Cloud-Resolving Simulations of DYNAMO Convection and Cloud Characteristics Associated with the Madden-Julian Oscillation

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Monday, 5 January 2015: 2:30 PM
229AB (Phoenix Convention Center - West and North Buildings)
Cora L. Virgei, Iowa State University, Ames, IA; and X. Wu

Despite the progress in the understanding of the Madden-Julian Oscillation (MJO) by observational and theoretical studies, the MJO simulation remains a major challenge for general circulation models (GCMs) and numerical weather prediction models. GCM simulations suggest that the trigger condition for deep convection as well as the representation of shallow convection play a major role in the improvement of MJO simulations over the Indian Ocean and western Pacific. The simulated MJOs show a different phase relationship between the lower tropospheric wind, surface latent heat flux and precipitation in these two regions. However, the moist static energy builds up from the lower troposphere 15 to 20 days before the peak of MJO precipitation, and reaches the maximum in the middle troposphere near the peak of MJO precipitation over both Oceans. These are indications of preconditioning of the MJO as shown by some observational studies and the recharge-discharge theory. Due to the lack of observations over the Indian Ocean, the characteristics of convection and cloud population are poorly understood for the initiation and evolution of the MJO.

The convective processes and their relationship to the large-scale variables are complex over the Indian Ocean, partly because of the presence of organized convection and the strong interaction between convection and the large-scale motion. For example, the light wind, suppressed convection, and correspondingly high SST (~30oC) situation gives way, often rapidly, to strong westerlies (westerly wind bursts) associated with deep convection, which is often organized into mesoscale cloud clusters or larger superclusters. These convectively disturbed episodes are linked to intraseasonal oscillations, although the relationship between convection and intraseasonal oscillations remains far from clear. Modeling of cloud systems over the Indian Ocean is an effective way to study the physically complex and highly nonlinear coupling among the environmental moisture, cloud microphysics, diabatic heating, convective organization, and surface processes.

The objectives of this study are to perform long-term (covering three MJO events) cloud-resolving model (CRM) simulations forced by the large-scale temperature and moisture advection obtained from DYNAMO,and to examine MJO convection and cloud properties including deep and shallow convection, convective and stratiform precipitation.The CRM simulations in combination with DYNAMO observations offer a comprehensive strategy to facilitate development and evaluation of GCM parameterizations of MJO convection and cloud population associated with the initiation and evolution of MJO over the Indian Ocean. The CRM is forced by the evolving large-scale temperature and moisture advection for the period of 90 days (1 October through 29 December 2011) during DYNAMO. Two periods of strong temperature and moisture forcing and one period of weak forcing were observed during 90 days. CRM simulations forced by the large-scale forcing offers a yet more powerful way to obtain thermodynamically and dynamically consistent cloud, precipitation and latent heating products for investigating the evolution of MJO convection and clouds. We will present the cloud statistical analysis such as the joint PDFs of cloud-top temperature and height, the cloud frequency distribution as a function of cloud base and top heights, and the time series of cloud-top heighthistogramto characterize the evolution of convection and cloud population associated with three MJOs. The updraft and downdraft mass fluxes of deep and shallow convection, convective and stratiform precipitation, cloud populations of liquid and ice water contents, and latent heating profiles from the condensation-evaporation process, freezing-melting process, and deposition-sublimation process will be examined in related to the temperature and moisture forcing.