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    (a)–(d) A case of cloud and large-scale environment associated with a baiu front on 11 Jun 2006. (a) MTSAT-1R infrared brightness temperature at 0030 UTC (K; colors), JRA-25–JCDAS daily-mean sea level pressure (hPa; black contours), and JRA-25–JCDAS daily-mean horizontal winds at 925 hPa (m s−1; arrows). (b) JRA-25–JCDAS daily-mean specific humidity at 925 hPa (g kg−1; colors) and JRA-25–JCDAS daily-mean horizontal winds at 925 hPa (m s−1; arrows). (c) JRA-25–JCDAS meridional gradients of (K deg−1 latitude; colors) and (K; contours). (d) Surface weather chart on 11 Jun 2006 provided by the Japan Meteorological Agency (JMA). (e) Horizontal distribution of mean rainfall rates (mm h−1; colors) calculated with TRMM PR2A25 and mean calculated with JRA-25–JCDAS, which are averaged during June–July for 14 yr (1998–2011). The red thick line in (a) and the black thick lines in (b),(c),(e) show where = 345 K.

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    (a) Latitude–pressure cross section of JRA-25–JCDAS θe (K; colors), zonal wind (m s−1; black contours), and pressure vertical velocity (Pa s−1; white contours); (b) latitudinal distribution of mean rainfall rates (mm h−1) observed with the TRMM PR; and (c) latitudinal distribution of meridional gradients of . Variables are averaged from 122° to 135°E during June–July for 14 yr (1998–2011). In (a), white contours indicate −0.06, −0.05, −0.04, −0.03, −0.02, −0.01, and 0 Pa s−1 with dotted contours for negative values.

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    Time–latitude cross sections of (a),(b) TRMM PR mean rainfall rates (mm h−1; colors) and (c) TRMM PR convective rainfall ratios (%; colors), which are averaged from 122° to 135°E for 14 yr (1998–2011). The 5-day running-mean rainfall rates for total and convective rain are utilized in this figure. In (a),(c), contours indicate the JRA-25–JCDAS and thick contours show where = 345 K. In (b), the black line indicates mean reference latitudes of the baiu front, which is detected using = 345 K (JRA-25–JCDAS), and error bars indicate the standard deviation. In (c), only regions with mean rainfall rates > 0.1 mm h−1 are plotted.

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    Composite meridional distributions of (a) mean rainfall rates, (b) convective rainfall ratios, and (c) rainfall intensities. Composites are performed with their centers at latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). Positive (negative) latitudes are to the north (south) of the reference latitude. In (a),(c), the black thick line, black thin line, and gray thin line represent total rainfall, convective rainfall, and stratiform rainfall, respectively. Error bars indicate the 99% confidence interval for the mean values with Student’s t test.

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    (a) Composite histograms of rainfall rates in the northern (0°–7.5°; black bars) and southern (−7.5° to 0°; gray bars) regions of the baiu front. The ordinate indicates ratios (%) of the number of pixels with each rainfall rate to the number of total observational pixels in each region. The abscissa indicates rainfall rates in a dBR scale = 10 × log10(rainfall rates). Correspondence between dBR and rainfall rates (mm h−1) are as follows: −6 dBR = 2.5 × 10−1, 4 dBR = 2.5, 18 dBR = 63, and 24 dBR = 2.5 × 102. (b) As in (a), but for rainfall ≥ 18 dBR. Composites are performed with their centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011).

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    Composite meridional histograms of ETH for (a) total precipitation, (b) stratiform precipitation, and (c) convective precipitation. Colors indicate ratios (%) of the number of pixels with each ETH to the number of total observational pixels in each relative latitudinal bin. Composites are performed with thier centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). Positive (negative) latitudes are to the north (south) of the reference latitude.

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    As in Fig. 6, but for mean near-surface rainfall rates from each ETH.

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    Composite cumulative frequencies of effective radar reflectivity factor (Ze) by altitudes in the (a) northern (0°–7.5°) and (b) southern (−7.5° to 0°) regions of the baiu front. Composites are performed with their centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). Contours are set at the 50, 70, 80, 90, 95, 99, 99.5, 99.9, and 100th percentiles, starting from the left. Thick contours indicate the 90 and 99.9th percentiles.

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    Profiles of effective radar reflectivity factor (Ze) with top 0.1% near-surface rainfall rates with ETHs (a),(d) <8 km; (b),(e) 8–14 km; and (c),(f) 14–20 km on the (a)–(c) northern (0°–7.5°) and (d)–(f) southern (−7.5° to 0°) sides of the baiu front over the region from 122° to 135°E during June–July for 14 yr (1998–2011). The mean Ze profiles for all extreme profiles in each category are shown with thick black solid lines.

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    Examples for PFs to the (a),(c) north and (b),(d) south of the latitudes with = 345 K. Snapshots taken on (a),(c) 30 Jun 2003 and (b),(d) 27 Jun 2006. In (a),(b), colors indicate near-surface rain rates (mm h−1) ; thick lines indicate = 345 K; and mean positions of PFs are marked with crosses. In (c),(d), precipitation rates (mm h−1) are three-dimensionally indicated.

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    Composite rain contribution (%; colors) of PFs in terms of (a),(b) areas and maximum 20-dBZ heights and (c),(d) convective rainfall ratios and maximum 20-dBZ heights for the (a),(c) northern (0°–7.5°) and (b),(d) southern (−7.5° to 0°) regions of the reference latitudes. Composites are performed with their centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). In (a),(b), the abscissa indicates PF areas on the logarithmic scale.

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    (a) Composite profiles of specific humidity (g kg−1). (b) Composite profiles of θ (K), θe(K), and saturated θe (K). Black lines indicate the profiles in the southern region (−7.5° to 0°) of the baiu front, while gray lines indicate the profiles in the northern region (0°–7.5°). In (b), the lines indicate θ, θe, and saturated θe from the left to right. Composites are performed with their centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). Error bars indicate the 99% significant intervals for the mean values with Student’s t test.

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    Latitude distribution of the number of total pixels observed with the TRMM PR. All pixels are counted from 0° to 359.5°E from January 1998 to December 2011. The y axis runs from 2 × 108 to 1.6 × 109.

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A Contrast in Precipitation Characteristics across the Baiu Front near Japan. Part I: TRMM PR Observation

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  • 1 Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Japan
  • | 2 Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan
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Abstract

Contrasts in precipitation characteristics across the baiu front are examined with Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data near Japan during June–July (1998–2011). The vertical structure of atmospheric stratification differs between the tropics and midlatitudes. On an average, the baiu front is found around the latitude that roughly divides the midlatitude atmosphere from the tropical atmosphere. Precipitation characteristics are compared between the southern and northern sides of the reference latitude of the baiu front, which is detected with equivalent potential temperature at 1000 hPa of 345 K in terms of the boundary between the tropics and midlatitudes.

The results show that there are obvious differences in precipitation characteristics between the southern and northern sides. In the south, convective rainfall ratios (CRRs) are 40%–60%, which are larger than those in the north (20%–40%). Greater rainfall intensity and taller/deeper precipitation are also observed in the south. Moreover, the characteristics of precipitation features (PFs), which are contiguous areas of nonzero rainfall, differ between the southern and northern sides. In the north, wide stratiform precipitation systems with CRRs of 0%–40% and heights of 8–11 km are dominant. In the south, organized precipitation systems with heights of 12–14 km and CRRs of 30%–50% and those with very large heights (14–17 km) and CRRs of 50%–80% are dominant in addition to wide stratiform precipitation systems. These results suggest that the mechanisms to bring rainfall are different between the southern and northern regions of the baiu front.

Corresponding author address: Chie Yokoyama, Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8568, Japan. E-mail: chie@aori.u-tokyo.ac.jp

Abstract

Contrasts in precipitation characteristics across the baiu front are examined with Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data near Japan during June–July (1998–2011). The vertical structure of atmospheric stratification differs between the tropics and midlatitudes. On an average, the baiu front is found around the latitude that roughly divides the midlatitude atmosphere from the tropical atmosphere. Precipitation characteristics are compared between the southern and northern sides of the reference latitude of the baiu front, which is detected with equivalent potential temperature at 1000 hPa of 345 K in terms of the boundary between the tropics and midlatitudes.

The results show that there are obvious differences in precipitation characteristics between the southern and northern sides. In the south, convective rainfall ratios (CRRs) are 40%–60%, which are larger than those in the north (20%–40%). Greater rainfall intensity and taller/deeper precipitation are also observed in the south. Moreover, the characteristics of precipitation features (PFs), which are contiguous areas of nonzero rainfall, differ between the southern and northern sides. In the north, wide stratiform precipitation systems with CRRs of 0%–40% and heights of 8–11 km are dominant. In the south, organized precipitation systems with heights of 12–14 km and CRRs of 30%–50% and those with very large heights (14–17 km) and CRRs of 50%–80% are dominant in addition to wide stratiform precipitation systems. These results suggest that the mechanisms to bring rainfall are different between the southern and northern regions of the baiu front.

Corresponding author address: Chie Yokoyama, Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8568, Japan. E-mail: chie@aori.u-tokyo.ac.jp

1. Introduction

The baiu front, which is also called the mei-yu front in China and the changma front in Korea, is the stationary front, which forms between tropical and midlatitude air during early summer in East Asia. A large amount of monsoon rainfall is observed associated with the baiu front. It is characterized by large gradients of moisture. Gradients of temperature are relatively small around the baiu front, although they are remarkable in eastern Japan (e.g., Matsumoto et al. 1971). The baiu front appears around Taiwan and South China in May. Its average position gradually moves northward with time, as the subtropical high over the Pacific grows in its influence. In the instantaneous view, however, the position of the baiu front fluctuates in a meridional direction.

The baiu front establishes its quasi-stationary position over the islands of Japan and the adjacent ocean during early May to mid-July. During this period, a zonally elongated belt of cloud associated with the baiu front usually appears around Japan. Figures 1a–c show an example of the baiu front with infrared (IR) brightness temperature observed with the Multifunctional Transport Satellite-1R (MTSAT-1R) and large-scale environments on 11 June 2006. In this case, a belt of cloud associated with the baiu front extends from the south of Taiwan to eastern Japan. Brightness temperature is lower in the southwestern part of the cloud belt. Low-level (925 hPa) southwesterlies flow around the periphery of the anticyclone over the Pacific into the cloud belt (Fig. 1a). The southwesterlies transport low-level humid air, which originates in the subtropics, to the cloud belt (Fig. 1b). Figure 1c shows that the cloud belt appears in the region characterized by strong meridional gradients of equivalent potential temperature (θe) near the surface (1000 hPa). It is also shown that latitudes with θe at 1000 hPa = 345 K roughly correspond to those with large meridional gradients of . According to the Japan Meteorological Agency (JMA) surface weather chart on the same day (Fig. 1d), a stationary front and meso-α-scale disturbances, which occasionally appear along the baiu front as its internal structure, are analyzed around the region with large meridional gradients of (Figs. 1c,d).

Fig. 1.
Fig. 1.

(a)–(d) A case of cloud and large-scale environment associated with a baiu front on 11 Jun 2006. (a) MTSAT-1R infrared brightness temperature at 0030 UTC (K; colors), JRA-25–JCDAS daily-mean sea level pressure (hPa; black contours), and JRA-25–JCDAS daily-mean horizontal winds at 925 hPa (m s−1; arrows). (b) JRA-25–JCDAS daily-mean specific humidity at 925 hPa (g kg−1; colors) and JRA-25–JCDAS daily-mean horizontal winds at 925 hPa (m s−1; arrows). (c) JRA-25–JCDAS meridional gradients of (K deg−1 latitude; colors) and (K; contours). (d) Surface weather chart on 11 Jun 2006 provided by the Japan Meteorological Agency (JMA). (e) Horizontal distribution of mean rainfall rates (mm h−1; colors) calculated with TRMM PR2A25 and mean calculated with JRA-25–JCDAS, which are averaged during June–July for 14 yr (1998–2011). The red thick line in (a) and the black thick lines in (b),(c),(e) show where = 345 K.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

Since a huge amount of precipitation is brought to Japan during the baiu season (June–July; Fig. 1e), it is important to understand the variations in precipitation associated with the baiu front. For example, it is well known that heavy rainfall events often occur in the late baiu season. In previous studies, the time evolution and spatial distribution of precipitation amounts have been examined in different regions of East Asia (e.g., Chen et al. 2004; Okada and Yamazaki 2012). Utilizing Tropical Rainfall Measuring Mission (TRMM) satellite-borne Precipitation Radar (PR) data, Takayabu and Hikosaka (2009) showed that the precipitation regime around Japan changes just before the withdrawal of the baiu season; precipitation with high echo tops begins to increase from that time. In some studies, using the TRMM data, mesoscale properties of precipitation systems in the mei-yu season were also investigated over different regions: for example, South China, Taiwan, the South China Sea (Xu et al. 2009), and the eastern Tibetan Plateau (Xu and Zipser 2011). Zhang et al. (2006) also used Doppler data to examine characteristics of mesoscale precipitation systems associated with the mei-yu front over the east part of continental China.

In this study, we will statistically show a meridional contrast in precipitation characteristics across the baiu front. Here we would like to remark that the baiu front exists around the boundary that roughly divides the tropical atmosphere from the midlatitude atmosphere. Basically, there is a significant difference in the structure of atmospheric stratification between the tropics and midlatitudes. In the tropics, the moist static energy is large in both the lower and upper troposphere, with its local minimum in the midtroposphere on average, whereas in the midlatitudes the moist static energy increases with altitudes (Riehl and Malkus 1958). In terms of the global energy transport, the primary phenomena to redistribute the moist static energy differ between the tropics and midlatitudes. A widely accepted concept of the tropics is that deep convection plays the important role of a “hot tower” in lifting air parcels with large moist static energy from the lower troposphere to the upper troposphere (Riehl and Malkus 1958), although large dilution of updrafts by entrainment is emphasized recently (Zipser 2003). In the midlatitudes, on the other hand, transient disturbances (i.e., extratropical cyclones) are primarily in charge of horizontal and vertical energy transport. That means that deep convection is more essential in the tropics on average than in the midlatitudes, and precipitation characteristics are basically different between the tropical and midlatitude atmosphere. From this aspect, change in precipitation characteristics around Japan just before the withdrawal of the baiu season (Takayabu and Hikosaka 2009) may be related to the northward expansion of tropical air associated with the northward movement of the boundary between the tropics and midlatitudes. In this study, we examine how precipitation characteristics vary across the baiu front, which is the boundary between the tropics and midlatitudes.

Figure 2a shows that such a contrast in the atmospheric stratification between the tropics and midlatitudes can be expressed by a latitude–height section of θe around Japan in the baiu season (June–July). The 14-yr-mean environment fields across the baiu front over 122°–135°E in June–July are produced with the 25-yr Japanese Reanalysis (JRA-25)–Japan Meteorological Agency Climate Data Assimilation System (JCDAS) data (Onogi et al. 2007). Consistent with the above-mentioned meridional contrast in vertical profiles of moist static energy, the lower–middle troposphere is more convectively unstable in the tropics compared to the midlatitudes. On average, the baiu front, which is identified by the maximum of mean rainfall rates observed by the TRMM PR, is found around the latitude that roughly divides the midlatitude atmosphere from the tropical atmosphere (Figs. 2a,b). Matsumoto et al. (1971) also performed an airmass analysis that indicated the coincidence of the axis of the maximum cloud amount, related to the baiu front, with the boundary between the polar airmass and the monsoon or the tropical airmass. Figure 2a shows the typical environment around the baiu front, where the strongest zonal winds are located in the upper troposphere to the north of the baiu front and the maximum upward flows exist around and to the south of the baiu front.

Fig. 2.
Fig. 2.

(a) Latitude–pressure cross section of JRA-25–JCDAS θe (K; colors), zonal wind (m s−1; black contours), and pressure vertical velocity (Pa s−1; white contours); (b) latitudinal distribution of mean rainfall rates (mm h−1) observed with the TRMM PR; and (c) latitudinal distribution of meridional gradients of . Variables are averaged from 122° to 135°E during June–July for 14 yr (1998–2011). In (a), white contours indicate −0.06, −0.05, −0.04, −0.03, −0.02, −0.01, and 0 Pa s−1 with dotted contours for negative values.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

In addition, meridional gradients of are large near the latitude of the boundary between the midlatitude and tropical atmosphere, which is consistent with Ninomiya’s (1984) result where the baiu frontal zone (a stationary precipitation zone appearing in June–July around the islands of Japan) was characterized by a strong gradient of θe. Note that the latitude of the maximum mean rainfall rate does not agree very well with that of the maximum meridional gradient of over 135°–145°E (not shown), where low-level temperature has a relatively strong meridional contrast (e.g., Matsumoto et al. 1971).

The above-mentioned meridional contrast in the atmospheric stratification across the baiu front suggests that precipitation characteristics may also vary between the southern and northern sides of the baiu front. Takayabu and Hikosaka (2009) showed that the timing of the increase in contribution from precipitation with high echo tops coincides well with that of the increase in the lowest-level θe. Kato et al. (2003) examined the vertical cross sections of the structures of the baiu front in a meridional direction. They reported the importance of low-level moist air and upper-level dry air associated with the subsidence of the subtropical high to the south of the baiu front, which result in strong convective instability and occasional extreme rainfall.

The purpose of this study is to statistically and quantitatively examine the differences in precipitation characteristics in the southern and northern regions of the baiu front, by using the TRMM PR observational data. In addition, we investigate the characteristics of the contiguous areas of nonzero rainfall to elucidate what kind of precipitation systems is dominant to the south and north of the baiu front. To compare precipitation characteristics to the south of the baiu front with those to the north, we define the reference latitude of the baiu front, which will be mentioned in the next section.

We also aim to provide observational evidence to verify precipitation characteristics simulated by climate models, which will be addressed in a companion paper (Kanada et al. 2014, manuscript submitted to Climate Dyn., hereafter KTY). In recent years, there has been growing interest in the future changes of precipitation characteristics of the baiu front that accompany future climate changes. Some studies have analyzed the simulations of multiple models to evaluate precipitation related to the baiu front and the Asian summer monsoon in the current climate and tried to understand future changes that are expected to occur (Seo and Ok 2013; Seo et al. 2013; Sperber et al. 2013; Kusunoki and Arakawa 2012; Ninomiya 2009, 2011). In the companion paper (KTY), we will examine how precipitation characteristics across the baiu front are simulated in the climate models that are included in phase 5 of the World Climate Research Programme Coupled Model Intercomparison Project (CMIP5).

2. Data and methodology

a. Data

The TRMM PR2A25 version 7 product, in which orbital data of profiles of radar reflectivity and precipitation rates observed by the TRMM Precipitation Radar are stored, is used to examine precipitation characteristics. Since December 1997, the TRMM satellite has observed precipitation in the entire region between 35°S and 35°N, and the TRMM satellite-borne PR can observe three-dimensional precipitation over both oceans and land. The data have a horizontal resolution of 4.3 km (before an orbit boost in August 2001) or 5 km (after the boost) and a vertical resolution of 250 m up to 20 km. The mean rainfall rate, rainfall intensity, convective rainfall ratio (CRR; ratio of convective rainfall to total rainfall near the surface), echo-top height (ETH), and effective radar reflectivity factor associated with the baiu front are investigated. In this study, we use a term “mean rainfall rate” to represent the unconditional mean near-surface rainfall rates for the total observed pixels (i.e., all pixels regardless of existence of rainfall), while “rainfall intensity” represents conditional mean for only rainy pixels. We also define “mean convective (stratiform) rainfall rates” as unconditional mean convective (stratiform) near-surface rainfall rates for total observed pixels and “convective (stratiform) rainfall intensity” as conditional mean for only convective (stratiform) pixels. Echo tops are detected using the threshold of precipitation rate greater than or equal to 0.3 mm h−1 and rain-certain flags, which indicate the presence of some strong echoes above noise in clutter-free ranges.

We also utilize the TRMM radar precipitation feature (PF) level-2 data from the University of Utah database (Liu et al. 2008) to investigate the characteristics of precipitation systems. Precipitation features (PFs) are defined by grouping contiguous pixels with nonzero PR2A25 near-surface rainfall. The information of pixel-level measurements is compressed into characteristics of PFs (Liu et al. 2008). For example, by analyzing PF dataset, we can obtain a variety of information on each PF, such as the mean position, the number of rainy pixels, the rain volume (i.e., the product of the average rain rate in millimeters per hour and area in square kilometers), the convective and stratiform rain volume, and the maximum height with reflectivity of 20 dBZ. Previous studies show that the analysis of characteristics of PFs is useful for understanding what kinds of precipitation systems bring rainfall to different regions of the earth (e.g., Xu and Zipser 2012; Liu et al. 2008). In this study, we examine PFs or contiguous areas of rainy pixels in relation to the baiu front.

In addition, the JRA-25–JCDAS data (Onogi et al. 2007) are used as meteorological data. The dataset is provided with a horizontal resolution of 1.25° and 23 pressure levels from 1000 to 0.4 hPa. We use daily data, which are averaged from the original 6-hourly JRA-25–JCDAS data. In this study, the analysis period is from June to July over a 14-yr period (1998–2011).

b. Detection of the baiu front

As already mentioned in section 1, the baiu front uniquely exists at which the midlatitude and tropical atmosphere abut on each other. It is known that large meridional gradients of θe appear around the baiu front (e.g., Kato et al. 2003). Suzuki and Hoskins (2009) also used lower-tropospheric θe to examine the extent of warm moist subtropical air. In this study, we define the baiu front as the boundary between the midlatitude and tropics in terms of the atmospheric stratification. According to Figs. 2a and 2b, the boundary and the maximum rainfall amount are found around the latitude with = 345 K, which is also indicated in Fig. 3 of Takayabu and Hikosaka (2009). Therefore, in this study, we utilize this value as the reference latitude for the detection of the baiu front. The reference latitude of the baiu front based on this detection is also found near the latitudes with large meridional gradients of θe (Fig. 2c). In addition, Fig. 2a shows that θe near the surface is comparable to that in the upper troposphere (~300 hPa) to the south of the reference latitude of the baiu front. This structure characterizes the tropical stratification, and is considered as a zeroth-order approximation where hot towers play a role in bringing up high θe air in the lower troposphere to the upper troposphere. Thus, our detection of the baiu front appears to be reasonable in the current climate. The method used to detect the baiu front from large-scale conditions will be discussed in more detail in a companion paper (KTY).

Note that most of baiu fronts based on = 345 K are found in the region with large meridional gradients of θe near the surface around Japan (KTY). We confirmed that the reference latitude of the baiu front is found within the analysis region, which will be described in the next subsection, for most days of the analysis period. We also confirmed that areas with low IR brightness temperature appear around the reference latitudes. Thus, we can capture most of actual baiu fronts by using = 345 K.

Figure 3 shows climatological time–latitude diagrams of mean rainfall rates and CRRs from 1 June to 31 July for 14 yr, respectively. These are based on 5-day running-mean total and convective rainfall from 122° to 135°E. The = 345 K contour (i.e., the reference for the detection of the baiu front; thick contour) is located near the center of the precipitation region associated with the baiu front (Fig. 3a). The precipitation region moves northward together with the reference latitude of the baiu front. The position of the reference latitude is changed year by year and longitude by longitude with its standard deviation of approximately 1.5°–3°. Standard deviation is calculated by using daily positions of the reference latitudes of the baiu front at every longitude. The majority of variability falls within the major precipitation region associated with the baiu front until mid-July (Fig. 3b). On the other hand, Fig. 3c shows that CRRs are below (above) ~40% to the north (south) of the reference latitude of the baiu front, which indicates that precipitation characteristics can be distinguished by the reference latitude. The boundary to divide CRRs moves northward together with the reference latitude of the baiu front over time. Overall, the = 345 K contour can capture precipitation characteristics associated with the baiu front in the current climate.

Fig. 3.
Fig. 3.

Time–latitude cross sections of (a),(b) TRMM PR mean rainfall rates (mm h−1; colors) and (c) TRMM PR convective rainfall ratios (%; colors), which are averaged from 122° to 135°E for 14 yr (1998–2011). The 5-day running-mean rainfall rates for total and convective rain are utilized in this figure. In (a),(c), contours indicate the JRA-25–JCDAS and thick contours show where = 345 K. In (b), the black line indicates mean reference latitudes of the baiu front, which is detected using = 345 K (JRA-25–JCDAS), and error bars indicate the standard deviation. In (c), only regions with mean rainfall rates > 0.1 mm h−1 are plotted.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

c. Analysis method

Composite analysis is performed to examine meridional variations in precipitation characteristics near the baiu front. The longitudinal region for the analysis is fixed from 122° to 135°E. First, we linearly interpolate data in a meridional direction so as to find the latitudes where = 345 K. The reference latitude of the baiu front is determined daily as the latitude closest to = 345 K at every 1.25° in longitude. If multiple reference latitudes are found at the same longitude at the same time, only the southernmost reference latitude is utilized. Note that spatially continuous lines of = 345 K are used, and the meridional gradient of θe is negative at each point. The reference latitudes are found from 20° to 40°N. We then perform a composite analysis on precipitation characteristics with its center at the reference latitudes.

Here, we underline the necessity to correct meridional sampling biases due to the TRMM observation, since the TRMM satellite more frequently overpasses higher latitudes. Thus, we have to pay attention to the latitude of each pixel. When we perform composite analysis relative to the baiu front, we average values with weights, which depend on the latitude of each pixel, to correct the biases. The weight for each pixel is the number of total pixels observed at the latitude of the pixel. The details on the weighted average are described in the appendix.

We also note that the northern limit of the TRMM PR observation region is approximately 36°N. Figure 3 shows that, as the baiu front proceeds northward, precipitation tends to be outside of the PR observation region, especially during July. Thus, composites for the entire months of June and July may be affected by precipitation more during June than during July. A comparison of composites for June with those for July shows that they are similar to each other in terms of meridional contrasts of precipitation characteristics, although CRRs are larger over all latitudes in July than in June (not shown). Notably, CRRs to the north of the baiu front are significantly larger in July than in June, while those to the south of the baiu front do not make much difference between June and July (not shown). Hereafter in this study, the composites for the entire months of June and July are shown and discussed.

3. Results

a. Precipitation characteristics based on pixel data

In this section, we first analyze composite meridional distributions of TRMM PR pixel-based precipitation characteristics with its center at the reference latitude of the baiu front. Figure 4 shows the composites of mean rainfall rates, rainfall intensities, and CRRs. Note that positive (negative) latitudes are to the north (south) of the baiu front. In Fig. 4a, the mean rainfall rates for total precipitation are largest slightly to the south of the baiu front with the peak of 0.44 mm h−1. Mean stratiform rainfall rates are distributed in a nearly symmetric fashion around the baiu front.

Fig. 4.
Fig. 4.

Composite meridional distributions of (a) mean rainfall rates, (b) convective rainfall ratios, and (c) rainfall intensities. Composites are performed with their centers at latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). Positive (negative) latitudes are to the north (south) of the reference latitude. In (a),(c), the black thick line, black thin line, and gray thin line represent total rainfall, convective rainfall, and stratiform rainfall, respectively. Error bars indicate the 99% confidence interval for the mean values with Student’s t test.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

In Fig. 4a, significant differences between the south and north sides are found in the convective rainfall; mean convective rainfall rates are obviously larger to the south of the baiu front than to the north. This abundance of convective rainfall results in an increase in total rainfall to the south of the baiu front. It is clearly shown that CRRs gradually increase southward from the relative latitude of +2.5° (Fig. 4b). The CRRs are 40%–60% to the south of the baiu front, while they are below 40% to the north.

Figure 4c shows that both convective and stratiform rainfall intensities are greater to the south of the baiu front than to the north, and consequently the rainfall intensities for total precipitation are larger by more than 1 mm h−1 to the south of the baiu front compared to the north. Note that latitudes with large convective rainfall intensity correspond to the region where deep convective precipitation occurs frequently (as will be shown in Fig. 6c).

To more quantitatively examine differences in rainfall intensity between the southern and northern sides of the baiu front, a composite histogram of rainfall rates in the southern (−7.5° to 0°) region is compared with that in the northern (0°–7.5°) region (Fig. 5). The values are ratios (%) of the number of pixels with each rainfall intensity to the number of total observational pixels in each region. The abscissa in Fig. 5 indicates rainfall rates in dBR, where dBR = 10 × log10(rainfall rates). Rainfall weaker than (greater than or equal to) 4 dBR or ~2.5 mm h−1 more frequently occurs in the northern (southern) region (Fig. 5a). In particular, the frequency at the extreme end of the rainfall rate, ≥18 dBR (~63 mm h−1), is larger in the southern region than in the northern region (Fig. 5b). The frequency of such extremely heavy rainfall ≥18 dBR in the southern region is about 2 times as large as that in the northern region. In the next subsection, extreme rainfall will be shown in more detail.

Fig. 5.
Fig. 5.

(a) Composite histograms of rainfall rates in the northern (0°–7.5°; black bars) and southern (−7.5° to 0°; gray bars) regions of the baiu front. The ordinate indicates ratios (%) of the number of pixels with each rainfall rate to the number of total observational pixels in each region. The abscissa indicates rainfall rates in a dBR scale = 10 × log10(rainfall rates). Correspondence between dBR and rainfall rates (mm h−1) are as follows: −6 dBR = 2.5 × 10−1, 4 dBR = 2.5, 18 dBR = 63, and 24 dBR = 2.5 × 102. (b) As in (a), but for rainfall ≥ 18 dBR. Composites are performed with their centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011).

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

In addition to the rainfall rate and CRR, ETH also provides important information on the activity of cumulus convection. Figure 6 shows the composite meridional histograms of ETHs. The values are ratios (%) of the number of precipitation-detected pixels with each ETH to the total observational pixel number in each relative latitudinal bin. Note that the color scale of Fig. 6c is different from those of Figs. 6a and 6b. In Fig. 6b, stratiform precipitation has slightly higher echo tops to the south of the baiu front than to the north. The histogram of echo top for all-observed precipitation is almost similar to that for stratiform precipitation because of its observation number (Figs. 6a,b).

Fig. 6.
Fig. 6.

Composite meridional histograms of ETH for (a) total precipitation, (b) stratiform precipitation, and (c) convective precipitation. Colors indicate ratios (%) of the number of pixels with each ETH to the number of total observational pixels in each relative latitudinal bin. Composites are performed with thier centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). Positive (negative) latitudes are to the north (south) of the reference latitude.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

Most significant differences between the southern and northern sides of the reference latitude are found in ETHs for convective precipitation (convective ETHs; Fig. 6c). For both shallow and deep precipitation, convective ETHs occur more to the south of the baiu front than to the north. The peak of shallow convective ETHs around 3–5 km is intensified to the south of the baiu front, and it is significant that the deeper peak of convective ETHs around 6–8 km is found only in the region from the reference latitude to −5°. Consistent with Fig. 4, the increase of the occurrence frequency of higher echo tops for both convective and stratiform precipitation suggests that precipitation to the south of the baiu front is more vigorous and may be associated with mesoscale convective systems.

Note that the observed convective precipitation associated with the baiu front has modest ETHs, which suggests that it is associated with mesoscale convective systems. It is consistent with case studies of mesoscale precipitation systems around Japan’s Southwest Islands (Shinoda et al. 2009) and around Japan’s Okinawa Island during the baiu season (Oue et al. 2010, 2011). Zhang et al. (2006) observed mesoscale convective systems, which have convective cells of both modest heights and deeper heights, along the mei-yu front over the downstream region of the Yangtze River in China. Kato et al. (2007) also showed using objective analysis data that shallower levels of neutral buoyancy are often found around the islands of Japan.

Figure 7 shows the composite meridional distributions of mean near-surface rainfall rates from each ETH. Mean near-surface rainfall rates, which are binned into each ETH, are calculated in each latitudinal bin, following the method described in the appendix. Larger values of mean near-surface rainfall rates from higher echo tops to the south of the baiu front are shown for total, convective, and stratiform precipitation (Figs. 7a–c). There is a large contribution to the near-surface rainfall from echo tops around 7–9 km. Precipitation with higher echo tops is generally stronger than that with lower echo tops, so that the larger rainfall contribution to the south of the baiu front in Fig. 7 shifts upward compared to Fig. 6. For convective precipitation, Fig. 7c shows only small signals around 3–5 km, while the distinct shallow peak is shown in Fig. 6c. It means that the convective precipitation with ETHs of 3–5 km occurs more frequently but has less contribution to the total rainfall.

Fig. 7.
Fig. 7.

As in Fig. 6, but for mean near-surface rainfall rates from each ETH.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

It is also worth noting that deep convective precipitation occurs less frequently and has a smaller contribution to the near-surface rainfall in the region from −5° to −10° of the reference latitude, compared to the region just to the south, from 0° to 3° of the reference latitude (Figs. 6c, 7c). This indicates that the region from −5° to −10° corresponds to the subtropical high pressure area, which overlies and suppresses deep convection to the south of the baiu front. Thus, precipitation characteristics that are observed to the south of the reference latitude, such as CRRs of 40%–60% and large ETHs, are not in the tropics but are actually associated with the baiu front to the north of the subtropical high.

As mentioned in section 1, there are some differences in the environmental fields between the region to the west of around 135°E and that to the east of the longitude. Finally, in this subsection, we would like to note similarities and differences between the regions of 122°–135°E and 135°–145°E. After examining precipitation characteristics in the eastern region (135°–145°E) in the same manner as the above-mentioned analyses for the western region (122°–135°E), we confirmed that the meridional contrasts in precipitation characteristics across the reference latitude are also found in the eastern region (not shown). However, some pronounced differences between the two regions are also observed: smaller mean rainfall rates, lesser rainfall intensity, smaller CRRs, and smaller frequency of high echo tops are observed around and to the south of the reference latitude in the eastern region than in the western region (not shown).

b. Extreme rainfall

The occurrence of extreme rainfall significantly affects the society. Thus, it is important to examine as to how the characteristics of extreme rainfall change across the baiu front. Figure 8 shows composite cumulative frequencies of effective radar reflectivity factor (Ze). Here, the cumulative frequency indicates the accumulated frequency from 15 dBZ to a given Ze with respect to each altitude.

Fig. 8.
Fig. 8.

Composite cumulative frequencies of effective radar reflectivity factor (Ze) by altitudes in the (a) northern (0°–7.5°) and (b) southern (−7.5° to 0°) regions of the baiu front. Composites are performed with their centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). Contours are set at the 50, 70, 80, 90, 95, 99, 99.5, 99.9, and 100th percentiles, starting from the left. Thick contours indicate the 90 and 99.9th percentiles.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

Large values of Ze are found at higher altitudes in the southern (−7.5° to 0°) region of the baiu front compared to the northern (0°–7.5°) region. For example, 10% of the profiles in the southern region have Ze > 25 dBZ at 14-km height, whereas less than 5% of profiles satisfy this criterion in the northern region. This result is consistent with Figs. 47, where stronger and deeper convective precipitation is observed to the south of the baiu front than to the north. It is suggested that large ice particles are lifted to higher altitudes in the southern region than in the northern region.

Next, we sort all Ze profiles with the near-surface rainfall rates, and define profiles with top 0.1% near-surface rainfall rates as those that are associated with extreme rainfall. The threshold value of near-surface rainfall rates is 16 mm h−1 greater in the southern region (80 mm h−1) than in the northern region (64 mm h−1). Figure 9 shows extreme Ze profiles in the northern (0°–7.5°) and southern (−7.5° to 0°) regions of the baiu front. All 1750 (2360) profiles in the northern (southern) region are categorized into three groups according to their ETHs.

Fig. 9.
Fig. 9.

Profiles of effective radar reflectivity factor (Ze) with top 0.1% near-surface rainfall rates with ETHs (a),(d) <8 km; (b),(e) 8–14 km; and (c),(f) 14–20 km on the (a)–(c) northern (0°–7.5°) and (d)–(f) southern (−7.5° to 0°) sides of the baiu front over the region from 122° to 135°E during June–July for 14 yr (1998–2011). The mean Ze profiles for all extreme profiles in each category are shown with thick black solid lines.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

It is shown that extreme rainfall often comes from moderately high echo tops and lower echo tops in both regions (Figs. 9a,b,d,e). In each panel of Fig. 9, reflectivity significantly increases with decreasing altitude above a height of ~5 km and continues to increase at lower rates below the height. The rate of increase in reflectivity below 5 km is larger for the profiles with lower echo tops. Sohn et al. (2013) demonstrated that the heavy rainfall associated with warm-type rain occurs over the Korean Peninsula during summer, and it could be possibly formed through collision/coalescence processes below the melting layer under extremely humid conditions. It is suggested that the same may occur for extreme rainfall with echo tops below 8 km in our analysis period and region.

More importantly, profiles with ETHs exceeding 16 km are found only in the southern region (Figs. 9c,f), which indicates that there are more profiles with large Ze values at higher altitudes in the southern region than in the northern region. There are profiles of extreme rainfall with echo tops reaching beyond 14 km even in the northern region. However, there are more profiles of extreme rainfall with echo tops exceeding 14 km in the southern region (8.1% of all profiles with extreme rainfall) than in the northern region (2.6%). Above the freezing level, large Ze values indicate the existence of large ice particles or supercooled liquid raindrops, which are produced by strong updrafts. It is suggested that more vigorous precipitation occurs more frequently in the southern region compared to the northern region. Note that the similar result was obtained, when we used the common threshold value of 75 mm h−1, which was in the top 0.1% near-surface rainfall rates in the whole (−7.5° to 7.5°) region of the baiu front, for the northern and southern regions. It is also worthy to note that there is a difference in heights of tropopause between the northern and southern regions of the baiu front. Composite dynamical tropopause height detected with potential vorticity of 2 PVU (i.e., 2 × 10−6 K m2 kg−1 s−1) is about 16.8 km in the southern region of the baiu front, which is 2.5 km higher than that in the northern region (about 14.3 km).

c. Characteristics of PFs

In the previous subsections, precipitation characteristics based on TRMM PR pixels were examined. There is a possibility that mechanisms to bring rainfall may have different characteristics between the south and north sides of the baiu front. Finally, in this subsection, the characteristics of PFs, which are contiguous areas of rainy pixels, are compared between the southern and northern sides of the reference latitude of the baiu front. With the analysis of PFs, we can know what kind of precipitation systems is dominant to the south and north of the baiu front. This analysis is helpful in understanding how it rains in each region.

As an example of PFs, the snapshots of PR2A25 near-surface rainfall rates on 30 June 2006 and 27 June 2003 are shown in Figs. 10a and 10b, respectively. The reference latitude of the baiu front is indicated with black contours in Figs. 10a and 10b. The simple average latitudes and longitudes of rainy pixels of PFs are marked with crosses. The PF in the former case has an area of approximately 1.6 × 105 km2, CRR of ~14%, and a maximum height (defined with 20 dBZ) of 8.75 km. On the other hand, the easternmost PF in the latter case has an area of approximately 4.6 × 104 km2, CRR of 39%, and a maximum height of 13 km. Other two PFs in Fig. 10b have CRRs exceeding 60% and maximum heights exceeding 13 km. The three-dimensional structures of these PFs are shown in Figs. 10c and 10d.

Fig. 10.
Fig. 10.

Examples for PFs to the (a),(c) north and (b),(d) south of the latitudes with = 345 K. Snapshots taken on (a),(c) 30 Jun 2003 and (b),(d) 27 Jun 2006. In (a),(b), colors indicate near-surface rain rates (mm h−1) ; thick lines indicate = 345 K; and mean positions of PFs are marked with crosses. In (c),(d), precipitation rates (mm h−1) are three-dimensionally indicated.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

To statistically examine the differences in the characteristics of PFs between the southern (−7.5° to 0°) and northern (0°–7.5°) regions of the baiu front, the rainfall contribution of PFs to the total rainfall in each (southern or northern) region is examined in terms of their areas and maximum heights (Figs. 11a,b). Similar diagrams are utilized to examine PF characteristics over the tropical Pacific (Yokoyama and Takayabu 2012) and those associated with tropical cyclones (Thatcher et al. 2012). Note that maximum heights defined with 20 dBZ are utilized in this study, while maximum storm heights from the TRMM 2A23 algorithm are utilized in Yokoyama and Takayabu (2012). Together with this diagram, we introduce new diagrams for the rainfall contribution of PFs in terms of their CRRs and maximum heights (Figs. 11c,d).

Fig. 11.
Fig. 11.

Composite rain contribution (%; colors) of PFs in terms of (a),(b) areas and maximum 20-dBZ heights and (c),(d) convective rainfall ratios and maximum 20-dBZ heights for the (a),(c) northern (0°–7.5°) and (b),(d) southern (−7.5° to 0°) regions of the reference latitudes. Composites are performed with their centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). In (a),(b), the abscissa indicates PF areas on the logarithmic scale.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

Figures 11a and 11b show that PFs with areas larger than ~103.5 km2 have the major rainfall contribution in both the southern and northern regions. Notably, these PFs are different in their maximum heights and CRRs. The large PFs are roughly divided into three ranges of maximum heights (i.e., 8–11 km, 12–14 km, and 14–17 km; Figs. 11a,b). The three groups of PFs, which are divided in terms of maximum heights, also differ according to CRRs: CRRs tend to be larger in a group with larger maximum heights, although there are overlaps in CRRs of the three groups (Figs. 11c,d). We show different characteristics of PFs in the northern and southern regions of the baiu front in more details below.

In the northern region, PFs with maximum heights of 8–11 km and small CRRs of 0%–40% account for the largest contribution to total rainfall (Figs. 11a,c). These PF characteristics in the northern region suggest that the large-scale lifting of air is dominant to the north of the baiu front, producing stratiform clouds and precipitation. In addition, PFs with relatively large maximum heights of around 14 km are found with CRRs of around 40% and 50%–60% in the northern region, which means that, although these deep and large precipitation systems are not dominant to the north of the baiu front, they are occasionally observed.

In the southern region, there is a significant rainfall contribution from the large (approximately ≥103.5 km2) PFs with maximum heights of 12–17 km and CRRs of 30%–80% (Figs. 11b,d). These PFs have characteristics similar to organized precipitation systems, such as mesoscale convective systems. Interestingly, large PFs in the southern region are divided in terms of maximum height and CRRs (Figs. 11b,d). Figure 11d shows that PFs with very large heights of 14–17 km tend to have CRRs of 50%–80%, which are larger than CRRs (30%–65%) of PFs with heights of 12–14 km. It is suggested that these different characteristics of PFs may correspond to the two types of mesoscale convective systems (large PFs with moderate heights of 8–14 km and large PFs with very tall heights of 14–20 km) over the tropical Pacific mentioned in Yokoyama and Takayabu (2012).

In the southern region, there is also a rainfall contribution from the PFs with maximum heights of around 10 km and CRRs less than 40%, which are similar to PFs that are dominant in the northern region. Note that, among these wide stratiform PFs, the contributions from PFs with very large areas of 105–105.5 km2 are not as large in the southern region as in the northern region. In addition, there is a relatively large contribution from PFs with areas smaller than 103.5 km2, heights smaller than 8 km, and CRRs exceeding 60% in the southern region. In short, dominant characteristics of PFs in the southern region of the baiu front are different from those in the northern region, which causes the differences in the way that it rains in the two regions.

4. Summary and discussion

In this study, we examined precipitation characteristics associated with the baiu front. First, we found that precipitation characteristics to the south of the baiu front, which is defined from = 345 K, are obviously different from those to the north. Second, it was shown that the meridional contrast in precipitation characteristics across the baiu front is attributed to differences in the characteristics of dominant precipitation systems.

To the north (0°–7.5°) of the baiu front, stratiform rainfall dominates with CRRs of 20%–40% and relatively weak rainfall intensities. Tall convective precipitation is significantly less to the north of the baiu front than to the south (−7.5° to 0°). Stratiform precipitation also has slightly lower echo tops to the north of the baiu front than to the south. These precipitation characteristics indicate that convective activity is relatively moderate to the north of the baiu front. Moreover, PFs with maximum heights of 8–11 km and CRRs of 0%–40% have the major contributions to the total rainfall in the northern region. It is suggested that the large-scale lifting of air is dominant to the north of the baiu front. The fact that these lower and stratiform-dominated precipitation systems are more dominant than mesoscale convective systems results in relatively moderate precipitation characteristics in the northern region of the baiu front.

To the south of the baiu front, convective rainfall has a larger contribution to the total rainfall with CRRs of 40%–60%, compared to the north. These values are comparable to the stratiform rainfall ratios of ~(40%–70%) [i.e., CRRs of ~(30%–60%)] for organized precipitation systems such as mesoscale convective systems in the tropics (e.g., Takayabu 2002; Yokoyama and Takayabu 2008, 2012). Both convective precipitation and stratiform precipitation are relatively strong and tall in the southern region of the baiu front. In particular, the distribution of ETHs for convective precipitation has a large contrast across the baiu front, and both shallow precipitation and deep convective precipitation more frequently occur in the southern region than in the northern region.

According to the analysis of PF characteristics, wide stratiform-dominated precipitation systems, which are similar to dominant precipitation systems in the northern region, are also found in the southern region. More importantly, large precipitation systems, which are divided in terms of maximum height with reflectivity of 20 dBZ and CRR, are found in the southern region. Precipitation systems with very large (14–17 km) heights tend to have CRRs of 50%–80%, while those with moderate (12–14 km) heights tend to have CRRs of 30%–65%. These precipitation systems correspond to organized precipitation systems such as mesoscale convective systems. In summary, it is concluded that the main reason why precipitation characteristics in the south are different from those in the north is the abundance of organized precipitation systems in the south.

It should also be emphasized that the above-mentioned characteristics of precipitation systems in the south can be identified by using three parameters: namely, the area, maximum height, and CRR. In particular, we found that organized precipitation systems to the south of the baiu front can be divided in terms of maximum height and CRR. It is suggested that these different characteristics of precipitation systems may correspond to the two types of organized precipitation systems over the tropical Pacific mentioned in Yokoyama and Takayabu (2012). In this study, we found that characteristics of organized precipitation systems are better divided by introducing CRR. We leave the cause of the differences between these two-type organized precipitation systems for future studies.

In addition, the frequency of rainfall ≥18 dBR (~63 mm h−1) in the southern region of the baiu front is about 2 times as large as that in the northern region. Top 0.1% near-surface rainfall rates in the south are greater than 80 mm h−1, which is 16 mm h−1 greater than the threshold in the north. For extreme rainfall, there are more profiles with large reflectivity at higher altitudes in the southern region than in the northern region. More vigorous precipitation occurs in the southern region compared to the northern region.

We here discuss these differences in precipitation characteristics between the south and north sides of the baiu front in relation to the atmospheric stratification (Fig. 12). Specific humidity in the lower troposphere is higher in the southern region of the baiu front than in the northern region (Fig. 12a), resulting in larger θe near the surface in the southern region (Fig. 12b). Consequently, lower–middle troposphere is more convectively unstable to the south of the baiu front than to the north (Fig. 12b). We also calculated the convective available potential temperature (CAPE) by lifting up air at 1000 hPa. As a result, the composite CAPE in the southern region of the baiu front is about 1.5 × 103 J kg−1, while that in the northern region is about 71 J kg−1. It is clearly shown that the environment to the south of the baiu front is more favorable for the development of deep convection. In addition, it is suggested that tropical air, which is favorable for the development of mesoscale convective systems, occasionally flows into the southern region of the baiu front.

Fig. 12.
Fig. 12.

(a) Composite profiles of specific humidity (g kg−1). (b) Composite profiles of θ (K), θe(K), and saturated θe (K). Black lines indicate the profiles in the southern region (−7.5° to 0°) of the baiu front, while gray lines indicate the profiles in the northern region (0°–7.5°). In (b), the lines indicate θ, θe, and saturated θe from the left to right. Composites are performed with their centers at the latitudes with = 345 K in the region from 122° to 135°E during June–July for 14 yr (1998–2011). Error bars indicate the 99% significant intervals for the mean values with Student’s t test.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

In this study, we used a simple reference for the detection of the baiu front to show the meridional contrast in precipitation characteristics across the baiu front. It means that precipitation characteristics at a given place can drastically change with the position of the baiu front. This has important implications for the study on changes in precipitation characteristics associated with the baiu front in future climate. Our definition of the baiu front based on the structure of atmospheric stratification in the current climate may not be directly applied to numerical model outputs for future and current climates. In the companion paper (KTY), we will discuss how the baiu front can be defined in models in the current climate and how the models simulate precipitation characteristics across the baiu front.

Acknowledgments

This work is supported by the Environment Research and Technology Development Fund (2A-1201) of the Ministry of the Environment, Japan, and the 6th RA of the Japan Aerospace Exploration Agency (JAXA) Precipitation Measuring Mission (PMM) science. The authors would like to express their gratitude to three anonymous reviewers for their very helpful comments. The Radar Precipitation Feature Level-2 data were provided by the University of Utah TRMM database. The JRA-25 data were provided from the cooperative research project of the JRA-25 long-term reanalysis by the JMA and the Central Research Institute of Electric Power Industry. The JMA surface weather chart was obtained from TENKI (the Bulletin Journal of the Meteorological Society of Japan in Japanese). The MTSAT-1R data used in this study were received by the JMA; Weathernews Inc.; the Earthquake Research Institute, the University of Tokyo; and Takeuchi Laboratory, the Institute of Industrial Science, the University of Tokyo, and they were processed and provided by the Center for Environmental Remote Sensing (CEReS), Chiba University. The VAPOR (http://www.vapor.ucar.edu) and the GrADS are utilized for the graphics. The authors also thank Dr. A. Hamada for helping them with using the VAPOR.

APPENDIX

Correction of the TRMM Meridional Sampling Bias

As shown in Fig. A1, the number of TRMM PR–observed pixels is significantly larger at higher latitudes, which means that the TRMM data have the meridional sampling bias. Bias-corrected values composited relative to the reference latitude of the baiu front are obtained by averaging with weights,
ea1
where k indicates individual pixels in the ith relative latitudinal bin and the jth absolute latitudinal bin, Ri indicates a bias-corrected value that is composited in the ith relative latitudinal bin, rijk indicates the value of the kth pixel in the ith relative latitudinal bin and jth absolute latitudinal bin, and wij indicates the weight for rijk. Here, relative latitude is the latitude relative to the baiu front, whereas absolute latitude is the geographical latitude. The weight is 1/allpixij, which is the inverse of all pixel numbers in the ith relative latitudinal bin and the jth absolute latitudinal bin where the rain pixels are located.
Fig. A1.
Fig. A1.

Latitude distribution of the number of total pixels observed with the TRMM PR. All pixels are counted from 0° to 359.5°E from January 1998 to December 2011. The y axis runs from 2 × 108 to 1.6 × 109.

Citation: Journal of Climate 27, 15; 10.1175/JCLI-D-13-00350.1

For the unconditional and conditional means, the denominators of (A1) are written as follows:
eq1
and
eq2
Here, rpixij represents rain pixel numbers observed by the PR in the ith relative latitudinal bin and jth absolute latitudinal bin and nlat denotes the number of absolute latitudinal bins in the ith relative latitudinal bin. On the other hand, a numerator of (A1) for both unconditional and conditional means is written as follows:
eq3
The mean rainfall rate and histograms of rainfall rates and ETHs are calculated using the equation for the unconditional mean, while the rainfall intensity is calculated using that for the conditional mean. Convective rainfall ratios are calculated by dividing bias-corrected mean convective rainfall rates by bias-corrected mean rainfall rates. A similar correction is applied to the analyses of PFs.

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