Joint Seminar: Tropical climate and the diurnal cycle of convection

The diurnal cycle of moist convection has been studied for many decades, yet we remain in the dark on certain critical aspects of this fundamental process, particularly as it manifests over the open oceans away from land influences.  In this talk we will entertain the notion that this diurnal cycle may act as a nonlinear forcing mechanism to the climate system – a notion motivated by observations from the recent DYNAMO field experiment (Dynamics of the Madden–Julian Oscillation; MJO).  These observations reveal a pronounced diurnal cycle during the suppressed phase of the MJO, i.e., the period leading up to MJO initiation or onset.  This diurnal cycle is characterized by afternoon cloud deepening, which coincides with reduced large-scale subsidence and increased moistening of the troposphere each afternoon.

A cloud-model experiment will be described in this talk, wherein coupling with large-scale circulation is parameterized and nonlinear “rectification” by the diurnal cycle is isolated by varying the length of the diurnal cycle (12 h, 24 h, and 48 h), or by removing it altogether.  The experiment reveals that the longer the diurnal cycle, the greater the diurnal cloud deepening, tropospheric moistening, and rainfall; and in turn, the more rapid the day-to-day increase of cloud depth and rainfall.  This feedback arises from the diurnal variation of convective instability, which covaries with the diurnal variation of low-level humidity.  This finding indicates that the diurnal cycle may indeed act as a nonlinear forcing mechanism to the climate system, which must therefore be validated and improved in models, to the measure that this nonlinear rectification process impacts the greater climate system.  More work is required to assess if this or a different diurnal rectification mechanism manifests in other regimes, and to quantify its impact on the greater climate system and its predictability.




15:15 h


Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg


James H. Ruppert, Max Planck Institute for Meteorology


Chao Li

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