About

Larry Di Girolamo is a recipient of the William T. Pecora Award, which is presented annually to individuals or teams who use satellite or aerial remote sensing to make outstanding contributions toward understanding the Earth, including land, oceans, and air. His work supports educating the next generation of scientists, informing decision makers, and aiding natural or human-induced disaster response. The award recognizes his contributions in areas such as remote sensing, disaster response, aerial and satellite remote sensing, decision making, and decision support related to man-made disasters. This recognition was granted by the National Aeronautics and Space Administration and the U.S. Department of the Interior in 2019. Larry Di Girolamo is affiliated with the National Center for Supercomputing Applications (NCSA) and works within the fields of Climate, Meteorology, and Atmospheric Sciences.

Research topics

• Climatology
• Geography
• Environmental science
• Meteorology
• Geology
• Oceanography
• Atmospheric sciences
• Physics

Selected publications

• Investigation of supercooled cloud droplet evaporation through very high-resolution numerical modeling, with implications for ice nucleation

  2026-03-14

  articleOpen accessSenior authorCorresponding

  Cloud droplet temperature plays a key role in fundamental cloud microphysical and radiative processes. The supercooled droplet temperature and lifetime can impact cloud ice and precipitation formation via homogeneous freezing and activation of ice-nucleating particles through contact and immersion freezing. While most observational and modeling studies often assume droplet temperature to be spatially uniform and equal to the ambient temperature (Ta), this assumption may not always be valid, particularly when droplets experience strong relative humidity (RH) gradients at cloud boundaries.For a wide range of ambient conditions, we model the coupled heat and mass transfer between the droplet and its environment and quantify the decrease in droplet temperature (ΔT) from that of the far-away ambient temperature (Ta), and the increase in droplet lifetime due to reduced droplet surface temperatures, compared to Maxwellian diffusion-limited evaporation estimates. ΔT is found to increase with Ta, and decrease with increase in ambient relative humidity (RH), and pressure (P). For a prescribed environment and assuming the droplet has infinite thermal heat conductivity, ΔT was typically 1-5°C lower than Ta, with highest values (~10.3°C) for very low RH, low P, and Ta closer to 0ºC. For higher RH and larger droplets, droplet lifetimes can increase by more than 100s compared to the diffusion-limited evaporation approach, which ignores droplet cooling. The steady state temperature of evaporating droplets can be approximated by environmental thermodynamic wet-bulb temperature. Radiation was found to play a minor role in influencing droplet temperatures, except for larger droplets in environments close to saturation. If we resolve the spatiotemporally varying thermal and vapor density gradients near the evaporating droplet, results demonstrate a higher subsaturation-dependent decrease in the droplet temperature as well as the envelope of air in the vicinity of the droplet surface. For an ambient environment specified far away, with Ta = -5°C, RH = 10%, 40%, and 70%, the decrease in droplet temperatures due to evaporative cooling is ~ 24, 11, and 5°C, respectively and the evaporatively cooled droplets survive longer compared to previous estimates. The implications of evaporative cooling and increased lifetimes of supercooled cloud droplets on potential enhancement of ice nucleation near evaporating cloud edges, such as cloud-top generating cells, and especially for moderately supercooled ambient temperatures, are discussed. The importance of using accurate droplet temperatures to improve activated ice nuclei number concentrations from existing primary ice nucleation parameterization schemes, especially in sub-saturated environments, is highlighted. Finally, using high-resolution direct numerical simulations of moderately supercooled cloud boundaries, we discuss the impacts of droplet evaporative cooling on the evolution of supercooled droplet size distributions, which critically impacts ice nucleation.

  PublisherDOI

• Decadal changes in atmospheric circulation detected in cloud motion vectors

  Nature · 2025-07-09 · 3 citations

  articleOpen access1st authorCorresponding

  Changing atmospheric circulations shift global weather patterns and their extremes, profoundly affecting human societies and ecosystems. Studies using atmospheric reanalysis and climate model data1–9 indicate diverse circulation changes in recent decades but show discrepancies in magnitude and even direction, underscoring the urgent need for validation with independent, climate-quality measurements3. Here we show statistically significant changes in tropospheric circulation over the past two decades using satellite-observed, height-resolved cloud motion vectors from the Multi-angle Imaging SpectroRadiometer (MISR)10,11. Upper tropospheric cloud motion speeds in the mid-latitudes have increased by up to about 4 m s−1 decade−1. This acceleration is primarily because of the strengthening of meridional flow, potentially indicating more poleward storm tracks or intensified extratropical cyclones. The Northern and Southern Hemisphere tropics shifted poleward at rates of 0.42 ± 0.22 and 0.02 ± 0.14° latitude decade−1 (95% confidence interval), respectively, whereas the corresponding polar fronts shifted at rates of 0.37 ± 0.31 and 0.31 ± 0.21° latitude decade−1. We also show that the widely used ERA5 (ref. 12) reanalysis winds subsampled to the MISR are in good agreement with the climatological values and trends of the MISR but indicate probable ERA5 biases in the upper troposphere. These MISR-based observations provide critical benchmarks for refining reanalysis and climate models to advance our understanding of climate change impacts on cloud and atmospheric circulations. Analysis of cloud motion vectors from the Multi-angle Imaging SpectroRadiometer over the past two decades indicates an acceleration and poleward shift of atmospheric circulation, and comparison with ERA5 reanalysis winds suggests probable ERA5 biases in the upper troposphere.

  PublisherOA PDFDOI

• Validating Machine Learning Retrievals of the Cloud‐Top Droplet Effective Radius Over Oceans That Account for 3D Radiative Transfer Effects

  Journal of Geophysical Research Atmospheres · 2025-11-25

  articleOpen accessSenior author

  Abstract Bispectral retrievals of the droplet effective radius ( r e ) from instruments such as MODIS are widely utilized to study cloud microphysics in marine boundary layer clouds. These retrievals are known to have systematic errors due to cloud heterogeneity. Here, we develop a neural network regression to retrieve cloud‐top r e at a solar zenith angle of 30° and nadir viewing using MODIS. The neural network regression is trained on 3D radiative transfer simulations of quasi‐adiabatic stochastically generated clouds and corrects relative errors in r e with respect to cloud‐top with an r 2 of 0.88. The neural network regression produces unbiased retrievals of cloud‐top r e against large eddy simulation cloud fields where the bispectral retrieval has biases reaching +100%. The neural network regression reduces retrieval biases against airborne observations of cumulus from CAMP 2 Ex from +100% to +40%, and marginally improves already good consistency against stratocumulus sampled during VOCALS. A cross‐comparison technique for assessing statistical remote sensing retrievals is introduced. The neural network regression explains 63% and 91% of the variance in the differences between MODIS 1.6 and 2.1 μm retrievals for Overcast and partially cloudy pixels (PCL) and 42% and 76% for the 2.1 and 3.7 μm differences, respectively. Residual spectral inconsistency is partially attributed to precipitation‐sized particles using radar observations. Regional averages of the operational MODIS r e exceed the cloud‐top r e predicted by the neural network by +50% for Overcast pixels in the tropics and a consistent +70% for PCL pixels. Errors in bispectral retrievals due to heterogeneity are nonrandom at both the cloud and climate scale.

  PublisherOA PDFDOI

• Improved Understanding of how Kinematic and Thermodynamic Environmental Changes Impact Modeled Overshooting Top Characteristics

  2025-07-15

  preprintOpen accessSenior author

  Overshooting tops (OT) are domed protrusions of deep convective updrafts that extend past the anvil of a cumulonimbus. Recent work has shown that certain characteristics of an OT such as its depth may be sensitive to the thermodynamic environment in the upper troposphere/lower stratosphere (UTLS). What remains unknown is the extent to which the characteristics of the tropospheric updraft, such as its size and intensity, influence an OT compared to the UTLS thermodynamic environment. This study uses numerical simulations of supercell thunderstorms to test the relative influences of kinematic and thermodynamic environmental changes on updraft characteristics and ultimately, OT area and depth. Results show static stability in the UTLS is important to modulating depth, but not area. Tropospheric vertical wind shear, which is important for controlling the size of a supercell updraft, is shown to be important for the area of the OT but not its depth.

  PublisherOA PDFDOI

• The Spatial Area and Other Attributes of GOES-16 Overshooting Tops as Indicators of Potential Hail

  Monthly Weather Review · 2025-08-18 · 1 citations

  article

  Abstract Recent studies using idealized simulations suggest that storms that generate large hail should exhibit deep and wide overshooting tops (OTs). Our work herein extends these and related studies to explore possible relationships between observed OT characteristics and hail size observed at the ground. All hail reports from 2018 through 2022 across the contiguous United States were organized using a grid-hour approach. An OT detection algorithm applied to GOES-16 data was used to find the nearest OT to each selected report. OT area (OTA), OT depth (OTD), and OT volume (OTV) were quantified and statistically related to hail size. OTA tended to exhibit a statistically significant difference across the three hail size groups (nonsevere, severe, and significant severe), with a decrease with increasing hail size. OTD also tended to exhibit a statistically significant difference across the hail size groups, with an increase with increasing hail size. A maximum expected hail size (MESH)-based report proxy was used to explore possible dependencies of these results on hail reports; the general tendencies of increased hail size for decreased OTA and increased OTD were also found using this proxy. Such tendencies were additionally found using a dataset limited to hail associated with supercells. Radar-derived OTA for this limited dataset was also explored and tended to increase with hail size. Finally, possible relationships between hail size and area of a proximal, midtropospheric radar reflectivity core were evaluated and found to be positive and statistically significant. Applications of these findings for risk assessment and operational forecasting are possible but will require further analyses.

  PublisherDOI

• Validating Machine-Learning Retrievals of Cloud Droplet Effective Radius Over Ocean that Account for 3D Radiative Transfer Effects

  2025-05-20

  preprintOpen accessSenior author

  Bi-spectral retrievals of droplet effective radius (re) from instruments such as MODIS are widely utilized to study cloud microphysics in marine boundary layer clouds. These retrievals are known to have systematic errors due to cloud heterogeneity. Here, we develop a neural-network regression to correct for pixel-by-pixel errors in retrieved re using four features available in the MODIS L2 product. The neural-network regression is trained on 3D radiative transfer simulations of quasi-adiabatic stochastically generated clouds and corrects relative errors in re with respect to cloud-top with an r2 of 0.88. The neural-network regression produces unbiased retrievals of re against Large Eddy Simulation cloud fields where the bi-spectral retrieval has biases reaching +100% in cumuliform conditions. The neural-network regression reduces retrieval biases against airborne observations of cumulus from CAMP2Ex from +100% to +40%, and marginally improves already good consistency against stratocumulus sampled during VOCALS. A cross-comparison technique for assessing statistical remote sensing retrievals is introduced. The neural-network regression explains 63% and 91% of the variance in the differences between MODIS 1.6 µm and 2.1 µm retrievals for Overcast and Partially Cloudy Pixels (PCL) and 42% and 76% for the 2.1 µm and 3.7 µm differences, respectively. Residual spectral inconsistency is partially attributed to precipitation-sized particles using radar observations. Regional biases in the operational MODIS re are predicted that reach +50% for Overcast pixels in the tropics and are consistently +70% for PCL pixels. Errors in bi-spectral retrievals due to heterogeneity are non-random at both the cloud and climate scale.

  PublisherOA PDFDOI

• Improved Understanding of How Kinematic and Thermodynamic Environmental Changes Impact Modeled Overshooting Top Characteristics

  Geophysical Research Letters · 2025-10-31

  articleOpen accessSenior author

  Abstract Overshooting tops (OTs) are domed protrusions of deep convective updrafts that extend past the anvil of a cumulonimbus. Recent work has shown that OT depth and area may be sensitive to the thermodynamic environment in the upper troposphere/lower stratosphere (UTLS). What remains unknown is the extent to which the UTLS influence on OT characteristics competes with the influence of the tropospheric updraft. This study uses numerical simulations of supercell thunderstorms to test the relative influences of kinematic and thermodynamic environmental changes on updraft characteristics and ultimately, OT area and depth. Results show static stability in the UTLS is important for modulating depth, but not area. Tropospheric vertical wind shear, which is important for controlling the size of a supercell updraft, is shown to be important for the area of the OT but not its depth.

  PublisherOA PDFDOI

• Can evaporative cooling of water droplets play a role in enhancing ice formation at moderately supercooled cloud boundaries?

  2025-03-15

  preprintOpen accessCorresponding

  Cloud droplet temperature is an important parameter influencing cloud microphysical and radiative processes. The supercooled droplet temperature and lifetime impact cloud ice and precipitation formation via homogeneous freezing and activation of ice-nucleating particles through contact and immersion freezing. While most observational and modeling studies often assume droplet temperature to be almost equal to the ambient temperature (Ta), this assumption may not always be valid, particularly when droplets experience strong relative humidity (RH) gradients at cloud boundaries.This study investigates the evolution of temperature and lifetime of evaporating, supercooled cloud droplets considering initial droplet radius (r0) and temperature (Tr0), and environmental relative humidity (RH), ambient temperature (Ta), and pressure (P). The time (tss) required by droplets to reach a lower steady-state temperature (Tss) after sudden introduction into a new subsaturated environment, the magnitude of ΔT = Ta - Tss, and droplet survival time (tst) at Tss are calculated. The temperature difference (ΔT) is found to increase with Ta, and decrease with RH and P. ΔT values are typically 1–5 K lower than Ta, with highest values (~10.3 K) for very low RH, low P, and Ta closer to 0°C. Results show that tss is < 0.5 s over the range of initial droplet and environmental conditions considered. Tss of the evaporating droplets can be approximated by the environmental thermodynamic wet-bulb temperature. Radiation was found to play a minor role in influencing droplet temperatures, except for larger droplets in environments close to saturation. The implications for ice nucleation in cloud-top generating cells and near cloud edges are discussed. Using Tss instead of Ta in widely used parameterization schemes could lead to enhanced number concentrations of activated ice-nucleating particles (INPs), by a typical factor of 2–30, with the greatest increases (>100) coincident with low RH, low P, and Ta closer to 0°C. The findings corroborate the hypothesized mechanism of potential enhancement of ice nucleation at cloud boundaries, such as cloud-top generating cells and for ambient temperatures close to 0°C. The importance of using accurate droplet temperatures to improve existing primary ice nucleation parameterization schemes, especially in sub-saturated environments, is highlighted.The impacts of droplet evaporative cooling on droplet lifetimes are compared with Maxwellian pure diffusion-limited evaporation approach under similar conditions. For higher RH and larger droplets, droplet lifetimes can increase by more than 100 s compared to those with droplet cooling ignored. Larger droplets (r0 ~ 30–50 µm) can survive at Tss for about 5 s to over 10 min, depending on the subsaturation of the environment. The impacts of droplet evaporative cooling on evolution of drop size distributions, using high-resolution direct numerical simulations of moderately supercooled mixed-phase cloud boundaries, are discussed.

  PublisherDOI

• Errors in stereoscopic retrievals of cloud top height for single-layer clouds

  Atmospheric measurement techniques · 2025-07-10 · 1 citations

  articleOpen accessSenior authorCorresponding

  Abstract. Multi-angle stereoscopic methods are a promising means for retrieving high-resolution cloud volumes and their temporal evolution. Stereoscopic retrievals assume that light emerges from localized points on a surface. We assess the errors introduced by this assumption using synthetic measurements at various wavelengths, solar-viewing geometries, and spatial resolutions generated by applying a 3D radiative transfer model to an ensemble of 841 cloud fields in (8 km × 8 km) domains of varying fractional cover, cloud top bumpiness, microphysics, and optical depth. We show that stereoscopic retrievals of cloud top height (CTH) have biases that vary from −175 to +20 m as the cloud edge extinction profile becomes sharper and absorption increases, all when mean visible cloud optical depth is greater than 5 and with little dependence on instrument resolution between 50 and 250 m. Stereo CTH fields are smoother than the ground truth when CTH variability is concentrated at small spatial scales, viewing angles are oblique, and absorption is weak. We attribute this effect to both the smoothing effect of multiple scattering, which is stronger at wavelengths with weak absorption, and the ill-posed nature of the retrieval in the presence of non-uniform CTH over the stereo-matching window. The standard deviation of stereo CTH errors increases from 25 to 200 m as the standard deviation of CTH increases to 200 m over the 8 km × 8 km domain. More than 50 % of stereo retrievals from two different 50 m resolution stereo viewing pairs of (0°, +38°) and (−38°, 0°) are consistent to within 30 m over 500 m × 500 m regions for clouds with a standard deviation of CTH of less than 200 m. We analyzed airborne lidar observations and found that 75 % of shallow cumulus clouds and all stratocumulus clouds have standard deviations of CTH of less than 200 m over 8 km transects. These results support the application of time-differenced stereoscopic cloud top height retrievals for the remote sensing of high-resolution cloud dynamics as well as macrophysics.

  PublisherOA PDFDOI

• “Godzilla,” the Extreme African Dust Event of June 2020: Origins, Transport, and Impact on Air Quality in the Greater Caribbean Basin

  Bulletin of the American Meteorological Society · 2025-05-14 · 7 citations

  articleOpen access

  Abstract In June 2020, the tropical Atlantic and the Caribbean Basin were affected by a series of African dust outbreaks unprecedented in size and intensity. These events, informally named “Godzilla,” coincided with CALIMA, a large field campaign, offering a rare opportunity to assess the impact of African dust on air quality in the Greater Caribbean Basin. Network measurements of respirable particles (i.e., PM 10 and PM 2.5 ) showed that dust significantly degraded regional air quality and increased the risk to public health in the Caribbean, the southern United States, northern South America, and Central America. CALIMA examined the meteorological context of Godzilla dust events over North Africa and how these conditions might relate to the greatly increased dust emissions and enhanced transport to the Americas. Godzilla was linked to strong pressure anomalies over West Africa, resulting in a large-scale geostrophic wind anomaly at 700 hPa over North Africa. We used surface-based and columnar measurements to test the performance of two frequently used aerosol forecast models: the NASA Goddard Earth Observing System (GEOS) and Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) models. The models showed some skills but differed substantially between their forecasts, suggesting large uncertainties in these forecasts that are critical for issuing early warnings of health-threatening dust events. Our results demonstrate the value of an integrated approach in characterizing the spatial and temporal variability of African dust transport and assessing its impact on regional air quality. Future studies are needed to improve models and to track the long-term changes in dust transport from Africa under a changing climate.

  PublisherOA PDFDOI

Frequent coauthors

• Guangyu Zhao

  University of Illinois Urbana-Champaign

  31 shared

• Robert M. Rauber

  25 shared

• Steven Platnick

  Goddard Space Flight Center

  19 shared

• Greg M. McFarquhar

  17 shared

• Stephen W. Nesbitt

  University of Illinois Urbana-Champaign

  16 shared

• Sagnik Dey

  16 shared

• Yulan Hong

  University of Illinois Urbana-Champaign

  13 shared

• Jesse Loveridge

  Colorado State University

  13 shared