Diagnostic Analysis Of An Extreme Low-Temperature Weather Event In North China On 4 And 5 January 2003

On 22 December 2002, a heavy snowfall occurred in a widespread area of the north China. Associated with this snowfall covered the land surface continuously, an extremely low temperature weather event colder than -10 ℃ was observed from 25 December 2002 to 10 January 2003 in the north China. From 4 to 5 January 2003, a large area with the low-temperature colder than -15℃ which covered 6 to 8 ten thousands km~2 was observed. On 5 January 2003, a low air temperature colder than -20℃ was observed in several regions. The Yellow River was frozen about 1300 kilometers in length, and the newly-frozen sections were mainly concentrated in the downstream regions of the Yellow River, such as Dongping Lake, Binzhou and Jinan of Shandong Province. This extreme low-temperature weather event had produced significant effects on the social life, and attracted the concerns of the news-media.In this paper, we utilized MICAPS (Meteorological Information Comprehensive Analysis and Process System) and AVN (Aviation Analysis) data to analyze the synoptic characteristics of this rare low-temperature weather event prior to and during its occurrence. It is shown that the surface was controlled by a strong Mongolian cold high pressure system whose maximum value was higher than 1050 hPa before its occurrence. At that time, the sky was clear, cloudless and calm. Only weak breeze was observed at night or dawn. At upper-level, the eastern Asia was occupied by one trough and ridge. At the level of 850-1000 hPa, most area of the continent of China was controlled by strong Mongolia cold high pressure, which carried cold air to move southeastward constantly. The north China was behind low and in front of high pressure. This region was obvious for its baroclinicity because the contour of temperature was almost in S-N direction, and the isotherm was in E-W direction. The temperature gradient was about 20℃ per 10-latitude.The time section chart of wind fields and T-lnP chart of Jinan station from 00UTC 4 to 00 UTC 5 January 2003 was analyzed. It is shown that above 925 hPa the winds were in north to northwest direction, and the temperature inversion layer existed under 500 hPa. The whole layers were stable energy areas, and the thickness of temperature inversion layer approached the thickest at 00 UTC 5 January 2003.The vertical-cross sections of vertical velocity and relative humidity across Lingxian station from 00 UTC 4 to 00 UTC 5 were also examined. It is indicated that in the upper-level the downdraft was dominated. The minimum of the vertical speed appeared near the surface. The vertical velocity at 00 UTC 5 was smaller than that at 00 UTC 4. The region where the air temperature was colder than -15 °C was identical to the area where vertical velocity approached to zero. At 00 UTC 5, the relative humidity at upper levels dropped more obviously than that at 00 UTC 4 January 2003The budget of surface air temperature change was diagnostically analyzed by using thermodynamic equation and radiation theory. It was shown that this extreme low-temperature weather event was caused by the joint effects of the radiation cooling, the cold advection of temperature and the cooling associated with the vertical transportation of temperature. Among them, the surface radiation cooling and the cold advection of temperature played a major role, and the role of cooling associated with the vertical transportation of temperature was relatively weak.