Studies On Multiscale Characteristics During The Evolution Of A Dust Event

In this study,a dust event occurred in northwest of China on March 27,2007 was selected and the circulation,the dust distribution characteristics and the evolution of meteorological elements in Minqin during this process was analyzed with multiple observations.Based on the analysis,WRF-Chem?V3.4?model was used to compare the performance of different Planetary Boundary Layer?PBL?parameterizations namely,the YSU,MYJ,QNSE,MYNN2.5 and BouLac PBL schemes,over the dust event.Then the detail of the evolution characteristics of this dust event was analyzed using the best of the five simulations.Finally,the multiscale meteorological characteristics during the dust event was studied by the Ensemble Empirical Mode Decomposition?EEMD?method.The main conclusions are as follows:?1?A largescale dust event occurred in northwestern China from 06:00 UTC to15:00 UTC on March 27,2007.A strong sandstorm process lasting nearly 3 hours was detected in Minqin Station.Accompanied by low ground temperature,high humidity,high air pressure,and sharp wind speed,this process was closely affected by the combined effects of the unique terrain in Minqin,the rapid movement of 700 hPa and850 hPa short wave troughs,and the transit of a cold front at surface.During this process,changes in near-surface wind speed and PM₁₀ concentration had a good consistency,showing the characteristics of a double-peak steep rise and steep decline.Also,there was an adjustment process between the pressure field and reverse variation between temperature and relative humidity.?2?Surface observations were used for evaluating model performance over the“3.27”dust event with different PBL namely,the YSU,MYJ,QNSE,MYNN2.5 and BouLac PBL schemes.It is shown that simulations with the five PBL schemes can successfully reproduce the evolution of the dust event.The YSU and BouLac schemes produced higher surface friction velocity,10-m wind speed,2-m air temperature and surface PM₁₀ concentration and lower 2-m relative humidity,thus simulating stronger weather processes than those of the MYJ,QNSE and MYNN2.5 schemes.These results indicate that different boundary layer schemes affect the dust emission flux and PM₁₀concentration through different simulation effects of friction velocity.The dust emission flux and PM₁₀ concentration tend to increase with higher friction velocity.Therefore,the dust event was enhanced under the comprehensive function of high friction velocity,high temperature as well as low humidity in near-surface layer in the afternoon.As a result,the dust event simulated by the BouLac scheme was the strongest one while the weakest one was outputted by the QNSE scheme.Observations from Minqin meteorological station were used to validate the simulated results over Minqin region.Statistical analysis of the five simulations shows that the QNSE scheme simulated better PM10 concentration,the BouLac scheme performs well for 10-m wind speed and the YSU scheme resulted in the best model performance for simulating air temperature and relative humidity at 2m.Overall,the YSU scheme was concluded as the best PBL scheme for the simulation of the dust storm and the QNSE scheme was the worst one.?3?Using data from the simulation with the YSU scheme to analyze the evolution characteristics during the“3.27”process,it was found that there was an upper jet stream center over the dust area,where the momentum downward transportation was promoted by the stronger,wider and lower wind center.As the near-surface meridional northerly wind increased rapidly and moved southeastward,and the zonal wind band with wind speed over 12 m·s^-1 extended to the near-surface layer,dust-raising process was induced,which resulted in this“3.27”process including the Minqin sandstorm.During the sandstorm,the dust column extended upward to about 4200m with the highest surface PM₁₀ concentration up to 9647?g·m^-3 under favorable dynamic and thermal conditions.?4?Meteorological data from the ground observation and the simulation with YSU PBL scheme were decomposed into three IMFs?IMF1-3?with average periods of 0.3h,1h and 2h respectively and one RES,using the EEMD method in Minqin.Res describes the diurnal variation of meteorological elements,while IMFs represent the oscillatory components of data at high frequencies.Among the three IMFs,IMF1 may relate to the turbulent process in the boundary layer,IMF2 and IMF3 represent typical mesoscale processes.According to the decomposition results of the observed and simulated ground PM10 concentration,10m wind speed and 2m temperature in Minqin,it is found that the fluctuation amplitude of IMFs was small,but its average periods was nearly equal to that of the observed IMF,and the high-frequency signals in the temperature field were ahead of signals in the wind field,which indicated that dust-raising process in the boundary layer was closely related to the near-surface thermal instability factors.According to the decomposition results of the simulated horizontal wind speed,vertical speed,disturbed pressure and disturbed potential temperature over Minqin,RES showed the downward transmission of upper westerly momentum and the upward movement during the enhancement period of the dust storm in Minqin,and the subsidence movement during the weakening period of the storm.Signals in IMF1mainly appeared in the developed stage of the dust storm with a relatively low height.Signals in IMF2 mainly appeared in the development and enhancement stage.Signals in IMF3 were throughout the development stage,and the signals of IMF2 and IMF3developed vertically up to 300hPa.These characteristics may indicate that there were inverse-scale developments in the evolution of the dust storm in Minqin.?5?Simulated meteorological data from the simulation with YSU PBL scheme in D02 domain were decomposed into three IMF fields characterizing mesoscale disturbances and one RES field characterizing largescale changes by the EEMD method.Res field describes the diurnal variation of meteorological elements,while IMFs represent the oscillatory components of data at high frequencies.During the“3.27”dust event,the RES field showed the largescale characteristics of the low-level jet belt moving eastward and southward,and the cold zone to the south.And the eastern edge of the jet stream area was accompanied by strong disturbed kinetic energy.The IMF field showed mesoscale disturbance during the dust event.Following the movement of the dust area,signals in the IMF field decomposed from each element moved and diffused southeasterly in 700hPa.The signals were mainly located on the northeast side of the dust area,extending outside the dust area.Signals in IMF1 field had an average period and wavelength of about 0.35h and 35km respectively with the phase velocity between 20-40 m·s^-1.Signals in IMF2 field had an average period and wavelength of about 0.87h and 46km respectively with the phase velocity between 15-40 m·s^-1.Signals in IMF3 field had an average period and wavelength of about 2.24h and 134km respectively with the phase velocity between 10-30 m·s^-1.Based on these analyses,IMF3 was closer to the gravity wave in terms of the life time,period and phase speed,which means this mode may be relevant to the gravity wave.IMF1 and IMF2 may belong to the gravity waves at smaller scales,which requires further analysis.