Morphology of breeze circulations induced by surface flux heterogeneities and their impact on convection initiation
Rochetin, Nicolas ; Couvreux, Fleur ; Guichard, Françoise
This study analyses the role of breeze circulations induced by a surface sensible heat flux heterogeneity on deep convection initiation. Large-eddy simulations are used to disentangle the processes at play in a typical case of daytime triggering of deep convection over a semi-arid land. We show that the presence of a realistic surface sensible heat flux heterogeneity leads to an earlier triggering of convection and induces a strong determinism in the triggering location at the beta-mesoscale (i.e. ?50 km). The transition to deep convection consists of three consecutive stages, each one corresponding to a specific mode of interaction between (i) the boundary-layer thermals (small-scale), (ii) the breeze circulation (mesoscale) and (iii) the background wind (synoptic scale). These stages are both interpreted thermodynamically and morphologically. All along the transition phase, the boundary-layer growth acts to slow down the background wind, which strengthens the breeze circulation. The breeze evolves towards a circular shape which optimizes moisture convergence and cloud formation just prior to triggering. The presence of wind shear leads with a more asymmetric shape of the breeze in the afternoon, associated with a preferential triggering on the down-shear side of the breeze circulation.
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