Maintenance of convection above a cold undercurrent in a mesoscale

convective system

School of Earth and EnvironmentINSTITUTE FOR CLIMATE AND ATMOSPHERIC SCIENCE

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.

Δ p WDA

Conclusions - 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

- 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

Conclusions (continued)