The storage term in eddy flux calculations

Formal derivation of the storage term in eddy flux estimates of surface exchange shows that it is just the difference between instantaneous concentration profiles at the tower measured at the beginning and end of the averaging period, T, divided by T. As such, the storage term is likely to be a poorer estimate of the true concentration change in a representative volume around the tower than the time-averaged eddy flux is of exchange from the surface patch. This is because instantaneous profiles are easily biased by a single gust. To avoid this, many workers compute the storage from time-averaged concentration profiles centered on the beginning and the end of the averaging period. Here we show that this procedure underestimates the storage by at least 50% in most conditions with larger errors occurring when the integral time scale of the turbulence is much smaller than the averaging time T. Reducing the averaging time to the minimum required to smooth out the influence of individual gusts does not improve things significantly.