ABSTRACT
The effects of topography, wind shear, and zonal wind speed on dryline formation and evolution are investigated using a three-dimensional nonhydrostatic mesoscale model. Rather than conduct a case study, a parameter study is performed to examine factors that control the depth and strength of the dryline circulation.
This study ascertains that the potential for convective storm formation is greatest in those environments in which the lower-tropospheric westerly wind around 1 km MSL is weak. This configuration results in flow nearly parallel to the north-south oriented dryline boundary. Under these conditions subsiding westerly flow and ERL formation did not strengthen the capping inversion above the eastern CBL. In addition, moist parcels from near the surface are able to reside within the dryline updraft resulting in higher mid-tropospheric relative humidity subsequently increasing the likelihood of convective storm initiation.
Hydrostatic pressure falls and subsidence develops to the east of the dryline as the westerly winds around 1 km MSL are increased. The downsloping westerly flow strengthens and lowers the capping inversion. In addition, the relaxed horizontal pressure gradient thereafter reduces the convergence and frontogenetical collapse along the dryline. Consequently, the dryline circulation height decreases, the dryline updraft becomes weaker and air parcels resident within the updraft detrained at lower elevations.
Simulations using cloud-resolving horizontal resolution develop long, narrow horizontal bands of counter-rotating circular motions within the unstably stratified convective boundary layer. These circulations are analogous to horizontal convective rolls discussed in previous boundary layer studies. In environments containing weak westerly winds the study results are inconclusive as to whether the convective bands modulate the dryline subsequently increasing the potential for convective storm formation. However, as the westerly winds are increased the interactions between the horizontal circulation bands and the dryline account for a considerable amount of the variability along the dryline.