NOCTURNAL BOUNDARY-LAYER REGIMES

This study analyzes turbulence data collected over a grassland site in the nocturnal boundary layer. Examination of the dependence of the no cturnal boundary layer on stability suggests three regimes: a) the wea kly stable case, b) a transition stability regime where many of the va riables change rapidly with increasing stability and c) the very stabl e case. The value of z/L where the downward heat flux is a maximum def ines the stability boundary between the weakly stable and transition r egimes, where L is the Obukhov length. In the present analysis, the do wnward heat flux reaches a maximum at z/L approximately equal to 0.05 for 10 m, although comparison with other data indicates that this is n ot a universal value. For weaker stability, the heat Bur decreases wit h decreasing z/L due to weaker temperature fluctuations. In the transi tion stability regime, the heat flux decreases rapidly with increasing stability due to restriction of vertical velocity fluctuations by the increasing stratification. For weakly stable conditions, the variance s scale according to Monin-Obukhov similarity theory. For very stable conditions, the variances are contaminated by non-turbulent horizontal motions and do not follow the scaling laws. An alternative length sca le based on variances is developed which explains more of the variance of the transfer coefficients compared to the Obukhov length.