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Journal Article

Diurnal circulation adjustment and organized deep convection


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


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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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
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.