School of Earth and Environment INSTITUTE FOR CLIMATE AND ATMOSPHERIC SCIENCE. Maintenance of convection above a cold undercurrent in a mesoscale convective system.
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.
- 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 (continued) convection plus dual slantwise circulations (possible causes?);
- 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