The Relationship between the Typhoon Storm in Fujian and Circulation Circumstances

The cases of landing and impact of the heavy rain of typhoon in Fujian are analyzed. From the research of the mainly weather systems which caused heavy rain and the distribution characteristics of atmospheric circulation situation field, we conclude that on the basis of Fujian special geographical location and terrain, there are four types (easterly jet, warm-type sheer, inverted V-shaped trough of typhoon, westerly trough) are conductive to the circulation patterns of causing the heavy rain of typhoon. Through studying a number of typical model cases of heavy rain of typhoon, we can reveal the mechanism which causes the heavy rain of typhoon by the mostly influencing systems in different circulation backgrounds and the causes of the heavy rain of typhoon.


Introduction
Heavy rain of typhoon (referring to the tropical cyclone over the intensity of tropical storms, the same below) is a major summer disaster in Fujian.The loss caused by the heavy rain of typhoon is particularly serious.In particular, the typhoon circulation system in the interaction of the environmental field, as well as certain special topographical conditions in Fujian leads to the development of MCC to generate mesoscale rainstorm.Although the generation of heavy rain of typhoon is related to the typhoon intensity and structure of its own, moving path, terrain, atmospheric circulation background and many other factors, the circulation situation field plays a decisive role.A certain circulation situation field not only affects the movement path of the typhoon, the structure and in-

The Circulation Background Classification of Heavy Rain of Typhoon
The generation of heavy rain of typhoon is related to the typhoon intensity and structure of its own, moving path, terrain, atmospheric circulation background and many other factors.In particular, whether heavy rain appears in the periphery spiral clouds band before typhoon's landing and in the rear of clouds after typhoon's invading the mainland after landing is more closely related to atmospheric circulation.In different circulation backgrounds, the influencing systems helping to causing the heavy rain of typhoon are different and the mechanism which caused heavy rain and the occurrence regions of the heavy rain of typhoon are also different.Therefore, we classified the cases of landing and impact of the heavy rain of typhoon in Fujian, according to the environments which can cause heavy rain, as well as the system conductive to the formation of the heavy rain of typhoon in such environment.They can be divided into four types (easterly jet, warm-type sheer, inverted V-shaped trough of typhoon, westerly trough).The same typhoon in different precipitation periods, if the environmental field changes, can be attributed to different types.and specific topographic conditions may help spiral rainbands of typhoon develop sharply (Figure 1).

Different Circulation Backgrounds on the Role of Heavy Rain of Typhoon
In such a situation field, the heavy rain of typhoon mainly appear in northcentral coastal areas of Fujian, which are located in the right of the forward direction of typhoon to the south side of subtropical high.The heavy rain of northeast Fujian caused by spiral rain bands of typhoon and the core of typhoon rain group occurred in this situation.From historical data analysis, it can be found that the maximum precipitation of the typhoon landing the central and southern Fujian, are mainly located in the southeast site of Jiufeng Mountain and Taimu Mountain which are located in north-central coastal area of Fujian.

The Topographic Conditions under the Effect of Easterly Jet Play a
Vital Role in Enlarging the Intensity of the Heavy Rain First of all, the particular terrain in northeast area plays a significant role in the increase of typhoon precipitation.It includes: frictional convergence of the coast and the role of the terrain uplift.According to the research, the increased precipitation caused by terrain depends on two factors: one is low-level wind speed.
The stronger is wind speed, the greater is the increase.The other is the warm and humid extent of the air.The warmer and the more humid is the air, the greater is the role of terrain plays on precipitation.The northeast coast of Fujian is the rugged hills, where frictional convergence of the coast wind is particularly prominent, is often one of the larger areas of heavy rain.And there is Jiufeng Mountain and Taimu Mountain about 1500 meters above sea level in the northeast of Fujian.They are presented the northeast-southwest direction and parallel to the coastline.In the situation field of be higher in the north and lower in the south, the east-southeast airflow of the north of typhoon and Mountains of northeast Fujian are orthogonal, and frictional convergence and the role of terrain uplift is very noticeable.In particular, Zherong, which is located halfway up Taimu Mountain, is often the "Rain vortex" of typhoons and storms in such situations.Such as the 2005 "Haitang" Typhoon, the daily rainfall of Zherong has broken the historical extreme record.As Taimu Mountain is presented the NNE-SSW direction, and is almost perpendicular to the spiral clouds band of "Haitang" typhoon.The role of blocking and forced uplift of mountain brings the occurrednce and development of convective clouds of the spiral clouds band.
In 2016, Typhoon "Megi", the daily precipitation of Shouning and Pingnan on September 28th refreshed the historical extreme values of the station since the weather records.The terrain of Ningde has features of high in the northwest and low in the southeast.The precipitation of the process in Ningde City is monitored.The strong rainfall area corresponds well with the topography of Ningde.
In the same way, the eastward and southeastern jets were forced to rise uphill in the windward slope of the mountain, which strengthening the rising movement of warm and moist air, and the precipitation increased significantly [3].

The Converging Point of Two Airs Is Conducive to the Occurrence
of Heavy Rain When typhoon enters the Taiwan Strait, the easterly airflow of the east of typhoon will form two airflow dues to the impact of the terrain of Taiwan.The airflow will converge once again in Fujian, the west side of Taiwan.In a case of cold air southward invasion, air meeting point not only provides strong convergence of water vapor in rainfall areas, but also forms dynamic and thermal conditions which are conducive to development of strong convective clouds.It has stimulated the development of strong convective clouds, and led to the occurrence of heavy rain in the converging point.The heavy rain processes of 0519 Typhoon "Longwang" [4], 1111 Typhoon "Nanmadol" [5], 1513 Typhoon "Soudelor" [6] and 1601 Typhoon "Nepartak" [7] have similar circulation characteristics (Table 1).
1111 Typhoon "Nanmadol" heavy rainstorm process [5].It can be seen from the hourly precipitation variation in the extremely heavy rain area (Putian and Linqiao) and the analysis of the hourly ground wind field evolution from the north to the south of the Upwind Square on the east side of the mega rainstorm area (quoted from Figure 2 in [5]).On the night of August 31, the southerly wind on the east side of the low pressure circulation of 1111 Typhoon "Nanmadol" was strengthened.In the process of the gradual strengthening of the southerly wind, the wind direction on the north side of the coast of Putian has reversed, and the northerly wind has shown a tendency to gradually press southwards.The convergence zone of the northeast wind, easterly wind, and southerly Journal of Geoscience and Environment Protection  The heavy rain of such situations mainly occurs in southern region of Fujian.
We especially pay attention to the strengthening of the southwest monsoon of the South China Sea and the stability of South Asia High under this situation.

Stable Continental High Is Conductive to Typhoon Convolution, and a Southeasterly Wind Jet Appears in the East to the Northeast
Side of Typhoon After the Typhoon "Bilis" landed on the map at 08 h 500 hPa on the 15th (quoted from Figure 3(d) in [8]), Lake Balkash and northeastern China were trough areas.The Baikal Lake between the two troughs was a deep warm ridge.The subtropical high was strengthening and extending into the west, and the west ridge point reached 100˚E.The main body was in the south of the Japanese island and the center reached 592 dgpm.The Hetao area was on the south side of the stable warm ridge and continued to strengthen under the superposition of warm advection and positive height.The coastal part had a breaking tendency under the action of the short wave trough in the north; the 575 dgpm low pressure in the Bay of Bengal lies at 22˚N and 90˚E."Bilis" lay between two high pressures.The northwestern side was a strong continental high, with northeasterly winds, southerly winds on the east, and southwesterly winds from the Bay of Bengal on the west and south.The environmental field was conducive to slower movement and the maintenance of cyclonic circulation.
This showed that after the landing of strong tropical storm "Bilis", continental high and the Western Pacific high were very strong and stable."Bilis", which was under the block of continental high, moved slowly westward in the south side of continental high."Bilis" low long stayed in the southern Jiangxi Province and a southeasterly wind jet maintained in the east to the northeast side of typhoon.Central and southern Fujian Province was in convergence zone of southeast wind and southwest wind in the periphery of low, and was conducive to the emergence of heavy rain.The activities of westerly trough were in the north of 35˚N, and there was no cold air invasion of typhoon.

The Strengthening of the Southwest Monsoon of the South China
Sea Provides Adequate Moisture and Energy for Rainfall Areas With the typhoon had moved westward inland, subtropical high westward extended following the typhoon.There was a strong southwesterly jet in Taiwan and the Taiwan Strait, which was in the periphery of the subtropical high.The wind speed of stream field at 500 hPa had reached 20 m/s or more.Westward extension of the subtropical high produced more obvious allobaric gradient, so that the large wind speed zones generated the super-geostrophic phenomenon.At the same time, the time that "Bilis" had effecting, was precisely the strong period of southwest monsoon.The southwest monsoon of the South China Sea blew 25˚N.The strong southwest monsoon of the South China Sea not only provided a sufficient latent heat to maintain strength after Bilis's landing, but also transported adequate moisture and energy over rainfall areas of southern China.Journal of Geoscience and Environment Protection

South Asia High Provides Strong Divergence for Rainfall Areas
When "Bilis" landed South Asia High at 200 hPa strengthened southward.
(quoted from Figure 3(b) in [8]) From the Qinghai-Tibet Plateau to the eastern part of South China, it was governed by a huge amount of high pressure in South China.The high-pressure center of 1264 dgpm was located on the west side of Hetao."Bilis" was near the center of the bottom of the South Asian High.The southeast coastal areas of Southern Fujian and Eastern Guangdong were in the Northeast outflow of the southeast of South Asian High."Bilis" and the rainfall areas of South China were the strong divergence zone.The subsidence effect of the centre of South Asia High was beneficial to the stability of continential high.
South Asia High provided strong divergence for "Bilis" and rainfall areas and led to low-level convergence and the ascending motion strengthened, not only was conductive to the maintaince of low circulation "Bilis", but also was beneficial to the occurrence and development of MCC of the South China coast.It was a major reason for heavy precipitation after Bilis's landing.
The southeast coastal areas of Southern Fujian and Eastern Guangdong were in the Northeast outflow of the southeast of South Asian High."Bilis" and the rainfall areas of South China were the strong divergence zone.With the strengthening and the stretching eastward and jumping northward of South Asia High, upper-level easterly jet jumped northward.Its entrance region was located over the area of low-level southwest jet.A special northeast-southwest monsoon circulation cell was formed.High-altitude areas of divergence were just to the left of low-level jet axis and the top of the rear of "Bilis".The coupling of upper-level and low-level jet streams had an important contribution to continuous heavy rain.

Situation Field Characteristics
Before the typhoon (come from Western Pacific but move into South China Sea or come from South China Sea) landing Fujian, a inverted V-shaped trough of typhoon will form between the Taiwan Strait and the border area near 115 ˚ E between Jiangxi and Fujian Provinces.The two sides of inverted V-shaped trough of typhoon are made of southeasterly jet and northeasterly jet and form a strong cyclonic curvature.The southeasterly jet is a warm and moist air conveyor and produces sufficient water vapor and energy for heavy rain.The mesoscale heavy rain appears in front of the southeasterly jet of the east side of warm wet inverted V-shaped trough in low altitude.The convergence of wind speed and the air orthogonal with the mountain are conducive to the occurrence and development of MCC (Figure 3).

The Steady Water Vapor Transport of Deep Wet Layer
When the typhoon center is in the 20 -22˚N, 114 -116˚E near South China Sea, the central part of Guangdong, the rain clusters of mesoscale heavy rain often Journal of Geoscience and Environment Protection appear in southeast Fujian in northeastern periphery of typhoon.This is mainly due to that the coastline along the Fujian coast is near the north-south direction.
The high-altitude of northeast of typhoon prevails east-southeast wind, which will help the sea clouds moving along the easterly airflow and affecting southern Fujian, the rain clusters of mesoscale heavy rain appear in the northeastern part of the typhoon; and northeast of the typhoon center is usually active district of spiral rain bands of typhoon and the zone contact with the inverted trough of typhoon and the cold air, especially the typhoon circulation orthogonal with mountains is likely to develop rain clusters of mesoscale of heavy rain, and such rain clusters often occur in some special terrain of Daiyun Mountain and the east of Bo-Ping Ling Mountain in the south-central of Fujian.For example, a big rainstorm case in southern Fujian coast by 9908 typhoon (Figure 4).1308 Typhoon "Cimaron" and 1407 Typhoon "Hagibis" (quoted from Figure 4 [9]), which had similar paths and produced extremely heavy rainstorms in Southern Fujian [9].

The Invasion of Cold Air behind Westerly Trough
Due to the rapid increase of southeast airflow of the eastern side of Typhoon inverted trough and the eastward shift of the westerly trough, the cold air behind upper-level trough forces the southeast airflow of low-level high temperature and high uplift, and results in the occurrence and development of strong convective weather of unstable regions.Heavy rain occurs in the places that typhoon inverted trough and westerly trough combine, atmosphere turns from stable stratification to unstable stratification, instability of energy rises rapidly and baroclinicity is strongest.

Situation Field Characteristics
When the typhoon moves closer to the coast and at the same time the westerly

The Effect of Lee Waves of High-Altitude North-West Airflow
Promotes the Development of Low-Level Cyclone "Choiwan" typhoon went northward into the westerlies, high-altitude of Fujian prevailed northwest airflows.The terrain of Fujian is high in the west and low in the east, and the mountain range has a northeast~ southwest trend.The effect of lee waves which generated by northwest airflow climbing Mount Wuyi promoted the formation and development of orographic depression.We can conclude from surface pressure field and 3 hours allobaric field from 19 th to 20 th that, when the high-altitude prevailed the northwest airflow, surface katallbaric centre was mainly located in the southeast of Mount Wuyi and the south of

Typhoon Moves Northward into the Westerly Trough and Speeds
up the Cold Air Southward In the early stage of the occurrence of heavy rain, at 20:00 on September 19, at 500 hPa the subtropical areas were under the control of the two high-pressure, and the continental high was large and strong.The Middle and Upper of Fujian was under the control of the same northwest airflows.Under normal circumstances, this was an adverse weather conditions resulting in the heavy rain.However, with typhoon moving northward into the westerly trough, the trough locating in East Asia deepened, and there were obvious adjustments of circulation in East Asia.The whole westerlies moved southward, the continental high had weakened rapidly, and the extent of southward invasion of north cold air strengthened.At 8:00 on 20 th , the negative variable temperature zones of the surface moved southward, 24-hour variable-temperature zero line moved to the north side of Mount Wuyi.At 20:00 on 20 th , variable-temperature zero line moved to the Fujian coast.The cold air southward played an important role in the occurrence of heavy rain.Because the uplift effect of frontal surface and the

Figure 4 .
Figure 4.The situation field at 500 hPa of 9908 typhoon.
115˚E and its ridge line is located in the vicinity of 30˚N -32˚N.There is obviously positive height change in the northwest area of ridge line.As the typhoon moved northwest, a strong easterly jet would form between the north of typhoon and the south of subtropical high, and it makes flow field generate asymmetric structure.The strong wind speed convergence 3.1.EasterlyJet3.1.1.Situation Field CharacteristicsThis type's circulation background shows that: subtropical high is zonal and its Journal of Geoscience and Environment Protection west ridge point is in 105˚E -

Table 1 .
Precipitation extremum of several circulation similar typhoon processes.