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How do ocean warm anomalies favor the aggregation of deep convective clouds?

Abstract : We investigate the role of a warm sea-surface temperature (SST) anomaly(hot-spot of typically 3 K to 5 K) on the aggregation of convection using cloudresolving simulations in a non-rotating framework. It is well known that SSTgradients can spatially organize convection. Even with uniform SST, the spon-taneous self-aggregation of convection is possible above a critical SST (here295 K), arising mainly from radiative feedbacks. We investigate how a cir-cular hot-spot helps organize convection, and how self-aggregation feedbacksmodulate this organization. The hot-spot significantly accelerates aggrega-tion, particularly for warmer/larger hot-spots, and extends the range of SSTsfor which aggregation occurs, however at cold SSTs (290 K) the aggregatedcluster disaggregates if we remove the hot-spot. A large convective instabil-ity over the hot-spot leads to stronger convection and generates a large-scalecirculation which forces the subsidence drying outside the hot-spot. Indeed,convection over the hot-spot brings the atmosphere towards a warmer tem-perature. The warmer temperatures are imprinted over the whole domainby gravity waves and subsidence warming. The initial transient warmingand concomitant subsidence drying suppress convection outside the hot-spot,thus driving the aggregation. The hot-spot induced large-scale circulation canenforce the aggregation even without radiative feedbacks for hot-spots suffi-ciently large/warm. The strength of the large-scale circulation, which definesthe speed of aggregation, is a function of the hot-spot fractional area. At equi-librium, once the aggregation is well established, the moist convective regionwith upward mid-tropospheric motion, centered over the hot-spot, has an areasurprisingly independent of the hot-spot size.
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Submitted on : Friday, December 6, 2019 - 5:08:57 PM
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Sara Shamekh, Caroline Muller, Jean-Philippe Duvel, Fabio d'Andrea. How do ocean warm anomalies favor the aggregation of deep convective clouds?. Journal of the Atmospheric Sciences, American Meteorological Society, 2019, ⟨10.1175/JAS-D-18-0369.1⟩. ⟨hal-02397865⟩

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