| This paper firstly summarized and discussed the current international points on main metal layer,sporadic metal layer and diurnal observational results,and then introduced principles and systems of atmospheric laser radars in detail.In the third part,the change rates and characteristics of Na main layer were got based on the Beijing high temporal resolution lidar data.In order to research the rapid Na change rates,the primary terms in the Na continuity equation were respectively evaluated under normal mesospause conditions.In the fourth part,we reported a set of double sporadic layer events observed by Fe and Na lidars over Wuhan,China.The detailed contents are below:1.Summarize and discuss the current international points of main metal layer,sporadic metal layer and diurnal observational results.Focus on the small scale characteristics of main layer as density change rates,characteristic time of metal atoms,relationship of boundary altitude for Fe and Na,the different phenomenon of model and observational result for Fe and Na profile structure.The formation mechanism of sporadic metal layer is also discussed.As to the diurnal observation of metal layer,the Fe lower boundary has significant relationship with solar elevation angles.2.This part expounds the principle of the lidar system under different scattering mechanism,especially Beijing and Wuhan resonance fluorescence lidar system,data inversion algorithm and the possible error sources.3.Based on the 6 s resolution Na lidar measurements during～395 hours on 47 different nights from May to November 2011 in Beijing(40.2°N,116°E),China,it was found that the Na density at altitude 83-98 km always exhibited strong short-term fluctuations.The magnitude of the mean absolute increase/decrease rates for these short-term fluctuations ranged from～8 to～16 cm-3s-1.Their profiles were close to each other with the increase rate being slightly larger than that of decrease rate at most altitudes.This coincided with the earlier observations that the Na layer column abundance mostly tends to have a slow net increase during night(Chen and Yi,2011)·The characteristic time for the short-term Na density fluctuations had a magnitude ranging from 46 to 118 s at altitudes between 83 and 98 km,which was apparently shorter than the Brunt-Vaisala period(-5 min).The gravity wave seems difficultly to induce observed Na fluctuations.The conventional gas-phase chemistry and transport terms are possibly too small to produce large local transient Na change rates.4.In this part,we report a set of double sporadic layer events observed by Fe and Na lidars over Wuhan,China.The two sporadic metal layers above normal layer were named as upper and middle sporadic metal layers,respectively.In these events,the upper,middle and normal Fe layers presented altitude separately.There were nine double sporadic Fe events observed in 163 nights during 2010-2013.Eight of the nine events were observed in summer.The maximum ratios of peak density for upper and middle sporadic Fe layers to normal Fe layer were up to-375%and-225%,respectively.The peak altitudes of upper(middle)sporadic Fe layers were in the range of 102-107 km(95-98.5 km).The double sporadic Fe layers lasted more than 2 hours.Interestingly we found density enhancement occurred simultaneously in upper,middle and normal Fe layers on two events.On the nine Fe events,there existed five nights of co-located Na lidar observations.We found double sporadic Na and Fe layers simultaneously appeared.They presented similar structures,altitudes and temporal variations in all five compared events.A little different from Fe,the middle sporadic Na layer was not separated with Na main layer maybe for the wide altitude range of Na main layer.The ratios of upper(middle)sporadic Fe and Na peak values were in the range of 6.6-52(0.57-6.58).While the exact formation mechanism responsible for double sporadic metal layers is still unclear,some possible explanations and corresponding observations are discussed. |
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