Evaluation of multiple surface irradiance-based clear sky detection methods at Xianghe—A heavy polluted site on the North China Plain

Surface irradiance measurements with high temporal resolution can be used to detect clear skies,which is a critical step for further study,such as aerosol and cloud radiative effects.Twenty-one clear-sky detection(CSD)methods are assessed based on five years of 1-min surface irradiance data at Xianghe—a heavily polluted station on the North China Plain.Total-sky imager(TSI)discrimination results corrected by manual checks are used as the benchmark for the evaluation.The performance heavily relies on the criteria adopted by the CSD methods.Those with higher cloudy-sky detection accuracy rates produce lower clear-sky accuracy rates,and vice versa.A general tendency in common among all CSD methods is the detection accuracy deteriorates when aerosol loading increases.Nearly all criteria adopted in CSD methods are too strict to detect clear skies under polluted conditions,which is more severe if clear-sky irradiance is not properly estimated.The mean true positive rate(CSD method correctly detects clear sky)decreases from 45%for aerosol optical depth(AOD)≤0.2%to 6%for AOD>0.5.The results clearly indicate that CSD methods in a highly polluted region still need further improvements.

Cloudy Sky Conditions Promote Net Ecosystem CO_2 Exchange in a Subtropical Coniferous Plantation across Seasons

Dynamic changes in solar radiation have an important influence on ecosystem carbon sequestration,but the effects of changes caused by sky conditions on net ecosystem CO2 exchange(NEE)are unclear.This study analyzed the effects of sunny,cloudy,and overcast sky conditions on NEE using carbon flux and meteorological data for a subtropical coniferous plantation in 2012.Based on one-year data,we found no seasonal variation in the light response curve under various sky conditions.Compared with sunny sky conditions,the apparent quantum yield(α)and potential photosynthetic rate at a light intensity of 150 and 750 W m^-2(P150 and P750)under cloudy sky conditions increased by an average of 82.3%,217.7%,and 22.5%;αand P150 under overcast sky conditions increased by 118.5%and 301%on average.Moderate radiation conditions were more favorable for maximum NEE,while low radiation conditions inhibited NEE.In most cases,when the sunny NEE was used as a baseline for comparison,the relative change in NEE(%NEE)was positive under cloudy sky conditions and negative under overcast sky conditions.The average maximal%NEE under cloudy sky conditions was 42.4% in spring,34.1% in summer,1.6% in autumn and –87.3% in winter.This study indicates that cloudy sky conditions promote photosynthetic rates and NEE in subtropical coniferous plantations.