Eddy flux measurements using an instrumented powered glider

An instrumented powered glider was used to measure eddy fluxes of heat and water vapour within the planetary boundary layer in summer over southern England. The errors involved in the method are analysed; it appears that individual 25km runs can give heat fluxes to±15W m −2 and latent heat fluxes to ±25W m −2 . The significance of individual runs was tested for internal consistency, against budget methods and, for heat fluxes, against tethered balloon measurements. With a well‐marked inversion and winds more than 5m s −1 , heating rates agreed within 0.2K h −1 . With light winds the discrepancies were larger; these are attributed to sampling problems. Latent heat fluxes could not be compared with budgets usefully, mainly because of the great variability encountered early in the day. The humidity measurements show the gradual establishment of the mixed layer. They show that sampling of the mean state of the boundary layer by single ascents, or of fluxes by 25km runs, may give misleading results. In light winds and deep boundary layers the heat flux carried by scales less than 3.5km was reduced, whereas the larger‐scale latent heat flux was increased, in each case by a factor of 2.