On the Origin of the Depressions in Southern China C. S. YAO Institute of Meteorology, Chungking, China N THE PAST, due to the lack of the As further stated by Fochler-Hauke I inland meteorological records some the depressions appear to form some- meteorologists, such as Froc(1) times in eastern Kweichow and even and Sung (2), in discussing the cy- in further west toward Yunnan. clones of China only emphasized their C. W. Tu (6) made an effort to divide characteristics and neglected to dis- the Yangtze depressions into two cuss their origins. Although some classes. The birth place of the first meteorologists in the Far East occa- class is the region between Szechuan sionally give their ideas or assump- and Sikong while the other class tions concerning the origin of the so- comes from Annam or from the per- called Yangtze depressions they have manent low of the Indian Ocean. no strong evidence to demonstrate Furthermore, Tu gave a detailed their points. treatment of origin of the depressions P. Chevalier (3) was first to state south of the Yangtze, using the theory that a certain number of the storms of Exner. which visit the Lower Yangtze is As the number of meteorological formed in Yunnan, Szechuan and stations in Yunnan, Sikong and even Tibet. Tibet has been increased of late, we S. W. Sung(2), by tracing the tracks now know that the earlier ideas of of the extratropical cyclones of China the meteorologists in the Far East from 1921 to 1930, has arrived at the are inconsistent with the facts as conclusion that most of the cyclones found later. From the analysis of the of the Yangtze type develop in the recent weather maps we are con- region between latitude 20 °N and the vinced that the depressions appearing Yangtze and occasionally to the south in Southern China are mostly formed of 20°N. over the Lake-Basin south of the About the origin of the Yangtze de- Middle Yangtze (and hence known as pressions, Fr. E. Gherzi(4) expresses "Lake-Basin cyclones"), and a small his opinion that their formation south number of them is to be attributed to of the Yangtze is mostly due to the the western disturbances from India.> northward pushing of the equatorial known as the "SW depressions." air from Tonkin. The Lake-Basin cyclones have been G. Fochler-Hauke (5), by a line of fully discussed in our previous, argument similar to those of Cheva- papers (7), the main conclusion being lier, Froc and Gherzi, has shown that that they are developed in two man- in the winter the depressions in South ners:—the first is due to the tongue- China seem to be formed in the prov- shaped deformation of the cold front inces of Kwangsi, Hunan and Kiangsi, over the Lake-Basin when the cold mostly after the passage of a secon- air masses outbreak from Siberia; the dary high pressure through the second is ascribed to the influence of Yangtze Valley; in other words, after an old Siberian cyclone propagating a "high" stagnating over the Yangtze along the Arctic front. Valley has shifted eastward and the However, we also note that SW following moist, warm southern winds China in general is not a region of of the Gulf of Tonkin invade the in- cyclogenesis. In SW China the cold land of China, the depressions are front is more or less retarded and no likely to form south of the Yangtze. cyclonic storm can be developed from

Unauthenticated | Downloaded 10/01/21 04:19 PM UTC the free wave in this region. There- case of a stationary front the winds in fore we ask whether there may be the prefrontal region often make a some special manner of orographical much larger angle with the isobars cyclogenesis in SW China. Yunnan, and thus often blow with much larger together with the adjoining regions, forces than the winds behind it (i. e. is a table land, a high and relatively in the cold air), which diminish grad- flat region. Invading cold air is at ually. This northward or northeast- first so shallow that the cold front is ward frontal deformation must be dynamically unimportant, though fin- originally due to the pushing of warm ally considerable cold air often ac- air masses. It is now recognized that cumulates over the region, but in any since there is no supply of kinetic case it does not give rise to cyclogene- energy derived from the mere push- sis in Yunnan. Next we consider the ing of the cold air masses (shearing), province of Szechuan, the famous the wave resulting therefrom is stable "Red-Basin." Here, since the long and and will not become a full cyclone, lofty Tsingling ranges extending to even though it may propagate along the north, cut off the Pc air mass in the front. However, it myist be noted lower layers of the atmosphere, the that in SW China the stable waves temperature, pressure and wind di- formed by the pushing of warm-air rection all show no sharp changes masses are mostly motionless. during the passage of a cold front. When a high pressure that has been The air currents, at least within the stagnating over Central China finally lower layers, move very slowly due to shifts eastwards a barometric low at the combined effect the Tsingling and once makes its appearance over SW the large friction of the rough sur- China as a result of the prevailing face farther south, in Szechuan it- warm air masses. One might assume self, where these currents undergo a that this is a nascent cyclone devel- much further modification in the lower oped over SW or W China, as sug- layers due to prolonged contact with gested by some meteorologists in the a warm surface. As a result the front Far East. However, fresh Pc air over Szechuan is always diffuse. Why often breaks out from the north just then do we explain that the Yangtze after the preceding Chinese extension depressions as originating in Szech- of the Siberian high pressure has uan? At last, let us examine the shifted eastwards and amalgamated mountainous region of Kweichow. with the subtropical high-pressure Here again the cold air is everywhere cell. When the new cold front ap- retarded in the lower layers by the proaches Central China a wave-distur- mountains, so that the cold front is bance often forms over the Lake- often quasi-stationary. No orographi- Basin south of the Yangtze Valley, cal cyclogenesis other than the insig- according to recent studies by +he nificant deformations takes place, as author. Thus, from a first glance at over the Lake-Basin south of the the weather charts, it might appear Middle Yangtze. that a cyclone assumed to have de- When the cold front becomes sta- veloped somewhere else over SW China tionary in a position from SW China had already arrived over the Lake- northeastwards to the Lower Yangtze Basin, whereas it actually formed in any cyclonic wave is most likely the Basin itself. (2) Considering that formed in SW China by the north- SW China is situated within a rela- ward or northeastward acceleration of tively cyclone-free belt of the globe a segment of the front. For in the and is sheltered from the western in-

Unauthenticated | Downloaded 10/01/21 04:19 PM UTC fluences by the Tibetian plateau, de- intensified again over the Pacific. pressions coming from the SW of From their positions on the morning China are naturally very rare. Es- weather maps their tracks are drawn pecially owing to the great height of in Fig. 1 and their distributions in the the plateau of Tibet, it appears that separate months for the period of the European depressions can visit 1929-1938 are shown in the following China only by an indirect course by table:— way of India in the guise of Indian "western disturbances." Thus Nor- Jan. Feb. Mar. Apr. Total mand(8) points out that the Indian 1929 0 0 0 0 0 1930 0 0 2 1 3 "western disturbances" after passing 1931 0 0 0 0 0 1932 1 1 0 0 2 eastwards across Persia and Northern 1933 0 0 0 0 0 India often move into China. In view 1934 0 0 0 0 0 1935 0 1 1 0 2 of the not so high mountains of Burma 1936 1 1 0 0 2 1397 0 0 0 0 <> and southern Yunnan, it is indeed 1398 0 0 0 0 0 quite reasonable to expect that In- Total 2 3 3 9 dian "western disturbances" can ap- X pear in the Lower Yangtze. By ana- In January, mostly due to blocking lyzing the weather maps of China and by the cold air masses over China, those in India Weather Review from there were only two occasions when 1929 to 1938 we find that the notion the "western disturbances" crossed is quite true, though these cases are China to intensify again over the comparatively rare. Pacific. In February and March, due In this ten-year period the author to decreased anticyclonic activity in found only nine Indian "western dis- China, the disturbances in the form of turbances" which crossed China and small vortices can more easily ap-

FIGURE 1.

Unauthenticated | Downloaded 10/01/21 04:19 PM UTC 354 BULLETIN AMERICAN METEOROLOGICAL SOCIETY [Vol. 21 proach China by way of South Yun- cyclone of the Arabian Sea can travel nan, widespread rain or snowfalls in over the Indian Peninsula or the cy- Central China and thunderstorms in clone of the Bay of Bengal cross South China often being associated Lower Burma. The ability of the with them. After April, the "western tropical cyclones sometimes to travel disturbances" take more northern long distances over the land may be tracks and are mostly destroyed in attributed to the help of the favor- the western Himalayas or Kashmir, able prevailing winds. wherefore very few of them can enter When the cyclones of the Bay of China. Bengal advance upon upper Burma The "western disturbances" are they have penetrated into the sub- ordinarily less intense than the de- tropical region where the westerlies pressions of higher latitudes and the prevail. Therefore, by analogy it precipitation accompanying them is seems that the tropical cyclones of the small in amount. Frequently they die Bay of Bengal might travel through in Northern India. But rather strong China by virtue of the westerly pre- disturbances can continue into Burma vailing winds. Within the ten-year where they generally fill up, merely period studied by us, one such case accentuating the pressure trough in has indeed occurred, in October, SW China with some rainy weather 1935 (9). In this case, due to warm in Yunnan. Especially in the mid- air invasion in the upper levels and winter few of these disturbances will the drift of a high pressure area cross China as well defined cyclones, eastwards, unusually rainy weather but rather in the form of low pressure prevailed in SW China before the troughs, notwithstanding that they Bengal cyclone arrived in China. The can give rise to the widespread snow- main source of energy to support this falls south of the Yellow River. cyclone in travelling over SW China It must be remembered that an In- was the moist-lability liberated from dian "western disturbance" after en- the cold air mass in rising over the tering China is always either a flat mountains. Owing to the drawing in vortex in the dying stage or a trough of a Pc cold front and the lesser of low pressure; hence their move- friction over the water surface, this ment is generally controlled by the cyclone turned into a violent typhoon- general pressure distribution and their like storm over the Gulf of Pe-chi-li. velocity may be anything from a few Though October is ordinarily a dry km/hr to more than 90 km/hr. month in China many stations re- In considering the SW depressions corded tremendous rainfalls reaching of China another question may arise: to 50 mm in a day during the passage Can the tropical cyclones of the Bay of this cyclone. In SW China the cy- of Bengal appear in China as the clone from the Bay of Bengal is only Chinese SW depressions? The general a small flat vortex; therefore the ve- circulation or pressure distribution locity of its movement is rather large, plays an important role in managing to which also the prevailing westerlies the direction of movement and main- contribute considerable acceleration. tenance of the tropical cyclone. For The tropical cyclones ordinarily example, the cyclones of the Bay of have poor ability to travel over the Bengal and the typhoons of the China land, especially mountains, and most Sea can pass through the Indian Pen- tropical cyclones from the Bay of insula and Burma respectively; on the Bengal travelling NE-wards are de- contrary, there is no record that a stroyed in Burma, causing only some

Unauthenticated | Downloaded 10/01/21 04:19 PM UTC light rain in Yunnan. Also, consider- (5) G. Fochler-Hauke: Monsune, Depres- sionen und Taifune Siidchinas, Gerl. Beitr. z. ing the very long trajectory from the Geoph., v. 43, 1935. Bay of Bengal to Central China it (6) C. W. Tu: The formation and origin of cyclones in China, Met. Mag., (Nanking), would of course be rare for a full cy- 1936. clone to survive the passage even if (7) C. S. Yao: The stationary cold fronts of Central China and the wave disturbances mountains were lacking. developed over the Lake-Basin, Mem. of Met. Inst., Chung-king, 1939; Die stationaren LITERATURE Fronten und die Seebassin-Zyklonen in der chinesischen "Mai-U-Periode", Zeit. fiir angw. (1) L. Froc: L'atmosphere en Extreme- Met., Heft 7, 1939 ; Analysis of a Lake-Basin Orient. Paris, 1920. cyclone, Met. Mag., Chungking, 1939. (2) S. W. Sung: The extratropical cy- clones of Eastern China and their character- (8) C. W. B. Normand: The weather of istics, Mem. of Met. Inst.. Nanking, 1930. India, Proc. Indian Science Congress Associa- (3) Quoted from L. Froc's "L'atmosphere tion, Calcutta, 1937. en Extreme-Orient". (9) C. S. Yao: Regeneration einer Bengal- (4) P. E. Gherzi: fitude sur la pluie en enzyklone uber China, Met. Zeit., Heft 11, Chine. 1928, Zika-wei. 1939.

Thermometer Shelter for Trial Use in the Antarctic* ARNOLD COURT Jr. Meteorologist, U. S. Antarctic Service, West Base, Antarctica Smaller than any in general use by case while on the march and as obser- the Weather Bureau are the ther- vation box while in trail camp. mometer boxes designed for use by Measuring 5 by 5 by 15 inches, the trail parties of the United States An- box is made of ^-inch plywood and is tarctic Service, whose two bases are so arranged that the lid is held fixed now operating in the south polar while the rest of the box may be regions. folded back upon it. A regular Towns- Antarctic exploration presents many end thermometer support is mounted problems of its own, and not the least upon this "lid", or base, and the two of them is that of obtaining reliable thermometers are held together by a temperature measurements by travel- loop of wire for safety while the box ing parties. is being transported. When the party halts the base is mounted on a special upright so that the thermometers are in their normal horizontal position. Two hooks hold the box slightly ajar, the resulting cracks together with a few holes pro- tected by metal baffles being sufficient to provide circulation under the usual windy conditions of the Antarctic. Preliminary tests indicate sufficient accuracy in the records obtained from instruments in this "Antarctic type" On a trip of several hundred miles shelter to warrant preferring their by dogsled every ounce counts, and to use over the old method of carrying carry a shelter of the usual size would thermometers loose and merely hang- be impossible. Because of this limita- ing them up in a convenient place for tion, a special box has been designed observation. by Weather Bureau meteorologists at- tached to the expedition, and is in- **Published by permission of the Executive Committee of the U. S. Antarctic Service, tended to be used both as a carrying May 3, 1940.

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