Longtime leader in his department and university made significant contributions to his discipline and also helped engineer the reorganization of the College of Arts and Sciences

Keith Julien, professor and chair of the Department of Applied Mathematics at the University of Colorado Boulder, died unexpectedly April 14 at Saint Joseph Hospital in Denver after a short illness.

Famous for his disarming laugh, clear-eyed judgment and potent intellect, Julien was a world-renowned scholar; an insightful and inventive researcher; an energetic, generous and productive collaborator; an engaging and effective teacher and mentor; a visionary administrator for the Department of Applied Mathematics; and a devoted husband, loving father and cherished friend.

Julien’s 33-year affiliation with CU Boulder began in 1991 as a postdoctoral research associate in JILA, where he served from 1991-94. After an advanced study postdoctoral fellowship from 1994-96 at the National Center for Atmospheric Research, he joined the Department of Applied Mathematics in 1997 as an instructor.

He moved up the ranks in the department, becoming an assistant professor in 1998 and earning tenure and a promotion to associate professor in 2003; he was named a full professor in 2008. Julien was elected department chair in 2015, a role in which he served until his passing.

Julien was an exceptional mentor, a leader on campus and an internationally recognized scholar who touched the lives of countless students, collaborators, colleagues and peers. His sudden passing is a shock to the department, the university and the wider academic community, his colleagues said.

Julien advised 12 PhD students and eight postdoctoral scholars, several of whom have become prominent scientific researchers. He inspired and skillfully bolstered the careers of applied math faculty and colleagues around the world.

Born June 12, 1965, Julien grew up in London, England, the second of four children of first-generation immigrant parents from Grenada. In addition to his studies, he was a vigorous and exceptional student athlete, accomplished in both soccer and cricket.

He also enjoyed music and played the electric bass. His academic trajectory took shape when he received his BSc degree in mathematics and physics with first-class honors from King’s College, University of London, in 1986.

Julien then moved to the University of Cambridge, where he received his Part III Certificate of Advanced Studies in 1987, on the basis of which he was accepted into the Cambridge doctoral program. He was awarded the J.T. Knight Prize in 1988 and received his PhD in applied mathematics and theoretical physics in 1991 for a dissertation titled “Strong Spatial Resonances in Convection,” studying under reknowned mathematician and physicist M.R.E. Proctor.

A visionary leader

Julien’s visionary leadership has left an indelible imprint on the Department of Applied Mathematics and the university. First, he substantially influenced the reorganization of the College of Arts and Sciences in 2022 with a new system that strengthens the authority of the three deans of division, an idea originally articulated in the Cumalat-Julien Academic Futures White Paper (2018).

Second, he played a major role in the development of multiple new or pending degrees, including the BA and BS in statistics and data science, the BS in applied mathematics and the professional MS in applied mathematics.

Third, for over a decade, Julien relentlessly worked to unify departmental space so that APPM PhD students would no longer be scattered through different buildings on campus, as they have been since 1989.

Even before serving as chair of applied mathematics, Julien led a committee to develop a plan for new facilities, which culminated in the CU Board of Regents’ approving on April 11 to construct a new shared Chemistry and Applied Mathematics facility; the approval came just three days before Julien’s passing. This new facility represents a major milestone for the department, with construction scheduled to begin this fall, and occupancy planned for late 2026.

Research focused on fluid dynamical phenomena

Julien’s applied mathematics research has had major influence on the understanding of fundamental geophysical and astrophysical fluid dynamical phenomena. He is recognized as a world expert in the instability, dynamics, evolution and simulation of important fluid processes, including rapidly rotating convection, magneto convection, fluid turbulence and the coherent structures that spontaneously appear in these flows.

Julien authored more than 80 research papers, which appeared in leading international journals, and his work is cited more than 4,000 times, according to Google Scholar.

One of his most consequential contributions to the field of geophysical and astrophysical fluid dynamics is the development of reduced or simplified partial differential equations valid in the limit of rapid rotation. Many important flows are strongly affected by rotation, such as thermal convection in the ocean, which regulates overturning rates that bear on climate change; convective flows in the outer core of the Earth, responsible for Earth's magnetic field; and flows in the Sun’s turbulent outer layer, which is an important region for solar magnetic activity, such as solar flares and coronal-mass ejections. 

In a seminal series of papers, Julien pioneered the development of multi-scale asymptotic methods and fast numerical algorithms to derive and simulate a reduced set of equations that approximate the governing Navier-Stokes equations for rapidly rotating convection. This enabled the exploration of extreme parameter regimes that are otherwise inaccessible to either state-of-the-art high performance computing hardware or laboratory investigation.

These developments led to the discovery of the spontaneous emergence of large-scale structures such as vortices and jets in turbulent rapidly rotating convection, predictions that were subsequently confirmed by direct numerical simulations of the full equations, albeit under much more modest conditions.

These advances attracted, in turn, a number of groups to this research area, in both experiment and theory, all motivated by Julien’s pioneering work. Together with many collaborators, Julien extended these ideas to the study of accretion disks in astrophysics, convection in a strong magnetic field, shear-flow instability, wind-driven circulation and, more recently, to ocean mixing by the doubly diffusive salt-finger instability.

Taken together, these applications demonstrate that highly anisotropic but fully three-dimensional turbulence is susceptible to instabilities generating large-scale coherent structures resembling those present in geophysical and astrophysical fluid dynamical systems.

Recognized widely by his peers

Julien received multiple awards recognizing his achievements, including CU’s Creative Research and Creative Works Junior Faculty Development Award in 1998 and Faculty Fellowship Award in 2004.

In 2017, he was elected Fellow of the American Physical Society, and in 2022 he was awarded the Kirk Distinguished Fellowship at the Isaac Newton Institute, University of Cambridge. In 2024, Julien was slated to be a principal lecturer for the Geophysical Fluid Dynamics Summer Program, Woods Hole Oceanographic Institution.

Through the years, Julien tirelessly served the scientific community. He co-directed two NCAR/IMAGe Theme-of-the-Year Programs for Geophysical Turbulent Phenomena in 2008 and Rotating Stratified Flows in 2012.

He served on the Committee of Visitors for External Evaluation of the Division of Ocean Sciences for the National Science Foundation in 2015 and 2019. In 2014, he co-organized a 14-week program on the mathematics of turbulence at the NSF Institute for Pure and Applied Mathematics (IPAM), UCLA, and since 2014 he served as an associate editor for the journal Nonlinearity.

In the summer of 2018, Julien co-organized an international workshop on rotating convection at the Lorentz Center, Netherlands, and he was serving as the lead organizer of a planned 2025 IPAM, UCLA workshop titled “Rotating Turbulence: Interplay and Separability of Bulk and Boundary Dynamics.”

He also served numerous times on scientific panels for NSF and NASA, and as principal investigator and/or co-principal investigator on research grants in the mathematical sciences, atmospheric sciences, solar physics and oceanic sciences.

Julien is survived by his loving wife, Susan; sons, Simon and Theodore; father, O’Neill, and mother, Agnes; older brother Kelvin; younger sisters, Sandra and Sherma; and a wide circle of friends.

Keith Julien’s colleagues and friends collaborated in the writing of this obituary.