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Journal Article

Diurnal circulation adjustment and organized deep convection

MPS-Authors
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Ruppert,  James H.
Hans Ertel Research Group Clouds and Convection, The Atmosphere in the Earth System, MPI for Meteorology, Max Planck Society;

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Hohenegger,  Cathy
Hans Ertel Research Group Clouds and Convection, The Atmosphere in the Earth System, MPI for Meteorology, Max Planck Society;

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Fulltext (public)

jcli-d-17-0693.1.pdf
(Publisher version), 2MB

Supplementary Material (public)

dc_agg_ruppert_hohenegger_jclimate2018.tar.gz
(Supplementary material), 39MB

Citation

Ruppert, J. H., & Hohenegger, C. (2018). Diurnal circulation adjustment and organized deep convection. Journal of Climate, 31, 4899-4916. doi:10.1175/JCLI-D-17-0693.1.


Cite as: http://hdl.handle.net/21.11116/0000-0000-0677-4
Abstract
This study investigates the diurnal cycle of tropical organized deep convection and the feedback in large-scale circulation. By considering gravity wave phase speeds, we find that the circulation adjustment into weak temperature gradient (WTG) balance occurs rapidly (<6 h) relative to diurnal diabatic forcing on the spatial scales typical of organized convection (≤500 km). Convection-permitting numerical simulations of self-aggregation in diurnal radiative–convective equilibrium (RCE) are conducted to explore this further. These simulations depict a pronounced diurnal cycle of circulation linked to organized convection, which indeed maintains WTG balance to first order. A set of sensitivity experiments is conducted to assess what governs the diurnal cycle of organized convection. We find that the “direct radiation–convection interaction” (or lapse-rate) mechanism is of primary importance for diurnal precipitation range, while the “dynamic cloudy–clear differential radiation” mechanism amplifies the range by ∼30%, and delays the nocturnal precipitation peak by ∼5 h. The differential radiation mechanism therefore explains the tendency for tropical heavy rainfall to peak in the early morning, while the lapse-rate mechanism primarily governs diurnal amplitude. The diurnal evolution of circulation can be understood as follows. While nocturnal deep convection invigorated by cloud-top cooling (i.e., the lapse-rate mechanism) leads to strong bottom-heavy circulation at nighttime, the localized (i.e., differential) top-heavy shortwave warming in the convective region maintains circulation at upper levels in daytime. A diurnal evolution of the circulation therefore arises, from bottom-heavy at nighttime to top-heavy in daytime, in a qualitatively consistent manner with the observed diurnal pulsing of the Hadley cell driven by the ITCZ.