Turbulence Intermittency and Nocturnal Disturbances in CASES-99

All the nocturnal disturbance events including density currents, solitary waves and internal gravity waves in CASES-99 (Cooperative Atmosphere-Surface Exchange Study) have been investigated. We found that all the density currents occurred under weak wind conditions, i.e. the wind at the 60 m tower was less than 4-6 m/s. The clear signal of the temperature drop associated with the density current could be at the upper level of the 60-m tower where the temperature change before and after the passage of the density current is large. The wind vector, water vapor, and CO2 concentration changes associated with the density current depend on the ambient conditions. Both density current and solitary waves could be associated with frontal passages. The density current could also result from drainage flows. A statistical study on relationships between turbulence intermittency and the nocturnal disturbances will be presented.