中文摘要：

通过分析武汉大学激光雷达在2001年4月到2004年12月的观测数据，研究武汉（30．5°N，114．4°E）上空背景Na层的长期（时间尺度大于1年）变化和夜间变化特征，并讨论了这种变化特征可能的产生原因。Na层长期变化的观测研究表明，背景Na层柱密度最大值出现在11月，大约是5月份最小值（1．6X100cm卅）的2倍；质心高度最大出现在8月，比年平均高度91．5kin高1．4km，最小值出现在5月，为91．2km；rms宽度平均值为4、5km，最大值和最小值分别出现在12月和3月；月平均Na层质心高度和均方根宽度都具有准半年周期的变化，但两者变化的相位相反，除了长期变化外，背景Na层还呈现出明显的夜间变化特征；除凌晨短暂的时间外，Na层柱密度随夜间时间增加，于0530LT达到最大值（2．9×100cm^-2）；峰值对应的高度随时间下降；质心高度随时间缓慢增加但在凌晨迅速下降基本恢复到入夜时的高度；整夜背景Na层平均柱密度、峰值对应的高度、质心高度和rms宽度起伏值分别为1．1×10^9cm^-2，3km，0．3km和0．8km。

英文摘要：

Abstract Based on the lidar observations from April 2001 to December 2004 over Wuhan, the long term （time scale longer than one year） and nighttime variations of background Na layer are presented, and their possible causes are also discussed. The observation results of long term variation show that Na abundance has a maximum value in November, which is approximately 2 times of the minimum value （1.6 × 10^9 cm^-2） in May; Na centroid height reaches a maximum value in August, which is about 1.4 km higher than the yearly average value （91.2 km）; the average value of rms width is 4.5 km, and its maximum and minimum values occur in December and March, respectively; Both of the Monthly averaged centroid height and rms width show a quasi-semiannual variation, but with contrary phase variations. Except the long term variations, the background Na layer also exhibits obvious nighttime variation： Na abundance increase with the nighttime except a short duration near the dawn time with a maximum value of 2.9 ×10^9 cm^-2 at 0540 LT; the height of peak value decreases with the nighttime; the centroid height of Na layer increases slowly with the nighttime until the time of 0520 LT, then it decreases rapidly to the value at the time of 2000 LT; the magnitudes of the fluctuations for the averaged Na layer abundance, height of peak, centroid height and rms width in the nighttime are 1.2 × 10^9 cm^-2, 3km, 0.3km and 0.8km, respectively.