Long-term memory and multifractality of downwelling longwave radiation flux at the Earth’s surface

We study for the first time the correlation properties and the multifractality of downward longwave radiation (DLR) temporal fluctuations, using the multi-fractal detrended fluctuation analysis. Our DLR data are taken from 40 stations of the Baseline Surface Radiation Network and span more than 20 years on temporal resolutions of a few minutes. In almost all stations, two different scaling regions are defined through the emergence of a crossover at around 10 days, similarly to earlier studies of other meteorological quantities. The crossover marks the transition from a persistent, non-stationary signal on weather scales to a more weakly persistent (and sometimes anti-persistent) behavior extending in the macroweather region between 2 months and at least several years. We observe typical multifractal behavior for DLR through the investigation of the generalized Hurst exponents and singularity spectra. Generally, the multifractality strength is larger for the weather regime compared to the macroweather. We find evidence that the shape of the probability density function of the DLR anomalies contributes to the multifractality strength for macroweather. The three Western Equatorial Pacific stations exhibit markedly different characteristics, with fluctuation functions closely following 1/ f noise between the scales of a few hours up to 2 months. These stations appear to be strongly affected by the ENSO. Also, their timeseries are barely long enough so that the ENSO shows as a semi-periodic signal on the large scales of their long-term memory. Their multifractality is not clearly defined and its strength is much smaller than that of the other stations.