氣候系統(tǒng)與氣候變化研究進展

氣候系統(tǒng)與氣候變化

氣候系統(tǒng)與氣候變化研究進展

2016年，氣候系統(tǒng)（極地氣象）研究所在氣候預測理論與方法、氣候系統(tǒng)模式研發(fā)以及極地氣候研究方面獲得了顯著進展。

1 氣候預測理論與方法

1.1 GPCP和CMAP資料在東亞夏季風降水年際變率上的不一致性及其改進方案

揭示了GPCP和CMAP資料在北半球夏季風降水年際變率上的不一致性，提出基于兩者算數(shù)平均減小該不確定性的方法和理由。GPCP和CMAP降水資料因覆蓋全球范圍和時間跨度較長而廣泛用于氣候監(jiān)測和氣候變率研究中。資料對比分析表明，盡管GPCP和CMAP資料均可描述北半球季風區(qū)降水的季節(jié)循環(huán)特征，但兩者之間仍然存在明顯的絕對誤差，這種差異在5—10月的西北太平洋（WNP）季風區(qū)最為明顯，表現(xiàn)為CMAP資料中WNP季風區(qū)夏秋季降水較GPCP資料數(shù)據(jù)偏多。就氣候平均夏季降水的空間分布而言，兩套資料的主要差異出現(xiàn)在WNP季風區(qū)的海洋上空，以及北非（NAM）和印度（IND）季風區(qū)的熱帶海域，這可能是CMAP資料使用了具有爭議的島嶼觀測資料。在年際尺度上，GPCP和CMAP資料在北半球五大季風區(qū)夏季降水年際變率方面大體一致，但兩者在IND和NAF季風區(qū)的差異顯著。GPCP和CMAP數(shù)據(jù)的一致性在近幾十年逐漸提高，特別是在NAM和IND季風區(qū)。使用GPCP和CMAP的算術平均來描述北半球季風區(qū)降水的年際變率能夠有效降低不同資料間的不確定性，特別是在1979—1997年的IND季風區(qū)和1998—2014年的NAF季風區(qū)（圖1）。（祝從文，劉伯奇）

1.2 東亞夏季風氣候季節(jié)內振蕩的特征和可能成因

該工作證實了東亞夏季風氣候季節(jié)內振蕩（CISO）的存在性，闡明了與CISO位相循環(huán)相關主要環(huán)流系統(tǒng)的耦合過程和東亞夏季風降水季節(jié)內變化的聯(lián)系。東亞夏季風（EASM）的氣候季節(jié)內振蕩以季風環(huán)流的垂向和經(jīng)向相互作用為特征，與夏季風雨帶的季節(jié)性北抬相聯(lián)系。EASM CISO在5—8月最為強盛。采用諧波分析和多變量EOF分析方法，從逐日氣候態(tài)（1981—2010年平均）風場、降水、非絕熱加熱以及海溫場的季節(jié)內變化分析了EASM CISO主模態(tài)特征。結果表明，EASM CISO存在的根本原因是東亞地區(qū)海陸熱力對比對太陽輻射年循環(huán)的非對稱響應，使得CISO變率中心位于我國東部至西北太平洋上空。在EASM盛行季節(jié)，非絕熱加熱Q1主要由凝結潛熱構成，Q1中心位于西北太平洋和東亞地區(qū)，因此Q1的水平梯度能夠在其中心的東北側制造負渦度源，其造成的背景上升運動有利于CISO的觸發(fā)。同時，Q1各成分之間的位相差也能夠維持EASM CISO。具體而言，EASM CISO開始于表面感熱加熱的加強，進而改變大氣對流穩(wěn)定度，調控季風對流及其凝結潛熱，而凝結潛熱的增加會令地面降溫，感熱減弱，如此循環(huán)往復，構成了EASM Q1 的CISO。

多變量EOF結果表明，EASM CISO的前兩個主模態(tài)的主要特征是850 hPa貝加爾湖附近蒙古氣旋、500 hPa西太平洋副高和200 hPa青藏高原上空南亞高壓的相互耦合。其中第1主模態(tài)表現(xiàn)為上述3個環(huán)流系統(tǒng)的同時加強，對應著東亞地區(qū)“3極型”降水異常，即梅雨鋒位于長江中下游至日本以南，而東北亞和西北太平洋的降水異常偏少。在EASM CISO的第2主模態(tài)中，蒙古氣旋和西太平洋副高加強并向西北方向傳播，而南亞高壓卻異常偏弱，與之對應的是亞洲到西太平洋地區(qū)的偶極型降水異常。而AGCM試驗證明，EASM CISO的第1主模態(tài)主要與SST對大氣環(huán)流的強迫作用有關，表現(xiàn)為東亞-太平洋地區(qū)逐日SST季節(jié)變化對大氣環(huán)流季節(jié)演變的驅動作用，但EASM CISO的第2主模態(tài)卻無法很好模擬，暗示該模態(tài)很可能與大氣對SST的強迫過程有關（圖2）。（祝從文，劉伯奇）

1.3 南海夏季風爆發(fā)年際變化特征及其海溫影響因子在1993/1994年前后的年代際差異

揭示了南海夏季風爆發(fā)過程的年際變化特征在1993/1994年前后存在顯著差異，指出春季南印度洋海溫和前冬ENSO事件分別是1993/1994年前后影響南海夏季風建立時間的關鍵因子。南海夏季風爆發(fā)時間具有明顯的年際變率。本研究依據(jù)南海夏季風爆發(fā)期間的動力熱力場和環(huán)流結構的年際變化，以1993/1994年為界，將南海夏季風爆發(fā)的年際變化劃分為兩種不同類型。在1980—1993年期間，南海夏季風爆發(fā)年際變化的特征是季風降水異常出現(xiàn)在南海北部，伴隨著顯著的低空緯向風異常，將其定義為“Ⅰ型”爆發(fā)，它與副熱帶環(huán)流系統(tǒng)的聯(lián)系更為緊密。在1994—2014年期間，南海夏季風爆發(fā)年際變化的特征變?yōu)橐詿釒Ъ撅L對流和高空緯向風異常為主，將其定義為“Ⅱ型”爆發(fā)。因此，南海夏季風爆發(fā)期間高、低空環(huán)流垂直耦合過程在這兩個時段也具有明顯差異。

影響南海夏季風爆發(fā)年際變化兩種類型的前期SSTA也截然不同，對“I型”爆發(fā)而言，1983—1993期間南海夏季風爆發(fā)早晚主要受春季南印度洋（40°～20°S，40°～110°E）SSTA影響，南印度洋暖SSTA會導致南海夏季風爆發(fā)異常偏晚。這是因為南印度洋暖SSTA能夠首先造成大尺度經(jīng)向海平面氣壓異常，令南印度洋出現(xiàn)低空異常輻合，而低空異常輻散卻位于南海地區(qū)，隨后異常低空反氣旋控制著南海北部，產(chǎn)生異常下沉運動，進而減弱南海地區(qū)的垂直東風切變和海陸熱力對比，最終抑制了南海季風對流的發(fā)生，令南海夏季風爆發(fā)異常偏晚。對“Ⅱ型”爆發(fā)而言，在1994—2014期間，前冬ENSO事件及其相關的春季熱帶印度洋SSTA是影響南海夏季風爆發(fā)遲早的主要海溫強迫。前冬暖ENSO事件發(fā)生后，能夠通過“大氣橋”造成春季熱帶印度洋的異常增暖。熱帶印度洋的異常增暖會激發(fā)出熱帶定常Kelvin波，令南海地區(qū)對流層中上部平均溫度的經(jīng)向梯度（MTG）減弱，從而推遲了MTG由冬到夏季節(jié)轉換的時間。此外，對流層高層由熱帶印度洋吹向南海的大尺度東風異常使熱帶印度洋的上升運動加強，但卻令南海南部的上升運動減弱，這將進一步抑制南海季風對流的建立，最終導致南海夏季風爆發(fā)異常偏晚。研究結果說明，ENSO和南海夏季風爆發(fā)時間之間的對應關系在1993/1994年前后發(fā)生了明顯改變。這可能是因為在1993/1994年之后La Ni?a事件的發(fā)生頻次明顯提高，而這又與ENSO事件的年代際調整有關（圖3）。（劉伯奇）

1.4 南亞高壓季節(jié)變化是南海夏季風爆發(fā)的可能前兆信號

研究發(fā)現(xiàn)高空南亞高壓的季節(jié)演變對南海夏季風的爆發(fā)具有指示意義，揭示了南海地區(qū)跨赤道季風環(huán)流建立的動力過程。南海夏季風爆發(fā)主要發(fā)生在每年的第28候（5月16—20日），以往的南海夏季風爆發(fā)研究主要基于對流層低層的大氣環(huán)流和下墊面熱狀況研究南海夏季風的爆發(fā)機理（例如，西太平洋副高的東西移動和局地海溫變化的影響）。利用NCEP/DOE R2大氣再分析資料計算和分析了南海夏季風爆發(fā)前后的大氣熱動力過程，發(fā)現(xiàn)南亞高壓在第27候的向東移動伸入南海上空很可能是導致南海夏季風爆發(fā)的主要誘因。第27～28候，隨著南亞高壓的東伸加強，南海上空出現(xiàn)正位渦平流，相應地局地高空上升運動加強，其導致的抽吸作用令低空西太平洋副高開始東撤出南海，季風槽和季風對流逐步建立；隨著南海季風對流的不斷加強，南亞高壓持續(xù)東伸發(fā)展，與之對應的高空暖中心和近海面暖中心的空間位相疊加，南海地區(qū)的對流層溫度垂直層結被破壞，從而滿足了環(huán)流的角動量守恒條件，令季風環(huán)流的下沉支能夠穿越赤道到達南半球，最終引起了跨赤道的季風經(jīng)向環(huán)流，將南、北半球季風系統(tǒng)相聯(lián)，南海夏季風最終于第29候完全爆發(fā)（圖4）。（劉伯奇）

2 氣候系統(tǒng)模式研發(fā)

2.1 東亞夏季降水結構模擬及其影響因子：2種不同類型大氣模式對比

當前氣候模式對東亞地區(qū)降水模擬存在普遍誤差：東南部氣候態(tài)降水偏少，北部地區(qū)氣候態(tài)降水偏多；模式普遍高估降水頻次，低估降水強度；對陸地午后日峰值的模擬普遍提前。研究指出，這些誤差并非孤立存在，而是具有一定的內在聯(lián)系。

研究對比了CAM5和SPCAM5模式的模擬結果，后者可以顯式模擬次網(wǎng)格云降水等物理過程。研究指出，與大部分氣候模式類似，CAM5具有以上誤差特點。其中，CAM5對華南地區(qū)午后以16:00—17:00為主的峰值模擬偏早，主要集中在14:00這一午后不穩(wěn)定最強的時刻。與此對比，SPCAM5緩解了華南地區(qū)的降水負偏差問題，降低了平均小時降水頻次，提高了降水強度。同時，在以午后降水峰值主導的地區(qū)，SPCAM5延遲了降水峰值時刻。

為了明確造成兩類模式模擬差異的原因，研究對模式的大尺度變量進行了小時尺度的源匯分析。針對Q1-Qrad的分析指出，SPCAM5在對流加熱隨時間演變的刻畫上具有更明顯的傾斜結構。針對Q2的分析指出，SPCAM5對降水峰值前的近地層變干信號具有更明顯的表征。這反映了SPCAM5對于午后對流發(fā)展的漸進過程，即對流由淺對流至深對流的轉換過渡階段具有更合理的描述。

結合二者結果可以得出以下的物理圖像。在CAM5中，每當有午后的不穩(wěn)定能量出現(xiàn)，模式作出消耗不穩(wěn)定能量的表現(xiàn)，開始降水，對流無法累積到一定的強度。因此，模式中以頻繁的弱降水主導，且降水峰值提前，在氣候平均態(tài)上表現(xiàn)為降水量偏低。在SPCAM5中，當有午后不穩(wěn)定能量出現(xiàn)，模式不會立刻產(chǎn)生消耗不穩(wěn)定能量的行為。模式能夠模擬出對流由淺對流發(fā)展為深對流的演變階段。淺對流云的存在使得模式不會快速形成降水，同時有助于不穩(wěn)定能量的累積。當不穩(wěn)定能量累積到一定程度時，降水峰值才出現(xiàn)。因此，降水峰值滯后，且峰值時刻降水量增強，降水頻次降低，強度提高，在氣候平均態(tài)上表現(xiàn)為降水量增多。因此，對流由淺至深轉化這一過程的合理模擬，使得SPCAM5能夠在一定程度上改善東亞地區(qū)降水在不同時間尺度上的特征。此外，由于降水峰值時刻對流更強，直接帶來的一個副效應是峰值時刻的大尺度上升運動在SPCAM5中更強，這反映了次網(wǎng)格尺度過程對大尺度動力環(huán)流的影響。（張祎）

2.2 青藏高原陡峭地形處降水的模擬誤差調節(jié)

針對大氣環(huán)流模式在東亞降水模擬存在的一大頑疾“高原南坡陡峭地形區(qū)的降水偏差”，本研究發(fā)現(xiàn)這一問題對大尺度動力過程更為敏感。研究通過設計一組敏感性試驗考察了這一問題。第1組試驗為標準AMIP型氣候積分的控制試驗。第2組試驗為敏感性試驗，其中，人為的引入了一個與空氣散度成正比的水汽散度項。這一項的引入使得空氣輻散區(qū)的水汽被驅散，輻合區(qū)的水汽被堆積，與地形周邊的散度效應恰好成對應關系。

通過長期氣候積分發(fā)現(xiàn)，這一項的引入減少了模式在陡峭高地形區(qū)原本高估的降水，增強了模式在低地形區(qū)原本低估的降水，對模式在陡峭地形附近的降水誤差呈抑制作用。此外，模式整體的降水頻次-強度結構亦發(fā)生了變化，在陡峭高地形區(qū)的降水頻次和強度值都有所降低。并且，總降水的主要變化來自于格點尺度降水和水汽含量的改變。這說明大尺度動力因子改變了局地的水汽結構，從而達到改變降水場模擬的結果。在非絕熱水汽傾向上，這一水汽散度項表現(xiàn)為對高地形區(qū)原始正平流傾向的抑制，而對低地形區(qū)原始平流傾向的增加，這與模式氣候積分結果相符。

為了能夠更好地考察這一水汽散度項對模式的影響，進一步開展了氣候模式的短期數(shù)值預報試驗。該試驗通過對模式進行短期的初始化預報構造模式的氣候態(tài)，以考察短期誤差和模式長期氣候誤差的聯(lián)系。研究指出，這一散度項所帶來的影響在很短的時間內即形成，是一個快過程變化。由于地形處的空氣輻合輻散，高地形處的水汽隨著該項的引入被“驅散”，低地形處的水汽隨著該項的引入被“補充”。陡峭地形處水汽隨大尺度動力因子的改變，影響了模式在該地區(qū)的降水特征。這一研究為消除模式在青藏高原地區(qū)的模擬誤差提供了依據(jù)。研究也揭示出大尺度動力因子對降水的影響。（張祎）

2.3 正二十面體大氣模式發(fā)展：傳輸與淺水方程求解器

為了適應面向未來的全球高分辨率大氣模擬，研究發(fā)展了基于正二十面體剖分后形成的球面準均勻網(wǎng)格上的大氣模式求解器。球面網(wǎng)格通過對一個規(guī)則正二十面體的三角邊進行逐次等分所形成，并采用Voronoi-Delaunay球面拓撲劃分構建為非結構型網(wǎng)格（區(qū)別于結構型正二十面體網(wǎng)格，不存在統(tǒng)一的拓撲規(guī)則）。求解器采用Arakawa-C網(wǎng)格，即質量位于單元格中心，正交預報速度位于單元壁。在羅斯貝半徑得以分辨的前提下，C網(wǎng)格由于對散度模態(tài)的模擬優(yōu)勢，非常適合于模擬以散度模態(tài)為主導的高分辨率下的大氣運動。同時，不同于Z網(wǎng)格，C網(wǎng)格無需求解橢圓方程，避免了全局通信，便于大規(guī)模并行。

傳輸和淺水方程包含了三維靜力原始方程的水平部分。首先在正二十面體網(wǎng)格上構造了一個守恒兩步保形平流方案（TSPAS），該方案是對Yu (1994年)兩步保形平流方案在非結構網(wǎng)格上的有限體積推廣。通過構造一個合適選取的預估參數(shù)，該方案可以靈活選擇Lax-Wendroff和迎風格式以實現(xiàn)在保持精度的前提下的正定平流傳輸。本研究還構造了國際上廣泛使用的通量糾正格式（FCT），并采用多個球面?zhèn)鬏斔憷▌傮w平移測試、變形流場測試等）對比了TSPAS和FCT方案。結果顯示，兩種方案都能夠很好地實現(xiàn)非結構網(wǎng)格上的正定平流傳輸，TSPAS對單調性保持略佳，而FCT對信號峰值的保持略佳。本研究還指出了TSPAS和FCT在構造理念上的差異。

在傳輸方程的基礎上，進一步構建了基于正二十面體C網(wǎng)格的球面淺水方程求解器。構造淺水方程，模式應滿足一些重要且潛在的物理約束。對于廣義形狀的非規(guī)則C網(wǎng)格，其中重要的一點是對與預報風場成正交的科氏力項的近似，以避免切向力產(chǎn)生虛假的能量源匯。采用可嚴格保持科氏力能量中性的矢量重構算法，以及保證能量轉換精確到時間截斷誤差的基本算子，構造了總能量守恒至時間精度的淺水方程求解器。通過采用多種時間積分方案，證實了這一求解器在能量方面的可靠性。該研究工作為后續(xù)研發(fā)奠定了基礎。（張祎）

2.4 一種可客觀評估模式降水的新方法及其適用性檢驗

在當前全球大氣環(huán)流模式分辨率尚不足以分辨云動力尺度云雨演變過程的情況下，由參數(shù)化方案計算得到的模式降水只能表征模式格點尺度的平均降水。降水存在極大的時空非均勻性，這使得模式模擬降水與臺站觀測降水（固定點）和高分辨率衛(wèi)星反演降水（小網(wǎng)格內的平均情況）難以進行直接比較，這一問題在評估降水小時尺度特征時會更為突出。本研究提出的應用一種新定義的區(qū)域降水事件（RRE）方法是客觀評估模式對小時尺度降水特征模擬能力的有效手段。為驗證該方法的適用性，首先比較了臺站觀測、小時融合降水產(chǎn)品（CMPA-Hourly）和兩套常用衛(wèi)星產(chǎn)品再現(xiàn)的中國中東部地區(qū)暖季（5—9月）小時降水特征。結果表明，與單站或單點比較不同的是，采用區(qū)域降水事件的方法比較不同源資料可以得到基本一致的降水頻次與強度的空間分布特征，只是衛(wèi)星反演降水在量值上與臺站觀測存在差異?；趩握净騿吸c發(fā)現(xiàn)的衛(wèi)星降水易于高估降水頻次而低估降水強度的問題在使用該方法時有顯著改進。同時，不同源觀測資料均顯示出暖季區(qū)域降水系數(shù)（RRC）分布的南北差異，即長江流域以南RRC相對較小，而以北區(qū)域則相對較大，表明該方法合理反映了暖季降水特性的區(qū)域差異，我國南方地區(qū)暖季對流降水頻繁，而北方地區(qū)系統(tǒng)性的區(qū)域性降水更為頻繁，這與此前研究的結論亦相一致。從日變化的角度看，衛(wèi)星反演的降水頻次和強度的日變化與臺站觀測降水也較為一致，基于區(qū)域降水事件考察不同資料間差異時未發(fā)現(xiàn)衛(wèi)星資料對午后降水峰值的明顯高估。整體而言，融合降水資料對中國中東部地區(qū)小時降水特征的再現(xiàn)要明顯優(yōu)于衛(wèi)星反演降水。除了不同源資料的一致性外，該方法也能給出不用源資料對降水特性再現(xiàn)的差別。分析發(fā)現(xiàn)，在中國中東部地區(qū)，代表有限區(qū)域內降水空間分布的RRC在夜間至上午達日最大值，表明這一時段區(qū)域內降水分布最為均勻。在大部分地區(qū)，RRC達到日峰值的時間較區(qū)域降水事件強度的峰值時間滯后幾個小時，表征了局地對流向層云降水轉變的過程。在午后RRC較小，表明局地對流活動頻繁，而對午后局地對流探測能力的不同可能是導致不同源資料對午后降水估計存在差異的重要原因之一。由于區(qū)域降水事件反映的是有限區(qū)域內的降水整體特征，采用該方法對不同源資料進行比較分析時衛(wèi)星資料高估午后降水的特征不明顯，以區(qū)域降水事件來分析衛(wèi)星降水產(chǎn)品小時尺度降水特征亦具有更高的可信度。研究結果表明，區(qū)域降水事件是一種可用于比較不同源降水資料小時尺度特征的更合理方法，后續(xù)研究將進一步應用該方法評估數(shù)值模式模擬的云和降水時空變化特征（圖5）。（陳昊明）

3 極地氣候研究

3.1 夏季北冰洋中心大氣邊界層垂直結構與海冰范圍變化的關系

最近的研究指出，北極海冰減少加強了北極大氣邊界層中海-冰-氣相互作用，特別是秋季至初冬季海/氣熱通量的增加和邊界層穩(wěn)定度的下降。1999年以來中國實施了6次北冰洋科學考察?？疾炱陂g，開展了對大氣垂直結構探測和海-冰-氣相互作用的觀測試驗，使我們對北極浮冰區(qū)不同海冰密集度的大氣邊界層特征有了初步認識。研究指出，北極海冰區(qū)的大氣逆溫層能有效地阻礙大氣與冰面之間的熱量及物質交換。北冰洋大氣邊界層可分為穩(wěn)定型、不穩(wěn)定型和多層結構等類型，并發(fā)現(xiàn)來自高空較強的暖濕氣流與冰面近地層冷空氣強烈相互作用會形成強風切變和逆溫、逆濕過程，從而導致北冰洋高緯度地區(qū)的大塊海冰破裂。隨著北極海冰的持續(xù)減少，2008年夏季中國第3次北極科學考察隊到達北冰洋85°N海域，第4次北極科學考察隊到達北極點附近（88.41°N），2012年第5次和2014年第6次北極科學考察隊都到達了80°N以北冰區(qū)，使我們得以在北冰洋中心區(qū)冰站獲取了GPS探空資料，為研究北冰洋高緯度對流層和邊界層結構提供了重要基礎。Ma等（2011年）和Bian等（2011年）分析了對流層和邊界層逆溫強度的變化特征，對北冰洋大氣層邊界層高度的變化特征提出了新認識。本研究利用中國第4～6次北極科學考察隊獲得的北極探空資料，對比分析北極夏季海冰面積變化對大氣邊界結構的影響，為研究北極海冰變化對大氣環(huán)流的影響機理提供觀測事實。

為了探索2010、2012和2014年夏季大氣垂直結構、邊界層高度參數(shù)差異的原因，分析了1979—2014年9月1000 hPa和850 hPa溫度與海冰范圍的變化關系，揭示出新的統(tǒng)計事實和它們的緊密關系，為深入研究北極海冰變化在全球氣候變化中的作用提供了重要依據(jù)。主要結果如下：

（1） 2012年與2010和2014年夏季北冰洋中心區(qū)的對流層頂、邊界層高度、溫度遞減率及風速和風向的垂直結構均在1 km以下存在明顯差異，2010年和2014年近地面存在明顯的逆溫結構，2012年逆溫層卻很少出現(xiàn)，其過程與探測區(qū)域周圍存在無冰海域和近地層氣流較強的混合作用有關。2010和2014年夏季邊界層高度與逆溫強度呈顯著的對數(shù)關系，相關系數(shù)分別為0.81和0.92，表明逆溫強度越強，邊界層高度越低。2012年二者對數(shù)關系相對離散，相關系數(shù)為0.56，邊界層高度為690 m，明顯高于2010年和2014年的邊界層高度，反映了9月海冰范圍的年際變化對大氣邊界層結構有重要影響。

（2）北極大氣垂直結構除了受大尺度天氣過程的影響外，與海冰覆蓋范圍的變化有直接關系：北冰洋中心區(qū)夏季的海冰面積越大，穩(wěn)定層結的天氣越多；反之對流性的天氣增多。通過分析1979—2014 年9月北極海冰范圍與1000 hPa和850 hPa溫度變化的關系，發(fā)現(xiàn)近30年北冰洋中心區(qū)1000 hPa和850 hPa的溫度變化呈顯著的升高趨勢，變化速率分別為1.3 ℃/10a和0.81 ℃/10a，與海冰范圍呈負顯著相關，相關數(shù)分別為0.83和0.74。結果表明，北極海冰減少，能夠引起1000～850 hPa高度的大氣層增溫，這是海-冰-氣相互作用的動力和熱力輸送結果。（丁明虎）

3.2 對2008—2013年間南極Dome A 表面物質平衡的再評估

2004/2005年，第21次南極考察在Dome A頂點安裝了自動氣象站，用于觀測該區(qū)域的氣象參數(shù)；2007/2008年，中國第24次南極考察隊在Dome A 30 km×30 km區(qū)域內布設了49個花桿，用于測量該地區(qū)的數(shù)字高程、冰流速和表面物質平衡；2010/2011年和2012/2013年分別對各觀測站點進行了復測。通過蒙特卡洛模擬，證明局部及區(qū)域尺度范圍內表面物質平衡至少需要利用12～20個花桿點建立可靠的估計，因此本研究的觀測方案可信度強。通過花桿觀測數(shù)據(jù)計算了該區(qū)域的凈物質平衡，為（22.9±5.9）kg/（cm2·a），遠低于包含Dome C、Dome F和南極點在內的南極冰蓋其他地區(qū)。結合氣象站觀測數(shù)據(jù)利用奧布霍夫-莫寧模型，模擬了該區(qū)域的升華和凝華狀況，發(fā)現(xiàn)Dome A區(qū)域的升華損耗為（2.22±0.02）kg/（cm2·a）、凝華損耗為（1.37±0.01）kg/（cm2·a），即大約有14.3%的降雪量通過凝華形式損耗，遠高于南極冰蓋平均狀況，這可能與該區(qū)域盛行下沉氣流有關，對南極冰蓋變化和雪冰芯研究具有重要的指示意義。另外研究發(fā)現(xiàn)，由于地形導致的下降風的原因，Dome A西部地區(qū)的冰蓋雪密度與表面物質平衡要高于其他區(qū)域。本研究還建立并分析了Dome A數(shù)字高程模型，確認2個山峰穹頂均可以作為東南極冰蓋的頂峰，修正了前人的結果。（丁明虎）

3.3 氣候模式和地面觀測中南極冰蓋表面質量平衡的對比

本研究利用3265個多年平均的站點觀測結果和29個逐年觀測的觀測數(shù)據(jù)對近些年出現(xiàn)的再分析資料和區(qū)域氣候模式產(chǎn)品（ERA-Interim、JRA-55、MERRA、PMM5、RACMO2.1和RACMO2.3）在南極地區(qū)物質平衡的空間分布和年際變率進行檢驗。

自第1次國際極地年（1957—1958年）以來，世界各國科學家積極參與國際橫穿南極科學考察計劃（ITASE）和南極物質平衡和海平面研究計劃（ISMASS），特別是國際極地年（2007—2009年）的一系列科學計劃，對南極冰蓋主要流域進行了大量表面物質平衡實地測量，采用的方法主要有花桿、超聲高度計（雪深儀），雪坑、冰/雪芯和探地雷達法。Vaughan等 (1999年) 最早對表面物質平衡觀測資料進行了編撰，建立了表面物質平衡空間數(shù)據(jù)庫，但是其中包含了很多不可靠的數(shù)據(jù)，影響了表面物質平衡的空間分析及氣候模式結果驗證，為此Magand等（2004年)建立了南極積累率實測數(shù)據(jù)質量控制標尺?；谠摌顺撸現(xiàn)avier等 （2013年）對收集整理的南極物質積累率數(shù)據(jù)集進行了甄別篩選，在此基礎上更新了該數(shù)據(jù)庫。本研究廣泛收集了冰芯、雪坑、自動氣象站、物質平衡花桿觀測資料，特別是國際極地年（2007—2009年）以來的最新研究成果，對Favier等（2013年）編撰的南極冰蓋表面物質平衡數(shù)據(jù)集進一步更新，建立了具有3550個位置觀測數(shù)據(jù)的經(jīng)質量控制的南極冰蓋多年平均表面物質平衡空間數(shù)據(jù)庫。該數(shù)據(jù)庫空間分布極不均勻，南極內陸和海岸許多區(qū)域仍然是數(shù)據(jù)空白區(qū)。

ERA-Interim、MERRA、CFSR、JRA-55和NCEP-2模式均能較好地模擬南極冰蓋表面物質平衡大尺度空間變化，與實測結果相關系數(shù)超過0.75，但是模擬表面物質平衡中尺度變化能力有限，如在昭和站-Dome F斷面，中山站-昆侖站斷面，蘭伯特冰川流域等 。所有這些再分析資料較好地再現(xiàn)了很多海岸區(qū)域表面物質平衡梯度，但是其模擬值通常偏高。JRA-25模式顯著高估了東南極高原表面物質平衡，而ERA-Interim過低估計了表面物質平衡?？傮w上來說，由于對邊界層潛熱通量的高估，NCEP-2過低估計了表面物質平衡。MERRA、JRA-55和CFSR模擬結果與觀測值比較一致，但是值得注意的是，所有再分析資料沒有包含風吹雪導致的消融過程，說明這3種再分析資料在一定程度上高估了表面物質平衡。

1979—2012年，冰蓋尺度上NCEP2、JRA-25、JRA-55、MERRA模擬的表面物質平衡呈顯著增加趨勢。CFSR和ERA-Interim模擬的表面物質平衡沒有顯著的變化趨勢，這與基于冰芯記錄重建的南極冰蓋表面物質平衡結果一致。ERA-Interim線性變化趨勢顯著性水平達不到0.1檢驗水平的區(qū)域最為廣泛。區(qū)域尺度上，盡管不同的再分析資料表面物質平衡趨勢大小和方向差異顯著，但是NCEP2、MERRA、JRA-25和JRA-55模擬的表面物質平衡在毛德皇后地海岸區(qū)域呈異常顯著上升趨勢，這與該區(qū)域的冰芯顯示1989—2007年積累率呈下降趨勢相左。在東南極70°～170°E區(qū)域內，再分析表面物質平衡變化資料趨勢相近，如蘭伯特冰川區(qū)和威爾克斯地島中部呈上升趨勢，而威爾克斯地島西部和維多利亞地島呈下降趨勢。需要指出的是，JRA-25和NCEP2在威爾克斯地島中部過高的上升趨勢（＞200 mm/a）令人懷疑。此外，除ERA-Interim模式外的Law Dome 顯著上升趨勢的再分析表面物質平衡與冰芯記錄結果相矛盾。在西南極冰蓋，1989—2009年，再分析表面物質平衡通常呈不顯著變化趨勢。在艾爾斯渥茲地（Ellsworth Land），ERA-Interim、CFSR、JRA-55和MERRA模擬的表面物質平衡呈上升趨勢，這得到了該區(qū)域附近Gomez 冰芯記錄的證實。所有再分析資料很好地再現(xiàn)了與羅斯海海冰范圍增加有關的威爾克斯地島表面物質平衡下降趨勢。

從格點尺度上線性回歸趨勢來看，PMM5、RACMO2.1和RACMO2.3模擬的表面物質平衡具有顯著性（p＜0.05）線性變化趨勢的區(qū)域都很有限。RACMO2.1模擬的表面物質平衡顯著性變化趨勢區(qū)域主要集中在毛德皇后地海岸區(qū)域和威爾克斯地島部分區(qū)域，而RACMO2.3 模擬顯著變化趨勢區(qū)域集中在威爾克斯地島和東南極內陸的部分區(qū)域，然而這些顯著趨勢區(qū)域并沒有得到冰芯記錄結果的證實。1979—2012年，南極冰蓋尺度上PMM5和RACMO2.1模擬的表面物質平衡沒有顯著的變化趨勢，但是RACMO2.3呈顯著的下降趨勢。29個具有區(qū)域代表性表面物質平衡實測序列與相應的PMM5、RACMO2.1和RACMO2.3模擬結果的相關分析表明，有15個點RACMO2.3模擬結果與實測表面物質平衡序列顯著相關，而RACMO2.1和PMM5模擬結果與實測顯著相關的點少于10個?？偟膩碚f，與再分析資料相比，區(qū)域氣候模式模擬的南極冰蓋表面物質平衡年變化趨勢可信度不高，這很可能是其內部由于沒有同化觀測數(shù)據(jù)限制任由模式自由演化導致的。未來在這些模式長期積分中引入松弛逼近方法或譜逼近方法等進行動力降尺度有望改進模擬表面物質平衡年際變化的能力。（丁明虎）

圖1 GPCP（a）和CMAP（b）資料計算的北半球五大季風區(qū)氣候平均降水的季節(jié)循環(huán)特征，以及兩者季節(jié)循環(huán)的絕對偏差(c)和兩者夏季降水的相對偏差(d)（紅點表示＞30%; 藍點表示＜-30%，虛線框表示兩者一致的季風區(qū)范圍，黑色等值線表示1.5 km的青藏高原范圍）Fig. 1 Climatological seasonal cycle of precipitation (1979–2014) calculated based on (a) GPCP and (b) CMAP (unit: mm d?1)products over the f ve monsoon regions as marked in (d). (c) Absolute difference in the averaged seasonal cycle between the two products (CMAP minus GPCP; unit: mm d?1). (d) Relative difference of the averaged summer precipitation between the two products (0.5 × (CMAP?GPCP) × (CMAP + GPCP)?1; red: ＞30%; blue: ＜?30%). The black contoured region is the 1.5 km topography of the TP

圖2 與東亞夏季風（EASM）氣候季節(jié)內振蕩（CISO）前2個主成分有關的季節(jié)內降水（陰影，mm/d，打點區(qū)表示通過90%信度檢驗）和850 hPa風場（矢量，m/s，通過90%信度檢驗的部分）的回歸場Fig. 2 Regressed intraseasonal anomalies of precipitation (shading, mm d-1; values exceeding 90% conf dence level are stippled)and 850 hPa winds (vectors, m s-1; vectors exceeding 90% conf dence level are plotted) associated with the first two MV-EOF modes of winds related to EASM CISO

圖3 不同年代影響南海夏季風爆發(fā)時間的春季海溫關鍵區(qū)(K)(a，b)及其季節(jié)變化(c，d)(a，c：1980—1993年；b，d：1994—2014年)Fig. 3 (a, b) Spring SSTA (K) and (c, d) its seasonal evolution (represented by the correlation coefficient between the regional mean SSTA and the SCSSM onset time) associated with the SCSSM onset time during (a, c) 1980–1993 and (b, d) 1994–2014,respectively

圖4 南海夏季風爆發(fā)進程示意：(a)第27～28候（上：360 K等熵位渦（陰影，PVU）和風場（矢量，m/s）；中：非絕熱加熱（陰影，K/d）、正位渦平流（等值線，10-5PVU/s）和垂直運動（矢量，加粗箭頭表示高空上升運動，10-2Pa/s）；下：OLR（W/m2））；(b)第28～29候（上：對流層上部非絕熱加熱（陰影，K/d）和氣溫（K）；中：非絕熱加熱（陰影，K/d）和季風經(jīng)圈環(huán)流（矢量，紫色箭頭表示跨赤道季風環(huán)流圈，m/s）；下：OLR（W/m2））Fig. 4 Schematic diagram of the SCSSM onset process. (a) Pentad 27–28 (upper panel: 360 K isentropical potential vorticity(shading, PVU) and winds (vectors, m s-1); middle panel: diabatic heating (shading, K day-1), positive advection of potential vorticity (contours, 10-5PVU s-1) and vertical motion (vectors, heavy ones represent the upper-level ascending, 10-2Pa s-1) over the SCS (averaged along 110°–120°E); low panel: OLR (W m-2)). (b) Pentad 28–29 (upper panel: diabatic heating (shading, K day-1) and air temperature (contour, K) in the upper troposphere (averaged bween 400–200 hPa); middle panel: diabatic heating(shading, K day-1) and meridional monsoon circulation (vectors, purple ones are for the cross-equatorial monsoon circulation, m s-1); low panel: OLR (W m-2))

圖5 2008—2013年暖季（5—9月）平均的區(qū)域降水事件強度（左）和頻次（右）的日峰值位相（當?shù)貢r間）分布（從上往下依次為臺站觀測(a～b)，CMPA-Hourly融合降水(c～d)，TRMM 3B42(e～f)和CMORPH(g～h)；灰色實線為長江和黃河的位置）Fig. 5 The diurnal phase (local solar time) of the 2008?2013 warm season (May?September) mean intensity (left column) and frequency (right column) of RRE from rain gauges (a-b), CMPA-Hourly (c-d), TRMM (e-f) and CMORPH (g-h) products. The locations of the Yellow and Yangtze rivers are marked by gray lines

Progress in Climate System and Climate Change Research

In 2016, the Institute of Climate System (Polar Meteorology) has achieved remarkable improvements in the fields of (1) theory and methodology of climate prediction, (2) development of climate system model, and (3)polar climate.

1 Theory and methodology of climate prediction

1.1 The inconsistency of interannual rainfall variability of boreal summer monsoon between GPCP and CMAP precipitation products and an improvement scheme

This work has revealed the discrepancy between GPCP and CMAP precipitation products in terms of the seasonal and interannual variations of boreal summer monsoon rainfall, and the arithmetic mean of the two products is shown to reduce such uncertainties. GPCP and CMAP precipitation products with global coverage and long record length have been widely used to monitor the climate status and study the climate variability.Our recent work has pointed out that although the GPCP and CMAP products are able to describe the seasonal cycle of rainfall in the boreal summer monsoon region, the absolute error between them is still evident. In particular, the difference in rainfall amount is largest from May to October in the western North Pacific(WNP) monsoon region, presenting more summer and autumn rainfall in the GPCP product. As to the spatial distribution of the climatological mean summer precipitation, the most prominent difference between the two datasets appears over the ocean portion in the WNP, North Africa (NAF) and Indian (IND) monsoon region. It is likely due to the controversial use of atoll gauges over the tropical Pacific in CMAP.

In the interannual timescale, the interannual variabilities of summer precipitation in GPCP and CMAP products are generally consistent over the boreal summer monsoon region, but the evident discrepancy appears over the IND and NAF monsoon regions. The consistency between CMAP and GPCP has increased in the recent decades, especially over the NAM and IND monsoon regions. Further analysis shows that the arithmetic mean of CMAP and GPCP products is an effective method to reduce the uncertainties. The improvement is mostly evident over the IND monsoon region during 1979–1997 and the NAF monsoon region during 1998–2014 (Fig.1). (Zhu Congwen, Liu Boqi)

1.2 The climatological intraseasonal oscillation in the East Asian summer monsoon and its possible mechanisms

The existence of climatological intraseasonal oscillation (CISO) in the East Asian summer monsoon(EASM) has been validated in this work. We have also revealed the CISO-related primary circulations and their coupling process, as well as its association with the EASM precipitation on the intraseasonal timescale.

The climatological intraseasonal oscillation (CISO) of the East Asian summer monsoon (EASM) is characterized by the vertical and meridional interactions of monsoon circulation, with a stepwise northward shift of front-related rain belt during boreal summer, particularly from May to August. To reveal the vertical structure and the internal modes of the EASM CISO, as well as their interaction with the surrounding SSTs, we conducted harmonic and multivariate empirical orthogonal function (MV-EOF) analyses on the climatological daily winds, rainfall, diabatic heating, and SST for the period 1981–2010. The EASM CISO exists not only in diabatic heating, primarily contributed by condensational heating, but also in rainfall and circulation. TheEASM CISO mainly results from the asymmetric response of the land-sea thermal contrast over East Asia to annual solar forcing. As a result, the center of the CISO variability is located over eastern China and the western North Pacific. During the EASM season, the Q1 is mainly associated with condensational heating and is centered over the WNP and East Asia. Thus, the horizontal gradient of Q1 produces a negative vorticity source to the north and east of the Q1 center. The resultant local background ascending motion favors the initiation of the CISO. Moreover, the EASM CISO can also be initiated and maintained by the different phasings of various diabatic heating (Q1) components. Namely, the CISO is initated by changes in surface sensible heat flux, which modulates the atmospheric convective instability, and thus the variations of monsoon convection and the condensational heating. The changes in condensational heating in turn would affect land surface temperature through radiative forcing. This eventually would lead to changes in surface sensible heating flux, and so on.

Results based on the MV-EOF analysis reveal that the first two modes of the EASM CISO are mainly characterized by the coupling of the Mongolian Cyclone (MC) around Lake Baikal at 850 hPa, the WNP subtropical High (WNPSH) at 500 hPa, and the South Asian High (SAH) over the Tibetan Plateau (TP) at 200 hPa. The first leading mode shows a simulteneous enhancement of the MC, WNPSH, and SAH, accompanied by a tripole rainfall anomaly of the strong Meiyu and Baiu fronts around the lower reaches of the Yangtze River and southern Japan, whereas the rainfall was suppressed over northeastern Asia and the WNP. The second leading mode, which indicates the eastward and northwestward propagation of the enhanced MC and WNPSH with the weakened SAH, is associated with a dipole of the rainfall anomaly, with abundant and deficient rainfall over the northeastern and southeastern Asia-Pacific regions, respectively. In this study, we also performed the AGCM simulations forced by daily SST from 1981 to 2010 using the ECHAM5.4 at the resolution of T63L31.The results show that the simulated first CISO leading mode is consistent with the observed, suggesting the critical role of seasonal variations of daily SST in the EASM CISO over the Asia-Pacific region. However, the AGCM runs failed to realistically reproduce the second CISO leading mode. The MC CISO is well reproduced by the AGCM, but the simulated CISO of the WNPSH and SAH is poor compared with the observations likely owing to the lack of feedback of SST to the atmospheric forcing (Fig. 2). (Zhu Congwen Liu Boqi)

1.3 The difference of interannual variation of South China Sea summer monsoon onset and itsrelated SSTs before and after 1993/1994

We have found the distinct differences of interannual variation of South China Sea summer monsoon(SCSSM) onset before and after 1993/1994. It is shown that the SSTA in the southern Indian Ocean in spring and the ENSO in previous winter are key factors in determining the SCSSM onset time before and after 1993/1994, respectively.

Based on their distinct thermodynamic field and circulation structures, we have identified different behaviors of the interannual variability of SCSSM onset in the periods 1980–1993 and 1994–2014. Our results suggest that the interannual variability of the SCSSM onset during 1980–1993 is characterized by rainfall anomalies over the northern SCS and evident anomalous zonal wind in the lower troposphere. This is defined as type-I SCSSM onset, presenting a closer association with subtropical systems. During 1994–2014, the interannual variability of the SCSSM onset is determined by the anomalies of tropical convection and the upper-tropospheric zonal wind, which is herein referred to as type-Ⅱ SCSSM onset. As a result, the vertical coupling between the upper- and lower-level circulations differs between type-I and type-Ⅱ SCSSM onsets on the interannual time scale.

The interannual variability of SSTAs in boreal spring is also distinct in the two types of SCSSM onsets.In the type-I SCSSM onset, the interannual variability of the SCSSM onset time is controlled by the springSSTAs in the South Indian Ocean (SIO, 40°–20°S, 40°–110°E) during 1980–1993, whereas it is affected by the winter ENSO and spring Tropical Indian Ocean (TIO) SSTAs in the type-II SCSSM onset during 1994–2014, on the interannual time scale. In the 1980–1993 period, the warm SIO SSTAs in spring can delay the SCSSM onset. In this process, the warm SIO SSTAs first produce a large-scale meridional dipole pattern, with anomalous convergence over the SIO and divergence over the SCS in the lower troposphere, bridging via the cross-equatorial flow near 90°E. Then, an anomalous anticyclone forms over the northern SCS, leading to the anomalous descent over the northern SCS and ascent over the land to its north. Subsequently, the vertical easterly shear and meridional land-sea thermal contrast weaken such that they suppress convection associated with the monsoon onset over the SCS and the SCSSM onset is postponed. During the 1994–2014 period, both winter ENSO events and spring TIO SSTAs are closely related to the interannual variability of the SCSSM onset time. A warm ENSO event during winter can induce the warm TIO SSTAs in the following spring via the atmospheric bridge. Subsequently, the meridional temperature gradient (MTG) in the mid- to uppertroposphere weakens over the SCS due to the tropical stationary Kelvin wave stimulated by the warm TIO SSTAs, and this delays the seasonal transition of the MTG from winter to summer. In addition, the anomalous large-scale westerly in the upper troposphere blows from the TIO to the SCS, with the anomalous ascending and descending branches over the TIO and SCS in the tropics, respectively. Consequently, the anomalous descent and weakened zonal easterly shear could suppress convection associated with the monsoon onset over the southern SCS, leading to the late onset of the SCSSM.

The present results also imply an abrupt change in the relationship between ENSO and the timing of the onset of the SCSSM before and after 1993/1994. In fact, the El Ni?o events of previous winters are followed by a late onset of the SCSSM in both periods, and the closer association between ENSO and the SCSSM onset time and the early SCSSM onset in the recent period after 1993/1994 may be attributed to the more frequent La Ni?a events, which may be related to the interdecadal adjustment of the ENSO after 1993/1994 (Fig. 3). (Liu Boqi)

1.4 The seasonal evolution of the South Asian High is a possible precursor for the onset of the South China Sea summer monsoon

The seasonal evolution of the South Asian High (SAH) has been suggested to be a precursor for the South China Sea (SCS) summer monsoon onset. We have also revealed the dynamical process that is responsible for the establishment of the cross-equatorial monsoon circulation over the SCS.

The averaged onset time of the SCS summer monsoon (SCSSM) is pentad 28 (16?20 May). Previous studies on the SCSSM onset have focused on the influences of the lower tropospheric circulation and the underlying thermal condition on the SCSSM onset. In the present study, we have used the NCEP/DOE reanalysis data to diagnose the thermal and dynamical processes during the SCSSM onset. It is found that the major incentive for the SCSSM onset is the eastward extension of the SAH to the upper troposphere of the SCS. During Pentad 27?28, the positive potential vorticity advection takes place over the SCS, along with the eastward development of the SAH. Accordingly, the upper-level ascending motion is enhanced, making the low-level western North Pacific subtropical anticyclone to withdraw eastward away from the SCS, followed by the initial formation of the monsoon trough and the onset of monsoon convection. Later, the monsoon convection over the SCS deepens gradually, resulting in the continuous eastward development of the SAH. The near-surface warm SST is overlaid by the upper-level warm center associated with the SAH. Such a thermal configuration implies a destruction of the vertical thermal stratification over the SCS, which satisfies the condition of absolute angular momentum conservation (AMC). As a consequence, the ascending motion over the SCS is able to cross the equator and reaches the Southern Hemisphere, and finally induces the closed cross-equatorial monsoon circulation that bridges the monsoon systems in the two hemispheres on Pentad 29 after the SCSSM builds up completely (Fig. 4). (Liu Boqi)

2 Development of climate system model

2.1 Comparing simulations and impact factors of precipitation characteristics over East Asia in two different atmospheric models

Current climate models have common biases in simulating precipitation over East Asia. They tend to underestimate the precipitation amount over southeastern China, overestimate the precipitation amount over northern China, and overestimate the precipitation frequency while underestimate the precipitation intensity,and often simulate the afternoon continental precipitation peak too early. These biases are not isolated problems but intimately correlated.

The study compared the CAM5 and Super-Parameterized CAM5 (SPCAM5) simulations of summer precipitation characteristics. SPCAM5 can explicitly simulate cloud and precipitation processes in a GCM’s grid column. Results show that CAM5 posesses those aforementioned biases. Especially, over southeastern China, the precipitation peak in CAM5 appears at around 14:00 local solar time, when the unstable condition reaches the maximum. However, in the observation, the precipitation peak occurs at around 16:00–17:00. In contrast, SPCAM5 alleviates the negative precipitation biases over South China, reduces the precipitation frequency while enhances the precipitation intensity. Moreover, over regions dominated by the afternoon precipitation peak, SPCAM5 delays the precipitation peak time by a few hours, closer to observtions.

To f gure out reasons for these differences in the model performances, we conducted a source/sink budget analysis by calculating the residuals of large-scale state variables. Results from the Q1-Qrad analysis indicate that SPCAM5 more evidently simulates the progressive evolution of convective heating, which exhibits a tilting structure in the pressure-time transect. The Q2 analysis indicates that SPCAM5 more evidently simulate the drying signal occurring before the intense precipitation starts. These reflect the fact that SPCAM5 better simulate the transition from shallow to deep convection.

Based on the aforementioned analysis, we can draw the following conclusions. In CAM5, when the unstable energy occurs in the afternoon, the model will quickly release the unstable energy through deep convection so as to stabilize the model atmosphere. In this case, precipitation appears quickly in the model and convection cannot be accumulated to certain strength. As a result, the model produces more frequent but less intense rain. In SPCAM5, the model will not instantly remove the unstable energy once the atmosphere becomes unstable. Instead, the model simulates a process in which convection progressively develops from shallow to deep convection. Shallow cumulus clouds inhibit precipitation and help the unstable energy to grow to a higher magnitude. At certain threshold, the precipitation occurs and reaches the maximum at a relatively late stage. Therefore, in SPCAM5, the peak is delayed and accompanied by more intense precipitation. The model atmosphere rains less frequently with higher intensity, and exhibits increased precipitation amount in terms of the climatology. Hence, it is the proper simulation of the transition from shallow to deep convection that helps SPCAM5 to improve the simulations of precipitation characteristics. Meanwhile, because convection becomes more intense near the peak time, a related effect is that the large-scale upward motion becomes stronger during the precipitation peak in SPCAM5. This reflects the impact of sub-grid scale convection on the large-scale dynamics. (Zhang Yi)

2.2 Regulating the precipitation errors along the steep slope of the Tibetan Plateau

A stubborn model bias over East Asia in GCMs, namely, the precipitation errors along the shouthern steepslope of the Tibetan Plateau, was investigated in this study. Results suggest that this problem is more closely related to the large-scale dynamics. Two contrasting model experiments were designed in this study. The first experiment is a typical AMIP style climate integration. In the second experiment, a moisture divergence term is artificially added to the model’s moisture transport equation. With this term added, moisture over the convergent area will be continuously accumulated, while moisture over the divergent area will be gradually removed.

Results show that this additional term well reduces the originally overestimated precipitation amount at the higher part of the steep slope, while enhances the originally underestimated amount at the lower part of the steep slope. Moreover, the entire precipitation frequency-intensity structure was also modulated. The changes in the total precipitation mainly result from the changes in the grid-scale precipitation and moisture contents.This reflects that the large-scale dynamics modulates the local moisture content and regulates the precipitation simulation. By checking the adiabatic moisture tendency, we confirm that this additional term inhibits the original positive moisture advection tendency at the high part, while enhances the tendency at the low part, in accordance with the differences in the simulated precipitaiton climates.

To better understand the impact of this term on the model performance, we conducted numerical weather prediction type experiments. Results show that this term quickly exerts an influence on the precipitation simulation, namely, it is a fast adjustment process. Moisture contents over the higher part are largely removed,while those over the lower part are significantly enlarged. The change of the moisture content in fluences the precipitation characteristics over the steep slope region. This study offers useful information for suppressing the simulation errors around the Tibetan Plateau. It also reveals the impact of large-scale dynamics on the precipitation simulations. (Zhang Yi)

2.3 Development of an icosahedral atmospheric model: Solvers of transport and shallow water equations

To accommodate the requirement of developing global high-resolution atmospheric models in the future,we are conducting research to solve the atmospheric model equations based on a quasi-uniform mesh obtained from a subdivided icosahedron. The spherical mesh is formed by recursively bisecting each triangular edge of a regular icosahedron. The mesh is further divided into the Voronoi-Delaunay diagram such that an unstructured mesh is achieved. This differs from the structured icosahedral mesh in that there is no uniform topological rule for the grid. The solver utilizes the Arakawa-C staggering grid system, namely, the mass is defined at the cell center and the normal velocity is located at the cell edge. When the Rossby deformational radius can be well resolved, the C-grid system is very suitable for simulating the resultant high-resolution atmospheric motions because of its advantage in depicting the divergent mode. Meanwhile, unlike the Z-grid, a C-grid model does not need to invert an elliptic equation, thus avoiding the massive global communication and favoring the massive parallel computation.

The transport and shallow water equations constitute the horizontal part of a global 3D model. We first built a conservative Two-step Shape-Preserving Advection Scheme (TSPAS). This can be viewed as a finitevolume and unstructured extension of the scheme in Yu (1994). By designing a properly defined pre-integration parameter, this method selects between the Lax-Wendroff and the upwind scheme to maintain a positive definite transport with the minimum loss ofinumerical accuracy. We also constructed a famous and widely used positive definite transport scheme called “Flux Corrected Transport (FCT)”, and compared TSPAS with FCT using various transport experiments. Results show that both schemes fulf ll the demand of a positive definite transport. TSPAS retains more monotone solutions and FCT maintains larger maxima of the transported signal.The work also pointed out difference in the designs behind the TSPAS and FCT.

Besides the transport equation, we further constructed a shallow water solver on the icosahedral C-grid.This needs to maintain some important underlying physical constraints. For an irregular C-grid, an important point is to approximate the tangent Coriolis term, which should not be an energy source or sink. We built a shallow water model based on a vector reconstruction algorithm that can rigidly maintain the energetic neutral Coriolis term, and a set of operators that conserve the total energy within the time truncation error. By testing different time integration schemes, we conf rmed that the solver perfectly fulf ll our requirement. This will be the basis of our further development. (Zhang Yi)

2.4 A new method proposed to objectively evaluate model precipitation and its evaluation using satellite retrieved precipitation

The current advanced global atmospheric model cannot explicitly resolve the evolution of cloud and precipitation at cloud scale. This is because the model simulated precipitation only represents the mean precipitation in the model grid box. Due to the great inhomogeneity of precipitation in nature, it is difficult to objectively compare the grid-box-mean precipitation in the model with rain gauge observed (in a f xed location)or satellite retrieved (in a much high spatial resolution) precipitation. In this study, a new defined method called regional rainfall event (RRE) is proposed to objectively evaluate the model simulated rainfall. To verify the effectiveness of the method, this study compares the hourly characteristics of warm season (May–September)rainfall among rain gauge observations, China Merged Hourly Precipitation Analysis (CMPA-Hourly), and two commonly used satellite products. Results show that the method largely eliminates the differences of rainfall characteristics among different observation measurements over central eastern China.The spatial distribution and diurnal variation of the RRE frequency and intensity are quite consistent among different datasets. All of the datasets present smaller (larger) regional rainfall coefficient (RRC) in south (north) of the Yangtze River.The values of warm season mean frequency and intensity of RRE are also similar compared with these datasets by using RRE, although the TRMM products overestimate the RRE intensity. The diurnal variations of RRE intensity and frequency are also comparable among different datasets, and the overestimation of afternoon peak is not obvious in both TRMM and CMORPH datasets while compared using RRE method. It is found that the spatial spread of rainfall, revealed by RRC, is more uniform during the nocturnal to morning hours over central eastern China. Over most regions in the central eastern China, the RRC reaches the diurnal maximum several hours after the RRE intensity peaks, implying an intermediate transition from convective to stratiform rainfall. In the afternoon, the RRC reaches the minimum, implying that local convections dominate in those hours, which could cause large differences between rain gauge and satellite observations. The RRC reaches the diurnal maximum more frequently in the afternoon in TRMM and CMORPH than in gauge and CMPA-Hourly data, implying the more organized afternoon rainfall systems in satellite products. Despite the similarity among different datasets, the CMPA-Hourly data show great advantages in reproducing gauge-observed mean state and diurnal variations of RREs compared with the TRMM and CMORPH datasets over the central eastern China. This indicates that the CMPA-Hourly dataset is more reliable for analyzing the sub-daily variations while considering the setailed rainfall characteristics as a regional event. Since RRE reflects the whole features of rainfall in a limited region rather in a f xed point or a single grid box, the widely-recognized overestimation of afternoon rainfall in satellite products is not obvious, and the satellite estimates are more reliable in representing sub-daily variation of rainfall in the RRE perspective. This study provides a reasonable method to compare satellite products with rain gauge observations in sub-daily scale, which also have great potential and will be further used to evaluate the spatio-temporal variation of cloud and rainfall in numerical models (Fig. 5).(Chen Haoming)

3 Polar climate research

3.1 Vertical structure of the summer atmospheric boundary layer in the central Arctic Ocean and its relationship with sea ice extent change

The atmospheric vertical structure and changed characteristics of boundary layer parameters, as well as their relationships with sea ice and temperature changes in the central Arctic Ocean (80°–88°N) are presented by adopting the GPS sounding data obtained from the 4–6th Arctic expeditions of China and NCEP(National Centre for Environmental Prediction) reanalysis data. Obvious differences are observed in terms of the tropopause height, the boundary layer height, the temperature inversion, and vertical structure of wind speed and direction in the central Arctic Ocean in summers of 2010, 2012, and 2014. These differences can be explained by the relationships between temperature and changes in sea ice extent in September from 1979 to 2014. In September 2012, the Arctic sea ice extent decreased by 44% with obvious warming. In September 2010 and 2014, it decreased by 22.6% and 17% with an obvious cooling, respectively. A comparison of the two processes shows that sea ice change has a significant in fluence on the vertical structure of the atmospheric boundary layer. In the recent 30 years, the temperatures at 1000 and 850 hPa in the central Arctic Ocean have displayed an obvious warming trend and are negatively correlated with the sea ice extent. These changes indicate that the continuous reduction of Arctic sea ice will continue the warming of the mid-lower troposphere.(Ding Minghu)

3.2 Re-assessment of recent (2008–2013) surface mass balance over Dome Argus, Antarctica

At Dome Argus, East Antarctica, the surface mass balance (SMB) from 2008 to 2013 was evaluated using 49 stakes installed across a 30 km by 30 km area. Spatial analysis showed that at least 12 and 20 stakes are needed to obtain reliable estimates of SMB at local scales (a few hundred square meters) and regional scales (tens of square kilometers), respectively. The estimated annual mean SMB was (22.9±5.9) kg m-2yr-1,including a net loss by sublimation of (2.22±0.02) kg m-2yr-1and a mass gain by deposition of (1.37±0.01) kg m-2yr-1. Therefore, about 14.3% of precipitation was modif ed after deposition, which should be considered when interpreting snow or ice core records produced by future drilling projects. The surface snow density and SMB in the western portion of Dome Argus are higher than in other areas. These differences are likely related to the katabatic wind, which is strengthened by topography in this sector. A new digital elevation model (DEM)of Dome Argus was generated, conf rming that both peaks of the dome can be considered as the summit of the East Antarctic Ice Sheet. The findings from this study should be valuable for validating SMB estimates obtained from regional climate models and DEMs established using remote-sensing data. (Ding Minghu)

3.3 A Comparison of Antarctic ice sheet surface mass balance from atmospheric climate models and in situ observations

In this study, 3265 multi-year averaged in situ observations and 29 observational records at annual time scale are used to examine the performance of recent reanalysis and regional atmospheric climate model products (ERA-Interim, JRA-55, MERRA, the Polar version of MM5 (PMM5), RACMO2.1, and RACMO2.3)for their spatial and interannual variability of Antarctic surface mass balance (SMB). The simulated precipitation seasonality is also evaluated using three in situ observations and model intercomparison. All products qualitatively capture the macroscale spatial variability of the observed SMB, but it is not possible to rank their relative performance because of the sparse observations at coastal regions with an elevation range from 200 to 1000 m. In terms of the absolute amount of the observed snow accumulation in the interior

Antarctica, RACMO2.3 f ts best, while the other models either underestimate (JRA-55, MERRA, ERA-Interim,and RACMO2.1) or overestimate (PMM5) the snow accumulation. Despite underestimated precipitation by the three reanalyses and RACMO2.1, this feature is clearly improved in JRA-55. However, because of changes in the observing system, especially the dramatically increased satellite observations for data assimilation, JRA-55 presents a marked jump in snow accumulation around 1979 and a large increase after the late 1990s. Although precipitation seasonality over the whole ice sheet is common for all products, ERA-Interim provides an unrealistic estimate of precipitation seasonality over the East Antarctic plateau, with high precipitation strongly peaking in summer. ERA-Interim shows a significant correlation with interannual variability of the observed snow accumulation measurements at 28 of 29 locations, whereas fewer than 20 site observations significantly correlate with simulations by the other models. This suggests that ERA-Interim exhibits the highest quality in capturing interannual variability of the observed precipitation. (Ding Minghu)