Abstract
We consider the statistical properties of solutions of the stochastic
fractional relaxation equation that has been proposed as a model for the
earth’s energy balance. In thisequation, the (scaling) fractional
derivative term modelsenergy storage processes that occur over a wide
range of space and time scales. Up until now, stochastic
fractionalrelaxation processes have only been considered
withRiemann-Liouville fractional derivatives in the context of random
walk processes where it yields highlynonstationary behaviour. For our
purposes we require the stationary processes that are the solutions of
the Weyl fractional relaxation equations whose domain is −∞ to t rather
than 0 to t. We develop a framework for handling fractional
equationsdriven by white noise forcings. To avoid divergences, wefollow
the approach used in fractional Brownian motion(fBm). The resulting
fractional relaxation motions (fRm) and fractional relaxation noises
(fRn) generalize the more familiar fBm and fGn (fractional Gaussian
noise). Weanalytically determine both the small and large scale
limitsand show extensive analytic and numerical results on the
autocorrelation functions, Haar fluctuations and spectra. We display
sample realizations. Finally, we discuss the prediction of fRn, fRm
which – due to long memories - is a past value problem, not an initial
value problem. We develop an analytic formula for the fRnforecast skill
and compare it to fGn. Although the large scale limit is an
(unpredictable) white noise that is attainedin a slow power law manner,
when the temporal resolutionof the series is small compared to the
relaxation time, fRncan mimick a long memory process with a wide range
of exponents ranging from fGn to fBm and beyond. Wediscuss the
implications for monthly, seasonal, annualforecasts of the earth’s
temperature.