云和降水扰动对黄土高原半干旱草地辐射收支及能量分配的影响

Effects of clouds and precipitation disturbance on the surface radiation budget and energy balance over loess plateau semi-arid grassland in China

地表辐射收支和能量分配对陆-气系统的反馈是气候模式中最重要的物理过程之一.认识半干旱地区云和降水的扰动对辐射收支和能量分配的影响规律,是提高数值模式中评估地表辐射收支和能量平衡参数化效果的关键环节.利用兰州大学半干旱气候与环境观测站2008年的观测资料,研究了云和降水的扰动对辐射收支各分量的削弱作用及对地表能量平衡的影响规律.年平均结果表明,多云状况可以作为年平均的气候背景;云和降水对短波辐射削弱最强,大气向下长波辐射随天空云量的增加而增强,地表向上长波辐射随着云量的增加而减小,净辐射占总辐射的比率受云和降水的影响较小.季节平均结果显示,短波辐射日积分量在生长季和非生长季均随云量的增加而降低,生长季云和降水对短波辐射的削弱作用明显强于非生长季.生长季,晴天、少云和多云时向上长波辐射差异不大,阴天时向下和向上长波辐射明显减小.非生长季,地表向上长波辐射受云和降水的影响较小,日积分量变化不大,向下长波辐射随云量的增多而增强.地表反照率具有明显的日变化和季节变化,冬季大,秋季小;地表反照率日变化呈不对称的"V"形分布.生长季,感热通量和土壤热通量随云量增多而减小;潜热通量在晴天、少云和多云状况下随云量增多而增大;阴天时受降水影响,净辐射的严重削弱导致了潜热通量大大降低.非生长季,少云时净辐射日积分量最大,晴天时的净辐射与多云和阴天状况接近;感热和潜热通量随云量的增多而减小,土壤热通量日平均积分值在非生长季为负.生长季,多云状况的能量闭合度最好,能量不平衡差额占净辐射的3.9%;阴天时最差,不平衡差额占净辐射的16.8%;晴天和少云状况不平衡差额约占净辐射量的7%.非生长季受积雪影响,能量不闭合差额明显大于生长季.

The feedback effect of land surface radiation budget and energy distribution on land-atmosphere system is one of the most important physical processes in climate models. Studying the effects of disturbance of cloud and precipitation on radiation budget and energy distribution is a crucial link to increase the parameterization effect of evaluating land surface radiation budget and energy balance in numerical models. Based on the data observed at the Semi-Arid Climate and Environment Observatory of Lanzhou University in 2008, the weakening influences of disturbance of cloud and precipitation on components in radiation budget and on land surface energy balance are studied. The annual average results showed that cloudy situation could be thought of as climatic background of annual average;weakening influences of cloud and precipitation on short-wave radiation are the strongest, and atmospheric long-wave radiation increases while surface long-wave radiation decreases with cloud amount increasing;the influences of cloud and precipitation on the ratio of net radiation to global radiation is small. The seasonal average results show that daily integral value of short-wave radiation decreases with cloud amount increasing both in growing season and in non-growing season, and weakening influences of cloud and precipitation on short-wave radiation in growing season are obviously stronger than in non-growing season. In growing season, there is no substantial difference among surface long-wave radiations on clear days, partly cloudy days and cloudy days, and atmospheric and surface long-wave radiation decrease apparently on overcast days. In non-growing season, the influences of cloud and precipitation on surface long-wave radiation are smaller, and its daily integral value changes a little, and atmospheric long-wave radiation increases with cloud amount increasing. The surface albedo has obvious diurnal and seasonal variation characteristics and it is higher in winter than in autumn;the diurnal variation curves of albedo present unsymmetrical“V”shapes. In growing season, sensible heat flux and soil heat flux decreased along with cloud amount increasing;latent heat flux increases with cloud amount increasing on clear, partly cloudy and cloudy days;due to the precipitation on obscure days, latent heat flux decreases greatly because net radiation is seriously weakened. In non-growing season, daily integral value of net radiation is greatest on partly cloudy days, and net radiation on clear days is very close to it on cloudy and overcast days;sensible and latent heat flux decreases with cloud amount increasing, average daily integral value of soil heat flux is negative in non-growing season. In growing season, energy closure degree on cloudy days is best, and imbalance energy accounts for 3.9%of net energy;energy closure degree on overcast days is worst, and imbalance energy accounts for 16.8%of net energy;imbalance energy accounts for 7%of net energy under both clear and partly cloud situations. Due to snow effect in non-growing season, energy closure degree in non-growing season is greater obviously than it in growing season.