ICTS

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Self-aggregation is the spontaneous transition from randomly distributed to organized convection despite homogeneous boundary conditions. Here I review the occurrence of this phenomenon in idealized numerical simulations and its physical mechanisms. I also discuss possible applications of self-aggregation to real-world observed phenomena such as the MJO and tropical cyclones.

We present the dynamical core of the Atmosphere model Aeolus 2.0, characterized by intermediate complexity. This model is grounded in the pseudo-spectral moist-convective Thermal Rotating Shallow Water (mcTRSW) framework with minimal parametrization over the full sphere. The Dedalus algorithm, renowned for its handling of spin-weighted spherical harmonics, manages the pseudo-spectral problem-solving tasks. We introduce an improved version of moist convection and a novel approach to refine the estimation of sea surface evaporation flux and the columnar bulk of humidity, pivotal components of the bulk aerodynamic scheme. The proposed scheme incorporates factors such as zonal wind velocity, variations in lower troposphere (potential) temperature, and free convection to enhance accuracy. Indeed, we demonstrate how the model enables the simulation of various atmospheric phenomena such as the Madden-Julian Oscillation and localized extreme heatwaves. For example, Aeolus 2.0 has facilitated the proposal of a novel theory for the genesis and dynamics of the MJO (Rostami et al., 2022). According to this theory, an eastward-propagating MJO-like structure can be generated in a self-sustained and self-propelled manner due to the nonlinear relaxation (adjustment) of a large-scale positive buoyancy anomaly, a depressed anomaly, or a combination of these. This occurs when the anomaly reaches a critical threshold in the presence of moist convection at the Equator. This MJO-like episode possesses a convectively coupled 'hybrid structure' consisting of a 'quasi-equatorial modon' with an enhanced vortex pair and a convectively coupled baroclinic Kelvin wave (BKW), exhibiting a greater phase speed than that of a dipolar structure on an intraseasonal time scale. Additionally, we demonstrate the model's capability to simulate extreme localized heatwaves in mid-latitudes (Rostami et al., 2024). This study examines the influence of large-scale localized temperature anomalies in mid-latitude regions on condensation patterns and corresponding circulation in various environments. Depending on the perturbation's characteristics, they can induce atmospheric instability, leading to precipitation systems such as rain bands and distinctive cloud patterns. The study also demonstrates the initiation of an anticyclonic high-pressure rotation in the upper troposphere due to heating on lower troposphere, resulting in an anisotropic northeast-southwest tilted circulation of heat flux.

I will present an overview of some minimal models of moist convection and demonstrate their utility in understanding stability and dynamics.

Every raindrop is formed by millions of cloud droplet collisions, and cloud turbulence is thought to be crucial in bringing about regions of extreme clustering, or caustics. Cloud droplets centrifuge out of vortical regions, and crowd into strain regions. We ask whether it is sufficient to pass through high strain to undergo caustics, and show that spending time in compressional strain is necessary.

A demonstration session will be conducted for the students to explain the BFS model fidelity to generate the convective coupling during monsoon season over India. The session will also highlight the impact of deterministic closure in the NWP model such as BFS as compared to reanalyses and observed rainfall.

Reversal of the sign of the Coriolis force across the equator creates a waveguide along the equator home to a wide spectrum of waves. Initially found as solutions to the shallow water model on the equatorial beta plane, they include Rossby waves, gravity waves, and mixed forms. I discuss the leading solution forms and their propagation mechanisms. Then, I give special focus to the Kelvin wave, a non dispersive disturbance of zonal wind. I show how Kelvin waves interacting with the mean atmospheric flow gives rise to the phenomenon known as the Madden Julian Oscillation.

The equatorial Pacific exhibits a clear seasonal cycle, with West Pacific SSTs being highest during boreal autumn and El Niño/Southern Oscillation (ENSO) events tending to peak during boreal winter. In this work, we use the concept of a monsoonal mode and idealized coupled simulations to show that the presence of a large land mass in the Northern Hemisphere can lead to these seasonal asymmetries. Specifically, warm air moving east from the Asian summer monsoon suppresses surface fluxes in the West Pacific, leading to increased temperature there during the following months. The warmth of the West Pacific in boreal autumn strengthens the Walker circulation and the zonal temperature gradient across the Pacific, leading to the growth of El Niño events during that season. In summary, the presence of the Asian monsoon north of the equator results in ENSO events preferentially growing during boreal autumn and peaking during boreal winter.

In comparison to other regions of the globe, the weather systems that affect precipitation in the tropics have received less attention. In this study, we investigate the structure, propagation, and thermodynamics of convectively coupled waves that impact precipitation in the tropical region. We found that slowly evolving tropical systems are “moisture modes,” i.e., moving regions of high humidity and precipitation that are maintained by interactions between clouds and radiation. The faster waves are systems that exhibit relatively larger fluctuations in temperature. Here we also documented that tropical waves could grow by extracting moisture from the Hadley cell, thereby weakening it. They also transport moisture to higher latitudes. Our results challenge the notion that the Hadley cell is the sole transporter of energy out of the tropics and instead favor a view where tropical waves are also essential for the global energy balance. They dry the humid regions and moisten the drier regions via stirring. Our results underscore the importance of water vapor in the governing dynamics of tropical waves, and the need to move away from dry theory as a basis to understand convectively coupled tropical motions.

A lecture discussing the role played by midlatitude eddies in determining the strength of the Hadley Cell.

Extreme precipitation and heat events are highly impactful — improving the precision of climate model projections of changes in these events is thus a key endeavor. Here we focus on two examples where fundamental understanding of process mechanisms can narrow uncertainty. 1) The processes that yield characteristic shapes for precipitation probability distributions provides a way of reducing  uncertainties in risk increases in face of interregional differences in probability increases of intense precipitation.  2) The frequency of hot days is known to increase under warming. However, heat wave durations are more challenging because daily temperature variations are correlated. Theory for autocorrelated temperature evolution above a threshold turns out to share common dynamics with precipitation. Climate model diagnostics informed by this dynamics show that probabilities of long heatwave durations have an unavoidable acceleration per increment of warming. The same analysis allows collapse of climate model future projections across regions onto scaling that can be compared with reanalysis warming in the historical period, reinforcing confidence in this effect.