Front line - Zensen (English spelling) front

Japanese: 前線 - ぜんせん（英語表記）front

The boundary between a dense (cold) air mass and a less dense (warm) air mass is called a front or frontal surface, and the line that is created when this intersects with the ground surface is called a front. The concepts of air masses, fronts, and low pressure systems were systematized in the 1930s by the Norwegian School of Meteorologists J. A. B. Bjerknes and others, and since then, this concept has become indispensable in weather analysis.

At the boundary between two air masses, the denser air mass tries to slip under the less dense air mass, but because of the Coriolis and centrifugal forces at work on the Earth's surface, the front is not horizontal to the Earth's surface but has an inclination. This inclination varies depending on the type of front - about 1/150 for a warm front and about 1/50 for a cold front - but generally the smaller the temperature difference between the two air masses or the greater the difference in the wind speed component parallel to the front, and the higher the latitude of the front, the greater the degree of inclination. The front is not a geometric surface, but rather the two air masses mix there, forming a transition layer several kilometers thick, so the front also has a width of several hundred kilometers. This band-like area is called a frontal zone. On weather maps, fronts are drawn on the warm air mass side of this frontal zone. The temperature changes discontinuously across the front, but the wind direction, wind speed, air pressure, and dew point temperature also change discontinuously and are often accompanied by areas of bad weather, making this a significant factor in weather changes.

[Yo Narumura]

Frontal Zone

The frontal zone has two meanings:

(1) Meteorological frontal zone: As mentioned above, this is a band-like region formed when the transition layer between two air masses intersects with the earth's surface.

(2) Climatological frontal zone: A region where climatological fronts are likely to form.

In the Northern Hemisphere, there are the Arctic frontal zone, the cold frontal zone, etc. Of these, the term "frontal zone" usually refers to (2). In winter, it moves along the east coast of the Asian continent, the east coast of the American continent, and the Mediterranean Sea, while in summer, it moves northward as a whole, forming the frontal zone in northeastern Russia, Canada, and northern Europe ( Figure A ).

[Yo Narumura]

Types of fronts

Fronts are usually classified according to the movement of the air masses associated with them: (1) a cold front, in which cold air masses push aside warm air masses; (2) a warm front, in which cold air masses retreat and warm air masses advance; (3) an occluded front, which is formed when a trailing cold front catches up with and overtakes a leading warm front; and (4) a stationary front, in which both air masses barely move.

Also, depending on the latitude at which they occur, they are divided into polar fronts, polar fronts, and equatorial fronts, but of these, the equatorial front forms at the boundary between air masses with almost the same properties, and does not have the properties of a front in the first place. Fronts that appear in the global atmospheric circulation, such as the North (South) Polar Front and the Polar Front, are called major fronts, but the Intratropical Convergence Zone may also be added to this.

Some fronts have different names depending on the situation of their movements:

(1) Ascending front: An active front with warm air rising along its front. Also called an anafront. Due to the prevailing updrafts, the front is active and is often accompanied by tall convective clouds and heavy precipitation.

(2) Downhill front: A front along which warm air descends. It is also called a katafront. The front is not active, clouds do not develop much, and precipitation is weak.

(3) Hidden front: A front that is difficult to see through ground observations due to local effects. It is also called a masked front. An active front breaks these local effects, so a hidden front is difficult to form.

(4) Secondary fronts: Fronts that occur within the same air mass, and are formed due to differences in the transformation process of the same air mass. For example, in the cold air behind a cold front associated with a developed low pressure system, a temperature difference occurs between the sinking and warming air and the air that was moving horizontally, making it easy for a secondary front to form.

(5) Seasonal fronts Lines on maps that connect the times, days, and periods when a certain phenomenon can be seen are called isolines, but because these lines resemble the movement of fronts seen on weather charts, they are sometimes called seasonal fronts. The cherry blossom front that connects the days when cherry blossoms bloom and the autumn foliage front that connects the days when autumn leaves turn red are types of seasonal fronts and are different from meteorological and climatological fronts.

(6) Smog front: A front on the edge of an area with high smog concentration. It is different from a so-called front, but when the source of smog is in a coastal area, the smog is carried by the sea and land breezes, so it often corresponds to a sea and land breeze front.

[Yo Narumura]

The appearance and disappearance of fronts

The process by which a front is newly generated or strengthened is called frontogenesis. It occurs when the isotherms become congested, such as when cold air and warm air gather in the flow field in the lower atmosphere. When the isotherms become congested to a certain extent, an updraft is generated in the upper atmosphere, and the convergence of the lower layers increases, further concentrating the isotherms and strengthening the front. At this time, if the conditions in the upper layers suppress the updraft, the front will not be strengthened. Areas where frontogenesis is likely to occur are also climatological frontal zones.

The process by which a front weakens or disappears is called frontolysis. This occurs when the air masses on either side of the front change and the temperature difference disappears, or when the conditions in the upper atmosphere become downdrafts, suppressing the updrafts that accompany the front and making it impossible to maintain its frontal properties.

[Yo Narumura]

Front and low pressure development

Most mid- and high-latitude lows (extreme cyclones) occur on polar fronts. The typical development of these is shown below ( Figure B ).

(1) First, a stationary front extending roughly east-west is formed, and westerly winds blow in the warm air mass south of the front, while easterly winds or weaker westerly winds than the warm air mass blow in the cold air mass.

(2) The air pressure drops in part of the front, forming a low pressure system, and the front begins to waver. The front on the eastern side is a warm front because warm air pushes aside the cold air, while the front on the western side is a cold front because cold air pushes aside the warm air.

(3) As the central pressure of the low pressure system drops, the undulations of the front also become larger. Because the cold front moves faster than the warm front, the warm area formed near the center gradually becomes narrower.

(4) Near the center of the low pressure system, the cold front catches up with the warm front, forming an occluded front, which pushes the warm air away from the ground and upwards, leaving a vortex of cold air on the surface. The drop in pressure stops, and the low pressure system gradually weakens. This type of low pressure system develops and develops one after another on the same front at intervals of several days, traveling several thousand kilometers in the meantime. Three or four low pressure systems may form a group from the time they form until they weaken, and this is called a low pressure system family.

If we model the development of a low pressure system, the cold and warm air masses are initially aligned horizontally on either side of the front, but eventually the cold air mass is on the bottom and the warm air mass is on the top. In this case, the center of gravity of the whole system drops, so the overall potential energy decreases. This decrease is converted into kinetic energy, which becomes the wind energy of the low pressure system ( Figure C ).

Large-scale frontal systems such as polar fronts have a close relationship with the jet stream in the upper atmosphere. The jet stream runs almost parallel to the front on the ground surface, but when a low pressure system that occurs on the front becomes blocked, it passes through the airspace above the blockage point.

The analysis and synthesis of the position and movement of fronts, their formation and dissolution, activity and weather conditions on weather charts and cross-sectional diagrams is called front analysis, and this analysis is inextricably linked to air mass analysis.

[Yo Narumura]

Major frontal zones in the Northern Hemisphere (Figure A)
©Shogakukan ">

Major frontal zones in the Northern Hemisphere (Figure A)

Stages of development of fronts and low pressure systems (Figure B)
©Shogakukan ">

Stages of development of fronts and low pressure systems (Figure B)

Low pressure and potential energy (Figure C)
©Shogakukan ">

Low pressure and potential energy (Figure C)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

密度の大きい（冷たい）気団と、密度の小さい（暖かい）気団との境界を、前面または前線面とよぶが、これが地表面などと交わってできる線を前線という。気団と前線、低気圧の概念は1930年代にノルウェーの気象学者J・A・B・ビャークネスらのノルウェー学派によって体系づけられ、それ以来この概念は天気解析に不可欠となっている。

　二つの気団の境界では、密度の大きい気団は密度の小さい気団の下に潜り込もうとするが、地球表面上ではコリオリの力と遠心力が働くので、前面は地表面に対して水平になることはなく傾きをもっている。この傾きは、温暖前線で約150分の1、寒冷前線で約50分の1と、前線の種類によって異なるが、一般に両気団の温度差が小さいか、風速の前線に平行な成分の差が大きいほど、また、高い緯度にある前線ほど傾く度合いは大きくなる。前面は幾何学的な面ではなく、そこで両気団が混じり合い、厚さ数キロメートル程度の転移層となっているので、前線も数百キロメートル程度の幅をもっている。この幅をもった帯状の領域を前線帯とよぶ。天気図上で前線は、この前線帯の暖気団側に引かれる。前線を横切って気温が不連続に変わるが、風向、風速、気圧、露点温度なども不連続に変わり悪天域を伴うことが多いことなどから、天気変化に重要な意味をもつ。

［饒村　曜］

前線帯

前線帯には次の2通りの意味がある。

(1)気象学的前線帯　前述のように二つの気団の転移層が地表面と交わってできる帯状の領域。

(2)気候学的前線帯　気候学的に前線ができやすい地帯。

　北半球では北極前線帯、寒冷前線帯などがある。このうち、普通、前線帯というと(2)をさす。冬季はアジア大陸東岸、アメリカ大陸東岸、地中海などが、夏季は全体として北に移動し、ロシア北東部、カナダ、北ヨーロッパが前線帯となる（図A）。

［饒村　曜］

前線の種類

前線は普通それに関連する気団の運動によって、(1)寒気団が暖気団を押しのけて進む寒冷前線、(2)寒気団が退き暖気団が進む温暖前線、(3)先行する温暖前線に後続の寒冷前線が追い付き、追い越すときにできる閉塞(へいそく)前線、(4)両気団ともほとんど動かない停滞前線、に分けられる。

　また、前線が発生する緯度によって、極前線、寒帯前線、赤道前線に分けられるが、このうち赤道前線は、性質のほとんど同じ気団の境界にでき、前線本来の性質はもっていない。北（南）極前線、寒帯前線のように、地球規模の大気大循環に現れる前線を主要前線とよぶが、熱帯内収束帯をこれに加えることもある。

　前線には、その動きの状況によって次のような名称をもつものもある。

(1)滑昇前線　前面に沿って暖気がはい上る動きの活発な前線で、アナフロントanafrontともいう。優勢な上昇気流のため前線が活動的で、背の高い対流性の雲や強い降水を伴うことが多い。

(2)滑降前線　前面に沿って暖気が下降している前線で、カタフロントkatafrontともいう。前線は活動的でなく、雲もあまり発達せず降水も弱い。

(3)隠れ前線　局地的な影響などのため、地上の観測ではわかりにくい前線。マスクドフロントmaskedfrontともいう。活発な前線はこのような局地効果を破るため、隠れ前線はできにくい。

(4)二次前線　同一気団内に生ずる前線で、同一気団の変質過程の相違によってできる。たとえば、発達した低気圧に伴う寒冷前線の後面の寒気内では、沈降昇温した空気と水平運動をしていた空気との間には温度差が生じ、二次前線ができやすい。

(5)季節前線　地図などの上に、ある現象がみられる時刻、日、期間などを結んだ線を等発現線というが、この線は天気図でみられる前線の移動に似ているので季節前線とよばれることがある。サクラの開花日を結んだ桜前線、紅葉日を結んだ紅葉前線は季節前線の一種であり、気象学上、気候学上の前線とは異なる。

(6)スモッグ前線　スモッグの濃度の高い地域の縁辺での前線。いわゆる前線とは異なるが、スモッグの発生源が海岸地方にあるとき、スモッグは海陸風に運ばれて移動するため、海陸風前線に対応することが多い。

［饒村　曜］

前線の発生と消滅

前線が新たに発生または強化される過程をフロントジェネシスfrontogenesisという。大気下層の流れの場が、冷たい空気と暖かい空気が集まってくるなど、等温線が混んでくる場合におこる。ある程度、等温線が混んでくると上空に上昇気流を生ずるようになり、下層の収束が増して等温線がさらに集中して前線が強化される。このとき、上層の状態が上昇気流を抑えるような場合であれば前線は強化されない。フロントジェネシスのおきやすい地域は、気候学的前線帯でもある。

　前線が弱まるか、消滅する過程をフロントリシスfrontolysisという。前線の両側の気団が変質によって温度差がなくなる場合や、上空の状態が下降気流となり、前線に伴う上昇気流が抑えられて前線としての性質を維持できなくなる場合におこる。

［饒村　曜］

前線と低気圧の発達

中緯度および高緯度の低気圧（温帯低気圧）は、ほとんど寒帯前線上で発生している。その典型的な発達状態を示すと次のようになる（図B）。

(1)初めにほぼ東西に延びる停滞前線が形成され、前線の南側の暖気団では西風、寒気団では東風もしくは暖気団の西風より弱い西風が吹いている。

(2)前線上の一部で気圧が下がり低気圧が形成され、前線が波を打ち始める。東側の前線は暖気が寒気を押しのけて進むため温暖前線、西側は寒気が暖気を押しのけて進むため寒冷前線となる。

(3)低気圧の中心気圧の降下につれ前線の波状も大きくなる。寒冷前線は温暖前線より早く進むため、中心付近に形成された暖域はしだいに狭くなる。

(4)低気圧の中心付近では、寒冷前線は温暖前線に追い付き、閉塞前線ができ、暖域の空気を地上から締め出して上空に押し上げ、地表は寒気だけの渦巻となる。気圧の降下は止まり、低気圧はしだいに衰える。この種の低気圧は同じ前線上に数日間隔で次々と発生、発達し、その間に数千キロメートルも移動する。発生から衰弱に至るまで3～4の低気圧が一つの集団をつくることがあり、これを低気圧家族という。

　低気圧の発達をモデル的に示すと、初め前線を挟んで水平方向に並んでいた寒暖両気団が、最後は寒気団が下、暖気団が上になる。この場合、全体の重心が下がることから、全体の位置エネルギーは減少する。この減少分が運動エネルギーに変わり、低気圧の風のエネルギーとなる（図C）。

　寒帯前線など大規模な前線系は、上空のジェット気流と密接な関係をもっている。ジェット気流は地表面の前線とほぼ平行に走っているが、前線上で発生した低気圧が閉塞すると、閉塞点の上空を通るようになる。

　天気図や断面図上で前線の位置や運動、発生・解消状況、活動と天気状態などを分析・総合することを前線解析というが、この解析は気団分析と表裏一体をなしている。

［饒村　曜］

[参照項目] | 気団 | コリオリの力 | ジェット気流 | 低気圧 | 天気図

北半球の主要前線帯〔図A〕

前線と低気圧の発達段階〔図B〕

低気圧と位置のエネルギー〔図C〕

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