Averaging Timescale for Near-Surface Turbulent Vertical Heat and Moisture Fluxes in the Convective Boundary Layer

With a new approach, we measure the averaging timescales

suitable for the eddy covariance estimates of the vertical transport of

heat and moisture by near-surface turbulent eddies in the convective

boundary layer (CBL). The turbulent vertical fluxes are dominated by

updrafts of warm and moist air, and downdrafts of cool and dry air.

Therefore, the cospectra between the fluctuations of vertical velocity

(w) and temperature (T) , between w and water vapor mixing ratio (r) ,

and between T and r should be all positive. From the timescale at which

any of the three positive cospectra does the sign change into negative,

one can evaluate that the vertical transport of heat and moisture by nonturbulent

process surpass the turbulent vertical fluxes. We apply this

cospectrum approach to 3.64-h long, 10-Hz time series of w, T and r

centered at the midday at the height of 5 m above ground level on 16 fair

weather days from the XPIA field campaign. The daily variation of the

midday broadly varies from 8 min to 41 min. The contribution of the nonturbulent

vertical heat flux to the 30-min mean flux (or the maximum

ogive flux) reaches up to 7 % (or 10 %). In terms of the vertical

moisture flux, on the minimum day, the 30-min mean flux explains only 87

% of the turbulent flux. Except this case, the non-turbulent contribution

to the 30-min mean flux are within 2 %. But with respect to the maximum

ogive moisture flux, the non-turbulent contribution goes up to 15 %. By

extending the estimation into the whole daytime, we observe a clear

diurnal evolution of . When the value rapidly grows in the morning, the

non-turbulent contribution to the maximum ogive vertical heat flux goes

up to 34 %, and the maximum ogive moisture flux to 18 % with 3.64-h

window centered at 0800 LT in the composite averaged over the 16 days.