Vol.22, NO.1, 2011
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Kong Qi, Zheng Yongguang, Chen Chunyan. Synoptic scale and mesoscale characteristics of 7·17 Urumqi heavy rainfall in 2007. J Appl Meteor Sci, 2011, 22(1): 12-22.
Citation: Kong Qi, Zheng Yongguang, Chen Chunyan. Synoptic scale and mesoscale characteristics of 7·17 Urumqi heavy rainfall in 2007. J Appl Meteor Sci, 2011, 22(1): 12-22.
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Synoptic Scale and Mesoscale Characteristics of 7·17 Urumqi Heavy Rainfall in 2007

  • 1.

    National Meteorological Center, Beijing 100081

  • 2.

    Xinjiang Meteorological Office, Urumqi 830002

  • Received Date: 2010-03-08
  • Rev Recd Date: 2010-07-22
  • Publish Date: 2011-02-28
    Abstract
  • Xinjiang is located in a semi-arid area, but a heavy rainfall event occurs unexpectedly along the Tianshan Mountains in Xinjiang during 13—18 July 2007. The circulation and the persistent mechanism of the low vortex as well as the mesoscale characteristics for the heavy rainfall in Urumqi during 16—17 July 2007 are analyzed and compared with one similar case in the same period of 1996, by use of the 1-hour and 1-minute precipitation data, geostationary satellite images, conventional surface and upper data, NCEP 1°×1° reanalysis data and the new generation Doppler radar data. The results are as follows: The heavy rainfall takes place under the favorable large-scale circumfluence. It is a large scale heavy rainfall which is related to the baroclinic disturbance. The central-Asian vortex is the major influential system, but the location, form and intensity are different from those of the cases in 1996. The atmospheric stratification turns unstable, but it is weaker comparing with the heavy rainfall in the eastern part of China. The characteristics of the stability are different during different rainfall periods, exhibiting the pattern of both stratiform and convective precipitation. The intrusion of the cold and dry air strengthens the instability of the atmosphere, and plays an important role for the heavy rainfall. The long maintenance of the vortex after the landing in Xinjiang is associated with the high PV anomaly (dry and cold air intrusion) in the upper-middle troposphere. Strong moisture convergence develops rapidly at lower level and the water mainly comes from the eastern part of the Tibetan Plateau, western part of Gansu Province and the north of the Southern Xinjiang Basin. The satellite and radar images show that there are obvious meso-γ-scale convective rainy cluster with characteristics of the echo pendency, vertical wind shear, but they are much weaker than those heavy rainfall systems in the east region of China. Radar radial base velocity products reveal that the mesoscale radial convergence may be the important trigger mechanism for the mesoscale convective rain cluster. Compared with the rainfall in eastern China, the rainfall in Xinjiang shows the characteristics of both synoptic scale and mesoscale. The water sources are less abundant and the convergence is more important. The instability of atmospheric stratification is not so strong without obvious low-level jet. The intrusion of the cold and dry air comes from the middle level of the troposphere. The mesoscale convective rainy-clusters have the characteristics of the mesoscale convective clouds, but the convection is not too intense as the cloud top black body temperature of the cloud clusters is not too low, and the vertical height of strong echo is not too high, either.
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  • Fig. 1  Rainfall distribution in Xinjiang for 15—18 July 2007 (a) stations of the rainfall exceeding over 10 mm from 08:00 16 July to 08:00 17 July (the shaded denotes the topographic height), (b) the evolution of the rainfall from 00:00 15 July to 00:00 18 July, (c) the evolution of the rainfall at Urumqi in July 2007

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    Fig. 2  Flow field of 500 hPa at 20:00 16 July 2007 and FY-2C infrared satellite image at 18:00 16 July 2007 (▲ denotes Urumqi)

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    Fig. 3  Meridional cross section along 87°E of the pseudo-potential temperature (solid lines, unit: K) and specific humidity (dashed lines, unit: 10-3 kg·kg-1) at 20:00 16 July 2007(the shaded denotes the topography) (a), time-height evolution of the horizontal wind field and the relative humidity (unit:%) over Urumqi (43.5°N, 87.4°E) during 14—17 July 2007 (b)

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    Fig. 4  Temperature advection (unit: 10-5 K·s-1) over Urumqi (43.5°N, 87.4°E) during 15—17 July 2007(a) and vertical cross section of the relative humidity (isolines, unit: %) and v-ω wind (streamlines) fields along 87.4°E at 20:00 16 July 2007 (the shaded denotes the topography) (b)

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    Fig. 5  FY-2C water vapor image at 18:00 16 July 2007 and streamlines, the isobar (dot lines, unit: hPa) and the potential vorticity (black solid lines, unit: PVU) at 330 K isentropic surface at 20:00 16 July 2007(▲ denotes Urumqi)

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    Fig. 6  Distribution of the horizontal moisture flux at 700 hPa (unit:g·cm-1·s-1·hPa-1) and divergence of moisture flux in the whole layer (gray area, unit: 10-2 g·cm-2·s-1) at 20:00 16 July 2007

    (the black area shows topography more than 3000 m)

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    Fig. 7  FY-2C TBB evolution and visible satellite image in July 2007

    (a) FY-2C TBB evolution during 16—17 July (unit: ℃; the black horizontal line denotes the position of Urumqi, the four black vertical lines denote the time that the precipitation per hour exceeding 5 mm) along 87.5°E, (b) enhanced visible satellite image at 18:00 16 July 2007 (▲ denotes Urumqi)

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    Fig. 8  Radar images observed at Urumqi for 18:46—18:51 16 July 2007 (a) reflectivity at 1.5° elevation, (b) radial base velocity at 1.5° elevation (the white arrows denote the wind directions), (c) vertical section of reflectivity and radial base velocity (isolines, unit:m·s-1) along the black solid line in Fig. 8a (azimuth: 122°; the shaded denotes topography)

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    • Received : 2010-03-08
    • Accepted : 2010-07-22
    • Published : 2011-02-28
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