From Glossary of Meteorology
In the Fourier or spectral analysis of two time series, a measure of the correlation between the two series as a function of frequency or wavelength, but ignoring any phase differences between the two time series.
It is defined as
where n is wavenumber (number of waves per total period of the time series), Co(n) is the cospectrum, and Q(n) is the quadrature spectrum.
Stull, R. B. 1988. An Introduction to Boundary Layer Meteorology. 666 pp.