Large-eddy simulation (LES) of open channel flow driven by an oscillating pressure gradient with zero surface shear stress was performed. The flow is representative of a tidal boundary layer in the coastal ocean under weak or no wind forcing. For a neutrally stratified water column, during peak pressure gradient forcing or peak tide, the flow develops large scale structures secondary to the mean flow consisting of streamwise-elongated, counter-rotating cells engulfing the bulk of the water column. These structures are similar to the classical Couette cells found in turbulent Couette flow driven by parallel no-slip plates moving in opposite direction. Application of a constant cooling flux of 200 W m−2 at the surface of the open channel flow with an adiabatic bottom wall leads to what we term convective supercells consisting of streamwise-elongated cells of greater intensity and cross-stream width than Couette cells. The signature of the convective supercells is observed even during times when the oscillating mean flow is decelerating, unlike the signature of Couette cells in the case without surface cooling. The signature of the cells is visualized and quantified in terms of instantaneous fields such as streamwise-averaged velocity fluctuations, streamwise velocity averaged over streamwise and cross-stream directions and turbulent structure revealed through depth trajectories of Lumley invariant maps. Investigation of these convective supercells is deemed important due to their strong influence on vertical mixing of momentum and scalars and their potential role in determining the wake behind turbines in tidal flows.