Quentin Nicolas

Postdoctoral Fellow, ETH Zürich

Email: qnicolas __at__ berkeley __dot__ edu
Github profile, Google scholar, CV

About

I am a postdoctoral fellow in the department of Environmental Systems Sciences at ETH Zürich, in Heini Wernli's group. My main research interest lies in climate dynamics, with an emphasis on the dynamics of the tropical atmosphere. I strive to develop simple theories to understand the distribution of rainfall in the tropics. I previously obtained my Ph.D. in the department of Earth and Planetary Science at UC Berkeley, working with William Boos. My thesis work focused on the organization of rainfall around mountains, termed orographic precipitation.

I am more broadly interested in geophysical fluid dynamics, and I have had the chance to dip my toes in the world of the geodynamo, working with Bruce Buffett on wave generation in Earth's outer core. I am also collaborating with Geoffrey Vallis on the physics of superrotation. I met Geoff during the Geophysical fluid dynamics summer program at WHOI, where I was a fellow in 2023.

Prior to my PhD, I earned a MS in applied mathematics at Ecole polytechnique, just south of Paris, France. There, I had my first research experience developing mathematical models of the human liver with Irene Vignon-Clementel at Inria Paris.

Main research projects

Tropical orographic precipitation

Tropical mountains are among the rainiest places on Earth. Previous theories for the enhancement of precipitation by mountains (termed orographic precipitation) focus on midlatitude regions, and assume that it results from the smooth uplift of prevailing winds by the topography. In the tropics, precipitating clouds are often associated with moist convection: this means they ascend much faster and much deeper than what the mountain uplift alone could cause. We proposed a theory which couples moist convection and mountain flow dynamics, allowing to understand the spatial distribution of orographic precipitation in the tropics.

Precipitation distribution around the Western Ghats of India in observations (left), as given by our theory (middle left), and as given by two previous theories for orographic precipitation (right). From Nicolas and Boos, 2024.

The theory rests on the simple idea that the seasonal-mean ascent forced by topography cools and moistens the lower free troposphere, a layer located roughly 1 to 4 km above the surface. This, in turn, increases the buoyancy of convective clouds: it is easier for them to rise through a cooler environment, and the entrainment of moister free-tropospheric air as they ascend is less effective at diluting them. We have subsequently applied this theory to understand the seasonal-mean rainfall distribution in several regions of Earth's tropics, and to understand how it changes as the background wind strenghten or weakens - a crucial question to project rainfall changes with global warming.
Relevant publications: Nicolas and Boos, 2022 | Nicolas and Boos, 2024 | Nicolas and Boos, 2025


Extreme wet and dry seasons in the tropics

Extreme weather events threaten human societies across the world, but are, by far, most devastating in the tropics. While tropical rainfall extremes have been widely studied at the daily timescales, there has been comparatively little work on seasonal-scale extremes. Are they caused by anomalously frequent wet days, or a few anomalously extreme events? What are their climatological drivers? Are they associated with large changes in the frequency of certain weather systems? My host group at ETH Zürich has tackled these questions over the past six years, with a focus on midlatitudes. I joined the group to address these questions in tropical regions.
One approach to understanding these is a plume buoyancy framework. Tropical extreme seasons, both wet and dry, are relatively well characterized by a simple measure of convective plume buoyancy (see figure). This measure only depends on seasonal-mean temperature and moisture in the boundary layer and lower free troposphere. Understanding what leads to seasonal extremes in these thermodynamic quantities (processes associated with surface fluxes, horizontal moisture advection, and others) may be easier than understanding directly what lead to rainfall extremes.

Distribution of the quantiles of B L (the plume buoyancy measure of Ahmed et al., 2020) for seasonal precipitation extremes at the 2 degree scale in the tropics (20S-20N). Wet extremes are widely associated with high quantiles of B L , and dry extremes with low quantiles.

Publications

Recent Presentations

Nice orographic cumuli (presumably forced by the background southwesterly wind) over Catalina island in November 2023