Numerical simulation of transitions in boundary layer convective structures in a lake-effect snow event

Numerical simulations are used to study transitions between boundary layer rolls and more cellular convective structures observed during a lake-effect snow event over Lake Michigan on 17 December 1983. Weak lake-effect nonrol l convection was observed near the eastern (downwind) shore preceding passa ge of a secondary cold front. After frontal passage horizontal wind speeds in the convective boundary layer increased. with subsequent development of linear convective patterns. Thereafter the convective pattern became more t hree-dimensional as low-level wind speeds decreased. Little directional she ar was observed in any of the wind profiles. Numerical simulations with the Advanced Regional Prediction System model were initialized with an upwind sounding and radar-derived wind profiles corresponding to each of the three convective structure regimes. Model derived reflectivity fields were in go od agreement with the observed regimes. These simulations differed primaril y in the initial wind speed profiles, and suggest that wind speed and shear in the lower boundary layer are critical in determining the linearity of c onvection. Simulation with an upwind-overlake wind profile, with strong low -level winds, produced the most linear model reflectivity structure. Fluxes and measures of shear-to-buoyancy ratio for this case were comparable to o bservations. Model sensitivity rests were conducted to determine the importance of low-l evel wind speed and speed sheer in determining the linearity of convection. Results are consistent with trends expected from ratios of buoyancy to she ar (but not proposed numerical threshold values). Eliminating all direction al shear from the initial wind profile for the most linear case did not red uce the degree of linearity, thus showing that directional shear is not a r equirement for rolls in lake-effect convection. Elimination of clouds (prin cipally latent heating) reduced the vertical velocities by about 50%. It wa s found that variations in wind speed shear below 200-m height played a maj or role in determining the degree of linearity of the convection.