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Maintenance of convection above a cold undercurrent in a mesoscale convective system

School of Earth and Environment INSTITUTE FOR CLIMATE AND ATMOSPHERIC SCIENCE. Maintenance of convection above a cold undercurrent in a mesoscale convective system.

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Maintenance of convection above a cold undercurrent in a mesoscale convective system

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  1. School of Earth and Environment INSTITUTE FOR CLIMATE AND ATMOSPHERIC SCIENCE Maintenance of convection above a cold undercurrent in a mesoscale convective system KA Browning1, JH Marsham 1, JC Nicol 2, AM Blyth 1 , U Corsmeier 3, N Kalthoff3, SD Mobbs1 , EG Norton4, DJ Parker1, FM Perry1, BA White1 and others. 1University of Leeds, UK. 2University of Reading, UK. 3Universität/Forshungzentrum Karlsruhe, Germany. 4University of Manchester, UK.

  2. Δ p WDA

  3. - Case study shows elevated convection with upright convection plus dual slantwise circulations (possible causes?); their slope decreased with time (1 in 4 to 1 in 9); the lower slantwise downdraught was main rear-inflow jet, RIJ - the RIJ did not penetrate to ground through cold undercurrent; instead it compressed the undercurrent where it impacted, and the undercurrent deepened ahead of it to form a gravity wave - the undercurrent gravity wave moved with the MCS - it was in a quasi-steady state for >2 hours - sfc press anomaly 2 mb over 25 km; explained hydrostatically - associated with a ground-rel wind-reversal up to 1000 m deep - lifting of streamlines by up to 1km (1.5 km descent behind) - strong shearing instability and mixing behind wave - rain evap gave less than 0.5 g/kg moistening and 1C cooling - the observed extended region of cooling (~4C) caused instead by shadowing from anvil canopy Conclusions

  4. Conclusions (continued) - The decrease in static stability with height and opposing low-level flow in the undercurrent, together lead to trapping/ducting, which increases amplitude of undercurrent wave and hence increases lifting to overcome CIN of source air for updraught - Possible generation mechanisms for undercurrent gravity wave: i impact of RIJ on cold undercurrent, as suggested by this study ii latent heat release in convection, Schumacher and Johnson 2008 iii evaporation cooling in undercurrent, Crook and Moncrieff 1988

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