The response of simulated nocturnal convective systems to a low-level jet

A recent study by Parker (2008) examined the dynamics of initially surface-based mesoscale

convective systems (MCSs) that encounter an increasingly stable boundary layer, akin to

what occurs with the onset of nocturnal cooling. The present study builds upon this work

by investigating the effect of adding a low-level jet (LLJ) to a simulated surface-based MCS

in concert with low-level cooling. This simulates, in an idealized sense, the environmental

transition experienced by an MCS that develops during the late afternoon and persists after

nightfall as the boundary layer stabilizes and the LLJ develops. The development, structure

and location of the simulated LLJ are based on past climatological studies of the LLJ in the

central United States. A variety of jet orientations are tested, and sensitivities to jet speed

and height are explored.

The primary impacts of adding the LLJ are that it alters the wind shear in the layers just

above and below the jet, and that it alters the magnitude of the storm-relative inflow in

the jet layer. The changes to wind shear have an attendant impact on low-level lifting as

suggested by Rotunno et. al (1988). However, the more significant impact on storm intensity

and longevity comes from the jet modulating the intensity of the storm relative inflow. When

oriented perpendicular and toward (away from) the storm, the LLJ enhances (diminishes)

storm intensity and longevity. In some cases, seemingly positive impacts on low-level lifting

due to shear generated by the jet can be offset by the negative impacts of reduced inflow.

The LLJ has a more significant impact on the simulated MCS when the jet speed is increased,

however the overall character of the impact remains unchanged. The shear-related impacts

on updraft strength are most significant when the jet is located proximal to the base of the

bore driving the elevated MCS, while the inflow modulation effects are most significant when

the jet is co-located with the layer of highest-CAPE air above the nocturnal stable layer.

Finally, simulations featuring an LLJ oriented parallel to a completely 3D MCS result in an

asymmetric precipitation distribution favoring the flank of the storm upon which the jet is

impinging.