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The effects of upstream relative humidity (RH) on low-level wind and precipitation patterns for low-speed, statically stable flows over a mountain are investigated using idealized two- and three-dimensional numerical-simulation experiments in which RH is increased from 0% to 100%. For RH less than some critical threshold, the flow upstream becomes less decelerated as RH is increased; for RH greater than this threshold, the flow upstream becomes more decelerated as RH is increased. This increasing deceleration with RH is due to locally enhanced static stability resulting from enhanced condensation near the freezing level. Analyses from the simulations indicate that the lifted condensation level and the height of the freezing level are significant control parameters for the upstream-flow deceleration in the steady-state solutions. Dimensional analysis using these control parameters (as well as others) brings forth new nondimensional parameters that are shown to enter into analytic formulas for the orographic upstream-flow deceleration in a moist atmosphere.

Observations and previous research of squall lines impinging on mountain ranges have revealed that the squall lines sometimes stall upstream of the mountains for several hours leading to copious accumulations of precipitation. It has been hypothesized that squall-line stagnation may be more prone to occur in flows where the Froude number (F = U/Nh, where U is the basic-state wind, N is the Brunt-Vaisala frequency, and h is the mountain height) is low. This hypothesis is tested herein through a series of idealized, two-dimensional experiments where a convective system was triggered upstream of a mesoscale mountain in conditionally unstable flow. For simulations with relatively low Froude numbers, stagnation of the preexisting convective system was not observed. In the simulations with high values of F, squall lines were noted to stagnate between 100 and 200 km upstream of the mountain. This result indicates that squall-line stagnation may be more favored for moderate to large values of F for conditionally unstable flow. The mechanisms leading to the formation of the stationary convective system upstream of the mountain in the unblocked flows were explored and it was found that evaporative cooling played a pivotal role in the stagnation of the squall line.

During intensive observation period (IOP)-8 of the Mesoscale Alpine Program, there was a stable layer of air in the lowest levels of the Po Valley and just upstream of the Apennines. In this study, the effects of the stable layer on the formation of convection in the southern Alpine region were investigated through a series of two-dimensional, idealized experiments. The goals of this study were twofold: 1) to determine if stable layer strength affected the placement of convection and 2) to test the notion that the stable layer during IOP-8 behaved as an effective mountain. To accomplish the first objective, three simulations were compared in which the strength of the inversion and low-level cooling in the Po Valley and upstream of the Apennines was varied. The results of these simulations show that the stronger the inversion and low-level cooling, the farther south the convection was positioned. Additionally, it was found that convection developed as a result of the formation of a broad region of moist instability over the stable layer. Cellular convection developed in this region of moist instability. The second objective was tested through a simulation where the cold pool was replaced by terrain (MMTN). As in the reference (or STB10) simulation, the upslope of the terrain in the MMTN simulation was characterized by a wide zone of moist instability. However, wave structures to the lee of the Apennines were markedly different in the STB10 and MMTN simulations. This led to different convective and precipitation patterns, with the MMTN simulation exhibiting heavier convection over the Po Valley while the heaviest convection in STB10 was upstream of the Apennines. These results suggest that, at best, the stable layer in the STB10 simulation can only be roughly approximated by terrain.

The dynamical impacts of an unusually strong stable layer that developed over the Po Valley and northern Ligurian Sea during Mesoscale Alpine Program (MAP) intensive observation period 8 (IOP-8) on the formation of convection over the Ligurian Sea are explored. Based on numerically simulated equivalent potential temperature, wind vectors, and by a trajectory analysis of parcels both beneath and above the stable layer, it is shown that the stable layer behaved as a material surface or “effective mountain” to the airstreams impinging on it from the south. Additional analyses show that the leading edge of the stable layer was collocated with maxima in upward motion and a strong positive moisture flux. Hence, it was further argued and demonstrated through inspection of soundings upstream of the cold dome and trajectory analyses that lifting by the stable layer enhanced convective activities over the Ligurian Sea. Finally, processes contributing to the maintenance of the stable layer during IOP-8 were explored. It was found that the differential advection of a warm, less stable air mass on top of a cooler, more stable air mass helped maintain the stable layer. The Ligurian Apennines made a secondary contribution to the stagnation of the cool air in the Po Valley by partially blocking this air mass from exiting the valley to the south. [ABSTRACT FROM AUTHOR]