Turbulence transport processes and parameterization in nocturnal boundary layers (Formerly 17B.2)

Turbulence transport processes and the bulk formula for momentum and sensible heat fluxes based on Monin-Obukhov (M-O) similarity theory are investigated over a relatively flat land using the data collected during the CASES99, and the results are compared to the data collected within and above canopy using the data collected during the CHATS. We found that deviations of the observed momentum flux from M-O similarity theory is mainly due to the height variation of the momentum flux under neutral/weakly stable conditions. We found that M-O similarity theory for momentum and sensible heat fluxes is only valid around 5 m above the ground. Below and above 5 m, the mixing length for momentum under neutral conditions is less than M-O values. Above the wall layer, the mixing length for momentum decreases with height and approaches a constant, indicating the z-less regime in the outer layer. As a result, the failure of the bulk formula for estimation of observed momentum flux is mainly due to the failure of M-O similarity theory in describing the vertical variation of the momentum flux under neutral conditions. Under weak mixing conditions, temperature advection plays an important role in temperature variations, and vertical temperature gradients may not be directly related to sensible heat flux, leading to the invalidity of M-O similarity theory. Eddy impingements were found close to the ground, both over the flat land, within the canopy, and somewhat above the canopy. Possible physical processes leading to the vertical variation of the momentum flux will be discussed.