大氣循環

局地環流

重新分配水熱

直接作用：輸送大氣環流中的熱能和水汽

間接作用：驅動大規模的洋流運動

搬運鬆散固體物質，改造地貌：侵蝕與堆積

物種傳播

緯度環流亦稱行星風系或氣壓帶風帶，地球上的風帶和喘流由三個對流環流（三圈環流）所推動：哈德里環流（低緯度）、費雷爾環流（中緯度）以及極地環流。有時候同一種環流（譬如低緯度）可以在同一緯度（如赤道）有數個同時存在，隨機地隨時間移動、互相合並與分裂。為了簡單起見，同一種環流通常當作一個環流處理。 ^[1] 

低緯度環流我們對低緯度環流運作的瞭解比較清楚。由喬治‧哈得萊（George Hadley 1685-1768）所記述的大氣環流模式，用以解釋貿易風（信風）的形成，與觀測到的非常符合。這是一個封閉的環流，由温暖潮濕空氣從赤道低壓地區上升開始，升至對流層頂，向極地方向邁進。直到南北緯30度左右，這些空氣在高壓地區下沉。部分空氣返回地面後於地面向赤道返回，形成信風，完成低緯度環流。

低緯度環流基本活動於熱帶地區，在太陽直射點引導下，以半年週期往返南北。

極地環流也同樣是一個簡單的系統。雖然相比赤道的空氣，這裏的空氣比較寒冷乾燥，但仍然有足夠熱力和水分進行對流，完成熱循環。本環流的活動範圍限於對流層內，最高也只到對流層頂（8公里）。往極地的氣流主要集中在空中，而赤道方向的氣流主要集中在地面。當空氣到達極地範圍，它的温度已經大大降低，在這高壓乾燥寒冷的地區下沉，受地轉偏向力影響向西偏轉，形成極地東風。極地環流的流出，形成呈簡諧波形的羅斯貝波。這些超長波在影響於中緯度環流與對流層頂間喘流的流向，扮演重要的角色。極地環流如散熱器般，平衡低緯度環流地區的熱盈餘，使整個地球熱量收支平衡。

可以説，在中高緯度地區，極地環流是影響這裏氣象的主要成因。雖然加拿大和歐洲在夏季會間中遇到暴風雨，在冬天從西伯利亞高壓區所帶來的寒冷才能感受到真正的嚴寒。實際上，就是因為極地高壓區的氣流，導致南極東方考察站在1983年錄得地球有紀錄以來最低氣温：攝氏零下89.2度。

低緯度環流與極地環流有着同一特點：兩者都是由於地表的温度而出現，直接與熱能相關。與此同時，其熱能特點蓋過其所產生的天氣現象。低緯度環流大量傳送的熱能，和極地環流巨形的吸熱能力，使除了特殊情況下，短暫氣象的效果不能被系統接收，也不能產生。在緯度30度至60度以外地區，根本不能感受到中緯度氣壓中心無休止地每天由低轉高再轉低的情況。

這兩個環流頗為穩定，雖然不時增強減弱，但是並不會完全消失。

中緯度環流由威廉‧費雷爾（William Ferrel 1817-1891）所提出的中緯度環流是一個次要的環流，依靠其餘兩個環流而出現。如一處於兩者之間的走珠軸承，因處於中緯度的渦旋（eddy）循環（高壓及低壓區）而出現。故本區時而又稱為“混合區”。在南面處於低緯度環流之上，在北面又漂浮在極地環流上。信風可以在低緯度環流以下找到，相同地西風帶也可以在中緯度環流下找到。 ^[2] 

與低緯度環流和極地環流不同，中緯度環流並不是真正閉合的循環，而重點卻在西風帶上。不像信風和極地東風那樣，有所屬的環流捍衞着它們在該區的主導地位。盛行西風沒有這樣幸運，常常聽命於經過的氣象系統。在上空通常由西風主導，但是在地表風向可以隨時突然改變。以北半球的參考系（觀點）而言，往北的低氣壓或是往南的高氣壓往往維持甚至加速西風的流速；但是經過當地的冷鋒可能扭轉這種情況。而往北的高氣壓帶來東風主導的氣流，常常持續數天。

氣團移動是中緯度環流底層特色之一。喘流吸收由地表低壓區上升的空氣，它所處的地方是影響氣團位置的原因之一（在天氣圖上可以見到地表低壓區是隨喘流移動的）。地表風整體的流向是從緯線30度至60度的。可是中緯度環流上空的流向尚未能完全界定，一方面因為環流本身處於極地環流與低緯度環流之間，沒有一個強烈的熱源或冷源推動對流，而另一方面地表渦漩也對上空環境造成不穩定影響。

Heavy rainfall, storm surges and strong winds brought about by tropical cyclones are an acute natural disaster which will result in severe damages of loss of lives and properties. Accurate forecasts and warnings on tropical cyclone activities are one of the major responsibilities for tropical cyclone forecasters and researchers. Tropical cyclone has been studied systematically in China in this half century including various topics of tropical cyclone motion and its forecasting techniques, formation and extratropical transition, structure and intensity change, tropical cyclone landfalling and abrupt change in this stage, sustaining and decaying over land, rain/wind distribution etc. The research achievements from Chinese Academy of Meteorological Sciences (CAMS) in this half century are reviewed for her 50 year anniversary. Other research works related to CAMS could be raised but it is not a comprehensive summary at all. The research methodology and technical strategy with CAMS are advanced in different era. Mathematical physical statistics, mechanism analysis with synoptic dynamical theory, numerical simulation, physical diagnostics and field experiment programs etc. are employed in the research programs and play an critical role to advance the tropical cyclone researches in the nation. Another characteristic of the research in CAMS is to integrate the theory with practice, to integrate the research with operational needs, to transfer the research achievements to operational forecast. One of the scientific objectives for CAMS is to raise the operational capacity of tropical cyclone forecasting. Several decades ago, scientists treated tropical cyclone as a particle without volume because people don't know much about the detail structure of it due to lack of data in the vicinity of storms. Motion forecast depends on the environmental steering derived from one layer barotropic model. Now motion forecast can be done by the high resolution global spectral model with data assimilation and bogussing tropical cyclone in CAMS. This technical variation cost 50 years or so. This also reflects the progress and advances of remotely sensed techniques and computer capacity in this time span. A serial field programs carried out by CAMS in the past 2 decades are helpful for the research programs on the sudden change in motion, intensity and rainfall of coastal typhoons and landfalling tropical cyclone studies. Relevant research achievements would be transferred to operational communities. Scientific cooperation and coordination especially the international cooperation are needed for the tropical cyclone research in CAMS. The technical cooperation between CAMS and operational centers are more important and beneficial for both sides. Some of the achievements are the common results of the cooperation, which should be explored and developed in the future research programs. ^[3]