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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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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.


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

Conclusions


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


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