An episode of intermittent turbulence in the nightime PBL during the JORNADA field study

In the early mourning hours of 21 April 2005 during the JORNADA field study, a wave-like disturbance was detected by microbarographs, sonic anemometers, thermocouples, a Doppler SODAR , and a tethersonde. The disturbance moved in the direction of about 200˚ with a speed of about 5 ms^-1, and persisted for about an hour. The pressure oscillation had an average period of about 4 minutes, and an average wavelength of about 1200 m. The wave-like structure was preceded by an increase in surface pressure of about 0.4 mb and a decrease in temperature of about 10˚. During the passage of the disturbance between 05:00 and 05:15 MST, the wind speed at 11 m AGL increased from about 1.5 ms^-1 to about 4 ms^-1 while the wind speed at 1.5 m increased from about 1 ms^-1 to only about 1.8 ms^-1. Between about 22 and 34 m AGL, the wind direction backed from about 57˚ to about 305˚ and the wind speed decreased from about 2 ms^-1 to about 1 ms^-1, and the gradient Richardson number between 11 and 1.5 m AGL decreased from about 10 to about 0.1. Friction velocity and downward heat flux at 11 m AGL jumped to large values for several minutes as the disturbance passed; however, at 1.5 these quantities changed only slightly. This paper presents these and other results in detail, and it is argued that the disturbance is most-likely a density current approximately 30-m deep with sufficient wind shear at its top to generate a wave instability.