During the Cooperative Atmosphere Surface Exchange Study field experiment held in 1999 (CASES-99), a wide variety of nocturnal boundary layer (NBL) phenomena were sampled with many different instrument platforms.Despite the the shallow (less than 0.5 degree) slope and local orographic relief (less than 30 m locally), density currents were frequently observed. Generally occurring under clear skies and weak winds, density currents were found to be as deep as 40 m, and were a source of waves and turbulence in the NBL.

Herein, we describe our analysis of two such events, with flows of ~ 2-4 m s^-1 and ~ 20 m depth, both arriving from an easterly direction. Observations from a heavily-instrumented 60 m tower show that these currents appear to be a source of gravity waves, and perhaps, Kelvin-Helmholtz billows. Additionally, turbulence activity is found following the density current head. Mixing and energy dissipation caused by these density currents are discussed.

Finally, a high-resolution modeling study is described. Simulations using CSU-RAMS with vertical grid spacing of O[1]m and horizontal grid spacing O[10]m are used to determine whether terrain undulations, relief and z_o variations along the density current path could be responsible for their initiation.