Tristan L'Ecuyer
University of Wisconsin-Madison · Environment and Resources
Active 1988–2026
About
Tristan L'Ecuyer is a Professor and the Director of the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin–Madison, within the Department of Atmospheric and Oceanic Sciences. His research centers on improving climate predictions by better understanding the processes that govern atmospheric energy balance and the global water cycle. He combines state-of-the-art remote sensing techniques, innovative analyses of satellite and ground-based datasets, coordinated regional field projects, and numerical modeling to achieve these goals. His current projects leverage these tools to document the causes of rapid Arctic climate change, establish the role of clouds and aerosols in Earth’s energy budget, understand the characteristics of shallow and deep convection, and document the global distribution of falling snow. Additionally, he works to extend this knowledge to broader applications of atmospheric sciences, including helping to realize the full potential of solar energy and applying Earth’s characteristics in the search for potentially habitable exoplanets. His research interests include climate and climate change, radiation and remote sensing, and atmospheric and cloud physics.
Selected publications
2026-04-17
articleBulletin of the American Meteorological Society · 2025-11-13
articleOpen accessAbstract Since 2014, space agencies have launched advanced meteorological imagers into the geostationary (GEO) orbit encircling Earth’s equator, known as the GEO-Ring. JMA, NOAA, and KMA launched imagers measuring 16 spectral bands with thermal resolutions of 2 km and full-disk coverage every 10 min. China Meteorological Administration’s (CMA’s) Fengyun-4A ( FY-4A) series, launched in 2016, observes 14 bands with 4-km thermal resolution and 15-min full-disk scans. In 2022, EUMETSAT introduced the Meteosat Third Generation (MTG) imager, offering 16 channels, 2-km thermal resolution, and 10-min full-disk coverage. Together, these satellites provide near-global coverage with improved capabilities over earlier generations. The 10–12 common channels across the latest imagers enable retrieval of diverse atmospheric variables at high temporal resolution. These data represent a substantial advance beyond the early 1980s when the International Satellite Cloud Climatology Project (ISCCP) was first developed. The challenge facing any new GEO-Ring project, such as one being planned as part of a next generation of ISCCP (ISCCP-NG), is to define a new baseline from these measurements and processing methods to extract meaningful information for the scientific community in the coming decades. This paper outlines the design of a GEO-Ring radiance project to support a future ISCCP-NG and many other applications and emphasizes the benefits compared to the B1 and B3 data used in ISCCP. Significance Statement Space agencies have made large investments in improving the capabilities of the geostationary (GEO) meteorological satellite imagers surrounding Earth’s equator in the so-called GEO-Ring. To facilitate the use of these data for scientific studies by a wide community, the Global Energy and Water Exchanges project (GEWEX) Data and Analysis Panel (GDAP) initiated a next generation of the International Satellite Cloud Climatology Project (ISCCP-NG). The original and pioneering ISCCP was formed when the first generation of the GEO-Ring was established in the early 1980s. This paper demonstrates the development of the new GEO-Ring dataset and how it may lead to an ISCCP-NG. The characteristics of the GEO-Ring data are described and their application to cloud, aerosol, and storm-tracking applications are illustrated.
Thermal Infrared Spectrometers for the Polar Radiant Energy in the Far‐Infrared Experiment (PREFIRE)
Earth and Space Science · 2025-09-26 · 1 citations
articleOpen accessAbstract The Polar Radiant Energy in the Far‐InfraRed Experiment (PREFIRE) was selected by NASA to fly two miniaturized Thermal InfraRed Spectrometers (TIRS) capable of distinguishing the spectral signatures of surface and atmospheric properties in Earth's polar regions. A trade study examining spectral sampling as well as separation of cloudy and clear scenery at 20 km scales highlighted the possibility to utilize ambient (uncooled) detector technologies in a miniaturized spectrometer that could facilitate low‐cost and rapid access to space. This work describes the design, implementation, testing and performance of two TIRS systems, as well as the challenges and acceptable limitations of the cost‐constrained effort, that feature the novel joining of compact thermopile array technologies with concentric imaging spectrometry methods. The TIRS systems presented here each have 2.7 kg mass, draw 4.3 W power, and provide spectral resolution of 1.71 m below 35 m sampled at 0.86 m increments.
2025-09-17
articleSenior authorThe thermal land-atmosphere interaction at the poles is modulated by a weak atmospheric greenhouse effect accentuating the impact of poorly characterized longwave surface emissivities on local energy balance. The PREFIRE mission is examining whether model biases in these interactions drive variability in projections of sea level rise, ice sheet loss and weather severity. Measurements of the spectral thermal emission to space are being made using two Thermal InfraRed Spectrometers (TIRS) aboard each of the twin NASA PREFIRE CubeSats, now in low-Earth orbit. These instruments provide us with broad observational bandwidth covering mid- and far-infrared wavelengths, and allow systematic sampling of polar conditions at sub-diurnal to seasonal scales. We will discuss the design, assembly and testing of these compact infrared sensors which required miniaturization of existing technologies including an ambient thermopile array detection system embedded in an Offner spectrometer. Challenges of calibration and geolocation for our CubeSat operations will also be presented.
Geophysical Research Letters · 2025-10-13 · 1 citations
articleOpen accessSenior authorAbstract Sulfate burden over the North Atlantic Ocean (NATL) exhibits strong seasonality despite no seasonality in anthropogenic sulfur dioxide (SO 2 ) emissions. However, the seasonality of sulfate aerosols over NATL has decreased since 1970, likely due to a reduction in the United States (US) SO 2 emissions following the Clean Air Act of 1970. We performed atmospheric chemistry and transport simulations to assess the impact of changing US SO 2 emissions between 1970 and 2010 on NATL sulfate burden and radiative forcing. United States SO 2 emission reductions weakened the seasonality in NATL sulfate burden by ∼17%, primarily due to a decrease in chemical production and transport in summer. These emission reductions caused a summertime radiative forcing (∼2 W m −2 ) twice as large as the wintertime forcing. Our findings highlight the complex, season‐dependent responses of sulfate burden and radiative effects to regional emission changes.
Journal of Geophysical Research Atmospheres · 2025-03-22 · 3 citations
articleOpen accessAbstract Spaceborne radars provide near‐global observations of clouds and precipitation, but ground clutter can result in a satellite radar blind zone as high as 2 km above the surface. As a result, satellite radars may underestimate snowfall from shallow clouds and incorrectly flag snow virga as snowfall at the surface. Ground‐based radar observations provide invaluable tools to assess satellite observations of clouds and precipitation. This study investigates snowfall regimes using observations from 2011 to 2021 at the Department of Energy Atmospheric Radiation Measurement North Slope of Alaska atmospheric observatory. Snowfall events identified in the Ka‐band ARM zenith radar (KAZR) are separated into regimes based on the cloud/precipitation layer characteristics: deep snowfall, shallow snowfall, and snow virga. The shallow snowfall regime accounts for nearly half of the regime occurrence (48%) followed by snow virga (28%) and deep snowfall (23%). However, more than half (62%) of the shallow snowfall is likely underestimated and/or undetected within the satellite radar blind zone. Snow virga is incorrectly flagged as snowfall for 7% of the total annual occurrence, but increases to 12% in October. The KAZR regimes and vertical structure are qualitatively compared to collocated CloudSat observations with snow certain/possible flags; the deep and shallow snowfall regime show similarities between the ground‐based and spaceborne radar observations. An assessment of observable snowfall occurrence and accumulation at varying reflectivity thresholds in KAZR and CloudSat provide a reference for detection characteristics for current and planned spaceborne radars.
Journal of Climate · 2025-06-18 · 2 citations
articleAbstract The high latitudes exhibit two distinct and recurring radiative states, often referred to as “transmissive” and “opaque.” These states, characterized by bimodality in surface longwave flux distributions, have a large influence on the surface energy balance and are ubiquitous across the Arctic. Since reanalyses are frequently used to supply surface radiative fluxes for Arctic applications, it is important to assess the extent to which reanalyses represent these states. This study uses satellite observations to evaluate how well the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), the fifth major global reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ERA5), and the Arctic System Reanalysis (ASR), version 2, capture these two modes. Each reanalysis demonstrates deficiencies in reproducing observed distributions of cloud radiative effect, often resulting in a noticeable absence of bimodality in surface longwave flux distributions. The ASR exhibits some improvement over the two global reanalyses, underscoring the value of regional reanalyses that more accurately resolve cloud processes. Idealized cloud forcing calculations provide insight into why these states are prevalent in the Arctic and indicate that too much ice at the expense of liquid water largely explains the absence of the opaque mode in ASR compared to observations. Given the non-Gaussian nature of polar longwave flux distributions, some traditional model evaluation methods, especially those relying on monthly means, likely miss errors in the representation of these states entirely. Thus, we urge model evaluators to consider the existence of distinct states in their assessments and, more generally, to adopt evaluation methods that can assess non-Gaussian distributions. Significance Statement Radiation plays an important role in the Arctic climate by modulating the surface temperature and the rate at which ice forms. Observations show that the Arctic alternates between periods with a large energy deficit (transmissive) and a small energy deficit (opaque). Due to the limited number of observation stations in the Arctic, models are often used to fill gaps where direct measurements are unavailable. These observation-assisted models are called “reanalyses.” This study assesses how well reanalyses capture the two dominant Arctic states. Understanding whether these states are accurately represented in well-constrained models may provide insights into our ability to predict the two states’ future evolution.
Temperature‐Dependent Optical Properties of Ice Crystals in the Far‐Infrared Regime
Geophysical Research Letters · 2025-06-18
articleOpen accessAbstract A database of temperature‐dependent hexagonal ice aggregate optical properties in the far‐infrared (FIR) spectrum is developed to support FIR missions, particularly the current Polar Radiant Energy in the Far InfraRed Experiment and the upcoming Far‐infrared‐Outgoing‐Radiation Understanding and Monitoring. Based on this data set, simulations of the brightness temperatures (BTs) in the 100–667 cm −1 FIR region are conducted for an anvil‐like ice cloud in a tropical atmosphere. The results show nonnegligible impact of ice cloud temperature on simulated BTs, which can be as large as 3 K due to the difference between fixed 160 or 270 K cloud temperature and the benchmark counterpart, varying in accordance with the ambient temperature profile for a cloud residing between 249.6 and 199.6 K. To enhance the accuracy of FIR radiative transfer modeling, it is recommended to incorporate temperature‐dependent optical properties of ice clouds.
Past, Present, and Future Arctic Radiative States Simulated by Polar-WRF
2025-10-17
articleOpen accessAbstract. Two recurring radiative states (“transmissive” and “opaque”) strongly modulate the Arctic surface energy balance through their control on downwelling longwave radiation (DLR). Because these states are primarily governed by cloud processes, many coarse-resolution models fail to capture their behavior. This study evaluates how well the Polar-optimized Weather Research and Forecasting model (PWRF) simulates present-day DLR distributions associated with these states and examines projected changes into the future. While most physics parameterizations mirror those of the widely used Arctic System Reanalysis (ASR), we test several advanced microphysics schemes and assess the impact of model resolution. Both the P3 and Morrison two-moment schemes (candidates for the next ASR version) overproduce the opaque mode, whereas the Goddard scheme used in ASR overproduces the transmissive mode. The opaque bias in P3 and Morrison arises mainly from excessive low-level, optically thick clouds over sea ice. Among all schemes, P3 best preserves the distinctiveness of the two radiative modes. Using this scheme, PWRF forced with end-of-century CESM1 output projects a shift toward more frequent opaque conditions, consistent with long-term observations at the North Slope of Alaska. While PWRF shows promise as a tool for dynamically downscaling climate model output, persistent cloud-related biases, especially over ice, warrant caution in future projections. Continued improvements in cloud representation are essential to obtain more quantitative insight into Arctic radiative regime changes.
The Role of Convective Intensity in Modulating Earth’s Radiative Balance
Journal of Climate · 2025-03-27
articleOpen accessSenior authorAbstract It has been suggested that deep convective vertical intensity may increase in regions that are becoming increasingly moist in response to warming sea surface temperatures, with corresponding impacts on anvil cloud development and their top-of-atmosphere radiative effects. This work seeks to document the observed relationships between convective core intensity and cloud optical thickness and altitude that are the primary drivers of how high clouds fundamentally influence Earth’s radiative energy budget. We employ a database of “convective objects” generated from 10 years of A-Train measurements to explore such relationships over the tropical ocean. In general, active convection has a negative net cloud radiative effect (CRE) over the tropical ocean regions. When sorting deep convective systems by a radar-based proxy for core intensity, the predominant radiative response is an increase in longwave (LW) CRE as cloud-top heights increase with intensity. Meanwhile, thin cirrus increases at the expense of thicker anvil cloud, which weakens the shortwave (SW) CRE for systems composed of mostly thin cloud. This offsets enhanced SW CRE in thicker cloud systems driving the ensemble of convective systems toward a more neutral net radiative effect at the top of the atmosphere as intensity increases. Significance Statement Studies suggest that the intensity of atmospheric deep convection might change in a changing climate, which could alter how deep convective clouds influence Earth’s energy budget. On average, the convective systems observed cool at the top of the atmosphere, but each individual system has largely differing cloud properties and energetic contributions. The most intense convective systems have a near-neutral or small cooling effect owing to higher cloud-top heights compared to less intense systems that have a much larger cooling effect. The ratio of thick to thin cloud amount helps explain the spread in the energy budget contributions by the most intense systems. Systems primarily composed of thick cloud have a cooling impact, while thin cloud systems contribute a warming. Finally, it is important to consider total energetic contribution when both high and low clouds are present, as they more often contribute a warming impact compared to systems with just high cloud.
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