The flux Richardson number Rf (also known as the mixing efficiency) for the
stably stratified atmospheric boundary layer is investigated as a function of the gradient
Richardson number Rig using data taken during two field studies: the Vertical
Transport and Mixing Experiment (VTMX) in Salt Lake City, Utah (October 2000),
and a long-term rural field data set from Technical Area 6 (TA-6) at Los Alamos
National Laboratory, New Mexico. The results show the existence of a maximum
Rf (0.4–0.5) at a gradient Richardson number of approximately unity. These
large-Reynolds-number results agree well with recent laboratory stratified shear layer
measurements, but are at odds with some commonly used Rf parameterizations,
particularly under high-Rig conditions. The observed variations in buoyancy flux
and turbulent kinetic energy production are consistent with the concept of global
intermittency of the atmospheric stable boundary layer.