Letter Weather image Few people associate south-east Asia with cloudy, sometimes dull, weather, but these conditions are often present in parts of southern China and northern Vietnam during the winter season. Extensive persistent areas of cloud develop east of mountain ranges that stretch north-south, marking the eastern edge of the Himalayas. The depth and extent of these cloud layers waxes and wanes day by day, as atmospheric conditions change. Altocumulus and altostratus layers typically affect western areas, whilst lower cloud layers are usually observed further east. Figure 1 is a pair of satellite images illustrating the cloud layers. An overcast layer of altostratus, stretching several hundred kilometres, can be seen to the east of the Daxue Shan and Wuliang Shan ranges of the eastern Himalayas and a shallower broken or overcast layer of altostratus and altocumulus east of the Phou Sam Sao, which forms the border between Laos and Vietnam. Further layers of altocumulus and stratocumulus can be seen as far east as Taiwan and the southern islands of Japan. The cloud-top temperatures of each of the areas of cloud over south-west China and northern Vietnam indicate that the tops of these layers were at around 5500 m and 3500 m, respectively. This gives a clue to their method of formation. The winter season across southern and
much of south-eastern Asia is dominated by the north-east Monsoon, which brings largely dry weather that is particularly cold across China. However, the Monsoon flow is comparatively shallow, despite deepening as it reaches the Himalayan mountain barrier. Above the north-easterly trade winds, the air is warm and comparatively humid, carried by predominantly westerly winds. The temperature contrast between the air masses yields a very stable flow. Complications arise due to the ranges of mountains. Cooling of the high-level air from the west causes extensive condensation east of the ridges and altostratus clouds form with a base near 3500 m. The northeasterlies are blocked as they reach high
ground, so the cloud does not affect areas to the west: Laos, Thailand and Burma. The height of the mountains also affects the height and depth of the cloud. Thus, the top of the cloud layers is a little above the height of the peaks of the Daxue Shan and Wuliang Shan, which reach between 3000 and 5000 m, as well as the Phou Sam Sao, which reaches around 2500 m. The cloud depth is likely to be determined in part by the strength of the high-level westerlies. In this case, wind speeds were around 30 m s–1 across the crest of the Daxue Shan (below the sub-tropical jet stream) and near 15 m s–1 across the crest of the Phou Sam Sao. Wind profiles, temperature lapse rates and indicated cloud depths
Weather – February 2007, Vol. 62, No. 2
Cloudy south-east Asia
Figure 1. Visible (a) and infra-red (b) satellite images from GOMS showing stratiform cloud across southern China and northern Vietnam at 0600 UTC on 3 February 2006. (Courtesy University of Dundee Satellite Receiving Station)
Figure 2. Radiosonde profiles for 3 February 2006 through the layer-cloud formations shown in Figure 1: (a) Nanning, Guangxi, China (22.6°N, 108.6°E) at 0000 UTC; (b) Huaihua, Hunan, China (27.6°N, 109.9°E) at 0000 UTC (only standard-level data were available from this site); and (c) Changsha, Hunan, China (28.2°N, 113.0°E) at 1200 UTC. (Courtesy University of Wyoming, Department of Atmospheric Science)
Weather image Weather – February 2007, Vol. 62, No. 2 56
for each of these cloud decks are shown by the radiosonde profiles from Nanning (Guangxi), Changsha (Hunan) and Huaihua (Hunan), China in Figure 2. Forecasters need to be aware of the persistence and development of these cloud sheets during the winter Monsoon season. Flights at medium and low levels may encounter hazardous weather conditions within the cloud. Both layers form in the wakes of mountain ranges and moderate turbulence may be expected locally around and within the clouds, in particular to the lee of the Daxue Shan, which lies below the mean position of the sub-tropical jetstream in winter. Infra-red satellite imagery (Figure 1(b)) gives cloud-top temperatures between around –13 °C in the lee of the Daxue Shan and around –2 °C in the lee of the Phou Sam Sao, so airframe and engine icing are also likely to be significant threats. Indeed, this persistent cloud may have been one of the main reasons for the high rate of aircraft losses on cargo flights “over the hump” during World War II.
The cloud depths indicate a broad area of overcast, locally dull, weather in centralsouthern China with outbreaks of slight rain or snow. Across northern Vietnam and southern coastal China, as well as the East China Sea, there are predominantly cloudy and rather cold conditions, although cloud depths are less and there are breaks in the cloud, allowing some brightness. A few slight showers associated with ‘warm’ cumulus and stratocumulus may be generated close to the coast by instability in the boundary layer over the warmth of the East China Sea. At other times stratus forms, bringing dull weather. This weather is frequently observed during the winter monsoon season (Atkinson, 1971). An interesting series of observations in March 2005 from the Australian School, Hong Kong, made during the Metlink project, illustrates the weather associated with the cloud sheet in southern coastal China (http://metlink.org/ data/observe.php?). Jim Galvin and Malcolm Walker
References Atkinson GD. 1971. Forecasters’ guide to tropical meteorology. University Press of the Pacific, Honolulu (2002 reprint) Raschke E, Ohmura A, Rossow WB, Carlson BE, Zhang YC, Stubenrauch C, Kottek M, Wild M. 2005. Cloud effects on the radiation budget based on ISCCP data (1991 to 1995). Int. J. Climatol. 25: 1103–1125.
Correspondence to: Jim Galvin, Met Office, FitzRoy Road, Exeter, Devon EX1 3PB e-mail: [email protected]