Mechanisms of Convectively-Induced Turbulence in the Vicinity of Strong Jet Streams

In this talk we discuss results from high-resolution numerical simulations of observed clear-air turbulence cases influenced by organized convection occurring in the vicinity of strong jet streams. The cases selected share the common attribute of occurrence during the midlatitude cold-season (November-March) when background conditions are baroclinic and jet streams are strong, but highlight the diverse mechanisms by which the associated convection can influence nearby turbulence.

In one case (9-10 March 2006) a turbulence outbreak over the central United States was concentrated within two separate regional clusters. In the northern cluster vertical shear at aircraft cruising altitudes (36-39 kft) was dramatically enhanced within the northwest edge of an anticyclonic outflow associated with large squall-type mesoscale convective system (MCS). In this sector of the MCS outflow the winds were approximately parallel to, and thus added significantly, to the synoptic jet. Here, the moderate-to-severe turbulence resulted from Kelvin-Helmholtz instability in the strong shear layer immediately above the outflow jet located several hundred km northwest of the ongoing parent convection. Farther south, moderate-to-severe turbulence occurred at cruising altitudes several km above shallower, but equally intense convection. Here, the shallow convection impinged upon a tropopause fold in the middle troposphere, and gravity waves were triggered above with wave-breaking and turbulence occurring as the waves encountered a critical level that was influenced by the vertical shear associated with the aforementioned larger-scale anticylonic outflow.

A second case (14-15 November 2011) featured turbulence at cruising altitudes (31-39 kft) over the North Atlantic south of maritime Canada where cirrus banding was widespread. This banding occurred over scales of approximately 1000 km to the south of antecedent deep convection associated with a surface front and strong upper-level jet stream. Over a broad horizontal region near the shallow (2-km-deep) simulated cirrus bands, criteria for both moist static instability and inertial instability were locally satisfied indicating the possibility that like in the previous case, multiple mechanisms could be contributing to the observed turbulence. Though the turbulence occurred several hundred km south of ongoing deep convection, a dry simulation suggests the important role of the convection in the turbulence generation as neither of the above instability criteria were satisfied and model TKE values were substantially lower than in the full-physics run that contained convection.

In a more recent case (17 March 2014) turbulence occurred at 18kft underneath a strong midtropospheric jet stream as the airliner was located over the northwestern Gulf of Mexico on descent into Houston Intercontinental Airport. The reported severe turbulence was located 150-300 km north of a wide band of relatively weak convection. Here, the relationship between the turbulence and convection appears to be more subtle than in the previous two cases. As in the previous cases, comparison between a full-physics control simulation and a dry simulation revealed an anticylonic outflow induced by the convection near a synoptic jet. However, the strength of this outflow was considerably weaker than in the previous cases and small Richardson numbers supportive of turbulence occurred beneath the strong midlevel synoptic jet even in the dry simulation. Nevertheless in the control run with convection, patches of low Richardson numbers extended a few hundred km farther northwest, closer to the location of the reported turbulence. This suggests the possibility that horizontally propagating gravity waves generated by weak convection could influence turbulence in environments of strong vertical shear near jet streams that are already close to supporting turbulence as indicated by small Richardson numbers. The hypothesis for this additional mechanism for convectively-induced turbulence near jet streams will be tested further with higher-resolution simulations.