Ridge rollers: Mesoscale disturbances on the periphery of cutoff anticyclones

Warm season continental anticyclones are frequently associated with heat waves and droughts. A less appreciated aspect of continental anticyclones is that mesoscale disturbances, known as “ridge rollers,” are often observed to circumnavigate the periphery of these anticyclones. These “ridge rollers” may trigger organized mesoscale convective systems (MCSs) with severe convection occurring on the poleward periphery of the anticyclone where a stronger upper-level jet, and associated jet-entrance region, provides enhanced ascent and deep-layer shear. The limiting factor is the presence (or absence) of a moist planetary boundary layer (PBL). The purpose of this presentation is to document the structure and evolution of heat wave-related continental anticyclones over the US and Australia during July 1995 and February 2004, respectively. Particular attention will be paid to the behavior of “ridge rollers” and their impact on MCSs along the periphery of these anticyclones.

Inspection of the 10-15 July 1995 US continental anticyclone shows that energy propagation and associated downstream development beginning just north of the Tibetan Plateau on 4 July resulted in amplification of the 500 mb height pattern over the US during the period 10-13 July. This amplification occurred in conjunction with the deepening of a cutoff cyclone over central Canada resulting in a strong upper-level jet (> 100 knots) along the Canadian-US border. As ridge rollers moved poleward on the western periphery of the anticyclone, they encountered the upper-level jet entrance region over the north-central US. With ample moist, unstable air present in the PBL (surface temperature (T) ~ 35∞C; dew point temperature (Td) ~28∞C), the “ridge rollers” were able to trigger convection. This convection quickly became severe and moved northeastward, eastward, then southeastward along the poleward periphery of the anticyclone on the poleward edge of the PBL moisture axis.

Inspection of the 7-22 February 2004 Australian continental anticyclone shows a progressive zonal pattern with a strong jet prevailing across southern Australia through 10 February. Individual short wave troughs embedded in the zonal flow allowed weak cold fronts to reach coastal southern Australia during this period. Downstream development resulted in ridging near 135∞E on 13 February, and subsequent troughing near 170∞E on 14 February. This amplification allowed the equatorward portion of the aforementioned trough to cutoff and move westward in the easterlies across northern Australia as a “ridge roller” on 15 February. Concurrently, the initial anticyclone underwent deamplification as an upstream anticyclone began to amplify near 120∞E in response to another bout of downstream development. The aforementioned “ridge roller” proceeded to move poleward on the upstream side of the “new” anticyclone, subsequently becoming entrenched in the westerlies (> 150 knots) by 20 February. Convection appeared to be limited in association with the “ridge roller” on the poleward periphery of the anticyclone, possibly due to drier PBL conditions (T ~ 33∞C; Td ~ 17∞C), and a higher level of free convection (LFC).

Any air of moist tropical origin that reaches southern Australia must traverse the arid continental interior, which is associated with deep mixed layers that favor the mixing out of low-level moisture. This contrasts with the US, where surface evapotranspiration processes can rapidly moisten the PBL, especially beneath the more common “caps” since the mixed layers originate over higher terrain to the west. Any northerly flow ahead of disturbances crossing southern Australia is likely to bring convection-killing desert air into the ascent region of any transient disturbance. These circumstances can change over coastal southeast Australia where a pre-disturbance warm, moist north-northeast flow off the warm Coral Sea can set the stage for MCSs to form.