Environment and Processes for Heavy Rainfall in the Early Morning over the Korean Peninsula during Episodes of Cloud Clusters Associated with Mesoscale Troughs

An investigation has been carried out using rainfall observation data, an analysis and forecast data by National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) to explain the environment and processes that lead to heavy rainfall in the early morning over the Korean peninsula during episodes of cloud clusters associated with mesoscale troughs (CCMTs). For this study, nine episodes with a maximum hourly rainfall amount in the early morning (i.e., 0300 – 0900 LST) are selected from seventeen heavy-rainfall episodes associated with CCMTs during 2001 – 2011. Case studies on two episodes have revealed that, for both episodes, 1) a low-level trough develops over eastern China and its coastal area during day time; 2) the strong southwesterly band (SWB; an area with wind speeds > 12.5 m s) on the pressure level of 925 hPa over the East China Sea, which is located southeast of the trough, strengthens and expands at night time toward the southwestern Korean peninsula; 3) the SWB supplies a large amount of moisture and increases convective instability over the southwestern Korean peninsula with a convection trigger mechanism (i.e., strong horizontal convergence); and 4) heavy rainfall occurs in the early morning over the southwestern Korean peninsula, where the exit region of the SWB is located. A mechanism for the SWB growth is presented. Furthermore, generality of the major results from the two case studies is verified using the results obtained for the composite fields of the nine CCMT episodes.


Introduction
Occurrences of heavy rainfall or rainfall maximum in the early morning during summer can be an important subject for weather forecast and the preparation to reduce the loss of lives and property that they may incur.Understanding of the environment and Sea.The precipitation amount in Japan exhibited a maximum between 1500 and 1800 LST over inland areas, while a maximum during 0300 -0600 LST prevailed at maritime stations on islands and peninsulas (Fujibe 1988).Over the Korean peninsula, Jung and Suh (2005) found that the early-morning peak appeared along the western and southern coastal regions, while both weak early-morning peaks and strong late-afternoon peaks were observed over the central inland region.
The causes and physics for rainfall maxima in the morning vary widely depending on the regional features and meteorological conditions around the region.Early-morning rainfall over southeastern China was related to early-morning maximum southwesterlies over southern China (Chen et al. 2009), late-night vertical differential thermal advection, and semidiurnal variations in land-sea differential radiative heating/ cooling (Huang and Chan 2011).Chen, T. et al. (1999) suggested that the early-morning maximum in Taiwan is caused by the combined effects of land breeze and nocturnal downslope flow.Oki and Musiake (1994) suggested that low-level convergence between predominant monsoon wind and offshore breeze of the local circulation and radiative cooling of cloud tops and the contrast with the surrounding cloudfree region are the mechanism for convective rainfall in the morning over Japan and Malaysia.In a study on diurnal variations in precipitation around western Japan during the warm season, Kanada et al. (2014) suggested that morning precipitation maxima around Kyushu were closely related to cloud systems that appeared over the East China Sea in the early morning and developed rapidly while traveling eastward.They also suggested that the diurnal cycle of moisture supplied by low-level winds can induce a diurnal cycle of convective cloud development.
Several studies suggested possible mechanisms of the morning peak of rainfall over the Korean peninsula.Roh et al. (2012) investigated diurnal variations of Korean summertime precipitation in 2009 using cyclostationary empirical orthogonal function (CSEOF) technique and suggested that nocturnal precipitation maxima were the result of rain band enhancement from instability due to radiative cooling at cloud top during the night time.However, supporting material for this effect of cloud top radiative cooling was not given in their study.The processes or mechanisms of early-morning peaks over the western and southern coastal areas of South Korea have not yet been studied in detail.
Heavy rainfall over the Korean peninsula during summer generally occurs over or near the northwestern edge of the western Pacific subtropical high (WPSH), where a wide band of southwesterlies can be found.As a meso-α-scale depression moves or develops along the northwestern edge of the WPSH, southwesterlies to the southeast of the depression become stronger (Shin and Lee 2015).Various studies have also found the importance of strong southwesterlies or low-level jets (LLJs) for heavy rainfall over the Korean peninsula (e.g., Kim et al. 1983;Hwang and Lee 1993;Chen, S. et al. 1999;Jung et al. 2018).
Cloud clusters (CCs) are a major type of heavy precipitation system over the Korean peninsula.Approximately 47 % of heavy rainfall events between 2000 and 2006 were associated with CCs (Lee and Kim 2007).A CC is defined as an oval-shaped cloudmass region, with an equivalent black-body temperature (TB) lower than -50°C, that is approximately larger than 100 km in diameter (Takeda and Iwasaki 1987;Lee and Kim 2007).Shin and Lee (2015) found that CCs over the Korean peninsula were associated with mesoscale depressions in the lower troposphere: 64 % of CCs were associated with meso-α-scale lows (CCMLs), which mainly originated from China, whereas 31 % of CCs were associated with mesoscale troughs, which mostly formed over the Yellow Sea and the west coast of the Korean peninsula.They used the term "mesoscale trough" for a mesoscale depression that did not develop into a system with a meso-αscale closed isobar.
Diurnal variations in hourly rainfall amount during heavy rainfall episodes associated with CCMTs (which are referred to as CCMT episodes hereafter) tend to show noticeable peaks in the morning (Lee et al. 2015).In addition, heavy rainfall during CCMT episodes mainly occurs over the southwestern part of the Korean peninsula within short time periods after the genesis of the CCMT; this makes it more difficult to prepare for them.Thus, it is important to understand the environment and processes leading to heavy rainfall in the early morning over the Korean peninsula for the forecast and preparation of heavy rainfall associated with CCMTs.
In this paper, an investigation has been carried out using rainfall observations, an analysis and forecast data by National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) to explain the environment and processes that lead to heavy rainfall in the morning over the Korean peninsula during CCMT episodes.Section 2 presents diurnal variations in the hourly rainfall amount over South Korea during CCMT episodes from 2001 -2011.In Section 3, case studies on two CCMT episodes are carried out to describe the diurnal variations in meteorological fields over East Asia and the physical and dynamical processes that lead to heavy rainfall in the early morning over the southern Korean peninsula.Generality of the major results from the case studies is also discussed.Finally, the summary and conclusions are given in Section 4.

Diurnal variations in rainfall during CCMT episodes
In this section, we examine the diurnal variations in rainfall amount over South Korea during heavy-rainfall episodes associated with CCMTs for the 11-year period of 2001 -2011.A heavy rainfall episode is defined by a 24-hour rainfall amount greater than 80 mm at a minimum of one station.Seventeen CCMT episodes have been selected for the 11-year period based on the definition of CCMTs by Shin and Lee (2015) (Table 1).It can be found from Table 1 that the maximum hourly rainfall amount occurs mostly during 0000 -1200 LST (LST = UTC + 9 hours) in 14 episodes, where 10 of them have maxima during 0300 -0900 LST (1800 -0000 UTC).Only 3 episodes show maximum rainfall in the afternoon.
Diurnal variation in the rainfall amount over South Korea is described using an hourly rainfall amount for the 17 CCMT episodes in Table 1 and also for the nine CCMT episodes (excluding CCMT-10) in which maximum of hourly rainfall amount occurs in the early morning (i.e., 0300 -0900 LST).The CCMT-10 episode is excluded, because a tropical depression is located over eastern and southeastern China during the episode.Rainfall data are acquired from automatic weather stations (AWSs) across South Korea, whose average total number is 613 during 2001 -2011.To obtain a total hourly rainfall amount over South Korea, the observed 1-hour rain gage data are interpolated into 0.1° × 0.1° horizontal grids over South Korea.Then, the total hourly rainfall amount is obtained by adding up the grid values over South Korea.The frequency of rainfall for each hour is obtained by counting grids whose hourly rainfall amounts are greater than 1 mm.Then, the mean intensity of rainfall for each hour is obtained by dividing the total hourly rainfall amount by the frequency of rainfall.
In this study, diurnal variations are represented by normalized hourly values.For example, the normalized hourly rainfall amount [NR (h)] is obtained as follows: where R (n, h) represents the total hourly rainfall amount for the n th episode at hour h, and nc represents the total number of episodes.The normalized hourly values for frequency and intensity are also obtained in the same manner.A zero value of NR (h) means that the hourly rainfall amount is the same as the 24-hour mean rainfall amount.In addition, values of 0.5 and −0.5 correspond to 150 % and 50 % of the 24-hour mean rainfall amount, respectively.The normalized hourly rainfall amount for the 17 episodes exhibits morning peaks during 0500 -0800 LST (2000 -2300 UTC) and an evening minimum from 2000 -2100 LST (1100 -1200 UTC) (Fig. 1a).The normalized hourly frequency of rainfall shows a similar diurnal variation compared to that for the rainfall amount, whereas the normalized hourly intensity of rainfall does not show a significant diurnal variation, although it is relatively large in the morning.The diurnal variations in rainfall associated with CCMT episodes shown in Fig. 1a are consistent with previous studies in terms of peak time (e.g., Lim and Kwon 1998;Jung and Suh 2005).
The tendency for large rainfall amount in the morning is much stronger for the nine CCMT episodes with rainfall maximum in the early morning, although the time of the morning peak is the same as that for the 17 episodes (Fig. 1b).At the time of the morning peak for the nine episodes, NR (h) exceeds 1.2.The normalized hourly frequency is in phase with NR (h).

Diurnal variations in meteorological fields
over East Asia that lead to heavy rainfall in the early morning over the Korean peninsula In this section, two case studies are presented to examine the diurnal variations in meteorological fields over East Asia and the processes that bring heavy rainfall over the Korean peninsula in the early morning.For this study, we use the NCEP CFSR analysis data, with a 0.5° × 0.5° horizontal resolution at a 6-hour interval, and hourly CFSR forecast data (Saha et al. 2010).In a recent study, Chen et al. (2014) examined the quality of reanalysis dataset in representing the diurnal cycle of meteorological fields over East Asia.Four reanalysis datasets (CFSR, MERRA, ERA-I, and JRA-55) were found to present similar patterns/structure and summer progress of the mean wind diurnal cycle, although the root-mean-square error was found to show a diurnal variation more evident in CFSR and MERRA than those for JRA-55 and ERA-I.
Rain rates obtained from the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) 3B42 data (Huffman et al. 2007) are also used for the analysis of diurnal variations.TMPA 3B42 data have been used to investigate the diurnal variation of precipitation over land and ocean over East Asia (e.g., Yuan et al. 2012;Choi et al. 2015).

Case 1
We have selected the episode of CCMT-06 as Case 1, in which heavy rainfall occurred from 1900 UTC 13 July to 0000 UTC 14 July 2004, with the maximum rainfall amount occurring at 2000 -2100 UTC over the southwestern part of the Korean peninsula (Fig. 2).
Constant pressure charts for 1200 UTC 13 July are shown in Fig. 3.At 925 hPa, the northwestern edge of the WPSH is over the East China Sea and southern Korean peninsula in the SW-NE direction, and a ridge extends from the WPSH to east of the Korean peninsula (Fig. 3a).An upper-level low at 500 hPa is found northwest of the Korean peninsula, and an upper-level jet can be found downstream of the northern Korean peninsula (Figs.3b, c).
The evolution of the cloud distribution is shown using geostationary operational environmental satellite (GOES) images (Fig. 4).After 0600 UTC, a convective system develops south of Shanghai and moves northeast of Shanghai at 1200 UTC.At 1800 UTC, convective cells develop off the southwestern coast of the Korean peninsula.These cells develop further as they move northeastward, and CCs develop over the southern Korean peninsula after 2100 UTC.  1 and (b) the 9 CCMT episodes in which the maximum of hourly rainfall amount occurs in the early morning (i.e., 0300 -0900 LST).All values are obtained using AWS data.
Figure 5 shows the geopotential height and equivalent potential temperature (θ e ) at 925 hPa and the hourly rainfall rate from the TMPA 3B42 data.The use of meteorological fields at 925 hPa is based on the fact that winds and moisture transport over the East China Sea tend to be strongest at 925 hPa in this study.At 0600 UTC 13 July, a meso-α-scale low was located over eastern China, where its trough axis extended southeastward (Fig. 5a).A weak trough was present over eastern China at 0000 UTC 13 (not shown), and a significant development occurred over the area during 0000 -0600 UTC.Scattered rainfalls were observed in the afternoon throughout China (Figs. 5a, b), but they mostly disappeared at 1800 UTC (Fig. 5c).A belt of high-θ e air (θ e > 344 K) extended from the southeastern coast of China to the southern Korean peninsula throughout the episode.
The trough over the eastern coast of China near Shanghai and Hangzhou (30.29°N, 120.16°E) was well maintained at 1200 UTC (Fig. 5b).At 1800 UTC, the trough axis over the east coast of China turned counterclockwise, and a relatively broad trough was found over the East China Sea (Fig. 5c).In addition, a mesoscale ridge appeared between this trough and another mesoscale trough over the west coast of the  southern Korean peninsula.A significant amount of rainfall was found off the southwestern coast of the peninsula at 1800 UTC.The mesoscale trough over the southwestern coast deepened further at 0000 UTC 14 July, and a large amount of rainfall was found over the southern Korean peninsula (Fig. 5d).
The diurnal variation in wind at 925 hPa is shown in Fig. 6.A wide-band southwesterly flow can be found over the sea between the southeastern coast of China and the southern Korean peninsula throughout the episode.At 0600 UTC 13 July, a strong southwesterly band (SWB) (black dashed line in Fig. 6; an area of wind speed > 12.5 m s −1 ) was located over the East China Sea southeast of the trough, which was located over the red-dashed area in Fig. 6a.The trough and SWB moved northeastward at 1200 UTC (Fig. 6b).The area of the SWB at 1200 UTC was smaller than that at 0600 UTC.At 1800 UTC, a broad trough was found over the East China Sea, with a mesoscale ridge located downstream, and the SWB became extensive and strong (Fig. 6c).The SWB extended from the East China Sea near (28°N, 123°E) to the southwestern coast of the Korean peninsula, where an area of significant convergence was found.A CC began to develop over this area at approximately 1900 UTC.At 0000 UTC 14 July, the northern part of the SWB expanded along the southern coast of the Korean peninsula, although strong convergence was still present over the southwestern coastal area (Fig. 6d).
Meteorological fields for 13 -14 July 2004 are found to show the following features: 1) the diurnal variation in convective rainfall over China; 2) the diurnal variation in the SWB over the East China Sea, including a significant SWB growth during 1200 -1800 UTC; and 3) strong rainfall over the southern Korean peninsula in the early morning.We examine the following processes to understand how heavy rainfall occurred in the early morning over the southern Korean peninsula: 1) the development of the trough over eastern China, 2) the growth of the SWB during 1200 -1800 UTC, and 3) the influence of SWB growth on the occurrence of heavy rainfall in the early morning over the Korean peninsula.

a. Development of the trough over eastern China
Figure 7a shows the profiles of height and virtual temperature changes during 0000 -0600 UTC 13 July averaged over the 5° × 5° boxed area shown in Fig. 6a.These profiles are obtained using the CFSR analysis data.The geopotential height decreased (increased) in the layer below (above) 650 hPa, as a temperature increase was found below approximately 350 hPa, with a maximum increase at the lowest level.The magnitude of the geopotential height decrease became larger toward the lowest level.This decrease in geopotential height may represent the trough development at low levels over eastern China during 0000 -0600 UTC from the state of a weak trough at 0000 UTC 13 July (not shown).
The cause of the height decrease at the lowest level during 0000 -0600 UTC is examined using the surface pressure tendency equation (PTE) formulated by Fink et al. (2012), which considers a vertical column of air from the surface to an upper boundary with pressure p 2 , which is chosen to be 350 hPa in this study: where p sfc represents the surface pressure, ρ sfc rep- resents the surface air density, φ p 2 represents the   6a.CFSR analysis data is used for the calculation of all terms, except for the diabatic heating rate (DIAB), which is obtained as the difference between the total and the sum of TADV and VMT terms in Eq. (3).
geopotential height at p 2 , R d is the gas constant for dry air, T v represents the virtual temperature, and g represents gravitational acceleration.The terms in Eq.
(2), from left to right, denoting the surface pressure tendency (dp), change in geopotential height at the upper boundary (dϕ), vertically integrated virtual temperature tendency (ITT), mass loss (increase) caused by surface precipitation (evaporation) (EP), and the residuum due to discretization (RES PTE ).
All terms in Eq. ( 2) are calculated over the box shown in Fig. 6a for area-or volume-averaged changes for 0000 -0600 UTC 13 July, during which the mean surface pressure decreased by 1.3 hPa (Fig. 7b).The ITT dictates the surface pressure decrease, which contributes −2.5 hPa per 6 hours.The negative ITT implies a warming of the atmosphere.The values of dϕ and EP are 1.0 and −0.5 hPa per 6 hours, respectively.A positive dϕ indicates an increase in the upper boundary height due to the warming of the atmospheric column.The value of EP is equivalent to the area-averaged accumulated rainfall amount of 5 mm.
The above calculation indicates that warming of the atmosphere is the dominant cause for the development of the trough over eastern China from 0000 -0600 UTC 13 July.Warming of the atmosphere can occur through several processes, such as horizontal temperature advection, vertical displacement of air parcels, and diabatic heating.To examine the cause of warming, the ITT term in Eq. ( 2) is expanded to: where T represents temperature; V h and ω represent the horizontal and vertical wind components, respectively; c p represents the specific heat capacity at a constant pressure; and Q represents diabatic heating.The terms on the right-hand side of the equation (from left to right) represent the effects of horizontal temperature advection (TADV), vertical motion (VMT), and diabatic processes (DIAB) on the column-integrated temperature tendency and the residual (RES ITT ).
All terms in equation ( 3) are integrated over the box volume and then averaged over 0000 and 0600 UTC 13 July.The data from the CFSR analysis are used for this calculation, except for DIAB, which is obtained by taking the difference between the total and the sum of TADV and VMT terms because the analysis data do not provide diabatic tendencies.The values of DIAB obtained this way are found to be consistent with the values obtained using the hourly CFSR forecast data, which provide the data for DIAB.
The above calculation indicates that diabatic heating (DIAB) is responsible for most of the warming in the atmospheric column, where it contributes −5.8 hPa per 6 hours to the surface pressure tendency for the period (Fig. 7b).By contrast, adiabatic cooling associated with ascending motion (i.e., the VMT term) results in an increase in surface pressure by 3.5 hPa per 6 hours.Positive temperature advection over eastern China (i.e., the TADV term) contributes to a small amount of surface pressure decrease.
To further find out diabatic processes that are important for trough development, vertical profiles of heating rates for various diabatic processes are obtained using hourly CFSR forecast data from 0000 -0600 UTC 13 July (not shown).The calculation suggests that trough development over eastern China for 0000 -0600 UTC is mainly due to deep convective heating and surface heating, which are mainly responsible for warming in the layer above and below 900 hPa, respectively.

b. Growth of the SWB during 1200 -1800 UTC
As found in Fig. 6, the strength and horizontal extent of the SWB increased significantly during 1200 -1800 UTC 13 July 2004.Here, we investigate the processes for the growth of the SWB during the 6-hour period using the hourly CFSR forecast data.
Geopotential height, horizontal divergence, and the SWB at 925 hPa are shown at 2-hour intervals in Fig. 8.The SWB grows with time in strength and horizontal extent during 1200 -1800 UTC.Its northern boundary passes over Jeju Island at approximately 1600 UTC and reaches the southwestern coast of the Korean peninsula at approximately 1800 UTC.The SWB also grows southwestward during 1200 -1600 UTC, and its southern boundary is maintained at about the same location (near 29°N, 123°E) after 1400 UTC.The maximum wind speed in the SWB is located at the southeast of the trough throughout the 6-hour period.
Figure 8 also shows a significant temporal variation in the 925 hPa height-contour pattern over the sea between the coast of eastern China (near Hangzhou) and the southwestern Korean peninsula.A broad, weak ridge is found over the sea downstream of the trough over the eastern coast of China at 1200 and 1400 UTC (Figs. 8a,b,respectively).This ridge changed into a broad, weak trough over the sea at 1600 UTC (Fig. 8c).After 1600 UTC, two mesoscale troughs developed over the area northeast of Shanghai and the southwestern coast of the Korean peninsula (Fig. 8d).These changes of height fields during 1600 -1800 UTC increase the height gradient along the SWB.
Note that the broadening of the trough over the sea occurred in approximately 4 hours.This fast broadening can be attributed to the continued production of positive vorticity, which is caused by the convergence that occurs northwest of the SWB and is consistent with the vorticity budget calculation (not shown).This area of convergence is located under the ridge located over the sea from 1200 -1400 UTC (Figs. 8a, b) and moves toward the southwestern coast of the Korean peninsula as the SWB grows northeastward, thus contributing to trough broadening ahead of the SWB.
As the fast growth of the SWB indicates, the flow along the SWB is not in geostrophic balance.To explain the SWB growth, temporal changes in wind over the SWB area are examined using the momentum equation for pressure coordinates: where f is the Coriolis parameter, and V a represents the ageostrophic wind (V a = V h -V g ).The first and second terms on the right-hand side of the equation represent the advection of momentum in the horizontal (HA) and vertical (VA) directions, respectively.
The third and fourth terms represent the Coriolis acceleration (CA) and friction (residual term, FR), respectively.The local tendency of wind is calculated from the hourly CFSR forecast data for times t and t - 1 h, and the terms on the right-hand side are obtained by averaging their values for t and t -1 h.
Figure 9 shows the results of the calculations at 925 hPa during 1400 -1500 UTC.The local tendency of wind (ΔV h /Δt ) shows the increase in southwesterlies or southerlies over most of the SWB and an especially large increase in southwesterlies over the northern part (and slightly ahead) of the SWB (Fig. 9a).These large increases indicate the northeastward growth of the SWB.The summation of terms on the right-hand side of Eq. ( 4) is very similar to the local tendency over and ahead of the SWB (Fig. 9b).The calculation also indicates that the CA is the major factor for the strengthening of the central and the southern SWB (Fig. 9d) and also for the southwestward growth during 1200 -1400 UTC (not shown).By contrast, the northeastward growth of the SWB is mainly due to horizontal momentum advection from the center of the SWB (Fig. 9c), and it is secondarily caused by the CA (Fig. 9d).Calculation indicates that magnitude of the friction is very small over the area of the SWB, which develops over the ocean (not shown).
During 1700 -1800 UTC, an increase in wind speed of the southwesterly is generally small over the central and southern SWB but significant over the northern part (Fig. 10a).Both horizontal momentum advection and the CA are important for the northeastward growth (not shown).Southeasterly ageostrophic winds prevail over most of the SWB except for the central area to the north of the maximum wind speed (Fig. 10b).The strong southeasterly ageostrophic winds over the northern SWB blow toward the area of height fall associated with the trough development over the southwestern coast of the Korean peninsula.The CA increases the southwesterly over the northern part of the SWB.The small change of winds over the southern part of the SWB in Fig. 10a is mainly due to the cancellation between the horizontal momentum advection and the CA (not shown).
In summary, the growth of the SWB during 1200 -1800 UTC is achieved mainly through ageostrophic processes in an environment of changing height fields over the East China Sea.The CA is found to be the major process for the strengthening and expansion of the SWB, while both the horizontal momentum advection and the CA are important for the northeastward growth of the SWB.
Throughout the case period, the core of the SWB is maintained at the southeast of the mesoscale trough (e.g., Fig. 6).This indicates that the geostrophic component is the major contributor to the existence of the SWB.At the same time, it is also found that the nocturnal growth of the SWB is mainly caused by the ageostrophic processes over the existing SWB, especially over the southern half and northern part of the SWB (e.g., Fig. 9).Thus, the mesoscale trough developed over eastern China during the day time can provide a site at which the SWB grows in strength and extent during the night time.
It is also important to understand what has initiated the ageostrophic growth of the SWB.In the present case, significant easterly ageostrophic winds appear over the upstream and southern part of the existing SWB after 1200 UTC, and then the nocturnal growth of the SWB proceeds as veering of the easterly ageostrophic winds enhances the southern part of the SWB (not shown).However, the causes for the easterly ageostrophic winds and the trigger mechanism for the nocturnal growth are not clearly understood yet, and further descriptions about them are not provided in this paper.Later, nocturnal growth of the SWB is described further by comparing it with those of LLJs over East Asia in previous studies.

c. Influence of SWB growth on the heavy-rainfall
environment over the southern Korean peninsula Figure 11 shows the vertical cross sections of the wind vector, wind speed, equivalent potential temperature (θ e ), and horizontal convergence along the SWB (i.e., along the line shown in Fig. 6a).At 0600 UTC on 13 July, the SWB is located over the area from 50 -450 km, and the belt of high-θ e air (θ e > 350 K) in the layer below 950 hPa is reaches approximately 750 km (Fig. 11a).The SWB decreased significantly, and the high-θ e air penetrated slightly further northeastward at 1200 UTC (Fig. 11b).At 1800 UTC, the SWB grew significantly in strength and extent, and the highθ e air at low levels reached the southwestern coast of the Korean peninsula (marked by ▲), where a zone of significant convergence was found in a deep layer (Fig. 11c).The θ e isopleths northeast of the coastline are nearly vertical, indicating convective mixing over the area.Convection also consumed a significant amount of moisture upstream, which can be noticed by the decrease in θ e during 1200 -1800 UTC in the layer below 950 hPa southwest of the 750 km region.
To demonstrate the influence of strong southwesterlies along the SWB on the atmospheric structure over the southwestern Korean peninsula, we have examined the temporal variations in the vertical profiles of θ e , convective available potential energy (CAPE) and vertical motion at the location marked by ▲ in Fig. 11.A significant increase in θ e can be found in the layer below approximately 800 hPa, with a maximum increase of 5 -6 K in the 950 -850 hPa layer during 1200 -1800 UTC (Fig. 12).This increase is mostly due to moisture transport caused by strong southwesterlies.At 1200 UTC, the air at the level of maximum θ e is found to have a CAPE of 490 J kg −1 , and its level of free convection (LFC) is approximately 740 hPa (Fig. 12a).The analysis indicates that the upward motion below the LFC at 1200 UTC is less than -0.1 Pa s −1 , which is too weak to lift an air parcel to its LFC.At 1800 UTC on 13 July, however, air parcels at low levels have larger CAPE values than those at 1200 UTC, and significant vertical velocities can be found (Fig. 12b).Let us consider the air parcels at 950 and 900 hPa whose LFCs are 910 and 890 hPa, respectively.The mean vertical velocities between the initial level and LFC are −0.55 and −0.7 Pa s −1 for the parcels at 950 and 900 hPa, respectively.If the air parcels at 950 and 900 hPa are lifted at their respective mean vertical velocities, they can reach their LFCs in 120 and 24 minutes, with CAPEs of approximately 1,148 J kg −1 and 921 J kg −1 , respectively.Furthermore, if we consider air approaching the convergence zone from the upstream area whose θ e is greater than that over the location of the profiles in Fig. 12, its CAPE can be significantly larger than those for the parcels considered above.This case study suggests that the fully extended SWB over the sea between the coasts of eastern China and the southwestern Korean peninsula brings large amounts of moisture and increases convective instability with the convection trigger mechanism (i.e., strong horizontal convergence) over the southwestern peninsula in the early morning.In a study of the diurnal cycle of a heavy rainfall corridor over East Asia, Chen et al. (2017) also found that nocturnal enhancement of LLJ over southeastern China resulted in the enhancement of moisture transport, low-level ascent, and elevated convective instability at the northern terminus of the LLJ.

Case 2
The second case study was carried out for the episode of CCMT-12, where heavy rainfall occurred over a wide area across the southern Korean peninsula (Fig. 13a), and the areal mean hourly rainfall amount was especially large during the morning hours of 2100 UTC 15 -0000 UTC 16 (0600 -0900 LST 16) July 2009 (Fig. 13b).
Constant pressure charts for 1200 UTC on 15 July are shown in Fig. 14.At 925 hPa, a synoptic-scale ridge extended from the WPSH and stretched out toward central China and the Korean peninsula, as a synoptic-scale cyclone was located northeast of the ridge (Fig. 14a).The horizontal temperature gradient at 925 hPa was weak over eastern China and the Yellow Sea.Upper-level troughs can be found east of the Korean peninsula (Figs.14b, c).At 200 hPa, relatively strong jet streams can be found over both the upstream and downstream regions of the Korean The evolution of the cloud distribution is shown using multifunctional transport satellite (MTSAT) images in Fig. 15.Mesoscale convective systems (MCSs) can be found at 0033 UTC on 15 July 2009 over the coastal area of the southern Korean peninsula.These MCSs are associated with a trough extended from the synoptic-scale cyclone to the northeast of the Korean peninsula.They move eastward and dissipate after approximately 1000 UTC.The convective systems continually develop near and to the southwest of Shanghai from 0600 -1300 UTC.New convective systems develop at approximately 1400 UTC over the southwestern coast of the Korean peninsula (not shown) and grow into MCSs over the southern Korean peninsula at 1733 UTC.Note that convective clouds are found across the Yellow Sea between eastern China (near 34°N) and the west coast of the southern Korean peninsula at 1733 UTC.The CC of the present episode developed at approximately 2100 UTC and grew further.It slowly moved eastward along the southern coast and maintained CC characteristics until it moved away from the southern coast at approximately 0300 UTC on 16 July.
A weak mesoscale trough was over eastern China at 0000 UTC on 15 July to the west of Shanghai (not shown).It grew significantly in size during the next 6 hours in the environment of a synoptic-scale ridge (Fig. 16a).The trough remained stationary until 1200 UTC, while a mesoscale ridge developed over the southern half of the Korean peninsula (Fig. 16b).It was intensified further northeastward at 1800 UTC (Fig. 16c).A weaker ridge was maintained over the Yellow Sea and the Korean peninsula at 0000 UTC on 16 July (Fig. 16d).
The SWB was located southeast of the trough, which was east of Shanghai at 0600 UTC (Fig. 16a).It grew significantly during the next 6 hours, and its northern boundary was near the southwestern coast of the Korean peninsula (Fig. 16b).According to the hourly CFSR forecast fields, the SWB weakened until approximately 0800 UTC and then grew with time toward the southwestern Korean peninsula (not shown).After 1800 UTC, the northern part of the SWB expanded eastward with time along the southern coast of the Korean peninsula until 0000 UTC on 16 July (Fig. 16d).Note that the SWB was significantly stronger and larger than that of the previous episode (CCMT-06).The stronger and more extensive SWB in this episode appears to be due to the trough development against the sustained ridge over the Yellow Sea, which can induce a stronger height gradient between the two disturbances.The trough development over eastern China during 0000 -0600 UTC 15 July is mainly due to surface and convective heating according to the calculation for surface PTE (not shown).The magnitude of the diabatic heating rate from 0000 -0600 UTC is smaller than that for Case 1 (CCMT-06) due to less convective heating during this episode.Deep convection persists over the area southwest of Shanghai during 0600 -1300 UTC and contributes to trough development over the coastal area near Shanghai and Hangzhou (Fig. 16b).
In this episode, the entire growth of the SWB occurs along the southeastern side of the quasi-stationary trough.Examination using hourly forecast fields has revealed that the processes of SWB growth during 0900 -1800 UTC are basically the same as those for the previous episode in that the Coriolis acceleration is the key process for the strengthening of the southern half of the SWB and horizontal momentum advection is the major process for the SWB growth toward the southwestern Korean peninsula (not shown).
The areal mean 1-hour rainfall amounts in this episode are especially large from 2100 UTC 15 July to 0000 UTC 16 July, which is mainly due to more intense rainfall over wider areas across the southern Korean peninsula, especially over the western half, where the CC grows further as it slowly moves eastward during the 3-hour period (Fig. 15).Possible factors for the large mean areal 1-hour rainfall amounts after 2100 UTC on 15 July 2009 are examined here using hourly CFSR forecast data from 1800 -2200 UTC (Fig. 17).While the synoptic-scale ridge is maintained over the Yellow Sea, a mesoscale ridge appears over the whole Korean peninsula after 2000 UTC.Horizontal convergence over the southwestern peninsula strengthens with time, reaching near maximum strengths (−16 × 10 −5 s −1 ) at 2100 and 2200 UTC (Figs. 17c,d,respectively).The strongest convergence at 2100 UTC is located over the area where the wind speed rapidly decreases toward the inland region.The weak winds over the inland area may be mainly due to the development of the mesoscale ridge over the middle Korean peninsula, which results in a weak height gradient over the inland area.Kim and Lee (2016) also found a similar development of the mesoscale ridge at surface level over the middle Korean peninsula in a similar synoptic situation and related the development to the blocking effect of the terrain along the east coast of the middle Korean peninsula (i.e., the Taebaek Mountain range) based on numerical experiments.These results indicate that the terrain effect may be a factor for the weak winds over the inland area and the stronger convergence ahead of the SWB.Another factor for the larger mean areal rainfall in the present episode could be the stronger southwesterly over the entire southern coast (Figs. 16d,17d), which brings larger amounts of moisture and convective instability over the southern peninsula.
The influence of strong southwesterlies on the heavy-rainfall environment for this episode (CCMT-12) is analyzed in the same manner as that for the previous episode (CCMT-06).The vertical cross section of wind speed and θ e along the SWB for 1800 UTC 15 July 2009 shows a more extensive SWB and a deeper layer of high-θ e air (θ e > 350 K) compared with those of the previous episode (Fig. 18).The northern parts of the SWB and the belt of high-θ e air were over the southern coast of the Korean peninsula (marked by ▲), where a zone of significant convergence was found in a deep layer below 700 hPa.
Figure 19 shows the vertical profiles of θ e and ver- tical motion at 1800 UTC at the shoreline (900 km), with a θ e profile at 800 km.The profile for θ e at 900 km shows a vertical structure that is neutralized by convection.The low θ e in the upper layer from 350 to 550 hPa is due to the horizontal advection of low-θ e air from the west.Convective instability should be transported from the upstream region to support the development of MCSs.Note that θ e for the low-level air at 800 km is 4 -6 K higher than that over 900 km.Let us consider an air parcel at 925 hPa over the location at 800 km, which travels at the level of strongest low-level wind.If the air parcel approaches the 900 km location conserving its θ e , it would be able to con- vectively rise with significant CAPE that can support the development of MCSs.As in the previous episode, the SWB provides the southwestern part of the Korean peninsula with sustained moisture transport and convective instability.
The two case studies presented in this section have revealed some important similarities between the CCMT-06 (Case 1) and CCMT-12 (Case 2) episodes in terms of the processes that lead to heavy rainfall over the Korean peninsula in the morning: 1) The low-level trough over eastern China and its coastal area is important for the strengthening of southwesterlies over the East China Sea; 2) the SWB grows with time toward the southwestern Korean peninsula during the night time; and 3) heavy rainfall occurs in the early morning over the southwestern part of the Korean peninsula, where the exit region of the SWB is located.However, some notable differences between the two episodes can also be found in the strength and horizontal extent of the SWB and the areal mean 1-hour rainfall amount during the morning hours (i.e., 2100 -0000 UTC).

Discussion
Generality of the major results from the case studies is examined against the results obtained using the composite fields of the nine CCMT episodes with rainfall maximum in the early morning.The composite fields at 925 hPa show characteristic patterns of geopotential height and winds, which are similar to those found in the case studies (Fig. 20).Throughout the 24 h period, a synoptic-scale cyclone is found to the north of the Korean peninsula, and the northwestern edge of the WPSH is over the southern Korean peninsula.The SWB is found over the East China Sea at the southeast of a weak mesoscale trough over Shanghai and Hangzhou area at 0600 and 1200 UTC (Figs. 20a,  b).It shows a significant growth during 1200 -1800 UTC, with its northern terminus reaching the southern coast of the Korean peninsula at 1800 UTC (Fig. 20c).
Calculations using PTE and the composite fields suggest that the development of trough over eastern China is mainly caused by the warming in the atmosphere, especially the surface and convective heating, as found in the case studies (not shown).Dynamical processes for the growth of the SWB during 1200 -1800 UTC in the composite fields are also examined using the momentum equation and the hourly CFSR forecast data (Fig. 21).As found in the case studies, the CA is responsible for the strengthening of the central and the southern SWB in the composite field, whereas the combined effects of the horizontal momentum advection and the CA bring the northeastward growth of the SWB (Figs. 21c, d).These agreements between the case and composite studies support the generality of the major results obtained in this study.
It is of a significant interest to find out how the present results compare with the findings of the previous studies concerning the nocturnal enhancement of LLJ in East Asia.The mechanism for the nocturnal SWB growth is similar to those for the nocturnal enhancement of LLJs over southeastern China (Chen et al. 2009;Xue et al. 2018) and off the coast of southeastern China (Du et al. 2015), in that the CA is the main mechanism for the growth of LLJs.In southeastern China, Xue et al. (2018) suggested that the diurnal variations in ageostrophic wind could be explained by Blackadar inertial oscillation theory (Blackadar 1957;Markowski and Richardson 2011).In a study of a coastal boundary layer jet (CBLJ) off the southeastern coast of China, where surface friction is relatively small and easterly ageostrophic winds toward the coast appear in the afternoon due to landsea thermal contrast, Du et al. (2015) suggested that the nocturnal maximum of the CBLJ was attributed mainly to the veering of offshore ageostrophic winds triggered by the disappearance of land-surface heating over the China plain after sunset.In the present study (in Case 1), the SWB occurs over the East China Sea and grows with time becoming supergeostrophic after 1300 UTC mainly due to a clockwise rotation of ageostrophic winds over and around the southern SWB (not shown) similar to that found by Du et al. (2015).However, an important difference can be found between the present and Du et al. (2015) studies in that the SWB growth occurs relatively far from the coast compared to that for Du et al. (2015), and consistent easterly ageostrophic winds at low levels are not found during the afternoon hours over the area between the coast and the SWB.In the present study on Case 1, significant easterly ageostrophic winds appear over the upstream and southern part of the SWB prior to the veering of the ageostrophic winds.However, the causes for the easterly ageostrophic winds and the trigger mechanism for the nocturnal growth are not clearly understood yet, and further studies are needed to explain them.

Summary and conclusions
An investigation has been carried out using observations and NCEP CFSR analysis and forecast data to explain the environment and processes that lead to heavy rainfall in the early morning over the Korean peninsula during CCMT episodes.For this study, nine episodes with the maximum hourly rainfall amount in the early morning (i.e., 0300 -0900 LST) are selected from 17 heavy-rainfall episodes associated with CCMTs during 2001 -2011.This paper presents 1) diurnal variation in the rainfall amount over South Korea during the CCMT episodes, 2) the diurnal variation in meteorological fields over East Asia and the processes that bring heavy rainfall in the early morning over the Korean peninsula based on case studies of two episodes.
Case studies on the episodes of CCMT-06 (Case 1) and CCMT-12 (Case 2) have revealed the diurnal variations in meteorological fields over East Asia and the processes that lead to heavy rainfall in the early morning over the southwestern Korean peninsula.It is found for both episodes that 1) a low-level trough develops over eastern China and its coastal area during the day time; 2) the SWB on the pressure level of 925 hPa over the East China Sea, which is located southeast of the trough, strengthens and expands at night time toward the southwestern Korean peninsula; 3) the SWB supplies a large amount of moisture and increases convective instability over the southwestern Korean peninsula with a convection trigger mechanism (i.e., strong horizontal convergence); and 4) heavy rainfall occurs in the early morning over the southwestern peninsula, where the exit region of the SWB is located.The growth of the SWB over the East China Sea is found to be the key process for heavy rainfall in the early morning over the southwestern peninsula.The growth is achieved mainly through ageostrophic processes in an environment of changing height fields over the East China Sea.It is also found that the dynamic processes of the SWB growth are basically the same for both episodes in that the CA is the key process for the strengthening of the southern half of the SWB. and both horizontal momentum advection and the CA are important for the SWB growth toward the southwestern Korean peninsula.
However, some notable differences between the two episodes can be found in the strength and horizontal extent of the SWB and the areal mean 1-hour rainfall amount over the southern Korean peninsula during the morning hours (2100 -0000 UTC).The stronger and more extensive SWB and the stronger convergence over the exit area of the SWB are suggested to be important for the large areal mean 1-hour rainfall amount for the morning hours in the CCMT-12 episode.
Additional analyses using the composite fields of the 9 CCMT episodes are found to support the generality of the major results from the case studies concerning the diurnal variation of meteorological fields and the dynamical processes of the SWB growth.
This study is mainly based on analysis data and does not provide detailed relationships and interactions between convection and environment.Numerical case studies may help further understand the role of these interactions in the diurnal variation of meteorological fields and the SWB growth.3 m s -1 h -1 3 m s -1 h -1 3 m s -1 h -1

Fig. 1 .
Fig. 1.Diurnal variations of normalized mean rainfall amount, frequency, and intensity over South Korea for (a) the 17 CCMT episodes in Table1and (b) the 9 CCMT episodes in which the maximum of hourly rainfall amount occurs in the early morning (i.e., 0300 -0900 LST).All values are obtained using AWS data.

Fig. 7 .
Fig. 7. (a) Vertical profiles of averaged changes in height (m, black solid line) and virtual temperature (K, red dashed line) during 0000 -0600 UTC 13 July 2004, and (b) results of the PTE calculation for 0600 UTC 13 July 2004.All values are the averages over the red dashed box in Fig.6a.CFSR analysis data is used for the calculation of all terms, except for the diabatic heating rate (DIAB), which is obtained as the difference between the total and the sum of TADV and VMT terms in Eq. (3).
Fig. 12. Vertical profiles of equivalent potential temperature (θ e , solid lines, K), saturated equivalent potential temperature (θ es , dashed lines, K), CAPE (shaded, J kg −1 ), and vertical velocity (solid lines in right figure, Pa s −1 ) from CFSR analysis data at the location marked by ▲ in Fig. 11 for (a) 1200 UTC and (b) 1800 UTC 13 July 2004.Thin solid line in the figures for θ e denotes the vertical path of the air parcel at the level of maximum θ e following the pseudoadiabatic process.

Fig. 19 .
Fig. 19.Vertical profiles of equivalent potential temperature (θ e , solid lines, K), saturated equivalent potential temperature (θ es , dashed lines, K), and vertical velocity (black solid lines in right figure, Pa s −1 ) from CFSR analysis data for 1800 UTC 15 July 2009.Red and blue lines indicate the soundings at 900 and 800 km locations in Fig. 18, respectively.

Fig. 20 .
Fig. 20.Composite fields at 925 hPa for (a) 0600 UTC, (b) 1200 UTC, (c) 1800 UTC, and (d) 0000 UTC.Geopotential height (m, black solid lines), wind vectors, and isotachs (m s −1 , red dashed lines) are obtained from CFSR 6-hour analysis data.Composite fields are obtained by averaging the fields over the 9 episodes with maximum hourly rainfall amount in the early morning.

Fig. 21 .
Fig. 21.Same as Fig. 9, except that the composite fields of the nine episodes are used for the calculations.