MEC, and NWP, eight typical precipitation cases that occurred around IGRA stations in the four regions are chosen from the merged data. Some basic information regarding these precipitation cases is listed in Table 2.

Table 2 Details of the precipitation cases chosen to assess the merged dataset
Region Type Station IGRA (yr.mon.day UTC) PR (yr.mon.day UTC)
SCB Convective Enshi (57447) (30.27°N, 109.48°E) 2011.07.21 2300 2011.07.21 2100
SCB Stratiform Chongqing (57516) (29.52°N, 106.4 8°E) 2008.06.14 2300 2008.06.15 0000
SEC Convective Ganzhou (57993) (25.85°N, 114.93°E) 2001.08.29 1100 2001.08.29 0900
SEC Stratiform Xiamen (59134) (24.45°N, 118.07°E) 2006.05.15 2300 2006.05.16 0100
MEC Convective Wuhan (57494) (30.63°N, 114.07°E) 2001.06.17 2300 2001.06.17 2200
MEC Stratiform Changsha (57679) (28.2°N, 112.97°E) 2006.07.26 1100 2006.07.26 1300
NWP Convective Naha (47936) (26.2°N, 127.68°E) 2006.08.06 0000 2006.08.06 0100
NWP Stratiform Chichi Jima (47971) (27.08°N, 142.18°E) 2007.05.21 0000 2007.05.20 2200
Note: SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific.
Figure 3 Contoured frequency by altitude diagrams (CFADs) of TRMM PR reflectivity for convective precipitation cases at stations: (a) Enshi, (b) Ganzhou, (c) Wuhan, (d) Naha, and for stratiform precipitation cases at (e) Chongqing, (f) Xiamen, (g) Changsha, and (h) Chichi Jima, in the four regions of East Asia (SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific).

The contoured frequency by altitude diagrams (CFADs) of TRMM PR reflectivity reflect the characteristics of the radar reflectivity frequency and storm top (Yuter and Houze, 1995; Lang et al., 2003). CFADs for the eight precipitation cases are presented in Fig. 3. The vertical structures of convective precipitation at Enshi, Ganzhou, Wuhan, and Naha stations are shown in Figs. 3ad, respectively. The PR reflectivity is larger than 50 dBZ within 2–5 km. Moreover, the vertical development of precipitation is deep at these four stations, and the storm tops are higher than 10 km. In contrast, the CFADs show vertical structures of stratiform precipitation at Chongqing, Xiamen, Changsha, and Chichi Jima stations, in Figs. 3eh. The PR reflectivity is less than 45 dBZ, and the storm tops are lower than 10 km. The vertical development of precipitation is shallow.

Figure 4 Skew-T-logp diagrams of precipitation cases at stations: (a) Enshi, (b) Ganzhou, (c) Wuhan, (d) Naha, (e) Chongqing, (f) Xiamen, (g) Changsha, and (h) Chichi Jima. Black solid lines show the temperature profiles and blue solid lines show the dew-point temperature profiles.

To explore the vertical structure of temperature and moisture inside convective and stratiform precipitating clouds, skew-T-logp diagrams are shown in Fig. 4. Clearly, the dew-point temperature profiles of the precipitation cases are close to the temperature profiles in the lower troposphere, which suggests that the atmosphere is close to saturation. However, the dew-point temperature profiles drift left during ascent, especially in the upper troposphere (Figs. 4ah). This suggests that the depression of the dew-point is small in the lower troposphere and large in the upper troposphere. In other words, the atmosphere is moister in the lower troposphere during the precipitation process.

3.3 Statistical analysis 3.3.1 Vertical structure of convective and stratiform precipitation

Characteristics of the vertical and thermal structures of precipitation inside precipitating clouds in the four regions in the rainy season from 1998 to 2012 are presented through statistical analysis in this section. Like Fig. 3, Figs. 5, 6 show twice-daily (0000 and 1200 UTC) CFADs for convective and stratiform precipitation in the four regions. According to Fig. 5, the storm top is higher than 10 km in the four regions, both at 0000 and 1200 UTC. The storm tops even exceed 15 km in SCB, SEC, and MEC at 1200 UTC. Notably, the storm tops in the land areas (SEC and MEC) are higher than those in the ocean area (NWP). Additionally, a large frequency of PR reflectivity, 20%–40%, gradually shifts to the right side of the CFADs from the storm top to 4 km. That is, radar reflectivity observed by PR becomes larger, because rain particles grow larger during descent (Houze, 1997; Fu et al., 2008). Compared with convective precipitation, stratiform precipitation develops more shallowly (Fig. 6). The radar storm tops are similar in the four regions. Moreover, from the height of the storm top to 10 km, the radar reflectivity of stratiform precipitation is mainly 20 dBZ. From 10 km down to 5 or 6 km, the radar reflectivity ranges from 20–40 dBZ, increasing with descent. Below 5 km, the distribution of the CFAD changes little with height. In conclusion, the maximum value of radar reflectivity occurs near 5 or 6 km. Therefore, the characteristics of the 0°C melting level (5–6 km) are evident. Furthermore, the storm tops are higher at 1200 UTC than at 0000 UTC (Figs. 5, 6) over the land regions (SCB, SEC, and MEC), for both types of precipitation, because peak rainfall of convective and stratiform precipitation occurs usually in the afternoon over land (Nesbitt and Zipser, 2003; Yu et al., 2007; Fu et al., 2017). Peak rainfall for convective precipitation occurs in the afternoon or evening (1200 UTC) over SEC and MEC; but over SCB, nighttime rain occurs frequently (Yu et al., 2007; Liu and Fu, 2010). Meanwhile, peak rainfall occurs in the morning over the ocean area (NWP) (Fu et al., 2012). Therefore, the storm tops are higher at 1200 UTC in SEC and MEC, and higher at 0000 UTC in NWP.

Figure 5 CFADs of TRMM PR reflectivity for the four convective precipitation cases (see Table 2) at (a–d) 0000 UTC and (e–h) 120 UTC in the four regions of East Asia (SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific).
Figure 6 As in Fig. 5, but for stratiform precipitation cases.

To analyze the magnitude of the near-surface rain rate, the corresponding probability density functions (PDFs) of the near-surface rain rate for convective and stratiform precipitation in the four regions are shown in Fig. 7. The results show that the distributions of the PDFs for convective precipitation are “wide”, and the maximum rain rate can reach 20 mm h–1 (Figs. 7a, c). In addition, the total area fractions of PDFs with a rain rate less than 5 mm h–1 for convective precipitation over the ocean region (NWP) are larger than those over the land regions (SCB, SEC, and MEC), whereas the total area fractions with rain rates larger than 5 mm h–1 over the land regions exceed those over the ocean region. The distributions of PDFs for stratiform precipitation are similar in the four regions, and the near-surface rain rates are less than 10 mm h–1 (Figs. 7b, d). As shown in Table 3, the mean near-surface rain rate at 0000 UTC in SCB, SEC, MEC, and NWP is 9.0, 6.3, 9.4, and 6.0 mm h–1, respectively, for convective precipitation; and 1.8, 2.3, 1.9, and 3.1 mm h–1 for stratiform precipitation. Additionally, the mean near-surface rain rate at 1200 UTC is 7.5, 8.0, 9.6, and 4.9 mm h–1 for convective precipitation; and 1.5, 1.9, 1.8, and 1.9 mm h–1 for stratiform precipitation. The maximum near-surface rain rate for convective precipitation occurs early in the morning in SCB (Yu et al., 2007; Liu and Fu, 2010). Thus, the mean surface rain rate at 0000 UTC is larger than that at 1200 UTC in this region. As a result, a higher storm top does not correspond to a larger near-surface rain rate in SCB. However, the storm tops for convective precipitation are higher at 1200 UTC than at 0000 UTC in SEC and MEC, which corresponds to larger near-surface rain rates at 1200 UTC. Similarly, a higher storm top corresponds to a larger near-surface rain rate at 0000 UTC in NWP. Therefore, the storm tops for convective precipitation increase with an increment in the near-surface rain rate in SEC, MEC, and NWP (Fu et al., 2005, 2012; Chen et al., 2016). As for stratiform precipitation, the near-surface rain rates at 0000 UTC are larger than the values at 1200 UTC in SCB, SEC, and MEC, which is a different result to that of convective precipitation. Consequently, the storm tops for stratiform precipitation do not increase with the near-surface rain rate over the land regions (SCB, SEC, and MEC).

Figure 7 PDFs of near-surface rain rate for (a, c) convective and (b, d) stratiform precipitation at (a, b) 0000 UTC and (c, d) 1200 UTC in the four regions of East Asia (SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific).
Table 3 Mean near-surface rain rate (mm h–1) for convective and stratiform precipitation at 0000 UTC and 1200 UTC in the four regions
0000 UTC 1200 UTC
Convective Stratiform Convective Stratiform
SCB 9.0 1.8 7.5 1.5
SEC 6.3 2.3 8.0 1.9
MEC 9.4 1.9 9.6 1.8
NWP 6.0 3.1 4.9 1.9
Note: SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific.

A rain rate profile exhibits the strength of the vertical air motion as well as the microphysical processes of precipitation, and the slope of the rain rate profile reveals the latent heat release (Fu et al., 2005; Liu et al., 2013). Figure 8 shows the rain rate profiles of convective and stratiform precipitation at 0000 and 1200 UTC. The mean storm tops for convective and stratiform precipitation in the four regions are coincident with the results in Figs. 5, 6. For convective precipitation (Figs. 8a, c), the rain rate increases towards the surface, which results from the increasing sizes of raindrops during descent (Liu and Fu, 2001; Fu et al., 2017). For stratiform precipitation (Figs. 8b, d), the rain intensity reaches its maximum rapidly below the 0°C melting level (5–6 km), and a larger value occurs in NWP. The shapes of the rain rate profiles for convective and stratiform precipitation inFigs. 8c, d are similar to those in Figs. 8a, b, respectively. However, the rain intensity of convective precipitation at 1200 UTC is larger than that at 0000 UTC in SEC and MEC. The rain intensity of stratiform precipitation at 0000 UTC is larger in the four regions, consistent with the results in Table 3.

Figure 8 Mean profiles of rain rate for (a, c) convective and (b, d) stratiform precipitation at (a, b) 0000 UTC and (c, d) 1200 UTC in the four regions of East Asia (SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific).
3.3.2 Atmospheric stability distribution for convective and stratiform precipitation

Unstable stratification of the atmosphere is necessary for precipitation (Adams and Souza, 2009; Lu et al., 2016), and the atmospheric Emagram is commonly used to analyze the atmospheric stability based on sounding observations (Sheng et al., 2003). In an Emagram, the two points where the stratification curve and state curve cross denote the level of free convection (LFC) and the level of neutral buoyancy (LNB), respectively. Generally, air parcels are influenced by the external environment and lifted adiabatically from the LFC to the LNB. The positive area integrated from the stratification curve to the state curve from the LFC to the LNB denotes the magnitude of unstable atmospheric energy. Moreover, the depth from the LFC to the LNB not only denotes the intensity of convective activity, but also the average magnitude of upward air buoyancy, which affects the formation and development of precipitation (Iturrioz et al., 2007; Zheng et al., 2008; Masunaga, 2012; Ratnam et al., 2013).

Figure 9 Positive area frequency (color shading) corresponding to the mean temperature profile (black solid line) and dew-point temperature profile (blue solid line) for the four convective precipitation cases (see Table 2) in the rainy season during 1998–2012 at (a–d) 0000 UTC and (e–h) 1200 UTC in the four regions of East Asia (SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific).
Figure 10 As in Fig. 9, but for stratiform precipitation cases.

Based on the principle of the Emagram, the positive area frequency is the two-dimensional PDF calculated between the stratification curve and state curve from the LFC to the LNB. Figure 9 shows the distribution of positive area frequency for convective precipitation, in which the tops of the positive area at 1200 UTC reach 150 hPa — higher than those at 0000 UTC (200 hPa) over the land regions (SCB, SEC, and MEC). This result implies that vertical air motions develop higher at 1200 UTC. Comparing Fig. 5 with Fig. 9 finds that the relationship between the vertical development of convective precipitation and vertical air motions can be analyzed and obtained. The results show that higher storm tops correspond to stronger upward motions over the land regions, which suggests that the vertical development of precipitation depends on strong upward motion due to unstable energy. Furthermore, the distribution of positive area frequency for stratiform precipitation (Fig. 10) is similar to that for convective precipitation (Fig. 9). However, the tops of the positive area for convective precipitation are relatively higher than those for stratiform precipitation, which corresponds to higher storm tops for convective precipitation in the four regions.

Additionally, the temperature profiles are close to the dew-point profiles in the lower troposphere (Figs. 9, 10). Specifically, the depression of the dew point is small, which indicates that the lower troposphere is moist in the four regions during the precipitation process. To reveal the difference in temperature and dew point in detail, Table 4 shows the mean depression of the dew point (DDP) near the surface in the four regions. The DDP is smaller at 0000 UTC than at 1200 UTC near the surface over the land regions (SCB, SEC, and MEC), for both convective and stratiform precipitation. That is, the atmosphere is moister at 0000 UTC over the land regions near the surface. The opposite result is found over the ocean region (NWP). Moreover, the DDP for stratiform precipitation is smaller than that for convective precipitation at the same point of time. This suggests that the atmosphere is moister during stratiform precipitation. Table 5 shows the DDP results for the middle troposphere (500 hPa), revealing that the DDP at this level is significantly larger than the DDP near the surface at the same time point and for the same precipitation type (except stratiform precipitation at 1200 UTC in SCB). Therefore, it can be concluded that the atmosphere is moister in the lower troposphere and drier in the upper troposphere during the precipitation process.

Table 4 Mean dew-point depression (°C) for convective and stratiform precipitation near the surface in the rainy season during 1998–2012 in the four regions of East Asia
SCB SEC MEC NWP
Convective Near surface (0000 UTC) 2.1 2.3 2.1 3.9
Near surface (1200 UTC) 5.7 3.6 4.3 3.0
Stratiform Near surface (0000 UTC) 2.0 1.9 1.9 3.1
Near surface (1200 UTC) 4.7 2.6 3.1 2.5
Note: SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific.
Table 5 Mean dew-point depression (°C) for convective and stratiform precipitation at 500 hPa in the rainy season during 1998–2012 in the four regions of East Asia
SCB SEC MEC NWP
Convective 500 hPa (0000 UTC) 3.4 6.8 7.2 10.2
500 hPa (1200 UTC) 7.2 6.4 7.6 11.0
Stratiform 500 hPa (0000 UTC) 2.5 3.7 4.0 5.6
500 hPa (1200 UTC) 4.2 3.1 4.2 6.0
Note: SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific.

Table 6 shows the average convective available potential energy (CAPE) for convective and stratiform precipitation at 0000 and 1200 UTC in the four regions. Generally, CAPE values for convective precipitation are larger than those for stratiform precipitation in the four regions. This result suggests that the atmosphere is more unstable when convective precipitation occurs. Therefore, the storm tops for convective precipitation are higher than those for stratiform precipitation, due to more unstable atmospheric movements. Moreover, CAPE values are larger at 1200 UTC than at 0000 UTC, for both types of precipitation, in SCB, SEC, and MEC; whereas, CAPE is larger at 0000 UTC than at 1200 UTC in NWP. This indicates that the atmosphere is more unstable in the evening (1200 UTC) over land regions, while the atmosphere is more unstable in the morning over the ocean region. Of note is that CAPE values over the ocean region (NWP) are larger than those over the land regions (SCB, SEC, and MEC). However, evaporative cooling occurs over the ocean and restrains the development of updrafts (Wu et al., 2013). Moreover, the higher surface elevation over land regions may give rise to dynamic forcing (Yu et al., 2014; Fu et al., 2017). Consequently, storm tops over land regions develop higher than those over the ocean.

Table 6 Mean convective available potential energy (CAPE; J kg–1) for convective and stratiform precipitation in the rainy season during 1998–2012 in the four regions of East Asia
SCB SEC MEC NWP
Convective CAPE (0000 UTC) 2469 3296 2807 3826
CAPE (1200 UTC) 3260 3385 3064 3097
Stratiform CAPE (0000 UTC) 2094 2637 2312 2980
CAPE (1200 UTC) 2473 2741 2548 2738
Note: SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific.
3.3.3 Temperature and relative humidity anomaly structure inside convective and stratiform precipitating clouds

The precipitation process is associated with the atmospheric temperature and relative humidity (Rosenfeld and Lensky, 1998). To examine the structure of temperature and relative humidity inside convective and stratiform precipitating clouds, the merged TRMM PR 2A25 and IGRA data are used to calculate the temperature and relative humidity anomaly from 12 h prior to the precipitation process to 12 h after.

Figure 11 CFADs of TRMM PR reflectivity for the four convective precipitation cases at (a–d) 0000 UTC and (e–h) 1200 UTC in the four regions of East Asia (SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific).
Figure 12 As in Fig. 11, but for stratiform precipitation cases.

The temperature anomaly relates to the vertical atmospheric motion during the period of precipitation (Sheng et al., 2003). That is, a positive anomaly suggests upward movement, whereas a negative anomaly suggests downdraft. As illustrated in Fig. 11 for convective precipitation, the temperature anomaly is negative in the lower troposphere, which corresponds to downdrafts, and the decreasing magnitude is 0–2 K. Notably, the cooling over the land regions (SCB, SEC, and MEC) is more significant than that over the ocean region (NWP) in the lower troposphere, because of the land surface discrepancy. A positive temperature anomaly causes updraft in the middle troposphere, and the strongest warming occurs at 300 hPa. This is because precipitation releases latent heat (Yang and Smith, 2000), and then the increasing temperature causes divergence that enhances the upward movement at this level. Compared with Fig. 11, the temperature anomaly for stratiform precipitation shows a similar pattern in Fig. 12. However, a significant positive anomaly for stratiform precipitation occurs higher in the middle troposphere, compared with convective precipitation, because latent heat released by stratiform precipitation is mainly concentrated in the upper troposphere (Fu et al., 2003). Stratiform precipitation has the characteristic of longevity. Thus, the decreasing temperature anomaly for stratiform precipitation is more significant than that of convective precipitation in the lower troposphere. In short, the temperature anomaly displays a cooling anomaly in the lower troposphere and warming anomaly in the middle troposphere, which is coincident with the latent heating structure and the vertical distribution of temperature reported in previous studies (Tao et al., 2010; Rapp et al., 2011; Mitovski and Folkins, 2014). This temperature anomaly pattern enhances the atmospheric stability in the middle troposphere (Folkins, 2013), which reduces positive buoyancy and inhibits the upward air motion related to precipitation.

Figure 13 CFADs of TRMM PR reflectivity for the four convective precipitation cases at (a–d) 0000 UTC and (e–h) 1200 UTC in the four regions of East Asia (SCB, Sichuan basin; SEC, Southeast China; MEC, middle-eastern China; NWP, Northwest Pacific).
Figure 14 As in Fig. 13, but for stratiform precipitation cases.

Figure 13 shows that a positive relative humidity anomaly appears 6 h prior to convective precipitation in the middle troposphere (700–500 hPa) in SCB, SEC, and MEC, but not in NWP, because moist convection over land regions is different from that over the ocean (Mitovski et al., 2010). The atmosphere is drier over land areas than over the ocean, and is sensitive to increasing moisture when precipitation occurs. Consequently, the increase in relative humidity over the land regions (SEC and MEC) is more significant than that over the ocean region (NWP). Figure 14 shows that the positive relative humidity anomaly pattern for stratiform precipitation is similar to that for convective precipitation in the middle troposphere, but the increase in relative humidity is more evident. In conclusion, a significant positive relative humidity anomaly appears in the middle troposphere (700–500 hPa) about 6 h prior to precipitation, and the increase in moisture is particularly evident for stratiform precipitation. The positive relative humidity anomaly in the middle troposphere generates negative buoyancy (Mitovski et al., 2010), which suggests that increasing moisture inhibits the vertical extent of precipitation in the four regions.

4 Summary and conclusions

In this work, a new quasi spatiotemporally synchronized dataset is developed based on IGRA and PR 2A25. Using the data, we focus on the link between the vertical structure of precipitation and the thermal characteristics of the atmosphere inside convective and stratiform precipitating clouds in the rainy season over East Asia. This provides an opportunity to better understand the thermal and dynamic processes involved in precipitation, and acts as a reference for the development of weather models. The results can be summarized as follows.

The vertical structure of precipitation shows that convective precipitation is associated with strong vertical air motions and occurs higher than 10 km. Generally, the radar storm tops for convective precipitation are higher than those for stratiform precipitation. The storm tops over the land regions (SCB, SEC, and MEC) are higher than those over the ocean region (NWP). Besides, the storm tops are higher at 1200 UTC than at 0000 UTC, for both precipitation types, over the land regions (SCB, SEC, and MEC); whereas the opposite result is found over the ocean region (NWP). In addition, the relationship between the storm tops and near-surface rain rates reveals that the storm tops for convective precipitation increase with an increment in the near-surface rain rate in SEC, MEC, and NWP.

Convective activity develops deeper for convective precipitation than stratiform precipitation in the four regions. Moreover, the atmospheric state is more unstable at 1200 UTC than at 0000 UTC in SCB, SEC, and MEC, which indicates that the atmosphere is more unstable in the evening over land regions. The opposite result is found in NWP. By combining the results for atmospheric stability and the vertical structure of precipitation, it can be deduced that unstable energy is correlated with the depth of precipitation development.

Furthermore, the temperature anomaly structures show a cooling anomaly in the lower troposphere and a warming anomaly in the middle troposphere, which would inhibit the vertical development of precipitation. Additionally, the increasing relative humidity is more significant prior to convective and stratiform precipitation in the middle troposphere and could inhibit the vertical extent of precipitation in the four regions.

Acknowledgments. We thanks the GSFC for providing the TRMM PR 2A25 data, the NCDC for providing the IGRA data, and the NOAA-CIRES Climate Diagnosis Center for providing the NCEP data. Additionally, we appreciate the constructive suggestions of the two anonymous reviewers.

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