Earth observation
Satellite observations of clouds, aerosols and trace gases help monitor climate and air quality. My work focuses on the radiative transfer behind those measurements, including cloud shadows and solar eclipses.
Atmospheric and planetary scientist · developer of MONKI
Dr Victor J. H. Trees
Dutch atmospheric and planetary scientist at KNMI and TU Delft working on Earth, Venus, and Earth-like exoplanets.
From polarised-light physics to planetary atmosphere observations. I develop and apply radiative-transfer computer codes to simulate total and polarised light in planetary atmospheres. A central part of my work is MONKI, a Monte Carlo radiative transfer code in Fortran that I built from scratch to connect polarised light scattering by 3D cloudy atmospheres with satellite and telescope observations.
Core expertise
Radiative transfer simulations of polarised light in planetary atmospheres
My work starts from the physics of light: scattering and absorption by atmospheric particles and gases, polarisation of light, reflection by land and ocean surfaces, and the geometry of real atmospheres. I turn that physics into numerical models and use those models to interpret reflected-light signals for Earth-observation satellites, Venus missions, and future observations of Earth-like exoplanets.
Research themes
More about my research →
Venus
Our nearest planetary neighbour, an Earth-sized world that evolved very differently. I simulate polarised reflected sunlight for ESA's EnVision mission, planned for launch in the 2030s, including challenging twilight and limb geometries.
Earth-like exoplanets
Exoplanets are planets beyond our solar system. I simulate reflected-light signals to test how future telescopes could recognise oceans, clouds, and possible habitability on distant Earth-like worlds.
Research highlights
Publication details →
Earth observation
Solar eclipses can rapidly dissipate shallow cumulus clouds
By restoring satellite data during solar eclipses, I found spaceborne evidence that shallow cumulus clouds over land respond rapidly when sunlight is partly reduced. This finding may have implications for proposed climate engineering ideas, because disappearing clouds can partly oppose the cooling effect of artificial dimming of sunlight.
Exoplanets
Polarisation of light can reveal oceans on Earth-like exoplanets
My exoplanet work showed how polarisation and colour variations of reflected starlight can carry signatures of liquid water oceans. Such a detection would be a milestone in the search for life beyond the solar system. The same simulations were later used in a study for NASA's Habitable Worlds Observatory.
Key software and methods
Software and methods →
MONKI · 3D Monte Carlo radiative transfer
Simulating polarised light in planetary atmospheres
MONKI is the central software development in my work: a Fortran code that I built from scratch to simulate total and polarised reflected light in realistic planetary atmospheres, including 3D cloud structures and multiple scattering.
DARCLOS · TROPOMI cloud-shadow detection
Detecting cloud shadows in satellite observations
DARCLOS identifies cloud-shadow scenes in TROPOMI observations. It supports the interpretation of aerosol and surface-reflectance products by flagging cases where 3D cloud geometry affects the measured radiation field.
Solar-eclipse correction · Eclipse experiment
Restoring satellite measurements during solar eclipses
I developed a correction method for satellite measurements during solar eclipses. This made it possible to use eclipses as controlled sunlight-perturbation experiments and revealed the rapid response of shallow cumulus clouds to reduced sunlight.
Stories
Read the stories →Recent news
All news →12 October 2026
MONKI paper published
After six years of development, our MONKI paper is now published in Geoscientific Model Development. It brings together the physics, numerical methods, and validation behind the code that I built from scratch in modern Fortran. MONKI, short for Monte Carlo KNMI, simulates how sunlight travels through planetary atmospheres: how it is scattered by molecules, aerosols, and clouds, absorbed by gases, and reflected from surfaces. Throughout these interactions, the code follows both the intensity and polarisation of the light.
A key part of this work was going back to the fundamental physics. I derived a new theoretical framework that allows MONKI to converge robustly even in thick atmospheres where light is scattered many times, such as that of Venus. MONKI is already being used to investigate 3D cloud effects in TROPOMI and SEVIRI satellite measurements and to predict signals for ESA’s upcoming EnVision mission to Venus. In principle, MONKI can be used to simulate reflected light signals from any planet.
27 May 2026
MONKI used at KNMI to study 3D cloud effects in weather satellite measurements
MONKI was used in a study led by Job Wiltink to quantify how three-dimensional cloud-radiation interactions affect satellite-based estimates of global horizontal irradiance: the amount of sunlight that reaches the surface on a horizontal plane.
Such 3D effects are often hidden within large satellite pixels. Light can be transported horizontally between cloudy and cloud-free parts of a scene, and cloud shadows can reduce the radiation that reaches the surface. As new geostationary weather satellites observe clouds with smaller pixels, these effects become increasingly important for interpreting and correcting satellite-based irradiance products.
The study combines MONKI with realistic cloud fields from MicroHH, a large-eddy simulation cloud model, and shows how pixel size, cloud structure, and horizontal photon transport shape the retrieved signal.
9–13 February 2026
Venus habitability workshop at the Lorentz Center
I spent the full week at the Lorentz Center in Leiden for the workshop “Roadmap to the Exploration of Venus Habitability”. The meeting brought together researchers working on Venus geology, interior structure, atmospheric evolution, and future mission concepts.
We discussed which observations and mission strategies are needed to understand how Venus evolved and whether it may once have been habitable. I contributed from the perspective of reflected-light modelling and argued for polarimetry as a way to constrain Venus’s clouds, hazes, and atmospheric structure.
