Abstract: The atmospheric downward longwave radiation has great significance to surface energy balance and water cycle. Based on the observation data of radiation flux from September 2018 to August 2019 on the northern shore of the Qinghai Lake, the multi-temporal scale change characteristics of radiation flux were analyzed, and the applicability of ten parameterized formulas of atmospheric emissivity on sunny days was compared. And on this basis, the influences of clouds on atmospheric downward longwave radiation were quantitatively evaluated. The results show that each component of radiation had obvious and similar diurnal variation with ‘single peak’ pattern on the northern shore of the Qinghai Lake, and the radiations were higher during the day and lower at night. The radiative components except for upward shortwave radiation (mainly affected by land surface albedo) varied following air temperature during the year. Among ten parameterized formulas of atmospheric emissivity on sunny days, the applicability of Brutsaert formula was the highest on the northern shore of the Qinghai Lake. The atmospheric downward longwave radiation gradually enhanced with increase of cloud fraction (C_f). The cloud enhancement effect was less than 10 W·m^-2 when C_f was less than 10%, and exceeded 60 W·m^-2 when C_f was more than 75%. The ratio between the observation value of atmospheric downward longwave radiation on cloudy sky and corresponding calculated value on clear sky increased with increase of C_f, and there was a very significant quadratic function relationship between them. When C_f was less than 10%, the mean ratio was only 1.04, while when C_f exceeded 75%, the ratio was more than 1.25, so their quantitative relationship could better reveal the influence of clouds on atmospheric downward longwave radiation in this area.

Key words: atmospheric downward longwave radiation, cloud fraction, atmospheric emissivity, the Qinghai Lake

摘要： 大气向下长波辐射对地表能量平衡、水循环等有重要影响。基于青海湖北岸2018年9月至2019年8月辐射观测资料，分析不同时间尺度的辐射通量变化特征，对比晴天条件下10个大气比辐射率参数化公式在该地区的适用性，在此基础上定量评估了云对大气向下长波辐射的影响。结果显示：青海湖北岸地区各辐射分量均呈现相似的“单峰型”日变化特征，辐射量白天较高、夜间较低；一年中，除向上短波辐射（主要受地表反照率影响）外，其余辐射分量均随气温而变化。晴天条件下大气比辐射率参数化Brutsaert公式在青海湖北岸地区的适用性最高。随着云份额数（C_f）的增大，青海湖北岸大气向下长波辐射逐渐增强，在C_f＜10%时，云增强效应低于10 W·m^-2；当C_f＞75%时，云增强效应超过60 W·m^-2。云天大气向下长波辐射观测值与晴天计算值的比值r随着C_f的增大而增大，两者存在极显著的二次函数关系，当C_f＜10%时，r平均为1.04；当C_f＞75%时，r＞1.25，该统计关系能够较好地揭示青海湖北岸地区云对大气向下长波辐射的影响。

关键词: 大气向下长波辐射, 云份额数, 大气比辐射率, 青海湖