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Mechanical forcing of convection by cold pools: collisions and energy scaling.
  • Bettina Meyer,
  • Jan Olaf Haerter
Bettina Meyer
Niels Bohr Institute, University of Copenhagen

Corresponding Author:bettina.meyer@nbi.ku.dk

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Jan Olaf Haerter
Niels Bohr Institute, University of Copenhagen
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Forced mechanical lifting through cold pool gust fronts can trigger new convection, and previous work highlights the role played by collisions between cold pools. However, as conceptual models show, the emergent organisation from two versus three colliding cold pools differs strongly. In idealised dry large-eddy simulations we therefore examine which of the two processes dominates. We simulate the spread of gravity currents and the collisions between two and three cold pools. The triggering likelihood is quantified in terms of the cumulative vertical mass flux of boundary layer air and the instantaneous updraft strength, generated at the cold pool gust fronts. We find that cold pool expansion can be well described by initial potential energy and time alone. Cold pool expansion monotonically slows but shows an abrupt transition between an axi-symmetric and a broken-symmetric state – mirrored by a sudden drop in expansion speed. We characterize these two dynamic regimes by two distinct power-law exponents and explain the transition by the onset of ‘lobe-and-cleft’ instabilities at the cold pool head. Two-cold pool collisions produce the strongest instantaneous updrafts in the lower boundary layer, which we expect to be important in environments with strong convective inhibition. Three-cold pool collisions generate weaker but deeper updrafts and the strongest cumulative mass flux and are thus predicted to induce the largest mid-level moistening, which has been identified as a precursor for the transition from shallow to deep convection. Combined, our findings may help decipher the role of cold pools in spatially organising convection and precipitation.
Nov 2020Published in Journal of Advances in Modeling Earth Systems volume 12 issue 11. 10.1029/2020MS002281