AGS-PRF: The Diurnal Cycle, Cloud-Radiative Feedbacks, and Large-scale Tropical Dynamics

This award supports a 2-year Postdoctoral Research Fellowship in which the PI will study the influence of the diurnal cycle on the Madden-Julian Oscillation (MJO). The MJO is a large region of convection and rainfall that forms over the Indian Ocean and propagates slowly eastward into the central equatorial Pacific. The MJO influences weather and climate worldwide, affecting the formation of hurricanes in the Gulf of Mexico, atmospheric rivers that produce floods and landslides on the US west coast, and the onset of El Nino events, among other impacts. Despite its importance and concerted efforts over several decades, the MJO has not yet been adequately explained, and weather and climate models have difficulty simulating and predicting it.

Research under this award addresses two aspects of the diurnal cycle, the first of which is the influence of the diurnal cycle of sea surface temperature (SST) in moistening the overlying atmosphere. When the MJO is in its 'suppressed' phase, in which fair weather prevails in the absence of organized convection and rainfall, SST has a relatively pronounced diurnal cycle. The diurnal cycle of SST is expected to result in greater surface evaporation and more vigorous vertical mixing of moisture throughout the lower atmosphere, with the overall moistening serving to promote the onset of the 'active' phase of the MJO. A suite of computer simulations is used to determine the importance of the diurnal cycle of SSTs for moistening the atmosphere and promoting MJO onset.

Further research considers the change in the diurnal cycle of cloudiness that occurs between the suppressed and active phases of the MJO. The smaller and less developed clouds of the suppressed phase occur preferentially in the afternoon, while the deep convective clouds of the active phase are most prominent at night. The PI hypothesizes that this shift in the time of maximum cloudiness has implications for the solar heating of the atmosphere that constitute a positive feedback, enhancing convection during the active phase and inhibiting it in the suppressed phase. Further computer simulations will be performed to test this hypothesis.

Finally, the PI will perform a simulation of an MJO event observed during the DYNAMO field campaign (see AGS-1022899) using a global high-resolution nonhydrostatic model (the ICON model). The results of the simulation will be used to further test the hypothesized effects of the diurnal cycle on the MJO.

As noted above, the work has societal broader impacts due to the worldwide weather and climate impacts of the MJO and the potential of this research to improve MJO forecasts. In addition, the work supports the PI at an early stage of his career, thereby promoting the future workforce in the research area. Also, the work is conducted at the Max Planck Institute of Meteorology (MPI-M) in Hamburg, Germany, under the supervision of Dr. Cathy Hohenegger. The host institution is among the world's top climate modeling centers, and scientific collaboration between a US PI and MPI-M scientists is expected to be beneficial to the US research community.

This award was supported with funding from the Office of International Science & Engineering as well as the Climate and Large-scale Dynamics Program.