The Interaction of Simulated Squall Lines with Idealized Mountain Ridges

Abstract

Numerical simulations of squall lines traversing sinusoidal mountain ridges are performed using the Advanced Regional Prediction System cloud-resolving model. Precipitation and updraft strength are enhanced through orographic ascent as a squall line approaches a ridge. The simulated squall line then weakens as it descends the ridge because some of the cold pool is blocked by the terrain, resulting in less lift along the gust front and weaker convective cells. The flow within the cold pool accelerates slightly and the depth of the cold air decreases owing to upstream blocking, transitioning the flow in the cold pool head from subcritical to supercritical, then back to subcritical at the bottom of the ridge. A hydraulic jump forms when the flow transitions the second time, enabling the development of a new convective line downwind of the mountain. These new updrafts grow and eventually replace the older updrafts that weakened during descent. This process results in the discrete propagation of a squall line just downstream of a ridge, resulting in the formation of rain shadows downstream from topographic features. Discrete propagation only occurs if a ridge is of sufficient height, however. This replacement process repeats itself if a squall line encounters multiple ridges. The risk of damaging winds from a squall line is greater on the lee side of ridges and on the top of high ridges. These terrain-forced intensity fluctuations increase with mountain height, because the higher terrain permits even less cold air to flow over it. A wider ridge results in a more gradual orographic enhancement and downslope-induced weakening.

A parameter-free dynamic alternative to hyper-viscosity for coupled transport equations: Application to the simulation of 3D squall lines using spectral elements

“…To further evaluate VMS+DC we apply it to the simulation of (I) the squall line of Weisman et al (1988) (WKR88, from now on) and of (II) the squall line over a flat terrain of Frame and Markowski (2006) (FM06, from now on). The two simulations are initialized with the same sounding given by Weisman and Klemp (1982) but di↵er in the domain size and in the intensity of the thermal perturbation.…”

“…5. Because Weisman et al (1988) do not report the values of either accumulated or instantaneous precipitation, we refer to the values of total accumulated rainfall reported by Frame and Markowski (2006). In spite of the di↵erent domain and resolution, the values of Frame and Markowski are still a valuable source of expected precipitation given a certain initial sounding.…”

“…In spite of the di↵erent domain and resolution, the values of Frame and Markowski are still a valuable source of expected precipitation given a certain initial sounding. In Frame and Markowski (2006), after 18000 s the precipitation ranges between 5 and 50 mm. In this study, at 7200 s the accumulated rainfall ranges between 0 and 35 mm (Fig.…”

“…This makes the simulation relatively smaller but without compromising the full development of the physical features of the squall line. Periodic boundary conditions are used at the four lateral boundaries (unlike in Frame and Markowski (2006), where periodicity is imposed along y only). The simulation is run until t f =18000 seconds.…”

Orographic Effects on Supercell: Development and Structure, Intensity and Tracking

“…Because the evolution of an MCS is critically tied to the behavior of the cold pool -the MCS is maintained by the continuous triggering of new cells by the cold pool -terrain-induced modifications of cold pool evolution and structure unavoidably affect the evolution and structure of the MCS. Frame and Markowski (2006) found that many MCSs weaken as they approach a mountain crest and then re-intensify in the lee of the mountain where a hydraulic jump develops in the outflow (i.e., where the outflow depth rapidly deepens). Markowski and Dotzek (2011) have investigated the effects of idealized terrain on supercells.…”