Ding Yihui

Monsoons over China

One: The Summer Monsoon in East Asia. Two: The Winter Monsoon in East Asia. Three: The Short-Range Fluctuations of Monsoons and Their Association with the Major Weather Events in China. Four: The Medium and Long-Range Fluctuations of Monsoons and Their Association with Floods and Droughts over China. Five: Effects of the Tibetan Plateau (Qinghai-Xizang) on the Monsoon. Six: Heat, Moisture and Energy Budgets over the Monsoon Regions of China and Some Aspects of Monsoon Dynamics.

Effects of the Qinghai-Xizang (Tibetan) plateau on the circulation features over the plateau and its surrounding areas

Areview of the effects of theTibetan Platean on circulation features over the plateau and its surrounding areas has been made, with a special emphasis upon the monsoon circulations in South Asin and East Asia. This includes estimates of heat sources, dynamic and thermal effects of the plateau, adn effects of the plateau on summer and winter monsoons. Major progresses made in this aspect by Chinese meteorologists have been specifically described and are compared with the achievements made by the meteorologists of other countries.

The summer monsoon onset over the tropical eastern Indian Ocean: The earliest onset process of the Asian summer monsoon

The onset process of the tropical eastern Indian Ocean (TEIO) summer monsoon (TEIOSM) and its relationship with the cross-equatorial flows are investigated via climatological analysis. Climatologically, results indicate that the earliest onset process of the Asian summer monsoon occurs over the TEIO at pentad 22 (April 15–20). Unlike the abrupt onset of the South China Sea (SCS) summer monsoon, the TEIOSM onset process displays a stepwise advance. Moreover, a close relationship between the TEIOSM development and the northward push of the cross-equatorial flows over 80°–90°E is revealed. A difference vorticity center, together with the counterpart over the southern Indian Ocean, constitutes a pair of difference cyclonic vortices, which strengthens the southwesterly wind over the TEIO and the northerly wind to the west of the Indian Peninsula from the end of March to late May. Therefore, the occurrence of the southwesterly wind over the TEIO is earlier than its counterpart over the tropical western Indian Ocean, and the cross-equatorial flows emerge firstly over the TEIO rather than over the Somali area. The former increases in intensity during its northward propagation, which provides a precondition for the TEIOSM onset and its northward advance.

The variation of the heat sources in East China in the early summer of 1984 and their effects on the large-scale circulation in East Asia

The distributions and daily variations of the apparent heat source (Q1 )and the apparent moisture sink (Q2) in East China in the early summer of 1984 have been estimated with the budget calculation method. It has been found that during this time period, there occurred three significant episodes of strong heating that corresponded to the three events of heavy rainfalls prior to, during and post to the onset of meiyu (plum rains). The peaks of Q1 were generally found at 200 hPa, with the heating rate of 6‡-10‡C/day observed, while the peaks of Q2 were located at about 700 hPa, with their magnitudes being 12‡-20‡C/day. The vertical distribution of Q1 and Q2 indicates the importance of eddy vertical flux. In other words, the convective activity plays a very important role in the processes of precipitation in East Asia in the early summer. This result is different from the finding obtained by Luo and Yanai (1984) in their calculation of the case of 1979. They pointed out that in the early summer of 1979 the continuous precipitation dominated the region of East China.Among the three terms of Q1 and Q2, the maximum contribution was made from the adiabatic term, which was caused by strong ascending motion. The adiabatic cooling produced by this term may compensate for the heating created by the condensation process.In addition, it has been revealed that the three significant heating processes were closely related to the seasonal transition from spring to summer in East China. One major synoptic event associated with it showed up in the sudden jump of the upper tropospheric, subtropical jet-stream from 30‡N to 40‡N. So did the planetary frontal zone in East China.

Study of the structure of a monsoon depression over the Bay of Bengal during Summer MONEX

Based on the data gathered during the Summer MONEX over the Bay of Bengal in July, 1979, a detailed observational study of the structure of a monsoon depression during the period of 3–8, July has been made. It has been revealed that the early disturbance of this depression was a mid-tropospheric cyclone. The subsequent rapid development was due mainly to the barotropic instability process of the basic zonal and meridional airflows.The cyclonic circulation of the depression extended in vertical upward to 500–400 mb level. Prior to the formation of the depression, the extremely strong westerly and northerly winds at the lower and middle levels, reaching the intensity of the low-level jet (22 m/s and 18.5 m/s, respectively) were observed. Post to the formation of the depression, a strong wind ring at the radius of 300–350 km from the depression center encircled the depression, with the wind maximum being at 850 mb. During this period, the maximum of the positive vorticity was of the magnitude of order of 10−4/s. The warm core at 400–300 mb was very remarkable. Finally, intrusion of the dry air over the depression may be an important factor leading to the weakening of the depression.

Transport of water vapor over North China during the drought period in summer of 1980

The features of water vapor transport, such as temporal evolution, vertical structure and spatial pattern, over North China during the prolonged drought in the summer of 1980 have been analyzed through computation of water vapor flux based on the once daily ECMWF grid-point data at 12GMT. The results indicate that it is unlikely that the characteristics of the atmospheric mass divergence, ascending motion and stratification stability in North China were completely opposite to those in the Yangtze River Valley, where the heavy flood occurred in the summer of 1980. It is major differences that the strong ascending motion and significant water vapor convergence overlap fairly well each other in the vertical levels in the Yangtze River Valley, while the maximum ascending motion is accompanied by water vapor divergence or weak water vapor convergence in North China. This vertical structure in North China results in insufficient water vapor supply and, therefore, little condensation and precipitation in the middle and upper atmosphere were produced. Additionally, a mode of monthly-scale low frequency oscillation can be found in water vapor flux, which is in correspondence to the fluctuation period of rainfall.

Climatic study on the summer tropical easterly jet at 200 hPa

The low latitude easterlies at 200 hPa in summer (May–October) is analysed climatically during the 13-year period from 1968 to 1980, with a special emphasis on the relationships between the anomalous tropical easterly jet Stream over South Asia and the low latitude atmospheric circulation, and also the summer monsoon precipitation in India. The compositing analysis shows that the tropical easterly jet stream over South Asia has five anomalous patterns at 200 hPa i.e. the western pattern, middle pattern, eastern pattern, two-branch pattern and multi-core pattern. Evidence has shown that the precipitaion in India anomalously increased during the anomalous period of the western pattern and the middle pattern, but reverse case is true in the eastern pattern. Some different anomalies of the precipitation in different area of India were found during the other two anomalous pattern.

A modeling study of climatic change and its implication for agriculture in China part I: Climatic change in China

The trends and features of China’s climatic change in the past and future are analysed by applying station observations and GCM simulation results. Nationally, the country has warmed by 0.3°C in annual mean air temperature and decreased by 5% in annual precipitation over 1951–1990. Regionally, temperature change has varied from a cooling of 0.3°C in Southwest China to a warming of 1.0°C in Northeast China. With the exception of South China, all regions of China have shown a declination in precipitation. Climatic change has the features of increasing remarkably in winter temperature and decreasing obviously in summer precipitation. Under doubled CO2 concentration, climatic change in China will tend to be warmer and moister, with increases of 4.5°C in annual mean air temperature and 11 % in annual precipitation on the national scale. Future climatic change will reduce the temporal and spatial differences of climatic factors.

Medium-range oscillations in the summer tropical easterlies at 200 hPa

By the use of space-time spectral analysis and band-pass filter, some of the features of the medium-range oscillations in the summer tropical easterlies (10°S –20°N) at 200 hPa are investigated based on a two-year (1980 and 1982) wind (u, v) data set for the period from may to September. Space-time power spectral analysis shows that the total energy of the westward moving waves was the largest and that of the standing waves and eastward moving waves was relatively small in the 200 hPa easterlies; the total energy of the eastward moving waves was at minimum at 10° N. Three kinds of the medium-range oscillations with about 50 day, 25 day and quasi-biweekly periods were found in the easterlies, which all show a remarkable interannual variations and latitudinal differences in these two years. The wave energy of zonal wind is mainly associated with the planetary waves (1–3), which all may make important contributions to the 50 day and 25 day oscillations in different years or different latitudes. The quasi-biweekly oscillation is mainly related to the synoptic waves (4–6). In equatorial region, the 50 day oscillation was dominant with a eastward phase propagation in 1982 while the dominant oscillation in 1980 was of 25 day period with a westward phase propagations in 1980. Both of them are of the mode of zonal wavenumber 1. Strong westward 50 day oscillation was found in 10° N–20° N in these two years. Regular propagations of the meridional wind 50 day oscillation were also found in the easterlies.The 50 day and 25 day oscillation of zonal wind all demonstrate southward phase propagation over the region of the South Asia monsoon and northward phase propagation near international date line, where are the climatic mean position of the tropical upper-tropospheric easterly jet and the tropical upper tropospheric trough (TUTT), respectively.

Temperature changes over Eurasia during the late summer of 1979

During the late summer of 1979, massive changes occurred in the distribution of temperature over Eur asia north of 15°N. At 300 hPa, zonal mean temperature averaged over Eurasia along 20°–25°N decreased sharply around 23 August. An abrupt decrease in 300 hPa zonal mean temperature also occurred over extensive mid-latitude zones (40°–55°N) around 18 August, i. e., about 5 days prior to the monsoon withdrawal over South Asia.The intensity and location of N−S oriented, vertical overturning underwent significant changes over Eurasia during the transition from summer to fall. Near 20°–25°N, zonal mean updrafts weakened considerably during the transition period (18–27 August.) Around 45°N, zonal mean downdrafts and the associated cooling (radiative) rate increased considerably during the transition period.Near 15°N, 300 hPa zonal mean temperature fluctuated nearly periodically with an approximate 40-day period. These fluctuations appear to be associated with a small imbalance between 40-day filtered adiabatic cooling (heating) and diabatic heating (cooling).