About

Leah Bertrand, affiliated with CIRES and the University of Colorado Boulder, is involved in research focused on Arctic cloud responses to warming. Her work utilizes spaceborne radar and lidar to quantify and communicate these responses, contributing to the understanding of climate change impacts in polar regions. Her research aims to improve the understanding of atmospheric processes related to Arctic warming, which is critical for climate modeling and prediction.

Research topics

• Computer Science
• Environmental science
• Geology
• Oceanography
• Climatology
• Meteorology
• Geography
• Atmospheric sciences
• Physics
• Mathematics
• Environmental resource management
• Earth science
• Astrobiology

Selected publications

• Detectability of phytoplankton biomass extremes using simulated satellite chlorophyll observations

  2026-01-01

  articleSenior author

  Extreme open-ocean phytoplankton events can influence marine ecosystems, yet their global occurrence, drivers, and consequences remain poorly understood. Most large-scale studies rely on satellite chlorophyll, which provides only a surface view, is affected by physiological variability, and is often missing due to clouds and low sunlight. Here, we use an Earth system model with a satellite chlorophyll simulator to test when and where vertically integrated phytoplankton biomass extremes align with satellite-detected chlorophyll extremes. Globally, about 10% of low and 19% of high phytoplankton biomass extremes are detected. The detection rate is the result of the combined impacts of missing data and extreme misalignment: only 34% of low and 56% of high detected chlorophyll extremes correspond with true biomass extremes, with the largest discrepancies occurring in the subtropical gyres. These findings highlight the need for caution when interpreting satellite chlorophyll as a proxy for phytoplankton biomass extremes.

  PublisherDOI

• Detectability of Phytoplankton Biomass Extremes Using Simulated Satellite Chlorophyll Observations

  Geophysical Research Letters · 2026-02-12

  articleOpen accessSenior author

  Abstract Extreme open‐ocean phytoplankton events can influence marine ecosystems, yet their global occurrence, drivers, and consequences remain poorly understood. Most large‐scale studies rely on satellite chlorophyll, which provides only a surface view, is affected by physiological variability, and is often missing due to clouds and low sunlight. Here, we use an Earth system model with a satellite chlorophyll simulator to test when and where vertically integrated phytoplankton biomass extremes align with satellite‐detected chlorophyll extremes. Globally, about 10% of low and 19% of high phytoplankton biomass extremes are detected. The detection rate is the result of the combined impacts of missing data and extreme misalignment: only 34% of low and 56% of high detected chlorophyll extremes correspond with true biomass extremes, with the largest discrepancies occurring in the subtropical gyres. These findings highlight the need for caution when interpreting satellite chlorophyll as a proxy for phytoplankton biomass extremes.

  PublisherDOI

• A novel model hierarchy isolates the limited effect of supercooled liquid cloud optics on infrared radiation

  Geoscientific model development · 2025-10-14

  articleOpen access

  Abstract. Clouds exert strong influences on surface energy budgets and climate projections. Yet, cloud physics are complex and often incompletely represented in models. For example, supercooled liquid cloud optics parameterizations are rarely incorporated into the radiative transfer models used for climate projections. Prior work has shown that incorporating these optics in longwave radiation calculations increases Arctic downwelling longwave fluxes by as much as 1.7 W m−2. Here we examine whether implementing supercooled liquid water optics in climate models for longwave radiation impacts global radiative fluxes and climate. We use a novel methodology that uses a hierarchy of dynamical constraints on the sequence of atmospheric states. In the model experiments with stronger dynamical constraints, we find that the supercooled liquid water optics increase Arctic downwelling longwave by 2.17–3.24 W m−2. In contrast, these optics increased Arctic downwelling longwave radiation by 0.36–0.68 W m−2 with dynamically unconstrained model experiments. While the optics impact was greater within the dynamically constrained models than in dynamically unconstrained models, the dynamically constrained models are also more idealized than the unconstrained models. In summary, we found a signal from supercooled liquid water optics; the influence of these optics for longwave radiation is small compared to the modeled longwave radiation variability. More broadly, this work demonstrates a novel framework for assessing the climate importance of a physics change.

  PublisherOA PDFDOI

• COSP-RTTOV-1.0: flexible radiation diagnostics to enable new science applications in model evaluation, climate change detection, and satellite mission design

  Geoscientific model development · 2025-08-13

  articleOpen access

  Abstract. Infrared spectral radiation fields observed by satellites make up an information-rich, multi-decade record with continuous coverage of the entire planet. As direct observations, spectral radiation fields are also largely free of uncertainties that accumulate during geophysical retrieval and data assimilation processes. Comparing these direct observations with Earth system models (ESMs), however, is hindered by definitional differences between the radiation fields satellites observe and those generated by models. Here, we present a flexible, computationally efficient tool called COSP-RTTOV (Cloud Feedback Model Intercomparison Project (CFMIP) Observation Simulator Package and Radiative Transfer for TOVS) for simulating satellite-like radiation fields within ESMs. Outputs from COSP-RTTOV are consistent with instrument spectral response functions, orbit sampling, and the physics of the host model. After validating COSP-RTTOV's performance, we demonstrate new constraints on model performance enabled by COSP-RTTOV. We show additional applications in climate change detection using the NASA Atmospheric Infra-Red Sounder (AIRS) instrument and observing system simulation experiments using the NASA PREFIRE mission. In summary, COSP-RTTOV is a convenient tool for directly comparing satellite radiation observations with ESMs. It enables a wide range of scientific applications, especially when users desire to avoid the assumptions and uncertainties inherent in satellite-based retrievals of geophysical variables or in atmospheric reanalysis.

  PublisherDOI

• Increasing wintertime cloud opacity increases surface longwave radiation at a long-term Arctic observatory

  Nature Communications · 2025-11-01

  articleOpen access

  Abstract As the Arctic warms, winter clouds are known and expected to change. Yet the extent to which these cloud changes amplify or dampen warming (cloud feedback) remains uncertain. This uncertainty results from systemic difficulties in modeling and observing Arctic low clouds. Surface-based observations avoid many of these difficulties. Here, we use two decades of surface-based observations (1998–2023) to constrain and explain longwave flux change during winter. We find that longwave flux into the surface is increasing and that this increase cannot be explained by direct impacts of temperature and greenhouse gases alone. Only when increasing cloud radiative effect (0.96 ± 0.64 W/m 2 /K) is considered can increasing longwave flux be explained. Cloud radiative effect increases due to increasing cloud opacity, which is driven equally by ice-only and mixed-phase clouds. The direct observational constraint from this work suggests that increasing cloud opacity drives increasing net surface radiation on Alaska’s North Slope during winter.

  PublisherOA PDFDOI

• Earth's Energy Imbalance More Than Doubled in Recent Decades

  AGU Advances · 2025-05-10 · 20 citations

  articleOpen access

  Abstract Global warming results from anthropogenic greenhouse gas emissions which upset the delicate balance between the incoming sunlight, and the reflected and emitted radiation from Earth. The imbalance leads to energy accumulation in the atmosphere, oceans and land, and melting of the cryosphere, resulting in increasing temperatures, rising sea levels, and more extreme weather around the globe. Despite the fundamental role of the energy imbalance in regulating the climate system, as known to humanity for more than two centuries, our capacity to observe it is rapidly deteriorating as satellites are being decommissioned.

  PublisherDOI

• Water sources and land capacitor effects stimulate observed summer Arctic moistening and warming

  Communications Earth & Environment · 2025-12-03

  articleOpen access

  Abstract The primary sources of recent summer Arctic moistening trends in reanalysis are uncertain, hindering attribution of observed Arctic warming due to radiative effects from water vapor changes. Here, we use a combined online numerical water tracer and circulation nudging approach in the Community Earth System Model to track the sources of water vapor beyond its initial sources. Trends in boreal summer large-scale circulation have driven moistening of the Arctic over recent decades, having a large impact on the Arctic radiative budget, accounting for 94% of the strengthening water vapor radiative feedback. We identify two key regions supplying the Arctic water vapor feedback: Northeast North America and western/central Eurasia. In both regions, anticyclonic circulations over the southwest Atlantic and eastern Europe move moisture from the tropical oceans poleward to high latitude land through precipitation in winter and spring. During summer, evapotranspiration over land releases this water vapor, and it is transported by winds into the Arctic. We refer to this sequence of terrestrial moisture storage and release as the land capacitor effect. Thus, the impacts of circulation changes on poleward moisture transport and land-atmosphere interactions over high latitudes represent the underlying mechanisms of the recent moistening and warming in the Arctic.

  PublisherOA PDFDOI

• Internal variability can overwhelm the forced precipitation response pattern over western North America

  2025-10-20

  articleOpen access

  State-of-the-art Earth system models project 21st-century winter precipitation trends of varying sign over western North America. We quantify the influence of internal variability on these precipitation changes in an initial-condition large ensemble from one global Earth system model. We decompose winter precipitation change into thermodynamic and dynamic components and find that thermodynamics explain the ensemble mean precipitation response. Yet, dynamics drive a large range of response patterns because of differences between individual ensemble members due to internal variability. To better understand and classify these patterns, we use a machine learning algorithm called self-organizing maps. While the forced precipitation response is a modest precipitation increase across the region, precipitation decreases over California in 28% of the members due to dynamics. Precipitation increases across California in the remaining members. These findings reinforce the notion that internal variability can overwhelm the forced precipitation response and lead to long-lived precipitation decreases in the future.

  PublisherOA PDFDOI

• COSP-RTTOV-1.0: Flexible radiation diagnostics to enable new science applications in model evaluation, climate change detection, and satellite mission design

  2025-02-04

  preprintOpen access

  Infrared spectral radiation fields observed by satellites make up an information-rich, multi-decade record with continuous coverage of the entire planet. As direct observations, spectral radiation fields are also largely free from uncertainties that accumulate during geophysical retrieval and data assimilation processes. Comparing these direct observations with earth system models (ESMs), however, is hindered by definitional differences between the radiation fields satellites observe and those generated by models. Here, we present a flexible, computationally efficient tool called COSP-RTTOV for simulating satellitelike radiation fields within ESMs. Outputs from COSP-RTTOV are consistent with instrument spectral response functions and orbit sampling, as well as the physics of the host model. After validating COSP-RTTOV's performance, we demonstrate new constraints on model performance enabled by COSP-RTTOV. We show additional applications in climate change detection using the NASA AIRS instrument, and observing system simulation experiments using the NASA PREFIRE mission. In summary, COSP-RTTOV is a convenient tool for directly comparing satellite radiation observations with ESMs. It enables a wide range of scientific applications, especially when users desire to avoid the assumptions and uncertainties inherent in satellite-based retrievals of geophysical variables or in atmospheric reanalysis.

  PublisherOA PDFDOI

• Mid-latitude clouds contribute to Arctic amplification via interactions with other climate feedbacks

  Environmental Research Climate · 2025-01-09 · 5 citations

  articleOpen access

  Abstract Traditional feedback analyses, which assume that individual climate feedback mechanisms act independently and add linearly, suggest that clouds do not contribute to Arctic amplification. However, feedback locking experiments, in which the cloud feedback is disabled, suggest that clouds, particularly outside of the Arctic, do contribute to Arctic amplification. Here, we reconcile these two perspectives by introducing a framework that quantifies the interactions between radiative feedbacks, radiative forcing, ocean heat uptake, and atmospheric heat transport. We show that including the cloud feedback in a comprehensive climate model can result in Arctic amplification because of interactions with other radiative feedbacks. The surface temperature change associated with including the cloud feedback is amplified in the Arctic by the surface-albedo, Planck, and lapse-rate feedbacks. A moist energy balance model with a locked cloud feedback exhibits similar behavior as the comprehensive climate model with a disabled cloud feedback and further indicates that the mid-latitude cloud feedback contributes to Arctic amplification via feedback interactions. Feedback locking in the moist energy balance model also suggests that the mid-latitude cloud feedback contributes substantially to the intermodel spread in Arctic amplification across comprehensive climate models. These results imply that constraining the mid-latitude cloud feedback will greatly reduce the intermodel spread in Arctic amplification. Furthermore, these results highlight a previously unrecognized non-local pathway for Arctic amplification.

  PublisherOA PDFDOI

Recent grants

• CAREER: Going Global - The Influence of Southern Ocean Albedo on Large-scale Climate Dynamics

  NSF · $844k · 2016–2022

• Collaborative Research: Uncertainty in Antarctic Climate Change Projections and the Role of Sea Ice, Clouds and Ocean Structure

  NSF · $244k · 2017–2021

• Collaborative Research: Assessing the Causal Influence of Atmospheric Opacity and Sea Ice on Arctic Warming in a Novel Circulation-controlled Framework

  NSF · $513k · 2023–2027

Frequent coauthors

• Hélène Chepfer

  Sorbonne Université

  84 shared

• Marika M. Holland

  64 shared

• Andrew Gettelman

  Pacific Northwest National Laboratory

  57 shared

• Arlan Dirkson

  University of Manitoba

  48 shared

• Alexandra Jahn

  45 shared

• Juan C. Acosta Navarro

  Johns Hopkins University Applied Physics Laboratory

  44 shared

• David A. Bailey

  NSF National Center for Atmospheric Research

  44 shared

• Tristan L’Ecuyer

  Cooperative Institute for Climate and Satellites

  40 shared

Labs

• Jen Kay's LabPI

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