| Although the frequency and intensity of dust weather(contain floating dust,blowing dust,and dust storm)have decreased in northern China in recent years,the occurrence of dust weather will still reduce air quality,threaten people's health and affect the global climate,with a more complex formation mechanism under the background of climate change.Thus,it is necessary to study the formation mechanism of dust storm in China,especially in high-incidence areas.In this paper,the datasets of daily dust surface meteorological observations in China and the datasets of MERRA-2 from 1980 to 2018 were employed to analyze the temporal-spatial distribution characteristics of dust weather in China.Then,seven spring dust storm events in South Xinjiang were selected to explain the formation of abnormal atmospheric circulation during the dust storm event from the perspective of transient eddy fluxes.Finally,a severe dust storm event on April 8,2001 over South Xinjiang was simulated by the WRF-Chem model.Main conclusions are as follows:(1)Study on the temporal-spatial distribution characteristics of dust weather by using two kinds of data shows that the large value center of dust column mass concentration(DCMD)and the high-incidence area of dust weather are located in south Xinjiang and central and western Inner Mongolia.Although average days of dust storm in South Xinjiang shows a decreasing trend,the 9-year moving average increase to different degrees after 2009.However,the DCMD value shows a trend of fluctuation increase.The number of days of dust storm is the most in the 1980s,followed by 2010s,and have little change in 1990s and 2000s.There is not significant inter-decadal change for the overall DCMD.The occurrence of dust storm in South Xinjiang shows a downward trend in 1980s,1990s and 2000s and an upward trend in the 2010s.The DCMD shows an increasing trend in four decades,but the increment gradually decreases.The contributions of dust index(DI)and DCMD values by empirical orthogonal decomposition(EOF)are above 45%,showing a regional consistency.(2)Focusing on seven spring dust storm events in South Xinjiang from 1980 to2018,the variation characteristics of DCMD from 2 days prior to 4 days lag to the establishment day of dust storm event(“Day-2”to“Day+4”)are summarized that DCMD begins to concentrate since“Day-2”,and reaches the maximum on“Day 0”including the intensity and range.Then the DCMD weakens and expands to adjacent and downstream areas in the following 4 days.The variation characteristics of wind speed of South Xinjiang from“Day-3”to“Day+2”is significant.The wind speed in the middle-high levels of the troposphere starts to increase at all levels of the troposphere since“Day-3”.The wind speed at 200 h Pa is above the jet stream standard(30 m·s-1)from“Day-2”to“Day 0”,and reaches the maximum on“Day-1”which is consist with the change characteristics of wind speed at 500 h Pa.At the lower levels of the troposphere(850 h Pa and below),the wind speed increase from“Day-2”,and reaches the largest value on“Day 0”.The timing of the high-level wind speeds of decreasing significantly and becoming below climatological mean is one day ahead of that at the lower-middle layers.Abnormal signals from the surface to the upper-level troposphere during the evolution of the dust storm event illustrate that on“Day-2”,the anomaly center of wind speed east of 60°E forms a new abnormal“negative north,positive middle and negative south”pattern at 200 h Pa.The middle positive anomaly strengths to the strongest and passes through the east subtropical JS center,forming a bulge and embracing the South Xinjiang region.The zonal wind speed over South Xinjiang increases to the jet stream standard.From“Day-6”to“Day 0”,the geopotential height anomaly at 500 h Pa shows a negative Eurasian(EU)teleconnection pattern and the Ural ridge reaches the maximum on the outbreak of the dust storm event.On“Day-4”,the anomaly center of the sea level pressure field in middle and high latitudes presents a“positive,negative”distribution.After that,the Siberian Highs(SH)strengths and approaches South Xinjiang region on“Day 0”,narrowing the intense pressure gradient,leading to wide-range of northerly gales and further causing the outbreak of the dust storm event.(3)By using the physical decomposition method,the formation of abnormal atmospheric circulation is explained during the dust storm event from the perspective of transient eddy fluxes.2 days prior to the outbreak of the dust storm events,the transient-momentum transport converges near 40°N and 70°N,and the wind speed of the subtropical jet stream and polar front jet stream weakens.South Xinjiang is located in the entrance area of the subtropical jet stream and polar front jet stream,and the transient eddy could obtain energy from the basic flow,which played an important role in increasing the wind speed of the zonal wind at 200h Pa.Comparably,transient eddy heat transport has a weaker influence on the change in the upper layer zonal wind.Through downward upper-level momentum derived from the upper-level jet stream,the lower-level wind increases,causing the 30 m·s-1contour below 400h Pa on“Day-1”.Additionally,transient-heat transport convergence approximately50-60°N is helpful for the establishment of the Ural ridge under the negative phase of EU teleconnection pattern.Although transient-momentum transport directly plays a negative role in development of the Ural ridge,the divergence of transient-momentum transport could indirectly favor the establishment of the Ural ridge mainly through decreasing the zonal wind.The establishment of the Ural ridge reinforces the SH and leads to low-level northerly gales.The verification results of Eliassen-Palm(E-P)flux are consistent with the characteristics of the transient momentum and heat flux transport.(4)A dust storm event in South Xinjiang on April 8,2001 is simulated by the WRF-Chem model.The results show that the WRF model can well simulate the circulation of main systems during the dust storm event.At 200 h Pa,the model simulates the zonal wind bulge at the northwest edge of the subtropical jet stream on April 6 and variation characteristics.The simulated zonal wind is the best on April 8.At 500 h Pa,the model simulated the evolution characteristics of the Ural ridge during the dust storm event.For sea level pressure field,the model simulates intensity and position of SH center,and the center strength error is less than 2 h Pa.The surface dust concentration simulated by WRF-Chem can well simulate the occurrence range,intensity variation and movement characteristics of the dust storm in terms of spatial distribution and time variation. |
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