Impact of aerosols and clouds on decadal trends in all-sky solar radiation over the Netherlands (1966a??2015)

摘要

pstrongAbstract./strong A 50-year hourly data set of global shortwave radiation, cloudiness and visibility over the Netherlands was used to quantify the contribution of aerosols and clouds to the trend in yearly-averaged all-sky radiation (1.81span class="thinspace"/span?±span class="thinspace"/span1.07span class="thinspace"/spanWspan class="thinspace"/spanmsupa??2/supspan class="thinspace"/spandecadesupa??1/sup). Yearly-averaged clear-sky and cloud-base radiation data show large year-to-year fluctuations caused by yearly changes in the occurrence of clear and cloudy periods and cannot be used for trend analysis. Therefore, proxy clear-sky and cloud-base radiations were computed. In a proxy analysis hourly radiation data falling within a fractional cloudiness value are fitted by monotonic increasing functions of solar zenith angle and summed over all zenith angles occurring in a single year to produce an average. Stable trends can then be computed from the proxy radiation data. A functional expression is derived whereby the trend in proxy all-sky radiation is a linear combination of trends in fractional cloudiness, proxy clear-sky radiation and proxy cloud-base radiation. Trends (per decade) in fractional cloudiness, proxy clear-sky and proxy cloud-base radiation were, respectively, 0.0097span class="thinspace"/span?±span class="thinspace"/span0.0062, 2.78span class="thinspace"/span?±span class="thinspace"/span0.50 and 3.43span class="thinspace"/span?±span class="thinspace"/span1.17span class="thinspace"/spanWspan class="thinspace"/spanmsupa??2/sup. To add up to the all-sky radiation the three trends have weight factors, namely the difference between the mean cloud-base and clear-sky radiation, the clear-sky fraction and the fractional cloudiness, respectively. Our analysis clearly demonstrates that all three components contribute significantly to the observed trend in all-sky radiation. Radiative transfer calculations using the aerosol optical thickness derived from visibility observations indicate that aerosola??radiation interaction (ARI) is a strong candidate to explain the upward trend in the clear-sky radiation. Aerosola??cloud interaction (ACI) may have some impact on cloud-base radiation, but it is suggested that decadal changes in cloud thickness and synoptic-scale changes in cloud amount also play an important role./p.