| The triggering and maintenance of rainstorms on the eastern region of the Helan mountains are significantly influenced by large-scale circulation,Low-Level Jet（LLJ）systems,and distribution of local terrain.Firstly,the characteristics of large-scale circulation situation and LLJ systems evolution during 20 heavy rainfall events on the eastern region of the Helan Mountains from 2009 to 2020 are summarized and analyzed based on the intensive observation data of precipitation and NCEP/NCAR reanalysis data.And then ideal numerical experiments are conducted by the em＿hill2d＿x module in the WRF model with environmental parameters of the 20heavy rainfall events,output of these experiments are used to explore circulation characteristics in the areas surrounding the Helan Mountains under different structured wind field conditions as well as the mechanism of the Helan Mountains’terrain on the formation of precipitation in this region.The mainly results as following:（1）Synthetic analyses on 20 cases heavy rainstorms which occurred on the eastern region of the Helan Mountains from 2009 to 2020 with relatively complete data show that precipitation centers are all located on the eastern region of the Helan Mountains and relatively evenly distributed of the selected cases,precipitation mostly occurs from the afternoon to night stage.The large-scale circulation backgrounds of selected heavy rainfall cases can be classified into two types:one is the heavy rainfall area located behind the trough ridge（hereafter“In front of upper-level trough”category rainstorm）,the other mainly influenced by the anomalous northwestern extension of the West Pacific Subtropical High（588&586 dagpm line）（referred as“Northwestern side of the subtropical high”category rainstorm process）.（2）Synoptic analyses of the two types of heavy rainfall processes indicate that the location of precipitation of the“In front of upper-level trough”is in the right side of westerly trough at 500 h Pa as well as the entrance of the upper-level jet stream,which have favorable dynamic conditions for formation of rainstorm;Besides,a strong water vapor convergence center appeared at 850 h Pa,abundant water vapor is transported to the heavy rainfall area by the southeast airflow at lower level.While the location of heavy rainfall in the“Northwest side of the subtropical high”process is on the right side of the upper-level jet stream axis,and the South Asian high belongs to the eastern type,The coupling of circulation systembetween the upper level with the lower levels is more significant than that of the“In front of upper trough”process,and this type rainstorm process occurs in the northwest edge of the subtropical high which convergence of water vapor at 850 h Pa is stronger than the former.（3）Statistical analyses of LLJs during evolution of 20 heavy rainfall processes at the eastern side of the Helan Mountain suggest rainstorm processes are almost accompanied with LLJ system.LLJs in the“Northwest side of the subtropical high”precipitation processes present the characteristic with“northward strengthening-southward weakening”,which axises mainly locate on the left side of the mountain,and wind direction of LLJs are mostly perpendicular to the mountain range.While reprensents of LLJs in the“In front of upper trough”precipitation processes are"eastward weakening",axises of LLJs appeared on the easttern side of the Helan Mountain,and wind direction of LLJs are mostly parallel to the direction of the mountain range.The role of LLJs in the former heavy rainfall processes is mainly reflected in the transportation of water vapor and the increase of wind speed in the windward slope,which plays an important role in triggering and maintaining this type of heavy rainfall through the development and evolution of mesoscale cyclones generated by the dynamic effect of LLJs for the latter.（4）Results of ideal numerical experiments D1 and D2 without water vapour（which means dry process）by em＿hill2d＿x module of WRF show that a longer wavelength and propagating mountain wave in vertical would be formed when vertical homogeneous airflow passes over a mountain.Stable vertical propagating gravity waves appear over the mountain,with longer wavelengths and no gravity wave breaking phenomen happened on downstream of the mountain at the larger Fr number.When the wind speed is smalle,there is a tendency to produce windward flow blocking as well as lee wave breaking,and the flow blocking phenomenon occurs under weaker wind speed conditions.Horizontally propagating lee waves would be formed at the condtion of wind shear exist in the lower layers of atmosphere,and the amplitude and wavelength of the lee waves become larger,and the propagation distance becomes farther.as the wind shear increased.（5）Water vapor and cloud microphysical processes have significant impacts on characteristics of topographic gravity waves.The moist air passing over the mountains produces vertical motion under the action of lee waves,the latent heat released by the phase change of water vapor further promotes the rapid upward movement of moist air during this process,which would induce wave breaking at the lee side,the propagation ability of gravity wave is weaker than that of under dry processes condtion.Simulation results show that the distribution of precipitation is mainly concentrated on the windward side under the conditions of homogeneous moist airflow,while which on the lee side is relatively less.When the upstream wind speed increases to 10m·s-1or more,the intensity of precipitation issignificantly increased and the location of precipitation gradually concentrates on the windward slope of the mountain top,while the precipitation intensity on the lee side is weaker than that at the windward side due to the stronger downslope wind and which is unfavourable for maintenance of precipitation.Characteristics of precipitation are significantly different in the presence of low-level wind shear,under this condition larger cloud water content values appeared on both the windward and lee sides of the mountain,and stronger precipitation centers locate on both sides of the mountain,especially cloud on the lee side has stronger vertical movement and extended height,which indicates deep convective characteristics.（6）The vertical distribution of wind field with easterly wind at low levels as well as westerly wind at upper levels is closest to the actual wind field vertical distribution during the heavy rainfall process in the eastern region of Helan Mountain.Simulated results under this scenario show that interaction between atmospheric circulationsat upper levels with which at low levels strengthen the updraft on the windward side and significantly reduce the water vapor content on the lee side,resulting in a significant reduction of precipitation on the lee side,the region of precipitation on the windward side is more concentrated around the mountain and the intensity of rainfall increased too.This is one of the main reasons for the significant differences of precipitation characteristics on both sides of the Helan Mountain. |
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