DSpace at VNU: Synoptic-scale physical mechanisms associated with the Mei-yu front: A numerical case study in 1999

To further understand the synoptic-scale physical mechanisms associated with the Mei-yu front system, the present study proposes another insight into the physical signifi-cance of the x-

Asia-Pacific J Atmos Sci., 48(4), 433-448, 2012

DOI:10.1007/s13143-012-0039-x

Synoptic-Scale Physical Mechanisms Associated with the Mei-yu Front:

A Numerical Case Study in 1999

Nguyen Minh Truong1, Vu Thanh Hang1, Roger A Pielke Sr.2, Christopher L Castro3, and Koji Dairaku4

1 Hanoi University of Science, Hanoi, Vietnam

2 CIRES, University of Colorado, Boulder, CO, U S A.

3 Department of Atmospheric Sciences, University of Arizona, Tucson, AZ, U S A.

4 Storm, Flood, and Landslide Research Department, National Research Institute for Earth Science and Disaster Prevention, Ibaraki, Japan

(Manuscript received 5 April 2012; revised 11 June 2012; accepted 30 June 2012)

© The Korean Meteorological Society and Springer 2012

Abstract: The Mei-yu front system occurring from 23 to 27 June

1999 consists of the Mei-yu front and the dewpoint front, which

confine a warm core extending from the eastern flank of the Tibetan

Plateau to the west of 145oE To further understand the

synoptic-scale physical mechanisms associated with the Mei-yu front system,

the present study proposes another insight into the physical

signifi-cance of the x-component relative vorticity (XRV) whose vertical

circulation is expected to tilt isentropic surfaces The XRV equation

diagnoses exhibit that the twisting effect of the planetary vorticity

(TEPV) is positive along the Mei-yu front and negative in the

dew-point front region, and tilts isentropic surfaces from south to north in

the Mei-yu frontal zone Conversely, the meridional gradient of the

atmospheric buoyancy (MGAB) tilts isentropic surfaces in the

opposite direction and maintains negative in the regions where the

TEPV is positive and vice versa Thus, the TEPV plays the role of

the yu frontogenesis, whereas the MGAB demonstrates the

Mei-yu frontolysis factor Both terms control the evolution of the

cross-front circulation The other terms show much minor contributions in

this case study The present simulations also indicate that the

weakening of the upper-level jet evidently induces the weakening of

the Mei-yu front and reduces the amplitude of the East Asia cold

trough Furthermore, the impact can also penetrate into the lower

troposphere in terms of mesoscale disturbances and precipitation,

proving that the upper-level jet imposes a noticeable top-down

influence on the Mei-yu front system

Key words: Mei-yu frontogenesis, frontolysis, twisting effect,

atmospheric buoyancy, ageostrophic twisting effect

1 Introduction

In the latest decades, a large amount of research has been

carried out to study the Mei-yu (or Baiu in Japanese)

phenom-enon since it often accompanies mesoscale disturbances and

torrential rain in the East Asian summer monsoon (EASM)

region (Shen et al., 2001; Kawatani and Takahashi, 2003;

Shibagaki and Ninomiya, 2005; Ninomiya and Shibagaki, 2007)

However, the studies may be divided into two common

frameworks: the Mei-yu season diagnosis and the Mei-yu front

diagnosis For the Mei-yu season diagnostic framework, for example, Sampe and Xie (2010) diagnosed the large-scale environment favorable for the Meiyu-Baiu season and found a close relation between the warm advection and upward motion, indicating the importance of the warm advection for the Meiyu-Baiu formation Besides, they proposed a hypothesis that the externally induced ascent helps to trigger convection by lifting air parcels, which in turn produces a positive feedback for the Meiyu-Baiu Kawatani and Takahashi (2003) analyzed the characteristics of large-scale circulations and the configurations

of numerical experiments, which could favor simulation of the Baiu front and the Baiu precipitation Wang et al (2003) used

a highly resolved regional climate model to simulate precipita-tion in the Mei-yu season from 26 April to 31 August 1998 Their 4-month simulations showed that rainfall associated with the Mei-yu front over the Yangtze River basin (26o-32oN, 110o

-122oE) was less convective Conversely, convective rainfall dominated in south China

For the Mei-yu front diagnostic framework, abundant re-search focuses on principal weather systems associated with the Mei-yu front For example, in 1998, the year after the strongest 1997/98 El Niño event in the 20th century, the Mei-yu front and accompanying weather disturbances caused severe flooding in the Yangtze River basin (Shen et al., 2001; Wang

et al., 2003; Qian et al., 2004) and have, therefore, received a lot of attention Chien et al (2002) verified the precipitation forecast skill of the MM5 model and exhibited that during the Mei-yu season in 1998, many mesoscale convective systems (MCS) developed along the front and moved toward Taiwan Zhang et al (2003) also used MM5 to depict conditions for the formation of mesoscale features embedded in a mature MCS, including lower-level jet, upper-level jet, mesolow, and meso-high etc A further investigation into the internal structures and evolution of the Mei-yu front was done by Chen et al (2006) who analyzed mechanisms producing lower-level jets, jet intensification, and the retreat of the Mei-yu front near Taiwan

In their study, the potential vorticity generated and latent heat released by MCS, along with the adjustment to geostrophic balance, were emphasized as the major mechanisms

So far, the Mei-yu studies have recognized the important roles

Corresponding Author: Nguyen Minh Truong, Hanoi University of

Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam

E-mail: truongnm@vnu.edu.vn

434 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

of the Tibetan Plateau, moisture sources, and western Pacific

subtropical high (WPSH) Yoshikane et al (2001) conducted

numerical experiments and concluded that the Tibetan Plateau

and mountains could significantly affect the Baiu front location,

lower- and upper-level jets, and Baiu precipitation, but the

fundamental structure of the front could be reproduced without

any orography Qian et al (2004) demonstrated that the

mois-ture flux from the Bay of Bengal played an essential role in

Mei-yu precipitation in 1998 The WPSH location might be

important in that it would decide where moisture comes from,

the South China Sea or Bay of Bengal (Cho and Chen, 1995;

Shen et al., 2001; Ninomiya and Shibagaki, 2007; Sampe and

Xie, 2010) However, all of the above studies did not explicitly

figure out the synoptic-scale physical mechanisms associated

with any particular Mei-yu front since they used subjective

analyses of the model output fields instead of using dynamic

relations that are analytically derived

Chen et al (2003, 2008) used the conserved Ertel’s potential

vorticity (PV) to diagnose two Mei-yu front cases in 1990 and

2003 Unfortunately, the PV is no longer conserved in diabatic

heating situations (Holton, 2004; Chen et al., 2008), while the

piecewise PV inversion technique (as well as the complete PV

equation) is complicated to interpret physical mechanisms

Another method diagnosing frontogenesis is the frontogenetical

function For example, Zhou et al (2004) used the

fronto-genetical function to diagnose the formation of a Mei-yu front

system in the La Niña year 1999, including the Mei-yu and

dewpoint front It is unfortunate that the function is a purely

kinematic approach (Bluestein, 1993; Chen et al., 2007) that

cannot describe some major physical processes such as the

transport by air parcels, development of transverse circulation,

and upper-level front (Chen et al., 2007) As cautioned by Chen

et al (2007), “one needs to be cautious in the interpretation of

frontogenetical function results because of the above

limita-tions, especially in frontal movement and evolution.” In other

words, starting with definition formulas containing the gradient

of potential temperature, the mathematic manipulations

de-veloped might then lead to relations showing the outward

appearance without addressing the inward essence of

fronto-genesis That is why a dynamic approach is desirable to

under-stand the large-scale dynamic mechanisms that help to anchor

the Mei-yu front as proposed by Sampe and Xie (2010)

Along with the vertical relative vorticity, the horizontal

vorticity equations are useful prognostic tools (Davies-Jones,

1991; Jung and Arakawa, 2008) However, in the EASM and

Mei-yu studies, most attention was paid to the vertical

component (Chen and Chang, 1980; Wang, 1987; Chang et al.,

2000; Chen et al., 2003), although the meridional streamline

might be favorable for the synoptic-scale horizontal vorticity

in the Mei-yu regions (Lau et al., 1988; Chang et al., 2000;

Chen et al., 2008; Sampe and Xie, 2010) Davies-Jones (1991)

described the frontogenetical forcing of secondary circulations,

but he used the hydrostatic approximation (i.e., the

atmo-spheric buoyancy is omitted) and did not figure out what are

major mechanisms for frontogenesis and frontolysis in any real case In such circumstance, the present study aims at reproduc-ing the synoptic-scale physical mechanisms for the formation and evolution of the Mei-yu front system by a case study in

1999 In the next section, the x-component relative vorticity (XRV) equation is given with the tilting effect Model configu-ration and data are described in Section 3 and numerical simulations are given in Section 4 Summary and concluding remarks are presented in Section 5

2 XRV equation and tilting effect

a XRV equation The motives for using the XRV equation to clarify the mechanisms for the formation and evolution of the Mei-yu front system derive from the evidence found that: 1) according

to the conceptual model of the Mei-yu/Baiu front by Ninomiya and Shibagaki (2007), maximum wind airflows are found along the northern and southern flank of the Tibetan Plateau at upper levels, which may then extend northeastward to Japan 2) the Mei-yu front is usually quasi-stationary, originates in south China or the Yangtze River basin, and also frequently extends northeastward to Japan 3) Davies-Jones (1991) indi-cated that the horizontal vorticity equation can be used to depict the frontogenetical forcing of secondary circulations, where the curl of the Coriolis force due to vertical shear of the horizontal wind (i.e., f∂v/∂z) may be important Thus, a close relation between the Mei-yu front, maximum wind airflows (Kawatani and Takahashi, 2003; Sampe and Xie, 2010), and horizontal vorticity is expected If so, the XRV equation needs

to be taken into account

As conventional, the x-, y-, and z-component of relative vorticity are respectively defined by

(1) Using two equations of the meridional and vertical wind without friction (Pielke, 2002), one may receive the XRV equation

(2) Here u, v, and w are the x-, y-, and z-component of velocity, respectively; B = g( /θ0) is the atmospheric buoyancy is the virtual potential temperature perturbation computed as the deviation from θ0 which is the reference state potential tem-perature at hydrostatic state, and f is the Coriolis parameter (Pielke et al., 1992; Pielke, 2002; Cotton et al., 2003) On the right-hand side of Eq (2), the first term describes the stretching effect (SERV), the second term is the twisting effect of relative vorticity (TERV), the third term is the twisting effect of the planetary vorticity (TEPV), and the last one represents the meridional gradient of the atmospheric buoyancy (MGAB)

ξ ∂ - ∂∂yw v

∂z -– , η ∂u - ∂∂z w

∂x -– , ζ ∂v∂x - ∂u

∂y -–

dξ dt - ξ ∂u

∂x - η ∂u

∂y - ζ ∂u

∂z -+

∂z - ∂B

∂y

=

30 November 2012 Nguyen Minh Truong et al 435

b Ageostrophic TEPV

If we define the zonal geostrophic wind (ug) by f∂ug/∂z = −∂B

/∂y (Holton, 2004), then the TEPV induced by the ageostrophic

wind is just

where ua denotes the zonal ageostrophic wind Equation (2) is

then rewritten by

(3)

used If we note that

then one may write

(4a) so

(4b)

It is obvious that the ageostrophic TEPV (or dynamic forcings)

entirely contributes to the evolution of the cross-front

cir-culation through the meridional component of the XRV

tendency that controls the tilting effect as described below

c Tilting effect

In zonal jet regions, strong vertical shear of the zonal wind

twists the planetary vorticity and favors positive XRV which in

turn supports northerly (southerly) wind increasing upwards

(downwards) to the north (south) of the jet in particular layers

Moreover, ascending (descending) motion is favored to the

north (south) of the jet, where the air is cooler (warmer) As a

result, the total effect of the TEPV tends to tilt isentropic

surfaces and make the atmosphere less stably stratified

Con-versely, the Earth’s gravity trends toward forcing cooler air to

sink to the north and warmer air to rise to the south of the jet;

i.e., negative MGAB tilts isentropic surfaces in the opposite

direction and resists the meridional wind tendency induced by

the TEPV (Fig 1) The contributions of the other terms can be

understood in a similar fashion Thus, a balance between the

XRV forcing terms may keep warmer air stationary to the

south and cooler air to the north of the jet, and make fronts

likely to form Any deviation from such balance might lead to

front strengthening or weakening, depending on whether the

XRV tendency is positive or negative If the right-hand side of

Eq (4a) is negative then southerly (northerly) wind to the

north (south) of the jet at upper (lower) levels is favorable and frontolysis may occur Note that equation (2) is an unique analytical expression containing the linear term of the MGAB (the Ertel’s potential vorticity and frontogenetical function are nonlinear to the gradients of potential temperature), which can presumably depict the Mei-yu front system as it appears in nature In other words, unlike the traditional approach using the horizontal gradient of potential temperature, the present study uses the MGAB to detect fronts since it is known that thermodynamic properties change suddenly across the frontal zones For example, Stonitsch and Markowski (2007) objec-tively defined a front in terms of the relative maximum in the magnitude of the horizontal velocity gradient tensor Although the MGAB tends to approach zero while making the atmos-phere stably stratified (i.e., frontolysis effect), its presence itself represents the presence of front

3 Model configuration and data

In the present study, the Regional Atmospheric Modeling System (RAMS version 4.4) is used to simulate the Mei-yu front system from 0000 UTC 23 to 0000 UTC 27 June 1999 The simulation period is chosen similar to that used by Zhou et

al (2004) The initial conditions for the RAMS simulations are specified by using the NCEP-NCAR Reanalysis data (Kalnay

et al., 1996) These data consist of horizontal wind, tempera-ture, relative humidity, and geopotential height on 17 isobaric surfaces with a horizontal grid interval of 2.5o× 2.5o The boundary conditions are updated every 6 h using the same data source A Barnes objective analysis scheme is used to inter-polate the initial data onto the model grids The interpolation operator for the updated lateral boundary conditions is imple-mented using a quadratic function The sea surface tempera-ture (SST) data is the weekly SST given by NOAA (Reynolds

et al., 2002)

Centered at 35oN-108oE, the domain of the present study respectively includes 207× 161 grid points in the zonal and meridional direction with a grid spacing of 45 km As shown

in Fig 2 the model topography may reach more than 5500 m above mean sea level (MSL) over the Tibetan Plateau The model grid contains 30 levels and is vertically stretched with a 1.15 ratio The lowest grid spacing is 100 m and the maximum

f ∂ua

∂z

-=TEPV MGAB+

dξ

dt

- d

dt

∂w

∂y

- ∂v

∂z

-–

∂x - η ∂u

∂y - ζ ∂u

∂z -+

∂z

=

f ∂ua

∂z

- J+ xy(u w, ) J+ xz(v u, )

=

Jmn(A B, ) ∂A

∂m - ∂B

∂n - ∂A

∂n - ∂B

∂m -–

≡

ua 1

f

-dv

dt

-=

d

dt

∂v

∂z

-–

⎛ ⎞ f ∂ua

∂z

- J+ xz(v u, )

=

d

dt

∂w

∂y

-⎝ ⎠

⎛ ⎞ J= xy(u w, )

Fig 1 Schematic description for the 3-D interaction between the TEPV and MGAB in an environment with strong vertical shear of the zonal wind Isentropic surfaces (lines) are tilted due to shears of vertical and meridional wind (arrows) in the XRV symbolic circle

436 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

vertical grid spacing is set to 1200 m The convective

para-meterization scheme (CPS) is the modified Kain-Fritsch scheme

described by Truong et al (2009) where a new trigger function,

closure assumption, and equation to compute updraft velocity

are developed The CPS is activated every 5 minutes The

explicit microphysical representation of resolvable

precipita-tion is the scheme developed by Walko et al (1995) The

model configuration and experiments are summarized in Table

1 where Ctrl and Jmod are the control and jet-modification run

(see Section 4), respectively In the following sections the Ctrl

run is discussed, otherwise the Jmod run is mentioned

4 Numerical simulations and discussions

a Mei-yu front evolution

To be consistent with Eq (2), virtual potential temperature is

used to represent the Mei-yu front system instead of equivalent

potential temperature as in some other studies, although

equiva-lent potential temperature should make the fronts look stronger

Figure 3 illustrates the evolution of the Mei-yu front system at

700 hPa from 23 to 26 June 1999 at 1200 UTC At the early

stage, a hot low occurs immediately to the southeast of the

Tibetan Plateau where the southwest wind is maximum and

blows toward Japan (Fig 3a) At the same time, a cold trough locates west of the Korean peninsula As time elapses, the cold trough comes out of the domain to the east and the Mei-yu front system starts to migrate toward southern Japan (Figs 3b-d), including two branches: the Mei-yu front and the dewpoint front (Zhou et al., 2004) The Mei-yu front extends from about

32oN-103oE to 36oN-145oE, while the dewpoint front clearly originates from 21oN-110oE, extends northeastward and merges into the Mei-yu front (Figs 3b and 3c), creating the Mei-yu front system As usual, the meridional gradient of virtual potential temperature along the dewpoint front is significantly weaker than along the Mei-yu front (Zhou et al., 2004) Except the hot low, there are mesoscale warm centers in the form of closed isotherms confined by the Mei-yu front system, which are aligned along the maximum westerly wind airflow (lower-level jet) The development of the Mei-yu front over southern Japan accompanies the eastward propagation of the cold trough and mesoscale warm centers, and the presence of midlatitude westerly wind (Figs 3b-d) It is also found that the develop-ment process of the Mei-yu front is not concurrent between the sections in China and over Japan while the southerly wind develops and blows throughout China

At 300 hPa the cold trough locates west of the Korean peninsula along with a ridge to the northwest of Japan to create

Fig 2 Domain and model topography in m above MSL

Table 1 Model configuration and experiments where jet-modification run is described in Section 4

Ctrl

30 November 2012 Nguyen Minh Truong et al 437

a short thermal wave across the East Asia shoreline at 1200

UTC 23 June (Fig 4a) On the next days, the thermal wave

propagates eastward and the Mei-yu front starts to develop

southward over Japan and adjacent seas (Figs 4b-d), similar to

that previously mentioned Note that the front-like section

along the northern flank of the Tibetan Plateau may not be

called “Mei-yu”, though the term is generally used in the

present study for simplicity At this level, the Mei-yu front is

much stronger, whereas the dewpoint front almost disappears

(see also Zhou et al., 2004) Along the dewpoint frontal zone,

the wind vector remains very light on the first two days, but is

accelerated on the last two days (Figs 4c and 4d) when the

Tibetan high circulation develops and the 700-hPa Mei-yu

front system becomes mature over the western Pacific, but

weakens in China (Fig 3c) It is evident from the figures that

the development of the Mei-yu front coincides with the

intensification of the maximum wind airflow (upper-level jet)

moving along the front The Mei-yu front system weakens on

26 June (Figs 3d and 4d) Basically, the Mei-yu thermal

patterns closely follow an ordinary structure in this case study

with a clear warm core (Chen et al., 2003, 2008; Ding and

Chan, 2005; Yanai and Wu, 2006) that extends from the eastern flank of the Tibetan Plateau to the west of 145oE The XRV distribution indicates that the Mei-yu front can neither develop nor last long where the XRV is negative (not shown) For the quasi-stationary Mei-yu sections, the XRV is nearly equal to zero

b Mei-yu precipitation

On the first day, the model gives a heavy rainfall band along the southern flank of the Himalayas (similar to Yoshikane et al., 2001), an extensive heavy rainfall region over all Burma, and a heavy rainfall region in southern and central China, which is associated with the Mei-yu front system A local maximum center can be found upstream of the Yangtze River valley (30oN-103oE) and another extensive heavy rainfall region covers all Japan and the Korean peninsula (Fig 5a), which might be induced by the wind convergence to the southeast of the cold trough (Fig 3a) Afterwards, the heavy rainfall regions extend northward in China, southward to the Indochina peninsula, and eastward over Japan and adjacent seas,

accom-Fig 3 Simulated virtual potential temperature and wind vector at 700 hPa on 23 (a), 24 (b), 25 (c), and 26 (d) June 1999 at 1200 UTC Contour interval is 1.5oK Shaded areas show the wind speed larger than 15 m s−1 Model topography higher than 3000 m above MSL is blanked

438 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

Fig 4 Same as Fig 3 except at 300 hPa Shaded areas show the wind speed larger than 45 m s−1

Fig 5 Total rainfall (mm) accumulated for 24 (a), 48 (b), 72 (c), and 96 (d) h model integration, starting from 0000 UTC 23 June 1999

30 November 2012 Nguyen Minh Truong et al 439

panying with the development of the Mei-yu front It is a fact

that heavy rainfall is frequently observed in northern Vietnam

during active Mei-yu periods, and therefore becomes a major

concern The evolution and distribution of the simulated

pre-cipitation and circulations show good agreement with the

Global Precipitation Climatology Project (GPCP) and

NCEP-NCAR Reanalysis data (Fig 6), except that the model seems

to overestimate rainfall near the Yangtze River valley and over

the tropical Indian Ocean where rain gauge data is very limited

to derive the GPCP data

Although the simulated precipitation spreads over all East

Asian regions on the first two days, it appears much narrower

and lighter over India (Figs 5a, 5b, 6a, and 6b) The reason for

this might be the convergence to the south of the Tibetan high

at upper levels (not shown), similar to Shen et al (2001),

which in turn might cause synoptic-scale subsidence

sup-pressing convection over these regions Thus, this case study is

a good example suggesting again that the East Asian summer

monsoon, which can be classified as a subtropical monsoon

system, is not a simple extension to the east of the South Asian

summer monsoon (Ding and Chan, 2005)

c XRV equation diagnoses Figure 7 represents the MGAB at 700 hPa from 23 to 26 June at 1200 UTC When the Mei-yu front system starts to develop, this term has positive increasing values in the hot low and along the dewpoint front, but remains negative in the

Mei-yu front region (Figs 7a and 7b), appearing consistent with the warm-core structure as mentioned At the latter stages, the MGAB prevails negative and is distributed consistently with the development of the Mei-yu front system That is, it decays

in China, but zonally broadens over Japan and adjacent seas (Figs 7c and 7d) It is not surprising that the MGAB is better organized in the Mei-yu front region than in the dewpoint front region

Contrary to the MGAB, the TEPV appears to be strong with opposite sign in the Mei-yu front regions at 700 hPa (Figs 8a and 8b) Thus, the westerly wind actually increases quickly with height along the fronts (e.g., Figs 3 and 4) and this itself

Fig 6 NCEP-NCAR reanalysis geopotential height (contours at every 20 m) and wind vector at 700 hPa on 23 (a), 24 (b), 25 (c), and 26 (d) June

1999 at 1200 UTC Shaded areas show GPCP precipitation similar to Fig 5

440 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

Fig 8 Same as Fig 7 except for the TEPV

Fig 7 MGAB at 700 hPa on 23 (a), 24 (b), 25 (c), and 26 (d) June 1999 at 1200 UTC Solid (filled with dark grey) and long dash (filled with light grey) contours respectively indicate positive and negative value at intervals of 3× 10−7s−1 with zero lines omitted Model topography higher than

3000 m above MSL is blanked

30 November 2012 Nguyen Minh Truong et al 441

Fig 9 Same as Fig 7 except at 300 hPa

Fig 10 Same as Fig 8 except at 300 hPa

442 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

supports the formation and evolution of the front as explained

by Fig 1 Along the dewpoint front region, the TEPV is

negative and remarkably dominates over the MGAB while the

front develops (Figs 8b-d) At 300 hPa, the distribution of the

MGAB follows a similar morphology to 700 hPa except in the

dewpoint front regions where it almost vanishes (Fig 9) This

means the Mei-yu front contains a larger meridional difference

in the atmospheric buoyancy than the dewpoint front in the

whole troposphere, which requires a large-scale dynamic source

to maintain such state As expected, the MGAB can be used to

detect the fronts as it becomes stronger and smoother with

height in the Mei-yu frontal zone Figure 10 intuitively shows

that the TEPV appears dominant or comparable to the MGAB

in the Eurasian continent and offshore along the Mei-yu front

(Figs 10a-c) except over the Korean peninsula, Japan and

adjacent seas when the front weakens (Fig 10d) At this point

of view, the TEPV is the required condition for the fronts to

form and develop with height (i.e., the dewpoint front weakens

with height) Conversely, the MGAB dominates while the

front sections gradually decay, showing the frontolysis effect

of the MGAB The SERV and TERV are much smaller in this case study (not shown) The common magnitudes of the terms

in the XRV equation are summarized in Table 2 It is note-worthy that these two forcing terms can be observed at zonal scales equivalent to the synoptic scale, their meridional scale, however, belongs to the mesoscale at which vertical velocity and precipitation are observationally large along the Mei-yu frontal zone (e.g., see also Kawatani and Takahashi, 2003; Chen et al., 2008; Sampe and Xie, 2010)

Figure 11 indicates that the ageostrophic TEPV may contri-bute roughly 50% to the TEPV (i.e., the thermal wind relation does not hold) when the front experiences quick changes in the intensity on the last two days In the light of ξ, the ageo-strophic TEPV serves to spin up or down the XRV that in turn would decide if the fronts could develop along the upper-level jet trajectories through the tilting effect For example, negative ageostrophic TEPV (Fig 11d) leads to the weakening of the front over the Korean peninsula, Japan and adjacent seas at

Fig 11 Ageostrophic TEPV at 700 (a, b) and 300 hPa (c, d) on 25 (a, c), and 26 (b, d) June 1999 at 1200 UTC Solid (filled with dark grey) and long dash (filled with light grey) contours respectively indicate positive and negative value at intervals of 3× 10−7s−1 with zero lines omitted Model topography higher than 3000 m above MSL is blanked at 700 hPa

Table 2 Common magnitudes of the terms in the XRV equation