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Effects of Topographic Perturbation on the Precipitation Distribution in Sichuan
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摘要: 引入一维加权平均的谱分析方法定量研究四川地形强迫对该区域降水分布的影响。结果表明：纬向地形和冬季降水谱峰锁相于同一波长（475.8 km），呈共振关系，地形与其他季节降水呈漂移关系，这与经向和纬向上环流变动有关，即冬季纬向环流占主导，纬向地形触发的大气波动对冬季降水策动作用大；夏季降水是各种不同尺度系统相互作用的结果，地形是重要因素之一。经向和纬向地形特征尺度分别为296.8 km和475.8 km，反映了地形强迫的中尺度特征，且纬向地形谱峰比经向大1个数量级，纬向强迫更明显。夏季降水谱峰比冬季大2个数量级，降水系统纬向特征尺度比冬季小约150 km，说明夏季在纬向地形强迫下，降水系统尺度减小的同时其强度大大增加，这在一定程度上可以解释中尺度对流性降水在夏季偏多。四川夏季最大降水位于雅安地区，其地形扰动比四川整体扰动更明显，故产生的降水也更大。夏季降水和经向地形锁相于同一波长（37.1 km），经向地形对雅安夏季强降水起关键作用。Abstract: Terrain characteristics can be accurately represented in spectrum space. Terrain spectra can quantitatively reflect effects of topographic dynamic forcing on the atmosphere. The one-dimensional weighted-average spatial spectral analysis method is used to explore topographic forcing on precipitation distribution in Sichuan. Results indicate that spectral distributions of terrain and winter precipitation in zonal direction present a typical resonance coupling pattern, while that of terrain and precipitation in other seasons drifts toward the smaller scale. In meridional direction, spectral distributions of terrain and precipitation in each season present the large-scale drift pattern. Different patterns are probably relevant to the change of circulations. In winter, due to strong zonal circulation and weak meridional circulation, atmospheric fluctuations caused by zonal topographic forcing show the most significant impact on precipitation. After that season, the zonal circulation weakens gradually in agreement with the decrease of zonal topographic forcing while the meridional flow enhances, leading to the increase of the damping of the zonal wind disturbance caused by terrain, and the pattern transforms from resonance to drift. Summer rainfall is produced by interaction among different scale systems, and terrain is one of the most important factors. The maximum topographic spectral energy in zonal direction is about an order of magnitude larger than that in meridional direction, implying that effects of topographic dynamic forcing are zonally stronger than that in meridional direction. Values of meridional and zonal topographic characteristic scales are 296.8 km and 475.8 km, respectively, which reflects the characteristic of the mesoscale topographic forcing coincident with the frequent mesoscale systems in Sichuan. The peak of the precipitation spectral energy in summer is about two orders of magnitude larger than that in winter and one order of magnitude larger than that in spring or autumn, and the characteristic scale in summer is about 150 km smaller than that in winter. It illustrates that the intensity of the zonal topographic dynamic forcing in summer is significantly increased when the scale of precipitation systems decreases, which explains the high frequency of mesoscale convective precipitation, and implies the significant impact of topographic dynamic forcing on atmosphere as well. The strongest summer precipitation in Sichuan is located at Ya'an, where larger-scale topographic perturbation is more significant than other region in Sichuan. The terrain spectra and summer precipitation spectra in meridional direction are phase-locked in identical wavelength (37.1 km), implying the critical role of terrain on the occurrence of heavy rainfall, and the effect of topographic dynamic forcing in meridional direction is dominant.
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Key words:
- spectral analysis;
- weighted average;
- terrain spectra;
- characteristic scale
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图 1 研究区域地形(填色)和年平均降水量(等值线，单位：mm)
Fig. 1 Distribution of topography(the shaded) and the climatological seasonal precipitation(the contour, unit:mm)
图 2 纬向地形谱和降水谱分布(图中数字表示谱峰的特征尺度，单位：km)
Fig. 2 Terrain spectra and precipitation spectra in zonal direction (numbers denote characteristic scales of peaks, unit:km)
图 3 经向加权平均(27°~35°N)的纬向地形高度剖面(填色)和降水剖面(等值线)分布
(P1，P3，P4代表雨峰位置，P2，P5代表雨谷位置)
Fig. 3 Profile of topography(the shaded) and precipitation(the contour) in zonal direction by meridional weighted average(27°-35°N)
(P1, P3 and P4 denote the location of precipitation peaks; P2 and P5 denote the location of precipitation valleys)
图 5 纬向加权平均(98°~108°E)的经向地形高度剖面(填色)和降水剖面(等值线)
Fig. 5 Profile of topography(the shaded) and precipitation(the contour) in meridional direction by zonal weighted average(98°-108°E)
图 6 对数坐标下四川地区的纬向和经向地形谱能
Fig. 6 Distribution of zonal terrain spectra and meridional terrain spectra in Sichuan in log-log coordinate
图 7 对数坐标下雅安地区的纬向和经向地形谱能
Fig. 7 Distribution of zonal terrain spectra and meridional terrain spectra at Ya'an in log-log coordinate
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