Cumulus clouds are so-named for their heaped, tower-like, appearance. They transport water vapour and energy from the surface to heights of several kilometres, and are crucial in the energy and mass balances in the atmosphere.

Entrainment is the process by which a fluid parcel not initially part of the flow becomes part of the flow. It is, for instance, the reason one feels a whoosh when a train rolls past.

Entrainment is a crucial part of the dynamics of cumulus clouds; in simple terms, the rate of entrainment decides how high the cumulus cloud can go and how much liquid water it contains when it gets there. These properties of cumulus clouds, in turn, decide the optical properties of clouds, and thus their influence on the radiative energy balance of the planet.

We study the entrainment in cumulus clouds using high-resolution numerical simulations, in collaboration with groups at IISc, Bengaluru and JNCASR, Bengaluru.

Our work has been featured in Physics Magazine.

Papers:

1) Vybhav and Ravichandran, Phys. Rev. Fluids 7 050501 (2022) https://doi.org/10.1103/PhysRevFluids.7.050501, Featured in Physics by Rachel Berkowitz DOI: 10.1103/Physics.15.s67

2) Singhal et al. (2021), in L. Venkatakrishnan et al. (eds.), Proceedings of 16th Asian Congress of Fluid Mechanics, https://doi.org/10.1007/978-981-15-5183-3_43

3) Singhal et al. (2023), forthcoming

4) Vybhav, Sreenivas and Ravichandran (2023), forthcoming

Pictured on the left, mammatus clouds are mysterious blob-like clouds that sometimes appear underneath layer couds called cumulonimbus anvil clouds. These clouds were first described about a hundred years ago, and have been seen in a wide range of settings. Why do these clouds occur only sometimes and not underneath all layer clouds? Why do the lobes appear smooth and laminar?

Several explanations for their blob-like shapes and smooth appearance have been proposed over the last century. It is plausible, if not likely, that no single mechanism explains all observations of mammatus, and that more than one of these mechanisms operates in different settings. One possible mechanism is the instability driven by the settling and evaporation of water droplets from the cumulus anvils into the sub-cloud dry air. We studied this mechanism in detail, and found that the size of the water droplets in the cloud determine whether mammatus clouds will form, thus making testable predictions.

Asperitas clouds are the latest addition to the list of cloud (sub-)types recognised by the World Meteorological Organisation (WMO). They are even more serendipitous than mammatus clouds, and are often associated with vertical shear (the horizontal wind velocity changing with altitude). We showed using numerical simulations that intermediate values of wind shear can lead to asperitas clouds in the same conditions in which mammatus clouds would occur.

Papers:

1) Ravichandran, Meiburg and Govindarajan, J. Fluid Mech. (2020), vol. 899, A27, doi:10.1017/jfm.2020.439

2) Ravichandran and Govindarajan, Phys. Rev. Fluids 7, 010501 (2022) https://doi.org/10.1103/PhysRevFluids.7.010501

Flow in clouds is driven by the buoyancy. This buoyancy is generated by the evaporation or condensation of water or water vapour. These thermodynamic processes are themselves controlled by the fluid dynamics. We study this coupling of the flow and the thermodynamics.

For example, we showed in the paper [2, below], that the fact that water droplets of finite size are expelled from vortices, and that water droplets act as nuclei for condensation, together mean that vortices can be colder than their surroundings. This in turn modifies the dynamics of the vortices [1], leading to the creation of smaller scale structures. Compare the figures at the top and bottom on the left, without and accounting for the effects of droplet inertia respectively.

Papers:

1) Ravichandran, Dixit, Govindarajan, Phys. Rev. Fluids (2017)

2) Ravichandran and Govindarajan, J. Fluid Mech. (2017)

How does rain form?

Water droplets in clouds start life as tiny aerosol particles a fraction of a micron in size. These aerosol particles grow by absorbing water vapour through deliquescence and condensation. Growth by vapour diffusion is rapid when the droplets are small, with the rate of growth varying inversely as the droplet's radius. Beyond a size of about 10 micron, growth by vapour diffusion is very slow. The settling velocities of droplets are proportional to the square of their radii, so these droplets do not settle out of the cloud.

Once a water droplet has (somehow) reached a size of about 50microns, its settling velocity is sufficiently large, and it collides with other smaller droplets on its way down, growing by coalescing with them and falling out as rain when they get to a size of about a millimetre. The formation of rain, in tropical cumulus clouds, is estimated to take only about 15 minutes.

A long-unsolved puzzle in cloud microphysics is to explain how droplets grow from about 10micron to about 50micron, in which range neither diffusive growth nor coalescence-driven growth are sufficiently rapid to explain the observed rapidity of rain formation.

This is the question we (my PhD advisor Rama Govindarajan and I) tried to answer in a series of studies from my doctoral work at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) Bengaluru, and the Tata Institute of Fundamental Research Centre for Interdisciplinary Sciences (TCIS) Hyderabad.

We start by noting that water droplets of finite size have a non-zero drag acting on them. This `Stokes' drag results in droplets getting thrown out of rapidly spinning regions--vortices--in turbulent flow and, as these droplets are thrown out, they can collect other smaller droplets and coalesce with them.

This is an example of caustics--the existence of multiply-valued droplet velocity in flow, and we show in our work that caustics-induced collisions-coalescence can explain rain formation.

Papers:

1. Attracting fixed points for heavy particles in the vicinity of a vortex pair, S. Ravichandran, Prasad Perlekar and Rama Govindarajan, Physics of Fluids, 26(1):013303, (2014), doi: 10.1063/1.4861395

2. Caustics and clustering in the vicinity of a vortex, S. Ravichandran and Rama Govindarajan, Physics of Fluids, 27(3), (2015), doi: 10.1063/1.4916583

3. Caustics-induced coalescence of small droplets near a vortex, P. Deepu, S. Ravichandran and Rama Govindarajan, Physical Review Fluids 2, 024305 (2017), doi: 10.1103/PhysRevFluids.2.024305

4. Droplet Collisions in Turbulence: Insights from a Burgers Vortex, L. Agasthya, J. R. Picardo, S. Ravichandran, R. Govindarajan, S. S. Ray, Physical Review E 99, 063107 (2019), doi: 10.1103/PhysRevE.99.063107

5. Waltz of droplets and the flow they live in, S. Ravichandran and Rama Govindarajan, Physical Review Fluids 7, 110512 (2022), Preprint: https://arxiv.org/abs/2207.00540