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T. N. Krishnamurti, Ruby Krishnamurti, Anu Simon, Aype Thomas, and Vinay Kumar

This chapter distinguishes the mechanism of tropical convective disturbances, such as a hurricane, from that of the Madden–Julian oscillation (MJO). The hurricane is maintained by organized convection around the azimuth. In a hurricane the organization of convection, the generation of eddy available potential energy, and the transformation of eddy available potential energy into eddy kinetic energy all occur on the scale of the hurricane and these are called “in-scale processes,” which invoke quadratic nonlinearity. The MJO is not a hurricane type of disturbance; organized convection simply does not drive an MJO in the same manner. The maintenance of the MJO is more akin to a multibody problem where the convection is indeed organized on scales of tropical synoptic disturbances that carry a similar organization of convection and carry similar roles for the generation of eddy available potential energy and its conversion to the eddy kinetic energy for their maintenance. The maintenance of the MJO is a scale interaction problem that comes next, where pairs of synoptic-scale disturbances are shown to interact with a member of the MJO time scale, thus contributing to its maintenance. This chapter illustrates the organization of convection, synoptic-scale energetics, and nonlinear scale interactions to show the above aspects for the mechanism of the MJO.

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T. N. Krishnamurti, Anu Simon, Aype Thomas, Akhilesh Mishra, Dev Sikka, Dev Niyogi, Arindam Chakraborty, and Li Li

Abstract

This study addresses observational and modeling sensitivity on the march of the onset isochrones of the Indian summer monsoon. The first 25 days of the passage of the isochrones of monsoon onset is of great scientific interest. Surface and satellite-based datasets are used for high-resolution modeling of the impact of the motion of the onset isochrones from Kerala to New Delhi. These include the asymmetries across the isochrone such as soil moisture and its temporal variability, moistening of the dry soil to the immediate north of the isochrone by nonconvective anvil rains, and formation of newly forming cloud elements to the immediate north of the isochrone. The region immediately north of the isochrone is shown to carry a spread of buoyancy elements. As these new elements grow, they are continually being steered by the divergent circulations of the parent isochrone to the north and eventually to the northwest. CloudSat was extremely useful for identifying the asymmetric cloud structures across the isochrone. In the modeling sensitivity studies, the authors used a mesoscale Advanced Research Weather Research and Forecasting Model (ARW-WRF) to examine days 1–25 of forecasts of the onset isochrone. Prediction experiments were first modeled during normal, dry, and wet Indian monsoons using default values of model parameters. This study was extended to determine the effects of changes in soil moisture and nonconvective rain parameterizations (the parameters suggested by the satellite observations). These sensitivity experiments show that the motion of the isochrones from Kerala to New Delhi are very sensitive to the parameterization of soil moisture and nonconvective anvil rains immediately north of the isochrone.

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