极区中层顶区域重力波谱的季节变化

Seasonal Variation of Gravity Wave Spectrum Near the Polar Mesopause

使用MAP/WINE和MAC/SINE两次试验中测量的25m高分辨率水平速度数据和1km低分辨率温度数据,研究极区中层顶区域重力波谱的季节变化.温度的直接测量使计算的谱振幅和Richardson数更接近真实大气.结果显示,极区中层顶区域水平速度垂直波数谱的斜率和振幅存在相当大的变率,这些大的观测变率用各种饱和模式及普适垂直波数谱不能解释.然而平均垂直波数谱显示了明显的季节变化,在夏季,平均谱具有饱和特性;在冬季,平均谱具有非饱和特性.这意味着饱和过程存在于夏季而不是冬季.因此,夏季比冬季应有更强的湍流.这个结果与湍流季节变化的观测大致一致.从Brunt-Vaisala频率N和水平风切变计算的Richardson数Ri剖面也显示出季节差异,Ri<1/4的动力不稳定区出现在夏季,而Ri>0.4的稳定区出现在冬季.这些不稳定区与夏季谱结合很好,而稳定区则与冬季谱结合很好.

This paper utilizes horizontal velocity data with a high resolution of 25 m and temperature data with a low resolution of 1 km measured during the MAP/WINE and MAC/SINE campaigns to study seasonal variation of gravity wave spectrum near the polar mesopause. Direct measurement of the temperature allows accurate calculation of the spectral amplitude and Richardson number. Results show that there is considerable variability in the slope and amplitude of the vertical wavenumber spectra of the horizontal velocity near the polar mesopause, which is not consistent with the predictions of various saturation models and the universal vertical wavenumber spectrum. However, the mean vertical wavenumber spectra reveal a significant seasonal variation: the mean spectrum in summer has saturated features, whereas the mean spectrum in winter has unsaturated features. This implies that the saturation processes are present in summer rather than in winter. Enhanced turbulence layer can therefore be expected to occur in summer compared to winter. This result is in rough agreement with the seasonal variation of turbulent energy dissipation rates. Richardson number Ri calculated from Brunt-Vaisala frequency N and horizontal wind shear S also reveals seasonal difference: dynamically unstable regions of Ri smaller than 1/4 are observed in summer, whereas stable regions of Ri larger than 0.4 are observed in winter. These unstable regions are found to associate well with the summer spectrum, whereas stable regions are found to associate well with the winter.