The Jack Rabbit (JR) field experiment, involving releases of one or two tons of pressurized liquefied chlorine and ammonia into a depression, took place in 2010 at Dugway Proving Ground, Utah, USA. The releases, of duration about 30s from a short pipe at a height of 2 m, were directed towards the ground. The dense two phase cloud was initially confined in a depression of 2 m depth and 50 m diameter. With wind speeds < about 1.5 m s(-1), the cloud stayed in the depression for 30-60 min, during which it was slowly detrained and moved downwind. At wind speeds > about 1.5 ms(-1), the initial cloud was not well-confined in the depression and moved downwind. Formulas suggested by Briggs et al. in 1990 in this journal satisfactorily predict the time durations of confinement. Sensitivity runs with the SLAB dense gas model show that the effect of a long confinement on maximum downwind concentrations is strongest near the depression. The model-predicted and observed maximum 20s chlorine concentrations agree within a factor of two most of the time, as long as the release times based on Briggs' theory are used. (C) 2012 Elsevier B.V. All rights reserved.