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 evident on the dynamic tropopause (DT, defined as the 1.5 potential vorticity unit (PVU) surface), known as "ridge rollers" (RRs), are often observed to circumnavigate the periphery of these anticyclones. RRs often originate as fractures from the equatorward ends of northeast-to-southwest oriented PV tails, and move westward along the equatorward periphery of continental anticyclones. As these RRs move poleward and then eastward around the upstream and poleward periphery, respectively, of the anticyclone they may interact with other subsynoptic-scale disturbances embedded in the westerlies on the poleward periphery of the anticyclone. RRs may be associated with convection along the anticyclone periphery with organized mesoscale convective systems (MCSs) occurring on the poleward periphery where the aforementioned upper-level westerly flow, and an associated jet-entrance region, provides enhanced ascent and deep-layer shear. A 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 RRs and their impact on the mode and severity of convection 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 on the DT) along the Canadian-US border. As RRs 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 (850 mb theta-e values approaching 360 K), along with an elevated mixed layer (850-500 mb lapse rates approaching 8 K/km) the RRs were able to trigger convection. This convection quickly became severe in the presence of large CAPE values (approaching 7000 J/kg) 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 135E on 13 February, and subsequent troughing near 170E 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 RR on 15 February. Concurrently, the initial anticyclone underwent deamplification as an upstream anticyclone began to amplify near 120E in response to another bout of downstream development. In response to this amplification, another trough amplified ahead of the second ridge resulting in a trough fracture producing three more RRs. These RRs proceeded to move poleward on the upstream side of the "new" anticyclone, subsequently becoming entrenched in the westerlies (> 150 knots on the DT) by 20 February. Convection appeared to be limited in association with these RRs on the poleward periphery of the anticyclone due to drier PBL conditions (850 mb theta-e values approaching 340 K), and weaker 850-500 mb lapse rates (approaching 5 K/km) when compared to the US case. This is likely because prefrontal northerlies in Australia have a source region over the arid continent whereas prefrontal southerlies in the US have a source region over the Gulf of Mexico. Surface evapotranspiration processes can also rapidly moisten the PBL in the US, especially beneath the more common "caps" since the mixed layers originate over higher terrain to the west. 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.