夜光云内小尺度重力波对冰晶粒径谱的影响规律研究

本文利用AIM卫星搭载的CIPS云图反照率和冰晶粒径数据,从中提取了2007/08南半球和2008年北半球共6489个小尺度重力波活动(波长5~150km范围)个例,对重力波区域与背景云层冰晶粒径谱进行对比分析,从而研究重力波对冰晶平均半径和谱宽的影响规律.结果表明,北半球重力波区域冰晶的平均半径和谱宽分别比背景云层小2.5nm和6.1nm,南半球则分别减小1.1nm和7.9nm.在随纬度的分布上,小于80°时,南北半球的平均半径扰动值均为负值,绝对值随纬度增大而减小,而大于80°时,负扰动转变为正扰动,且绝对值增加;谱宽扰动的绝对值也随着纬度增加而减小,但均为负值.在季节内随时间的分布上,南北半球重力波对冰晶平均半径和谱宽的扰动在始末阶段以负值为主,且绝对值较大,而在中期阶段正负值相当,且绝对值较小.这一特征与重力波引起冰晶粒径变化的振幅在纬度和时间上的分布趋势一致.重力波的波长均随纬度升高而减小,在季节的始末阶段较大,中期小,且南半球的平均波长和变化幅度都要明显大于北半球的,粒径扰动振幅随波长的变化率为南半球0.207nm·km-1,北半球的0.163nm·km-1.根据分析推断,重力波自身的扰动振幅应与其影响区域内的谱参数相对于背景云层的变化量有直接关系,且振幅越大,平均半径和谱宽的负扰动就越大.

Characteristics of perturbations induced by small-scale gravity waves on ice particle size distribution of noctilucent clouds

Recent research has shown that the occurrence frequency and brightness of polar mesospheric clouds (PMCs) have been gradually increasing since first recorded in 1883; the trend is likely related to global warming and associated anthropogenic activity. The increasing trend is somewhat under debate, however, as not all studies on the subject have reached similar conclusions. Analysis of the thermodynamic mechanism remains necessary to confirm the impact of climate change and the surrounding atmosphere on this trend. Upward-propagating atmospheric gravity waves (GWs) are a thermodynamic process with significant effect on the upper mesosphere, which were addressed in this work by examining characteristics of perturbations induced by small-scale GWs on the ice particle size distribution (PSDs) of PMCs.#br#Using the level_2 albedo images and particle effective radius data of both North (NH) and South (SH) hemispheres during 2007-2008 from the cloud imaging and particle size (CIPS) experiment, 6489 GW events were distinguished to calculate the corresponding PSDs. The differences of PSDs in the mean clouds formed in GW areas were employed to determine the effect of GWs with regard to latitude and throughout the PMCs seasons. Additionally, the amplitudes in particle radius and wavelength of GWs were used to explore the relationship between the GWs and clouds.#br#The mean radius and width of PSDs in GW areas were 2.5 nm and 6.1 nm, respectively; less than those of mean clouds in the NH season. The differences were 1.1 nm and 7.9 nm in the SH season. The radius perturbations induced by GWs were almost negative at latitudes below 80° for both hemispheres, and the absolute values decreased as latitude increased. These distributions reversed at latitudes above 80°. Similarly, the absolute width decreased as latitude increased. Both the mean radius and width were dominated by negative perturbations during the starting and ending stages of the seasons, but increased to positive values in the middle of the seasons. These characteristics are consistent with the distributions of amplitude in particle size induced by GWs regarding latitude and season. GW wavelengths also decreased as latitude increased, reaching larger values during the starting and ending stages but smaller values in the middle of both seasons. The magnitude of variations in the SH season was considerably larger than that in the NH season, and the change rate of perturbation amplitude in particle size varied with wavelength at 0.207 nm·km^-1 in the SH season and 0.163 nm·km^-1 in the NH season.#br#According to our results and those found in the literature, we deduced that the parameters of PSDs in GW areas have direct relationship with GW properties (e.g., amplitude and wavelength.) Large amplitude and wavelength create large negative perturbation for the mean radius and width. The results presented here further suggest that the micro physical processes under the effect of GWs on ice crystals can be further studied via the cloud micro physical model.