Some observations of turbulence structure in stable layers

Abstract

AbstractThree days of measurements in stable conditions (Monin‐Obukhov length L between 15 and 300 m) at the Boulder Atmospheric Observatory tower are presented. Winds came off the Rocky Mountains on two days and off the plains on the other day. Vertical profiles of the mean horizontal velocity and Brunt‐Väisälä frequency N, and the standard deviation of the vertical component, ow, fluxes, spectra, cospectra, quadspectra, correlations and length scales are examined in this study. the measurements indicate that wave motion was present on all occasions, but which varied in strength, from weak to strong. New theoretical arguments are tested, which suggest that where the Richardson number is less than one, the velocity gradient dU/dz rather than N determines the integral scales Lx(w) of the vertical velocity fluctuations, i.e. Lx(w) ≃ ow/(dU/dz). But theory also suggests that it is not this scale, but the larger buoyancy scale ow/N that determines the temperature variance and thermal diffusivity. On this basis, dimensionless parameters are developed for plotting the measurements of temperature variance and thermal diffusivity. They are found to be in reasonably good agreement with other observations and with theory, even though the relative strength of wave motion to turbulence varied considerably. Low frequency wave motion appeared to be associated with significant heat flux, a possibility that has been excluded or overlooked in some previous studies.