耿煥同,黃超,張偉,史達(dá)偉

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耿煥同①②*,黃超①,張偉①,史達(dá)偉①

① 南京信息工程大學(xué) 大氣科學(xué)學(xué)院,江蘇 南京 210044;

② 南京信息工程大學(xué) 濱江學(xué)院,江蘇 南京 210044

2014-11-06收稿，2015-12-14接受

國(guó)家自然科學(xué)基金資助項(xiàng)目(41201045;41430427);江蘇省自然科學(xué)基金資助項(xiàng)目(BK20151458)

利用1982—2009年全球熱帶云團(tuán)數(shù)據(jù)集、NCEP/NCAR再分析資料和英國(guó)Hadley中心海溫資料,并引入熱帶云團(tuán)生成率(Genesis Productivity,GP)來分析EP(Eastern Pacific)El Nio和CP(Central Pacific)El Nio事件與西北太平洋熱帶云團(tuán)發(fā)展的相關(guān)性。研究表明,1)夏秋兩季GP與Nio3指數(shù)在東南區(qū)(SE)均為顯著正相關(guān),在西南區(qū)(SW)僅秋季呈顯著正相關(guān);GP與EMI(El Nio Modoki Index)指數(shù)在夏季SE區(qū)域?yàn)轱@著正相關(guān),在秋季南中國(guó)海(South China Sea,SCS)區(qū)呈負(fù)相關(guān)。2)在EP El Nio年,夏季SE區(qū)域的GP增大與低層渦度、高層散度以及低層相對(duì)濕度的相對(duì)增大一致。夏季SW區(qū)域與秋季SE區(qū)域的GP增大與有利的高低空配置相關(guān),La Nia年則與之相反。3)在CP El Nio年,夏季SE區(qū)域的GP增大伴隨著低層渦度和高層散度的增加,同時(shí)與充足的水汽及弱風(fēng)切變相吻合;而秋季SCS區(qū)域的GP下降源于正渦度帶、正散度帶以及水汽帶的東移。

西北太平洋熱帶云團(tuán)熱帶氣旋生成率厄爾尼諾

熱帶氣旋的研究已經(jīng)取得了顯著進(jìn)展,但這些研究大多集中于熱帶氣旋生成后的移動(dòng)路徑和強(qiáng)度變化,很少關(guān)注其生成前的熱帶云團(tuán)發(fā)展變化規(guī)律。熱帶云團(tuán)(Tropical Cloud Cluster,TCC)是一種熱帶中小尺度對(duì)流系統(tǒng),能將能量從海面和低層大氣輸送至對(duì)流層上層(Houze,1982),并在有利的環(huán)境場(chǎng)配合下發(fā)展成為熱帶氣旋。對(duì)于熱帶氣旋生成問題,盡管先后提出了強(qiáng)調(diào)積云對(duì)流機(jī)制的第二類條件性不穩(wěn)定理論(Conditional Instability of Second Kind,CISK)以及強(qiáng)調(diào)海氣相互作用的海氣交換理論(Wind Induced Surface Heat Exchange,WISHE),但都還無法完全揭示熱帶氣旋的生成規(guī)律(Emanuel,1986;張文龍和崔曉鵬,2013)。

TCC的發(fā)展?fàn)顩r會(huì)直接影響熱帶氣旋的活動(dòng),Hennon et al.(2012)指出,相比其他大洋,西北太平洋TCC發(fā)展最為活躍,平均每年有12.4%的TCC最終發(fā)展成為熱帶氣旋。大尺度環(huán)流場(chǎng)是影響TCC發(fā)展和熱帶氣旋生成的重要因素。Gary(1968)認(rèn)為對(duì)流系統(tǒng)很難在缺乏持續(xù)對(duì)流和低層輻合的條件下產(chǎn)生,并最早揭示了熱帶氣旋生成的氣候條件(26 ℃以上的海溫、低層輻合、水汽輸送、條件性不穩(wěn)定層結(jié)和弱風(fēng)切變)。此后不斷有學(xué)者對(duì)此進(jìn)行驗(yàn)證(Demaria et al.,2001;Schumacher et al.,2009)。Fu et al.(2011)的研究表明,西北太平洋TCC的擾動(dòng)發(fā)展與低層渦度、散度、降水率、風(fēng)切變等因素相關(guān);Kerns and Chen(2012)則認(rèn)為低層渦度與輻合、弱風(fēng)切變以及低層水汽輸送更有利于TCC的發(fā)展。大量觀測(cè)資料顯示熱帶氣旋是由初始擾動(dòng)發(fā)展而來,Chen et al.(2008)指出西北太平洋大約有71%的熱帶氣旋由季風(fēng)槽中的擾動(dòng)產(chǎn)生。不僅如此,MJO(Maddan-Julian Oscillation)、Mixed Rossy-Gravity(MRG)波等均能觸發(fā)擾動(dòng)導(dǎo)致熱帶氣旋生成(Dickinson and Molinari,2002;李崇銀等,2012)。TCC作為一種初始擾動(dòng)的產(chǎn)物,其產(chǎn)生與發(fā)展同樣與季風(fēng)槽、東風(fēng)波、MJO、Rossby波、Kelvin波等大氣系統(tǒng)相聯(lián)系(Frank and Roundy,2006)。國(guó)內(nèi)學(xué)者很早就對(duì)TCC發(fā)展過程進(jìn)行了相關(guān)研究,方宗義(1982)通過分析擾動(dòng)云團(tuán)的高空環(huán)境場(chǎng)發(fā)現(xiàn)云團(tuán)環(huán)流中心與高空大尺度輻散關(guān)系密切。

1　資料和方法

文中熱帶云團(tuán)資料來自1982—2009年的全球熱帶云團(tuán)數(shù)據(jù)集(Global Tropical Cloud Cluster Dataset v01r01),該數(shù)據(jù)集通過分析紅外衛(wèi)星資料云頂亮溫?cái)?shù)據(jù)來確定和跟蹤熱帶云團(tuán)的移動(dòng)、強(qiáng)度等發(fā)展?fàn)顩r,時(shí)間分辨率為3 h,詳見Hennon et al.(2011,2012);海溫資料來自英國(guó)Hadley中心的1982—2009年1°×1°月平均資料(Rayner et al.,2003);熱帶氣旋資料取自1982—2009年IBTrACS西太平洋最佳路徑數(shù)據(jù)集(Knapp et al.,2010),將最大風(fēng)速大于17 m/s的時(shí)間和位置定義為熱帶氣旋生成的時(shí)間和位置。同時(shí),臺(tái)風(fēng)聯(lián)合預(yù)警中心(JTWC)、中國(guó)氣象局、日本氣象廳的熱帶氣旋最佳路徑資料也被用于對(duì)比。其他大氣資料來自1982—2009年美國(guó)NCAR/NCEP的月平均資料(Kalnay et al.,1996),水平分辨率為2.5°×2.5°,垂直層共有17層。

本文引入熱帶云團(tuán)生成率(Genesis Productivity,GP)來量化TCC發(fā)展?fàn)顩r,GP表示熱帶云團(tuán)發(fā)展成熱帶氣旋的百分比(Hennon et al.,2012),即一定時(shí)間內(nèi)熱帶氣旋的數(shù)量和TCC數(shù)量之比。

IEMI=I(SSTA)C-0.5×I(SSTA)W-0.5×I(SSTA)E。

(1)

其中:I(SSTA)C、I(SSTA)W、I(SSTA)E分別代表(165°E～140°W,10°S～10°N,)、(125°E～145°E,10°S～20°N,)、(110°W～70°W,15°S～5°N,)區(qū)域的海表溫度距平。

此外,為了區(qū)分西北太平洋不同區(qū)域不同季節(jié)TCC的發(fā)展?fàn)顩r,以120°E和140°E作為東西邊界、17°N作為南北的邊界將西北太平洋分成五個(gè)部分(圖1):南中國(guó)海(South China Sea,SCS)、西北區(qū)(NorthWestern,NW)、西南區(qū)(SouthWestern,SW)、東北區(qū)(NorthEastern,NE)和東南區(qū)(SouthEastern,SE)。并且對(duì)1982—2009年夏、秋兩季熱帶云團(tuán)的生成率情況分別進(jìn)行統(tǒng)計(jì)。

圖1　夏秋兩季熱帶云團(tuán)頻數(shù)分布和西北太平洋分區(qū)(南中國(guó)海(SCS)、西北區(qū)(NW)、西南區(qū)(SW)、東北區(qū)(NE)、東南區(qū)(SE))Fig.1　Spatial distribution of tropical cloud cluster frequency during summer and fall(the WNP is divided into the following sub-regions:South China Sea(SCS);NorthWestern(NW) WNP;SouthWestern(SW) WNP;NorthEastern(NE) WNP;and SouthEastern(SE) WNP)

2　熱帶云團(tuán)發(fā)展與兩類El Nio事件的關(guān)系

從氣候平均態(tài)來看,熱帶云團(tuán)在西北太平洋五個(gè)子區(qū)域發(fā)展?fàn)顩r并不相同。由表1可知,無論夏季還是秋季SE區(qū)的GP最小,NW區(qū)的GP最大,這是因?yàn)槲鞅碧窖蟮娜齻€(gè)主要的熱帶氣旋生成源地(南海東北部、菲律賓以東以及關(guān)島附近)位置偏西,而SE區(qū)TCC頻數(shù)較多、NW區(qū)TCC頻數(shù)較少。對(duì)于整個(gè)西北太平洋(Western North Pacific,WNP)區(qū)域而言,平均GP約為9.77%,夏季GP比秋季平均每年小2%左右。

表1　1982—2009年西北太平洋及其不同區(qū)域熱帶云團(tuán)平均生成率Table 1　The climatological GP of TCCs in the WNP and its five sub-regions during summer,fall and June to November from 1982 to 2009　%

夏、秋兩季的相關(guān)系數(shù)。從表2可以看出,夏季Nio3指數(shù)在SW和SE區(qū)與GP呈顯著正相關(guān);夏季EMI指數(shù)在SE區(qū)與GP同樣表現(xiàn)為顯著正相關(guān)。這表明,EP El Nio年夏季的TCC在SW和SE區(qū)更易發(fā)展,而La Nia年則相反;同時(shí)CP El Nio年夏季SE區(qū)的GP上升。

表2熱帶云團(tuán)生成率分別與Nio3、EMI、Nio4、Nio3.4指數(shù)的相關(guān)系數(shù)

Table 2Correlation coefficients of Nio3 index,EMI,Nio4 index and Nio3.4 index with the TCC GP in five sub-regions and the entire WNP during summer and fall

區(qū)域夏季秋季Ni?o3EMINi?o4Ni?o3.4Ni?o3EMINi?o4Ni?o3.4SCS0.1380.2190.4191)0.297-0.018-0.3861)-0.171-0.018NW-0.3580.128-0.154-0.238-0.1740.039-0.124-0.174SW0.5962)-0.1460.246-0.1840.0610.1350.0560.061NE-0.2850.1990.0200.176-0.1220.060-0.024-0.122SE0.4802)0.4481)0.5952)0.4351)0.5882)0.3370.5482)0.5882)WNP0.0960.3180.3180.2710.2550.0460.1800.245

注：1)表示通過95%置信度的雙側(cè)檢驗(yàn);2)表示通過99%置信度的雙側(cè)檢驗(yàn);下劃線表示以TCC頻數(shù)為控制變量的偏相關(guān)系數(shù)同樣顯著.

3　熱帶云團(tuán)發(fā)展與兩類El Nio相聯(lián)系的大尺度環(huán)流因素

從850 hPa的渦度合成距平場(chǎng)(圖2)來看,對(duì)于EP El Nio年(圖2a、圖2b),夏季SE區(qū)的正渦度帶一直延伸至菲律賓上空,而在其北部的NE區(qū)域和西部的SCS區(qū)域均存在一個(gè)寬闊的負(fù)渦度區(qū),致使該區(qū)域低層輻散增大。到了秋季,正渦度帶東移,SCS—SW一帶的負(fù)渦度區(qū)范圍加大,相反NE區(qū)正渦度范圍加大,輻合增強(qiáng)。同時(shí),結(jié)合200 hPa散度合成距平場(chǎng)(圖3a、圖3b)后發(fā)現(xiàn),夏季赤道以北的西太平洋區(qū)域高層存在一條正散度帶,正好與低層正渦度帶相對(duì)應(yīng)。因此,夏季SE和SW區(qū)低層輻合高層輻散的高低空配置有助于TCC的對(duì)流活動(dòng)增強(qiáng),促進(jìn)其發(fā)展。秋季SE—SCS一帶正散度同樣存在東移趨勢(shì),導(dǎo)致SW區(qū)負(fù)散度加強(qiáng),高層散度在SW區(qū)產(chǎn)生的季節(jié)性轉(zhuǎn)變抑制該區(qū)域的TCC活動(dòng)。

圖3　200 hPa散度(單位:10-7 s-1)合成距平場(chǎng)(a、c、e分別表示夏季東部型El Nio、中部型El Nio、La Nia;b、d、f分別表示秋季東部型El Nio、中部型El Nio、La Nia;陰影區(qū)表示通過95%置信度檢驗(yàn)的區(qū)域)Fig.3　Composite of 200 hPa divergence anomalies(units:10-7 s-1) during (a,c,d)summer and (b,d,f)fall of EP El Nio,CP El Nio and La Nia years(shading indicates the statistical significance of divergence at the 95% level)

圖5表示緯向風(fēng)切變合成距平場(chǎng)。從圖5a、圖5b可以看出,EP El Nio夏秋兩季在赤道以北西太平洋東側(cè)的緯向風(fēng)切變偏大,而在其北部存在一條風(fēng)切變減弱帶,這種環(huán)流配置在CP El Nio期間更為顯著,但弱風(fēng)切變范圍更廣。在La Nia年,西北太平洋中部大范圍區(qū)域夏秋兩季風(fēng)切變減弱,但與EP El Nio年相反,位于SE區(qū)域東南側(cè)的風(fēng)切變偏大。

圖4　700 hPa相對(duì)濕度(單位:%)合成距平場(chǎng)(a、c、e分別表示夏季東部型El Nio、中部型El Nio、La Nia;b、d、f分別表示秋季東部型El Nio、中部型El Nio、La Nia;陰影區(qū)表示通過95%置信度檢驗(yàn)的區(qū)域)Fig.4　Composite of 700 hPa relative humidity anomalies(units:%) during (a,c,d)summer and (b,d,f)fall of EP El Nio,CP El Nio and La Nia years(shading indicates the statistical significance of relative humidity at the 95% level)

圖5　緯向風(fēng)垂直切變(單位:m·s-1)合成距平場(chǎng)(a、c、e分別表示夏季東部型El Nio、中部型El Nio、La Nia;b、d、f分別表示秋季東部型El Nio、中部型El Nio、La Nia;緯向風(fēng)切變?yōu)?00 hPa與850 hPa緯向風(fēng)差值;陰影區(qū)表示通過95%置信度檢驗(yàn)的區(qū)域)Fig.5　Composite of vertical zonal wind shear anomalies (units:m·s-1) during (a,c,d)summer and (b,d,f)fall of EP El Nio,CP El Nio and La Nia years(shading indicates the statistical significance of vertical zonal wind shear at the 95% level;the vertical zonal wind shear is calculated as the magnitude of the zonal wind difference between 200 hPa and 850 hPa)

3.2熱帶云團(tuán)在風(fēng)切變偏大區(qū)發(fā)展的可能原因

一般而言,弱的風(fēng)切變有利于對(duì)流維持與發(fā)展,強(qiáng)風(fēng)切變不利于對(duì)流發(fā)展。但在兩類El Nio期間,SE區(qū)域存在大范圍強(qiáng)風(fēng)切變區(qū),這與相關(guān)分析結(jié)果相矛盾。而且El Nio年Walker環(huán)流減弱會(huì)導(dǎo)致中東部太平洋對(duì)流增強(qiáng)以及西太平洋對(duì)流活動(dòng)減弱(Wang et al.,2013),這與圖5西北太平洋東側(cè)風(fēng)切變減弱、西側(cè)增強(qiáng)是一致的。為分析高GP區(qū)域風(fēng)切變偏大的原因,表3給出了GP分別與850 hPa相對(duì)渦度、200 hPa散度、700 hPa相對(duì)濕度、緯向風(fēng)垂直風(fēng)切變的相關(guān)系數(shù)?？梢娤那飪杉維E區(qū)域低層渦度和高層散度與GP的相關(guān)性都非常顯著,這可能與TCC具有深厚輻合層的自身結(jié)構(gòu)特征相關(guān)(Williams and Gray,1973),其對(duì)流系統(tǒng)的維持非常依賴高低空環(huán)境場(chǎng)的配合。值得注意的是夏季SE區(qū)域的GP與所有因子都呈顯著正相關(guān),這表明由于El Nio年夏季SE區(qū)域的渦度、散度以及水汽條件的配置都非常有利TCC發(fā)展,即使強(qiáng)風(fēng)切變會(huì)抑制TCC對(duì)流活動(dòng),但GP仍然在SE區(qū)域增大。

表3熱帶云團(tuán)生成率與850 hPa相對(duì)渦度、200 hPa散度、700 hPa相對(duì)濕度、緯向風(fēng)垂直風(fēng)切變的相關(guān)系數(shù)

Table 3Correlation coefficients of 850 hPa relative vorticity,200 hPa divergence,700 hPa relative humidity and vertical zonal wind shear with the TCC GP in five sub-regions during summer and fall

區(qū)域夏季秋季850hPa相對(duì)渦度200hPa散度700hPa相對(duì)濕度緯向風(fēng)垂直風(fēng)切變850hPa相對(duì)渦度200hPa散度700hPa相對(duì)濕度緯向風(fēng)垂直風(fēng)切變SCS0.2190.3350.211-0.081-0.1260.5042)0.2870.125NW-0.138-0.257-0.058-0.2960.4371)0.2650.136-0.097SW0.5222)0.109-0.2850.0030.0700.2090.0130.054NE0.013-0.108-0.021-0.0240.1680.070-0.033-0.098SE0.7222)0.5132)0.5192)0.3791)0.5662)0.6902)0.0710.109

注：1)表示通過95%置信度雙側(cè)檢驗(yàn);2)表示通過99%置信度雙側(cè)檢驗(yàn).

4　結(jié)論與討論

2)夏季GP與EMI指數(shù)在SE區(qū)域具有顯著正相關(guān)關(guān)系,而在秋季的正相關(guān)關(guān)系卻并不顯著,并且SCS區(qū)域的GP與EMI指數(shù)在秋季呈負(fù)相關(guān)。造成該現(xiàn)象的主要原因是CP El Nio年夏季SE區(qū)域的低層渦度和高層散度增加,配合充足水汽與弱風(fēng)切變使得該區(qū)的GP增大,而秋季正渦度帶、正散度帶以及水汽帶的整體東移導(dǎo)致了SCS區(qū)域的GP下降。

上述結(jié)論表明不同區(qū)域海溫異常造成的大氣環(huán)流場(chǎng)異常變化,是兩類El Nio事件對(duì)TCC發(fā)展產(chǎn)生不同影響的原因。而且由于TCC自身具備的深厚輻合層的結(jié)構(gòu)特點(diǎn),其發(fā)展過程更為依賴有利的高低空環(huán)境場(chǎng)的配合(低層輻合和高層輻散)。這對(duì)進(jìn)一步認(rèn)識(shí)El Nio事件對(duì)TCC發(fā)展和熱帶氣旋的生成的影響有一定的價(jià)值。當(dāng)然,文中結(jié)論來自對(duì)大量觀測(cè)資料的分析,因此后續(xù)還需要在數(shù)值模式實(shí)驗(yàn)中加以驗(yàn)證。

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In order to improve the accuracy of tropical cyclogenesis forecasting,it is vitally important to study how Tropical Cloud Clusters(TCCs) develop into Tropical Cyclones(TCs),and the related climatic characteristics.TC genesis is poorly understood,since most previous studies have focused on the time after genesis.In the present study,global tropical cloud cluster data,NCEP/NCAR reanalysis data,Hadley Centre sea surface temperature data,and Genesis Productivity(GP) data (representing the ratio of TCCs that develop into TCs) were used to analyze the different impacts of two El Nio types on the evolution of TCCs in the Western North Pacific (WNP) from 1982 to 2009.The results indicate that GP is significantly and positively correlated with the Nio3 index in the southeastern sub-region of the WNP during summer.During fall,GP is significantly and positively correlated with the Nio3 index in the southwestern and southeastern sub-regions.The El Nio Modoki Index(EMI) has a markedly positive(negative) correlation with GP in the southeastern(South China Sea) sub-region during summer(fall).All these results are consistent with the large-scale environmental flow that affects TCC activity.Eastern Pacific(EP) El Nio events are associated with a significant increase in 850 hPa relative vorticity,200 hPa divergence and 700 hPa relative humidity in the southeastern sub-region during summer.The increase of GP in the southwestern sub-region during fall occurs because the relative vorticity and divergence are favorable for TCCs in EP El Nio years,whereas the opposite situation occurs during La Nia years.Central Pacific(CP) El Nio events enhance(suppress) GP in the southeastern sub-region(South China Sea) during summer(fall) because relative vorticity,divergence,relative humidity and vertical zonal wind shear are favorable(unfavorable) for the development of TCCs.In summary,the large-scale environmental flow anomalies,caused by El Nio events,create distinct effects on TCC activity in the WNP.Positive feedback is produced by CP El Nio events and EP El Nio events in the southeastern sub-region,but the effects of the two El Nio types are completely different in the southwestern sub-region and South China Sea.After comparing the different impacts of the two El Nio types on TCC activity,it was found that the changes in large-scale environmental factors produce different feedback effects on the development of TCCs in the WNP.Specifically,it is in the eastern part of the WNP that EP El Nio events affect TCC activity,but for CP El Nio events the impacts on the evolution of TCCs spread to the South China Sea.And it is in the South China Sea that the influences of the two El Nio types on TCCs are opposite.

Western North Pacific;tropical cloud cluster;tropical cyclone;genesis productivity;El Nio

(責(zé)任編輯：孫寧)

Relationship between the development of tropical cloud clusters and two El Nio types over the Western North Pacific

GENG Huantong1,2,HUANG Chao1,ZHANG Wei1,SHI Dawei1

1SchoolofAtmosphericScience,NanjingUniversityofInformationScience&Technology,Nanjing210044,China;2BinjiangCollege,NanjingUniversityofInformationScience&Technology,Nanjing210044,China

10.13878/j.cnki.dqkxxb.20141106002

*聯(lián)系人，E-mail:htgeng@nuist.edu.cn

引用格式：耿煥同,黃超,張偉,等.2016.西北太平洋熱帶云團(tuán)發(fā)展與兩類El Nio事件的聯(lián)系[J].大氣科學(xué)學(xué)報(bào),39(2):189-197.

Geng H T,Huang C,Zhang W,et al.2016.Relationship between the development of tropical cloud clusters and two El Nio types over the Western North Pacific[J].Trans Atmos Sci,39(2):189-197.doi：10.13878/j.cnki.dqkxxb.20141106002.(in Chinese).