Precipitation and Convective Characteristics of Summer Deep ...

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Precipitation and Convective Characteristics of Summer Deep Convection over East Asia Observed by TRMM WEIXIN XU Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, Maryland (Manuscript received 13 June 2012, in final form 5 November 2012) ABSTRACT This study examines precipitation and convective characteristics of summer deep convection for five distinct regions (plateau, foothill, lowland, south China, and ocean) in East Asia using 13 yr of Tropical Rainfall Measuring Mission (TRMM)-based precipitation features. Every region has its own unique features in terms of elevation, rainfall amount, and dynamic/thermodynamic environments. Results show that large, deep convective systems contribute the majority of precipitation totals over all regions except the plateau. Mixedphase precipitation processes are more important in the south China and the lowland regions than in the foothill and ocean regions. The plateau region also shows substantial dependence upon mixed-phase processes, though the mixed-phase region has a smaller depth than the other regions. Most metrics indicate that the south China region has the most intense storms, followed by the lowland, plateau, foothill, and ocean regions. However, ice scattering signatures do show that the ocean region is more ‘‘intense’’ than the foothill and plateau regions. Deep convective systems over the plateau are the smallest and ocean systems the largest, while storms over the foothill, lowland, and south China regions are in between. Alternatively, convective intensity (storm size) in all regions strengthens (decreases) from early summer to midsummer. Both regional and intraseasonal variations in convective intensity and morphology are mainly modulated by changes in the meteorological environment, such as the convective available potential energy, height of neutral buoyancy, total water vapor, and vertical wind shear.

1. Introduction In East Asia, the strong summer monsoon system provides substantial moisture, instability, and dynamic forces for the development of active convection. As a result, copious amounts of cloud, rainfall, and latent heating occur in this region, affecting the world’s largest population and providing important feedbacks to the climate system. The monsoon circulation, rainfall characteristics, and weather systems over East Asia have been extensively studied [see the summary in Ding and Chan (2005)], but storm development, convective characteristics, precipitation microphysics, and their regional (or interregional) variability less so [summary in Xu (2011)]. The goal of this study is to more fully understand convective and precipitation processes in

Corresponding author address: Dr. Weixin Xu, Earth System Science Interdisciplinary Center, University of Maryland, College Park, 5825 University Research Court, College Park, MD 207403823. E-mail: [email protected] DOI: 10.1175/MWR-D-12-00177.1 Ó 2013 American Meteorological Society

East Asia under various influence of the monsoon and topography. Tropical convection (especially deep convection) can be divided into three archetypical regimes: continental, monsoon, and oceanic (Xu and Zipser 2012). Continental convection is more intense, producing frequent lightning activity and having strong mixed-phase processes (or updrafts), while maritime convection mostly is weak to moderate (Zipser 1994; Mohr and Zipser 1996; Rosenfeld and Lensky 1998; Petersen and Rutledge 2001; Williams and Stanfill 2002). In contrast, monsoon convection is intermediate in intensity between continental and oceanic convection (Williams et al. 1992, 2002; Petersen et al. 2002; Cifelli et al. 2002; May and Ballinger 2007; Xu et al. 2009; Lang et al. 2010; Xu and Zipser 2012). Xu and Zipser (2012) pointed out that major regime variations in convective structures are related to the mixed-phase dynamics (updrafts) and microphysics, rather than the cloud depth or ice depth. In addition to continental-scale regime variations, seasonal (or intraseasonal) regional-scale variations in atmospheric forcing could result in different convective

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properties and structures (Petersen and Rutledge 2001; Petersen et al. 2002; Cifelli et al. 2002; Williams et al. 2002; Xu et al. 2009; Lang et al. 2010; Xu and Zipser 2012). In short, convective intensity and structures could be influenced by factors such as thermodynamics (Rosenfeld and Lensky 1998; Williams et al. 2002), aerosol loading (Rosenfeld and Lensky 1998; Williams and Stanfill 2002), and convective environment and morphology (Zipser 2003; Kelley et al. 2010). Convective characteristics may vary depending upon regional topographic features and particular atmospheric forcing. Differing configurations of atmospheric flows and topography can lead to variations in the intensity of convective outbreak (Velasco and Fritsch 1987; Zipser et al. 2006; Houze et al. 2007; Medina et al. 2010; Rasmussen and Houze 2011). For instance, some extreme convective hot spots are located over the foothills of steep terrain such as east of the Andes in southeastern South America and the southwest foothills of the Himalayas in South Asia (Zipser et al. 2006; Houze et al. 2007; Medina et al. 2010; Romatschke and Houze 2010). Over these hot spots, low-level moist flows are frequently capped by drier elevated mixed layers (e.g., downslope dry flows or drier air crossing the steep mountains) forming a capping inversion situation that continuously builds instability (Carlson et al. 1983; Garreaud and Wallace 1998; Houze et al. 2007). Under strong buoyancy situations, intense convection is then triggered by dynamic forcing such as frontal passage, drylines, diurnal heating, etc. Furthermore, different topographic and meteorological settings also contribute to different habits of storm growth, propagation, and duration, and thus to various convective properties and structures (Houze et al. 2007; Nesbitt et al. 2008; Romatschke and Houze 2010; Romatschke et al. 2010). In fact, summer convection and precipitation in East Asia is modulated by variations in topography from high terrain, slopes, and foothills, and to plains (Yu et al. 2007; Zhou et al. 2008; Xu et al. 2009; Xu and Zipser 2011). Precipitation increases stepwise from the high terrain of the eastern Tibetan Plateau to the plains of central China (also see Fig. 1). Precipitation episodes (or cloud clusters) are found to propagate downstream (eastward) from the eastern Plateau to eastern China (Wang et al. 2004, 2012; Xu and Zipser 2011; Bao et al. 2011). Because of the plateau’s strong thermal and dynamical forcing during summer, active cumulus, thunderstorms, and hailstorms frequently develop over the Tibetan Plateau (Uyeda et al. 2001; Qie et al. 2003; Zhang et al. 2008; Cecil and Blankenship 2012). To the southeast of the plateau, intense convection and lightning activity preferentially occur over southeast China and the foothills of the south China mountain range (Xu

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FIG. 1. Topography and rainfall map of East Asia: (a) elevations (shaded) and subregions selected for analysis, and (b) mean seasonal rainfall (May–August) during 1998–2010 based on the TRMM Multisatellite Precipitation Analysis (TMPA) monthly (3B43) rainfall product.

et al. 2009; Xu 2011). Earlier studies revealed that convection over the plateau is weaker than that over the south slopes and the East Asian monsoon region (Park et al. 2007; Houze et al. 2007; Romatschke et al. 2010; Luo et al. 2011). Similarly, lightning (hail) activity is quite frequent over the plateau, but the lightning flash rate (hail size) is relatively lower (smaller) than over other active convection regions in East Asia (Qie et al. 2003; Xu and Zipser 2011; Zhang et al. 2008; Xie et al. 2010). However, differences in the convective and microphysical characteristics between storms over and downstream of the eastern plateau are less known. In most of the Asian monsoon regions, convective intensity decreases significantly from the premonsoon to monsoon. Convection is stronger during monsoon breaks and the postmonsoon periods than during active monsoon periods (Kodama et al. 2005; Yuan and Qie 2008; Xu et al. 2009; Xu and Zipser 2012; Luo et al. 2013). Furthermore, convective intensity in key East Asian monsoon regions increases as the monsoon progresses northward (Xu 2011; Luo et al. 2013). Convective

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properties during the Indian monsoon show evident regional changes due to the influence of topography (Houze et al. 2007; Medina et al. 2010; Romatschke et al. 2010). Though convective properties in some specific regions of East Asia have been examined (Xu et al. 2009; Xu 2011; Luo et al. 2011, 2013), there are still important issues regarding the regional and intraseasonal variations. For example, how does elevation account for differences in convective properties and rainfall contribution (i.e., from the plateau to slopes, and to the plains)? How do convective characteristics respond to the extent, intensity, and seasonal progression of the monsoon? What are the land–ocean contrasts in convective properties during different phases of the monsoon? Long-term Tropical Rainfall Measuring Mission (TRMM) 3D observations of precipitating clouds provide a unique approach to address these issues. This study examines radar-based precipitation features over several regions in East Asia during summer. First, the rainfall contribution of storms with different convective properties in specific regions is documented. Next, storm horizontal extent, convective intensity, and vertical structures of precipitating systems are examined regionally and intraseasonally. Finally, interregional variability in the above-mentioned storm quantities are discussed and summarized.

2. Data and methods a. TRMM precipitation features This study uses 13 yr (1998–2010) of TRMM measurements, version 6, from four sets of instruments on board the satellite (Kummerow et al. 1998), including Precipitation Radar (PR), TRMM Microwave Imager (TMI), Lightning Image Sensor (LIS), and Visible and Infrared Scanner (VIRS). Measurements have been grouped into precipitation features (PFs) at the University of Utah (Liu et al. 2008). By definition, PFs are identified as PR-derived near-surface raining clusters. After a PF is found, measurements from different instruments (with different footprints) are collocated and grouped into its PR pixels with the adjacent-pixel method and parallax collection (Liu et al. 2008). In short, a PF includes not only three-dimensional radar reflectivity measurements but also ice scattering signatures from the TMI, lightning observations from LIS, and IR cloud-top temperatures from VIRS. Since most small features are very shallow and contribute little rainfall (Liu et al. 2008), only features greater than 400 km2 are considered for analysis in this study. The PF database includes a set of parameters developed as proxies for convective intensity (Liu et al.

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2008). This study uses several convective proxies for analysis, including radar echo top height, IR cloud-top temperature, microwave ice scattering signatures, and lightning flash rate. Radar echo tops include maximum heights of 20-dBZ (MAXHT20) and 40-dBZ (MAXHT40) radar reflectivity, while ice scattering signature represents the minimum polarization corrected temperature (PCT) at 85 GHz (MIN85PCT; Spencer et al. 1989). Minimum IR cloud-top temperature (MINIR) is defined as the coldest IR brightness temperature within a PF. Furthermore, lightning flash rate is determined by the frequency of total lightning flashes that occur in the feature during the LIS viewing time. In practice, MAXHT20 is an indicator of how high the updraft can loft precipitation-size ice particles (Liu and Zipser 2005). MAXHT40 and lightning flash rate are two of the best proxies for convection intensity (Zipser 1994; Zipser et al. 2006). When 40-dBZ radar echoes reach the mixed-phase region (usually 6–9 km), it indicates the presence of large hydrometeors (e.g., graupel and/or supercooled rain drops) at this level. This requires strong updrafts (or intense convection) to loft those large size particles well above the freezing level. This also involves active mixed-phase processes such as accretional growth (e.g., rimming) and freezing of raindrops supported by strong updrafts. The higher the MAXHT40 or lightning flash rate the stronger the updraft in the convective core (Zipser and Lutz 1994). MIN85PCT depends on the scattering of upwelling radiation by the lofted column of ice that is commonly known as the ice water path (Vivekanandan et al. 1991). On the other hand, MINIR only indicates the height of the cloud top, regardless of the cloud type (e.g., thunderstorm overshooting top, anvil cloud, or cirrus cloud). Both the horizontal and vertical structures of PFs are analyzed in this study. Horizontal areal coverage of PFs (continuous raining clusters) is used for precipitating system size. The vertical profile of radar reflectivity (VPRR) at the maximum value is used to represent the vertical structure of a precipitating system. Many studies have indicated that VPRR is a direct way of representing the vertical structure of the convective core of precipitating systems (Donaldson 1961; Zipser and Lutz 1994; Liu et al. 2008). The PF dataset also includes auxiliary National Centers for Environmental Prediction (NCEP) reanalysis data. NCEP vertical profiles (17 levels) of temperature, geopotential height, wind, total precipitable water, and humidity at 6-hourly and 2.58 resolution are interpolated into each PF location (Liu et al. 2008). Auxiliary NCEP profiles associated with PFs in each region are used to calculate a set of meteorological parameters including convective available potential energy (CAPE; pseudoadiabatic), level

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FIG. 2. Regional and intraseasonal variations in storm environment characteristics: (a) CAPE, (b) LNB, (c) total precipitable water in the atmospheric column, and (d) wind shear between 200 and 850 mb, except for the plateau (200–500 mb) and foothill (700–200 mb) regions. PLA, FTH, LOL, SOC, and OCN represent the plateau, foothill, lowland, south China, and ocean regions, respectively. Triangles/squares and error bars in (a) show median values and 75% quartiles, while those in (b)–(d) indicate mean values and half standard deviations.

of neutral buoyancy (LNB; pseudoadiabatic), total precipitable water (TPW), and vertical wind shear (e.g., the shear value between 200 and 850 hPa).

b. Definition of various regions and periods After the onset of the summer monsoon (mid-May), most of subtropical East Asia is under the influence of low-level warm and moist southerly or southwesterly monsoon flows (Chang 2004; Wang et al. 2005). The East Asian summer monsoon (EASM) system regulates a pronounced rainband extending from southern China to Japan (Ding 1992; Xu et al. 2009; Xu 2011). The key EASM region is located to the east of the Tibetan Plateau and includes eastern China, south China, the East China Sea, and Japan. The eastern Tibetan Plateau (.4 km; Fig. 1a) is too high to be directly affected by the monsoon flows (925–700 hPa; Chen 1994; Wang et al. 2005). However, the eastern slopes or foothills (1–2 km;

Fig. 1a) are on the edge of the southwesterly monsoon flows. As a result, summer rainfall in East Asia shows a stepwise (about 35 mm per degree longitude) decrease from the key monsoon region (e.g., central China) to the foothills and slopes of the Tibetan Plateau and the eastern Tibetan Plateau (Fig. 1b). However, the regional contrasts in precipitation and convective characteristics in East Asia are unknown. This study selects five subregions in East Asia (shown in Fig. 1a) for analysis and comparison on storm properties. These regions include the eastern Tibetan Plateau (plateau), eastern slopes of the eastern plateau (foothill), southern China to the east of the plateau (south China), low lands in the vicinity of the Yangtze River (lowland), and maritime region to the east of China (ocean). Regional variations in the storm environment in terms of thermodynamics (e.g., CAPE) and dynamics (e.g., vertical wind shear) are shown in Fig. 2.

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TABLE 1. Populations (percentages given in parentheses) of all precipitation features and systems with specific properties in the various study regions marked in Fig. 1a: plateau, foothill, south China, lowland, and ocean. Lightning PFs, deep PFs, and intense PFs refer to PFs with lightning flash rate .1 min21, MAXHT20 . 12 km, and MAXHT40 . 7 km, respectively. Regions Plateau Foothill Lowland South China Ocean

All PFs Lightning PFs 13 140 14 679 10 417 11 295 7622

1964 (15%) 1946 (13%) 2381 (21%) 2304 (22%) 571 (7%)

Deep PFs

Intense PFs

1977 (15%) 1321 (9%) 1793 (16%) 2075 (20%) 772 (10%)

1749 (13%) 1187 (8%) 1577 (14%) 1451 (14%) 304 (4%)

Generally, the EASM first becomes established over south China in mid-May before propagating northward (Chen 1983; Ding 1992; Ding and Chan 2005). By mid-June, the monsoon rainband jumps from south China and Taiwan to the Yangtze River (or the lowland region in this study). On the other hand, the westerly steering winds (300–500 hPa) at 208–308N diminish with the progress of the monsoon and vanish by mid-July (Murakami 1958; Murakami and Ding 1982; Chen 1993; Xu 2011). During early summer (May–June), lengthy mei-yu fronts occur frequently and repeatedly over the south China and lowland regions (Chen 1983; Xu et al. 2009; Xu 2011). In midsummer (July–August), frontal convection and precipitation is less frequent, but the monsoon reaches its peak stage. Based on the seasonal change in the tropospheric flow and thermodynamics (also shown by Fig. 2), the warm season is separated into early summer and midsummer for the study of intraseasonal variations in convective characteristics.

3. Regional variations in precipitation and convective properties This section describes the regional climatology of precipitation and convective properties of summertime deep convection. Specifically, the rainfall contribution by storm type, the convective intensity of the dominant systems, and the horizontal/vertical convective structures are investigated. A breakdown of the PFs for each of the regions is given in Table 1.

a. Rainfall contribution by storm type Figure 3 shows the rainfall fraction contributed by PFs categorized by various convective proxies. Note that the rainfall of a PF is defined as volumetric rain (rain rate multiplied by rain area) based on PR rainfall estimates. Generally, the majority of rainfall in most regions is contributed by large, deep convective storms, except for the plateau. For example, 60%–80% of rainfall over

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East Asia, except for the plateau region (20%), is due to large precipitating systems (area .10 000 km2; Fig. 3a). A large percentage of rainfall over the plateau, as well as other regions, comes from precipitating clouds with cold cloud tops (MINIR , 2508C or 223 K; Fig. 3b) or high precipitation tops (MAXHT20 . 12 km; Fig. 3c). In other words, rainfall over the plateau region is mainly from relatively small (area ,5000 km2) but deep precipitating systems, while monsoon rainfall over East Asia comes from large (area .10 000 km2) and deep systems. This is reasonable, since the atmospheric conditions over the plateau region are drier (Fig. 2c) and less influenced by the monsoon circulations than the key monsoon regions. For the plateau region, a substantial portion (40%) of rainfall comes from intense convection (e.g., MAXHT40 . 7 km; Fig. 3d); however, a much smaller (20%) rainfall amount originates from storms with strong ice scattering (e.g., MIN85PCT , 180 K; Fig. 3e) or active thunderstorms (e.g., flash rate .5 min21). Intense convection (e.g., MAXHT40 . 7 km) also contributes a substantial amount of rainfall (40%) in the key monsoon regions but less so in the foothill and ocean regions (Fig. 3d). For example, approximately 40% (20%) of rainfall over the plateau, south China, and lowland (foothill) regions comes from systems with 40-dBZ radar echoes reaching the mixed-phase region (e.g., 7 km). In this regard, foothill convection is closer to that of typical maritime (Xu and Zipser 2012) where only 10%–20% of rain comes from strong mixed-phase processes (MAXHT40 . 7 km). In addition, much less foothill rainfall is due to storms with strong ice scattering (e.g., MIN85PCT , 200 K; Fig. 3e) and frequent lightning activity (flash rate .5 min21; Fig. 3f) than key monsoon regions over land (i.e., south China and lowland). Even the ocean region has a higher fraction of precipitation with a strong ice scattering signature than the foothill region (low value of MIN85PCT; Fig. 3e). The foothill region also has 10%–20% less rainfall from deep convective systems (MAXHT20 . 12 km) than the plateau region. These unique characteristics of foothill convective systems and the possible causes will be discussed in the discussion section (section 5a).

b. Convective intensity measured by TRMM proxies Shallow and small precipitating systems occupy a large fraction of the PF dataset (Liu et al. 2008); median level convection has similar intensities across different meteorological regimes (Xu and Zipser 2012). Therefore, this study examines both deep convection (MAXHT20 . 12 km; Liu and Zipser 2005) and storms with intensities above the median values (Fig. 4). Most importantly, this group of PFs contributes the majority of rainfall (Fig. 3).

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FIG. 3. Cumulative distribution frequencies (CDFs) of rainfall fraction contributed by storms categorized by convective proxies: (a) size of precipitation feature, (b) minimum infrared brightness temperature, (c) maximum height of radar echo with 20 dBZ, (d) maximum height of radar echo with 40 dBZ, (e) minimum microwave brightness temperature at 85 GHz, and (f) lightning flash rate.

Convective intensity generally ranks in order from the south China, lowland, plateau, and foothill regions down to the ocean region (Fig. 4 and Table 1). Specifically, the south China region has the strongest convection as measured by radar echo (both 20 and 40 dBZ; Figs. 4a,b) tops and ice scattering signature (MIN85PCT; Fig. 4c). Using these metrics, lowland storms are relatively weaker than south China although south China lightning flash rates are slightly higher. Storms over the foothill and ocean regions exhibit the weakest convective intensity by the radar metrics (Figs. 4a,b). Plateau convection displays much stronger radar reflectivity profiles (mainly above the freezing level) than foothill convection (Figs. 4a,b), indicating stronger midlevel updrafts (Zipser 1994; Heymsfield et al. 2010). On the other hand, plateau

storms only show weak-to-moderate ice scattering signatures and low lightning frequency (Figs. 4c,d). However, ice scattering signatures at 85 GHz are influenced by the depth of the ice column, the vertical distribution of large ice particles, and the presence of supercooled cloud drops (Adler et al. 1991; Smith et al. 1992). Overall, deep convection over slopes of the plateau (i.e., foothill) has a weaker intensity (i.e., weaker midlevel updrafts) than both the upstream (i.e., plateau) and downstream (i.e., lowland) regions. For example, the top 10% of convective systems over the plateau and foothill regions have 20-dBZ radar echoes above 13.5 and 12 km, and 40-dBZ echoes above 7.5 and 6.5 km, respectively. Regional changes in thermodynamic conditions (Fig. 2) can provide some insights into the regional variations

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FIG. 4. Storm intensity (above the median) of precipitation features in different subregions. Intensity indicated by the (a) maximum height of radar echo with 20 dBZ, (b) maximum height of radar echo with 40 dBZ, (c) minimum microwave brightness temperature at 85 GHz, and (d) lightning flash rate.

in convective intensity and properties. For example, the region with the most intense convection (i.e., south China) happens to have the largest CAPE (Fig. 2a) and highest LNB (Fig. 2b) as well as a large amount of TPW (Fig. 2c). On the other hand, the tropospheric bulk shear parameter (Fig. 2d) might not be an efficient discriminator of convective intensity in East Asia, as south China (the most convectively intense region) has the lowest value. It is interesting that the plateau region shows much stronger convective intensity than the foothill region despite having smaller CAPE and less TPW, though a higher LNB. It should be noted that convection and precipitation in the foothill region peak in the evening or early morning (Yu et al. 2007; Chen et al. 2010; Xu and Zipser 2011; Yuan et al. 2012) and deep continental stratus dominates this region (Yu et al. 2004; Li et al. 2005, 2008).

c. Horizontal storm structures Figure 5 illustrates the horizontal structures (by effective width) of precipitating systems as a function of convective intensity. In general, the more humid regions would be expected to have larger storms regardless of convective intensity, except for the very weak storms (e.g., MAXHT20 , 5 km). Storm size increases with increasing convective intensity, especially for intensities measured by MAXHT20 and MIN85PCT. The smallest storms occur over the plateau region, the largest systems over the ocean, while the monsoon storms are in between. These regional contrasts may help to explain why convective systems over the plateau region produce far less lightning flashes (Fig. 4d) even though their convective intensity (as measured by radar echo) is close to other regions (e.g., lowland; Figs. 4a,b). For example,

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FIG. 5. Size (square root of the near-surface raining area) of precipitation features with specific storm properties: (a) maximum height of radar echo with 20 dBZ, (b) maximum height of radar echo with 40 dBZ, (c) minimum microwave brightness temperature at 85 GHz, and (d) lightning flash rate.

convective systems over the lowland region may contain more convective cells (or larger convective cores; Fig. 5) and, therefore, have larger precipitating areas and hence more lightning flashes (or higher flash rate; Fig. 4d).

d. Convective vertical structures Figure 6 shows vertical convective structures over different regions in terms of vertical profiles of maximum radar reflectivity (Donaldson 1961; Zipser and Lutz 1994; Liu et al. 2008). Deep convection over the south China, lowland, and the plateau regions has the strongest vertical profiles (higher radar values at each altitude) and ocean convection the weakest, while foothill systems are intermediate between these two groups (e.g., top 10%; Fig. 6a). Vertical radar profiles (top 10%–20%; Figs. 6a,b) of deep systems over the plateau are identical to those over the lowland and south China regions in the mixed-phase region (e.g., from 2108 to 2308C), indicating a similar ability of updrafts to loft large ice particles and supercooled liquid drops (Zipser and Lutz 1994; Heymsfield et al. 2010) into the

midtroposhere. Radar vertical profiles for the plateau region stay constant below 2108C, but profiles from other regions increase downward from an altitude of 2108C to the freezing level. This indicates that the cloud bases of convective systems over the plateau region are located near or above the 2108C level, since mixedphase processes (e.g., riming) are not expected below cloud base. Although deep systems over the plateau region exhibit greater radar echoes (3–5 dBZ) than foothill systems in the mixed-phase region, they produce less rain at the ground than foothill storms (Figs. 6a,b). Subcloud-base evaporation could account for the difference in surface rainfall over the plateau region. The dramatic regional differences (e.g., plateau vs foothill) in vertical precipitation structures may complicate satellite rainfall estimates (e.g., passive microwave estimates based on only ice scattering signatures). With decreasing convective intensity, foothill system vertical structures approach those over the ocean (Figs. 6b,c). Finally, it is not surprising that weak-to-moderate storms (50% in this study) in all regions are generally

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FIG. 6. Vertical profiles of maximum radar reflectivity as a function of NCEP temperatures for precipitation features in different subregions in percentiles of (a) top 10%, (b) top 20%, (c) top 30%, and (d) median.

close (less than 2-dBZ difference) in vertical structure (Fig. 6d). Plateau convection has lower ‘‘cloud tops’’ (e.g., the minimum TRMM detectable value of 17–18 dBZ) than south China and the lowland region, but higher echo tops than foothill and ocean convection (Figs. 6a–c). However, actual cloud tops over the plateau region can be lower than in the foothill and ocean regions, since true cloud tops consist of small ice particles whose radar return is lower than the TRMM-detectable value. Luo et al. (2011) showed that deep convection over the northwest Pacific Ocean has much higher cloud tops than that over the Tibetan Plateau. As previously noted, deep precipitating systems over the plateau region exhibit much weaker microwave ice scattering (MIN85PCT; Fig. 4c) and lower lightning flash rates (Fig. 4d) than those with similar radar echo

tops in other regions (e.g., lowland; Figs. 4a,b). The ice scattering signatures of plateau storms are even weaker than that of ocean systems. At first glance, these results sound contradictory. Nevertheless, 85-GHz signatures respond to the depth of the ice column, large ice particles, and presence of supercooled liquid drops (Adler et al. 1991; Smith et al. 1992), while radar reflectivity is more sensitive to particle size. As has been shown, convection over the plateau has shallower ice columns (e.g., cloud bases well above the freezing level and lower true cloud tops) and fewer large ice particles at lower layers (Figs. 6a,b). In addition, cloud drops might just start to grow in the lower part of the mixed-phase region (e.g., from 258 to 2108C), rather than frozen or nucleating into ice, due to the high cloud of the plateau’s convective systems (i.e., above 258C). As a result, the plateau’s convective storms might have a greater portion

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FIG. 7. As in Fig. 4, but for the periods of May–June (blue) and July–August (red).

of supercooled water in the mixed-phase region, which tends to offset through emission the scattering signal by graupel at 85 GHz. All of these factors might contribute to the weak ice scattering signatures and low lightning flash rates in the plateau region. However, the strong radar reflectivity profiles at midlevels (e.g., from 2108 to 2308C) in plateau convection might be due to a special vertical updraft structure. Since plateau systems have cloud bases near 2108C, these storms might have local peak updrafts at levels a few degrees below 2108C due to condensation, while updrafts inside storms of other regions might first peak near the freezing level (Heymsfield et al. 2010).

4. Intraseasonal variations in convective intensity and morphology The last section showed clear regional contrasts in precipitation characteristics, convective intensity, and

horizontal/vertical convective structures. This section compares early summer and midsummer convection in terms of convective intensity (Fig. 7) and horizontal storm size (Fig. 8). Intraseasonal changes in thermodynamics and dynamics were shown in Fig. 2.

a. Convective intensity In general, the intensity of deep convection as measured by different metrics in different regions strengthens when midsummer arrives (Fig. 7). Greater surface heating, a higher tropopause, more moisture, or deeper penetration of the monsoon in midsummer might be responsible for the intensification of convection. For example, midsummer shows much higher values of CAPE and LNB, and relatively larger TPW than early summer (Fig. 2). Larger CAPE and higher LNB allow deep convection to grow to a greater depth under similar large-scale or mesoscale settings. Of all the regions, the lowland experiences the greatest increase in

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FIG. 8. As in Fig. 5, but for the periods of May–June (blue) and July–August (red).

convective intensity. For example, for the top 20% of PFs, 20- (40-) dBZ echo tops rise from 9 (5) km in early summer to 12.5 (6.5) km in midsummer (Figs. 7a,b) while 85-GHz depressions deepen from 220 to 180 K (Fig. 7c). This is mainly due to the fact that the monsoon does not peak until midsummer in this region (Ding 1992; Ding and Chan 2005) leading to large intraseasonal changes in thermodynamic conditions (e.g., CAPE, LNB, and TPW; Figs. 2a,c). On the other hand, the south China region experiences much smaller intraseasonal changes in convective intensity, since the summer monsoon already arrives in this region as early as mid-May (Chen 1983; Ding 1992). For example, for the top 20% of convection, 20-dBZ echo tops increase 1 km and 85-GHz depressions increase by 10 K from early summer to midsummer. The plateau, foothill, and ocean regions experience a dramatic increase (20%– 30%) in precipitation tops (i.e., 20 dBZ; Fig. 7a) but relatively small changes (,10%) in ice scattering signature and lightning frequency (Figs. 7c,d). Over the plateau region, deep convection in midsummer produces even lower lightning flash rates than in early summer (Fig. 7d). A drier surface and thus higher

Bowen ratio over the plateau in early summer might be the cause of the greater lightning activity in early summer (Qie et al. 2003; Ma et al. 2003). Qie et al. (2003) and Ma et al. (2003) reported that the typical Bowen ratio on over the plateau is extremely high (about 9.0) in May when the surface is very dry, and decreases to 0.5 as the latent heat flux becomes larger than the sensible heat flux.

b. Horizontal storm size Figure 8 shows precipitation feature horizontal extent for specific regions and periods as a function of convective intensity. Results show that precipitating system size decreases substantially from early summer to midsummer. In general, intraseasonal variations in storm size (Fig. 8) are consistent with the changes in the magnitude of the vertical wind shear (Fig. 2d). The vertical wind shear in midsummer decreases dramatically as the upper-level jet shuts down after mid-July (Fig. 2d). Specifically, the key monsoon regions (lowland, south China, and ocean) experience the greatest decrease in storm size (Fig. 8). In early summer, mei-yu fronts occur frequently and produce large convective

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systems along the front (Ding 1992; Ding and Chan 2005). In contrast, there are far fewer mei-yu frontal systems in midsummer as mei-yu season ends after midJuly (Ding 1992; Ding and Chan 2005). As a result, weaker large-scale dynamic forcing (e.g., by frontal systems) and smaller vertical wind shear in midsummer cause a decrease in the horizontal area of precipitating systems. On the other hand, intraseasonal changes in deep convective storm size are trivial over the plateau and foothill regions, where the influence of the summer monsoon is less important.

5. Discussion on interregional variability This section discusses how convective properties and microphysical characteristics change due to various regional monsoon dynamics and topography. Comparisons are made between regions with different elevations (i.e., upland vs lowland regions), regions under different monsoon phases (i.e., south China vs lowland), and land–sea contrasts (i.e., coastal regions vs the ocean).

a. Upland versus lowland regions This subsection examines how elevation affects convective variability. There is a huge elevation drop (about 3 km) from the plateau (4.5 km) to the foothill region, and another drop (more than 1 km) from the foothill to the key monsoon regions (i.e., lowland and south China; 1.5 km; Fig. 1a). Deep convection over the plateau region seems to have strong midtropospheric updrafts that loft large hydrometeors into the mixed-phase region (e.g., 35–42 dBZ between 2108 and 2308C) but relatively low lightning flash rates and short ice water paths. Similarly, previous studies (Qie et al. 2003; Xu and Zipser 2011; Zhang et al. 2008; Xie et al. 2010) showed that lightning and hail activity are quite robust but lightning flash rates and hail size are relatively small. These could be attributed to smaller convective areas and higher cloud bases (above sea level) that may limit the area and depth of the mixed-phase region. As has been shown, vertical radar profiles over the plateau region remain constant below the 2108C level, indicating that the cloud base is near or above the 2108C level as mixed-phase processes (e.g., riming) cannot occur below the cloud base. As a result, deep convective systems over the plateau region have shallower mixed-phase depths, at least they lack the portion between 2108 and 08C when compared to their counterparts over the south China or lowland regions where cloud bases should be below the freezing level (see illustration in Fig. 9). Section 3c also showed that convective systems over the plateau region might contain fewer convective cells or smaller convective cores and therefore smaller

FIG. 9. Schematic diagram of convective structures and associated microphysics for deep convective precipitating systems in different regions.

precipitating areas. With this combination of factors, deep convective systems are expected to produce fewer lightning flashes and smaller hail compared to regions with slightly stronger convection (e.g., south China and lowland). Many studies have pointed out that convection over the plateau region is dramatically weaker than that over the southern slopes of the plateau or southern Asian monsoon region (Park et al. 2007; Houze et al. 2007; Romatschke et al. 2010; Luo et al. 2011). This study shows that deep convection over the slopes and foothills of the eastern plateau (foothill region) is substantially weaker than that over the plateau. Total precipitation in the foothill region originates more from storms without active mixed-phase processes. In fact, precipitation in the foothill region peaks in the evening or early morning when convective initiation or precipitation enhancement caused by a solenoid (or mountain valley) circulation occurs (Yu et al. 2007; Xu and Zipser 2011; Bao et al. 2011; Wang et al. 2012; Yuan et al. 2012). Over the foothill region, deep continental stratus clouds frequently develop (Yu et al. 2004; Li et al. 2005, 2008). Deep continental stratus hinders solar radiation from reaching the ground and causes lower surface air temperatures in the afternoon (Li et al. 2008). In addition, there is warm, moist advection in the mid- to upper-level troposphere over the foothill region (Chen et al. 2010). As a result, the development of high static instability and intense afternoon local thermal convection over this region are limited. The key monsoon regions (lowland/south China) have much stronger convection than the region along the periphery of the monsoon (foothill; Fig. 4). The stronger surface heating and deeper moist air mass in the

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lowland/south China region due to the lower elevation may contribute to the higher CAPE/TPW (Fig. 2a) and, therefore, deeper convection. Precipitating systems at the foothill region only resemble those in the lowerelevation regions (i.e., lowland/south China) in terms of storm size (Fig. 5). This might be explained by the fact that the foothill region is still under the control of the monsoon’s moist flows and large-scale dynamics.

b. South China versus lowland Both the south China and lowland regions are in the core of the EASM. The summer monsoon first peaks in the south China region before progressing to the lowland region. For example, monsoon rainbands or mei-yu fronts are first established in south China in mid-May and jump to the lowland region after mid-June. On average, the south China and lowland regions share similar convective characteristics, including rainfall contribution (Fig. 3), intensities (Fig. 4), storm size (Fig. 5), and vertical structures (Fig. 6). However, during the first stage of the monsoon (May–June), south China precipitating systems exhibit much stronger convective intensity (blue curves in Fig. 7) than the lowland region. As has been mentioned, during the first stage, thermodynamic conditions in the lowland region are less favorable for deep convection (Fig. 2). Lowland systems have a larger storm size than south China storms (Fig. 8, blue curves), since the steering winds and cold front dynamics are still strong in the lowland region during May–June. When the lowland region approaches its peak monsoon period, convection and lightning are more intense in the lowland than in the south China region (red curves in Fig. 7). Earlier studies also showed that convective intensity in the EASM increases progressively as the monsoon develops (Xu 2011; Luo et al. 2013). Luo et al. (2013) further pointed out that stronger convection in the Yangtze–Huaihe River basin (YHRB; lowland region in this study) compared to the south China region is due to the flatter land, higher frequency of surface fronts, and stronger intensity of low-level vortices in the YHRB.

c. Coastal regions versus the ocean In the summertime, the East China Sea ocean region is part of the key monsoon region. Monsoon rainbands or mei-yu fronts drop comparable amounts of rain over the ocean and over land (Xu et al. 2009; Xu 2011). However, the monsoon brings much more intense convection and lightning to coastal regions than to the ocean (Fig. 4). Similarly, this and previous studies show evident land–sea contrasts in convective intensity (with the land much stronger than the ocean), even between coastal regions and the ocean (Zipser 1994; Zipser et al.

2006; Xu and Zipser 2012). Strong surface heating might be one of the factors contributing to the land–sea convective difference. In addition, aerosol loading (Rosenfeld and Lensky 1998; Williams and Stanfill 2002) and storm morphology (Zipser 2003; Kelley et al. 2010) are thought to be major factors. In this study, a certain percentage (10%) of ocean convection is still quite deep (MAXHT20 . 12 km), has strong ice scattering, and produces significant amounts of lightning, though less than land systems (Figs. 4a,c,d). In fact, storms over the East Asian ocean region show stronger convective intensity and higher lightning flash rates than those over the open Atlantic Ocean or northeast Pacific (Zipser and Lutz 1994; Zipser et al. 2006; Xu 2011). This may be due to this region has warmer sea surface temperature and stronger large-scale dynamic forcing from such as monsoon troughs, mei-yu fronts, and intrusion of midlatitude systems.

6. Summary and conclusions This study finds substantial regional and intraseasonal variations in the precipitation and convective characteristics of summer deep convection in East Asia. Specifically, deep precipitating systems over the plateau region (eastern Tibetan Plateau) display active mixedphase microphysics processes (large echoes in the mixed-phase region), though they are small in horizontal extent and exhibit weak microwave (85 GHz) ice scattering signatures. The lightning flash rate of precipitating systems over the plateau is relatively low and possibly associated with their small storm size. Compared to the plateau, foothill (slopes and foothills of the eastern Tibetan Plateau) convection is weaker (with weaker echoes in the mixed-phase) and has lower precipitation tops (i.e., 20 dBZ). In contrast, systems over the south China region are the strongest (followed by the lowland region) in all of the convective proxies such as the maximum height of 20- or 40-dBZ radar echoes, depression of brightness temperature in both IR and microwave (85 GHz) channels, and lightning frequency. Ocean convection is the weakest according to most metrics except microwave ice scattering. Deep convective structures for different regions analyzed in this study are summarized in a schematic diagram (Fig. 9). Analysis also indicates that regional and intraseasonal contrasts in convective properties appear significantly consistent with changes in dynamic and thermodynamic conditions over a region (Fig. 2). In short, the major conclusions drawn from this study are listed as follows: d

A large fraction (40%–60%) of rainfall over East Asia originates from large precipitating systems (e.g., raining areas .10 000 km2) and deep precipitating systems

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d

d


(e.g., maximum heights of 20 dBZ . 12 km), except over the plateau where large systems only contribute 20% of the total rainfall; about 40% (20%) of the rainfall over the key monsoon region in East Asia (foothill and ocean) comes from storms with robust mixed-phased processes (e.g., 40 dBZ at 7 km) and strong lightning activity; plateau rainfall also significantly depends on mixed-phase precipitation processes. The intensity of deep convection over the plateau is weaker than that over low elevations in the East Asian monsoon region (e.g., south China or lowland), due to the special local environment (e.g., less CAPE, lower LBN, and drier); deep convective clouds over the plateau region are smaller in horizontal extent, have higher cloud bases and shallower mixed-phase depths, and produce less lightning flashes than the south China or lowland regions. Compared to the lowland or the plateau regions, convection in the foothill region has much weaker convective intensity in most metrics; in contrast, precipitating systems over the south China and lowland regions are the strongest, while ocean convection is the weakest as indicated by most proxies, except for microwave ice scattering where ocean convection is stronger than the plateau and foothill regions. In most regions, deep convection significantly increases in convective intensity but decreases in horizontal precipitation size from early summer to midsummer; this is mostly due to intraseasonal changes in environmental thermodynamics (e.g., an increase of CAPE, LNB, and TPW toward midsummer), large-scale dynamic forcing (e.g., fewer frontal systems), and vertical wind shear (weaker).

Acknowledgments. This research was supported by the NASA Earth Science and Space Fellowship program. Special thanks go to Professor Edward Zipser and Dr. Chuntao Liu at the University of Utah for providing the TRMM precipitation feature dataset and science discussions.

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