Introduction

The Mei-Yu front develops between spring and summer when warm, humid air from the Southwest Monsoon meets the cool, dry air from the weakening Northeast Monsoon. Since neither of the two air masses is strong enough to replace the other, the boundary between them remains stationery and leads to continuous rainfall. This stationary boundary is called the Mei-Yu front. In Taiwan, the weather phenomena associated with the Mei-Yu front is known as the “Mei-Yu”, which means “plum rains” in Mandarin.

Heavy rainfall during the Mei-Yu season (May through June) can cause various disasters such as flash floods and landslides. In Taiwan, the heavy rainfall associated with the Mei-Yu front is the second leading weather disaster following typhoons. The “612 Floods” in 2005 is the costliest Mei-Yu disaster that led to 13 fatalities and more than NTD $2.1 billion in property damage. The “609 Floods” in 2006 resulted in more than 700 mm of rain in the mountainous areas of southwestern Taiwan, where the Alishan Station recorded 811.5 mm in one day!

Introduction » Onset of the Mei-Yu Season

The onset of the Mei-Yu season signifies the beginning of Taiwan’s rainy season. Hung and Hsu (2008) concluded that the start of the Mei-Yu season in Taiwan typically follows the onset of the East Asian summer monsoon, when large-scale flow transports moisture northward from the tropics.

The figure below compares the moisture transport before and after the monsoon onset over East Asia. Before the monsoon onset, moisture transport is rather weak across the North Indian Ocean and the South China Sea (SCS). After the monsoon onset, moisture is carried from the Southern Hemisphere along the eastern African coast, eastward across the Indian and Indochina peninsulas, and then northeastward towards Taiwan. This meandering atmospheric river brings tropical moisture towards East Asia. If Central Weather Bureau (CWB) forecasters observe southwesterly flow that is strong and abundant in moisture, the CWB will broadcast the term “Southwesterly Flow” to warn people of the heavy rainfall potential across Taiwan.

The East Asian monsoon onset depends mostly on the upper-level circulation pattern responsible for the large-scale transport of low-level moisture. The animation below illustrates the atmospheric environment of the monsoon onset over East Asia. Low-level warm, moist air traveling east across the North Indian Ocean supports convection over East Asia. Above the Tibetan Plateau (about 200 hPa), northeasterly flow on the south side of an upper-level high transports air away from the convection and across the equator, where the air sinks due to the presence of subtropical high pressure systems. The self-sustaining, large-scale circulation system of the East Asian summer monsoon makes conditions favorable for convection across Taiwan.

Characteristics of the Mei-Yu Front over Taiwan

Taiwan is an island that is 394 km long and 140 km wide, and one-third of the island is covered by mountains. The peaks of the north-south oriented Central Mountain Range (CMR) span between 1500 m and 3900 m. In CWB's Mei-Yu forecast procedure, the island is divided into four forecast regions: the North, Central, South and East Areas (see map below). The first three regions comprise western Taiwan, where 90% of the population lives. Therefore, rainfall impacts are more significant in western Taiwan than in the East Area. In the CWB, forecasters need to determine whether heavy Mei-Yu rain will fall over the mountainous areas or plains of Taiwan, and also in which forecast regions.

The Mei-Yu front that impacts Taiwan has unique characteristics. As shown in the surface analysis below, while the front itself is a synoptic-scale feature, mesoscale features exist near the front (convective cells and mesoscale convective systems, or MCSs). From a planetary point of view, the “Southwesterly Flow” that interacts with the front is just one piece of the Asian Monsoon system. The Mei-Yu front combines atmospheric phenomena at multiple scales, which poses a great challenge in forecasting precipitation across Taiwan.

Characteristics of the Mei-Yu Front over Taiwan» 3-Dimensional Structure of the Mei-Yu Front

The climatology of precipitation during Taiwan’s Mei-Yu season has been studied quite extensively since the Taiwan Area Mesoscale Experiment (TAMEX) field campaign in 1987 (Kuo and Chen, 1990) and the establishment of a dense rain gauge network by CWB in 1993 (Hsu, 1998). Research conducted by Chen (2004) suggested six favorable conditions for the development of MCSs associated with the Mei-Yu front:

1. low-level warm air advection,
2. low-level convergence produced by the Mei-Yu front or frontal low,
3. low-level jets,
4. short-wave troughs at low- to mid-levels,
5. diffluence or speed divergence at mid- to upper-levels, and
6. potential instability.

Today, identifying these features and their contribution to heavy rain is part of the CWB operational forecast procedure (Wang et al., 2012). The conceptual animation below shows the atmospheric environment conducive for heavy rainfall over Taiwan during the Mei-Yu season.

The conceptual animation below shows the typical atmospheric characteristics of a Mei-Yu front in a three-dimensional space. Let’s explore these characteristics from the surface to the top of the atmosphere.

• Surface
  • A stationary front passes over Taiwan or is in its vicinity.
  • Mesolows may form in the vicinity of Taiwan. Chen (1992) concluded that there is a positive correlation between the frequency of heavy rain events (rainfall <50 mm per day) and mesolow presence. Note that this mesolow is either a stationary low due to the terrain effect near Taiwan (Wang and Chen, 2002), or a local mesolow along with the front, rather than the synoptic-scale low (extratropical cyclone) as shown in the animation.

• Low-Level (925-850 hPa)
  • In operations, CWB forecasters often use 850 hPa (about 1500 m above the surface) to see how Taiwan's terrain affects the atmospheric flow.
  • Convergence along a shear line can trigger convection.
  • A LLJ carrying warm, moist air can lead to significant precipitation in central and southern Taiwan when it encounters Taiwan’s terrain.
    • CWB forecasters identify a LLJ when the 850 hPa southwesterly winds exceed 25 kts near Taiwan.
    • The positive feedback between a LLJ, latent heat release, and mesolow development helps to reinforce flow over the CMR and contribute to orographic precipitation (Wang and Chen, 2002). This nonlinear mechanism is similar to the conditional instability of the second kind (CISK; Chen et al. 2003).
    • The temporal and spatial scales of a LLJ are smaller than the “Southwesterly Flow”.
  • Warm, moist air provides an environment conducive to convective instability.

• Mid-Level（700-500 hPa）
  • The area downstream of a trough is favorable for ascent.

• Upper-Level (300-200 hPa)
  • Generally, distinct diffluent flow over the right-entrance region of an upper-level jet streak can also result in an area of upper-level divergence.

If these features synchronize well, convection will be supported. Forecasters are interested in where the strongest convection is likely to occur. To learn how to identify potential areas favorable for intense convection, let’s explore the main atmospheric lifting processes that can make the environment favorable for convection and see what roles they play in the Mei-Yu front.

Characteristics of the Mei-Yu Front over Taiwan » Lifting Mechanisms Favorable for Convective Development

The main atmospheric lifting processes that lead to cloud formation and convection during a Mei-Yu event are the following:

Frontal Lift

Frontal lift is the collision of two air masses with different temperature and moisture characteristics, where warm air rises over cool air. Typically, convective clouds form to the south of a Mei-Yu front, especially during severe Mei-Yu events.

Convective Instability

CWB forecasters often use the theta-E lapse rate to determine the convective instability of the environment. Warmer temperatures and greater moisture increase the value of theta-E. When theta-E decreases with height, the atmospheric layer is convectively unstable. Although CWB forecasters need to determine how theta-E changes with height in different atmospheric layers, the forecasters usually check the value at 850 hPa (lower layer). Climatology for this region suggests that a theta-E value higher than 344 K suggests an environment with high convective instability between low and mid-levels.

To identify an area of high theta-e values, we can use total precipitable water (TPW) data. Since atmospheric water vapor is mostly concentrated below 700 hPa, higher TPW values could roughly suggest the presence of higher theta-e values in the lower levels of the atmosphere.

Orographic Lift

Orographic lift is the upward deflection of air after colliding with a mountain barrier. As the air mass hits the rising terrain, it is forced to move over the terrain. In the case of a Mei-Yu event, warm and moist southwesterly flow encounters the CMR to produce heavy rainfall over western Taiwan.

Upper-level Divergence

Upper-level divergence can be an important ingredient for the development or maintenance of convection.

Areas of upper-level divergence are often associated with an upper-level jet streak (divergence usually occurs in the right-entrance and left-exit regions) and/or upper-level diffluent wind flow. In operations, CWB forecasters often observe that strong convection is better connected to the area of distinct upper-level diffluent flow (defined by the angle in the spread of the wind vectors to be greater than 45˚) than the right-entrance region of a jet streak. Occasionally, forecasters find distinct upper-level diffluent flow aligning with the right-entrance region of the jet streak, which can produce enhanced lift and favorable conditions for convection.

Low-level Jet Lift

Positive shear vorticity is present on the left side of a westerly LLJ, and an area of speed convergence is found downstream of the LLJ core. Both factors, in addition to the advection of warm, moist air by the LLJ, will create an environment favorable for convection.

Mei-Yu Forecast Procedure in Taiwan

The Mei-Yu forecast procedure in Taiwan can be described by the following four steps:

1. Use satellite and radar to analyze the Mei-Yu front and the mesoscale convective systems (MCSs) associated with it.
2. Use NWP model guidance and the Mei-Yu Front Checklist to analyze the 3D structure of the atmosphere, and identify the Mei-Yu precipitation pattern associated with the front.
3. Use NWP model guidance to identify trends in the movement of the surface front.
4. Use QPF guidance to determine the possible time period and area of torrential rainfall.

The graphic below shows the detailed steps of the forecast procedure during the Mei-Yu season:

1. Analyze the current situation in the upstream using satellite data, radar, and soundings to find the presence and location of synoptic-scale features (e.g. surface front) and MCSs.
2. Analyze conditions from the top to bottom layer of the atmosphere. Look for the thermodynamic indicators of the Mei-Yu front using the Mei-Yu Checklist such as the LLJ, high 850 hPa theta-e values, mid-level trough, upper-level jet and diffluent flow. These indicators help forecasters predict what will happen when the front approaches Taiwan. Then, compare the synoptic-scale observations to their depiction in the model analysis.
3. Review the QPF guidance to identify the potential for large precipitation amounts and diagnose what mechanisms are triggering those amounts. Check other high-resolution models to identify similar mechanisms.
4. Prepare the Torrential Rain Advisory, if necessary.
5. Communicate the advisory to the Emergency Operation Centers (EOCs).
6. Communicate the advisory to the next forecast shift.
7. Issue torrential rain advisory, if necessary.
8. The new shift monitors radar and rain gauges and amends the advisory, if necessary.

In this module, we will focus on the first two steps in the procedure. The rest will be covered in a future module.

Mei-Yu Forecast Process in Taiwan » The Mei-Yu Checklist

Since 1997, CWB uses a checklist based on the work of Shieh (1996) to evaluate the synoptic environment and to assist with 12–48-hour heavy rainfall forecasts during the Mei-Yu season. The checklist currently contains a total of 20 items categorized into nine groups (Wang et al. 2012). Forecasters check for the items based on numerical model forecasts valid for a 12-hour period. As present in the NWP guidance, the possibility of heavy rain in a 12-hour period increases as more items are marked. Currently, CWB uses a threshold of 14 marked items to indicate a high chance for heavy rainfall events.

CWB’s Mei-Yu Checklist

(All items are inside the domain of 15–30˚N, 110–127˚E.)

Checklist Items	Obj	12h	24h	36h
(a) Surface Mei-Yu Front
between 20˚-28˚N and 118˚-124˚E.
Taipei (25.03˚N, 121.63˚E) is behind it within 100 km or in front of it within 200 km.
Kaohsiung (22.58˚N, 120.42˚E) is in front of it within 200 km.
(b) Humidity
850 hPa: Td >= 15℃
850 hPa: θe axis points toward Taiwan
700 hPa: (T-Td) <= 3℃
(c) LLJ (between 18˚-26˚N and 115˚-125˚E.)
Surface southwesterly flow of 10-20 kts
850 hPa southwesterly flow > 25 kts
700 hPa southwesterly flow > 30 kts
850 hPa southerly to southwesterly flow >=10 kts over northern SCS (north of 15˚N）
(d) Temperature
850 / 700 hPa: A cold tongue which is north of the shearline
Taiwan is in the diffluent area of the 1000-500 hPa thickness field.
(e) Shearline (between 22˚-28˚N and 114˚-127˚E)
850 / 700 hPa shearline
(f) Sub-synoptic system (along the coast of southeastern China or in northern SCS, east of 114˚E)
Surface / 850 hPa: mesolow
700 / 500 hPa: short-wave trough
(g) Pressure (near Taiwan)
Taiwan is in a low pressure zone.
Surface pressure < 1005 hPa
(h) Upper-level wind
300 / 200 hPa diffluent flow with angle > 45˚
Taiwan is under the right-entrance quadrant of the 300 / 200 hPa jet streak
(i) Stability
K-Index > 35
Total Checks

Question

Review the model forecasts and text information below. Use the Mei-Yu Checklist to determine the potential for severe precipitation across Taiwan and answer the questions below.

You are working on a potential Mei-Yu front precipitation event using CWB’s Mei-Yu Checklist. The objective analysis time is at 1200 UTC on June 13. The forecast is for 36 hours in 12-hour intervals. Within a domain between 15˚-30˚N and 110˚-127˚E, you have determined the following conditions:

• the 850 hPa dew point is over 15℃ from 0-36h,
• the difference between the temperature and the dewpoint at 700 hPa is 2℃ from 12-36h, and
• the K-Index is higher than 35 from 12-24h.

Please use the Mei-Yu Checklist to confirm the other items based on the NWP model guidance below.

CWB’s Mei-Yu Checklist

(All items inside the domain of 15–30˚N, 110–127˚E.)

Checklist Items	Obj	12h	24h	36h
(a) Surface Mei-Yu Front
between 20˚-28˚N and 118˚-124˚E.
Taipei (25.03˚N, 121.63˚E) is behind it within 100 km or in front of it within 200 km.
Kaohsiung (22.58˚N, 120.42˚E) is in front of it within 200 km.
(b) Humidity
850 hPa: Td>=15℃
850 hPa: θe axis points toward Taiwan
700 hPa: (T-Td) <= 3℃
(c) LLJ (between 18˚-26˚N and 115˚-125˚E.)
Surface southwesterly flow of 10-20 kts
850 hPa southwesterly flow＞25 kts
700 hPa southwesterly flow＞30 kts
850 hPa southerly to southwesterly flow>=10 kts over northern SCS (north of 15˚N)
(d) Temperature
850-700 hPa: A cold tongue which is north of the shearline
Taiwan is in the diffluent area of the 1000-500 hPa thickness field
(e) Shearline (between 22˚-28˚N and 114˚-127˚E)
850 / 700 hPa shearline
(f) Sub-synoptic system (along the coast of southeastern China or in northern SCS, east of 114˚E）
Surface / 850 hPa: mesolow
700 / 500 hPa: short-wave trough
(g) Pressure (near Taiwan)
Taiwan is in a low pressure zone.
Surface pressure < 1005 hPa
(h) Upper-level wind
300 / 200 hPa diffluent flow with angle > 45˚
Taiwan is under the right-entrance quadrant of the 300 / 200 hPa jet streak
(i) Stability
K-Index > 35
Total Checks	0	0	0	0

Mei-Yu Forecast Process in Taiwan » Four Main Factors for Severe Precipitation

Although there are many factors that contribute to heavy rain during the Mei-Yu season, CWB forecasters focus on several key elements based on their past forecast experience: the location of the surface front, low-level jet, low-level theta-e values and axis, and distinct upper-level diffluence (to detect area of divergence). The first three provide the main thermodynamic processes for convection and are key to determining the Mei-Yu precipitation pattern. Upper-level divergence associated with distinct upper-level diffluent flow results in lift. When these features synchronize well, heavy precipitation is likely. For more information on the significance of the checklist parameters to different basins in Taiwan, see Wang et al. (2012). Next, we will learn about the two main Mei-Yu precipitation patterns and the various forecast factors in each pattern.

Types of Mei-Yu Precipitation Patterns

Type-1

The 6-hour radar composite reflectivity animation and accumulated rainfall of a “Type-1” Mei-Yu precipitation event are shown in the tabs below. A strong “Type-1” event shows a linear convective system near the surface front that is producing extremely heavy rainfall (possibly exceeding 100 mm per hour). The linear convective system may persist for several hours.

Review the composite radar reflectivity loop and rainfall accumulation map above, and answer the questions below.

Type-2

The 6-hour composite radar reflectivity animation and accumulated rainfall of a “Type-2” Mei-Yu precipitation event are shown in the tabs below.

Review the composite radar reflectivity loop and rainfall accumulation map above, and answer the questions below.

Type-1+2

Often times, Type-1 and Type-2 events occur simultaneously. The 6-hour composite radar reflectivity animation and accumulated rainfall of a “Type-1+2” Mei-Yu precipitation event are shown below.

Determine the Mei-Yu Precipitation Pattern

Meteorologists try to compose a three-dimensional structure of the atmosphere using observations and numerical model maps at standard pressure levels. In order to forecast the Mei-Yu precipitation pattern and the areas at risk of precipitation, we need to review surface analyses, satellite data, observations, and NWP guidance to identify the areas of rising motion. Consider the following steps:

1. Identify the location of the surface front
2. Determine the moisture availability with low-level theta-e
3. Identify the presence and orientation of the LLJ
4. Identify the presence of upper-level divergence (associated with diffluent flow)
5. Determine the Mei-Yu precipitation pattern and locations at risk for heavy rainfall

Next, we are going to practice these steps using model analysis to identify the four main Mei-Yu parameters (surface front, low-level jet, low-level theta-e values and axis, and upper-level diffluence) and their ascent areas during past Mei-Yu events. Also, we will analyze the Mei-Yu precipitation patterns in 6-hour intervals (00, 06, 12, and 18 UTC). If a heavy-rain event occurred during the 00-06 UTC time frame, we will analyze the weather conditions at 00 UTC for the upcoming heavy-rain event.

Determine the Mei-Yu Precipitation Pattern » Case 1

Determine the Mei-Yu Precipitation Pattern » Case 1 » Location of the Front

Model Surface and 850 mb Analysis

Water Vapor and Surface Observations

Determine the Mei-Yu Precipitation Pattern » Case 1 » Moisture Availability

Please first answer the questions on the previous page.

What we know so far:

1. The surface front is located offshore from North Taiwan.

Determine the Mei-Yu Precipitation Pattern » Case 1 » Presence and Orientation of the LLJ

Please first answer the questions on the previous page.

What we know so far:

1. The surface front is located offshore from North Taiwan. The position of the 850 hPa shear line is close to the surface front.
2. At 850 hPa, an axis of high theta-e points to the offshore region of the North Area.
3. The available moisture is greater than what is suggested in the model analysis.

Determine the Mei-Yu Precipitation Pattern » Case 1 » Upper Level Divergence

Please first answer the questions on the previous page.

What we know so far:

1. The surface front is located offshore from North Taiwan. The position of the 850 hPa shear line is close to the surface front.
2. At 850 hPa, an axis of high theta-e points to the offshore region of the North Area.
3. The available moisture is greater than what is suggested in the model analysis.
4. The LLJ that is just south of the 850 hPa shear line is very strong (the maximum 850 hPa wind speed is up to 40 knots), resulting in strong positive shear vorticity between the LLJ and the shear line. The strength of the winds in the model analysis is similar to that in the wind profile.

Determine the Mei-Yu Precipitation Pattern » Case 1 » Determine the Locations at Risk of Heavy Rainfall

Please first answer the questions on the previous page.

What we know so far:

1. The surface front is located offshore from North Taiwan. The position of the 850 hPa shear line is close to the surface front.
2. At 850 hPa, an axis of high theta-e points to the offshore region of the North Area.
3. The available moisture is greater than what is suggested in the model analysis.
4. The LLJ that is just south of the 850 hPa shear line is very strong (the maximum 850 hPa wind speed is up to 40 knots), resulting in strong positive shear vorticity between the LLJ and the shear line. The strength of the winds in the model analysis is similar to that in the wind profile.
5. At 200 hPa, diffluence (especially when aligned with the right-entrance region of the upper-level jet streak) resulted in lift over the North Taiwan Strait.

Determine the Locations at Risk of Heavy Rainfall » Case 2

Determine the Locations at Risk of Heavy Rainfall » Case 2 » Location of the Front

Model Surface and 850 mb Analysis

Water Vapor and Surface Observations

Determine the Mei-Yu Precipitation Pattern » Case 2 » Moisture Availability

Please first answer the questions on the previous page.

What we know so far:

1. The surface front is located offshore from North Taiwan.

Determine the Mei-Yu Precipitation Pattern » Case 2 » Presence and Orientation of the LLJ

What we know so far:

Please first answer the questions on the previous page.

1. The surface front is located offshore from North Taiwan. The 850 hPa shear line is located just north of the surface front.
2. At 850 hPa, two axes of high theta-e point to Taiwan and the offshore region of the North Area.
3. The available moisture is greater than what is suggested in the model analysis.

Determine the Mei-Yu Precipitation Pattern » Case 2 » Upper Level Divergence

What we know so far:

Please first answer the questions on the previous page.

1. The surface front is located offshore from North Taiwan. The 850 hPa shear line is located just north of the surface front.
2. At 850 hPa, two axes of high theta-e point to Taiwan and the offshore region of the North Area.
3. The available moisture is greater than what is suggested in the model analysis.
4. The LLJ that is just south of the 850 hPa shear line is very strong, resulting in strong positive shear vorticity between the LLJ and the shear line. The strength of the winds over northern Taiwan are stronger in observations at 850 hPa (about 50 knots) than in the model analysis (about 35 knots).

Determine the Mei-Yu Precipitation Pattern » Case 2 » Determine the Locations at Risk of Heavy Rainfall

What we know so far:

Please first answer the questions on the previous page.

1. The surface front is located offshore from North Taiwan. The 850 hPa shear line is located just north of the surface front.
2. At 850 hPa, two axes of high theta-e point to Taiwan and the offshore region of the North Area.
3. The available moisture is greater than what is suggested in the model analysis.
4. The LLJ that is just south of the 850 hPa shear line is very strong, resulting in strong positive shear vorticity between the LLJ and the shear line. The strength of the winds over northern Taiwan are stronger in observations at 850 hPa (about 50 knots) than in the model analysis (about 35 knots).
5. At 200 hPa, diffluence (especially when aligned with the right-entrance region of the upper-level jet streak) resulted in lift over the North Taiwan Strait.

Summary

This module introduced learners to the impacts, climatology, 3D structure, and precipitation patterns produced by the Mei-Yu front over Taiwan. The front is a complex weather system with important features operating at different temporal and spatial resolutions. The module included a conceptual understanding of the Mei-Yu front and an overview of CWB's precipitation forecast procedure. Future modules will discuss the QPF guidance that forecasters can use to predict the Mei-Yu front event.

The four main parameters that influence the distribution and intensity of precipitation during a Mei-Yu event are: the location of the surface front, the strength and axis orientation of the LLJ, the strength of the low-level theta-e values, and the strength of the upper-level diffluence.

There are two basic Mei-Yu front precipitation patterns. The Type-1 pattern results in a severe linear convective system near the front. The Type-2 pattern produces precipitation over central and southern Taiwan (mostly in the mountains) due to the warm, moist southwesterly flow which is lifted by the mountains. The Type-1+2 combines both Type-1 and Type-2 precipitation patterns. Learners worked through several exercises to identify the synoptic and mesoscale features important for a Mei-Yu event, diagnosed the type of precipitation pattern and identified areas that could be impacted. With practice, forecasters will be able to increase their confidence in identifying Mei-Yu features and forecasting its precipitation impacts across Taiwan.

You have reached the end of the lesson. Please complete the quiz and share your feedback with us via the user survey.

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