About

Robert M Rauber is a professor emeritus in the Department of Climate, Meteorology & Atmospheric Sciences at the Illinois School of Earth, Society & Environment. His research falls within the disciplines of physical meteorology, radar meteorology, and mesoscale meteorology. He has been actively involved in collaborative efforts with other faculty members and scientists from various institutions. Rauber has a strong passion for field research, involving the use of sophisticated state-of-the-art instruments to study weather phenomena. He has participated as an investigator in thirty major field research programs, working extensively with conventional, dual-Doppler, and airborne radars, microwave radiometers, optical array and scattering probes, as well as other aircraft, ground-based, and satellite-based instruments. His research also involves the use of cloud and mesoscale models to simulate microphysical and mesoscale phenomena. Rauber encourages his students to produce publication-quality research, participate in national and international scientific conferences, and be involved in national field research programs. He has supported student participation through grants and collaborative research efforts, emphasizing the importance of exposure and self-development in the scientific community.

Research topics

• Environmental science
• Geology
• Meteorology
• Computer Science
• Geography
• Climatology
• Physics
• Atmospheric sciences

Selected publications

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

  2026-03-14

  articleOpen access

  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

• Invigoration Due To Cloud Seeding: New Observations Confirm an Old Hypothesis

  Geophysical Research Letters · 2026-03-16

  articleOpen access

  Abstract The potential for cloud seeding to induce dynamic changes that alter cloud structure beyond basic ice formation processes has remained theoretical. While previous research hypothesized the presence of dynamic responses in seeded clouds, this study presents the first direct observational evidence that seeding can generate buoyant forces strong enough to deepen and deform clouds. In situ aircraft measurements and W‐band radar analysis shows how the buoyant force increased cloud tops by hundreds of meters and induced secondary circulations that altered the cloud and precipitation structure. These dynamic changes triggered additional ice formation and precipitation not captured in current conceptual models. The results demonstrate that dynamic responses can be induced through glaciogenic seeding, representing foundational research that will significantly improve understanding of seeding mechanisms and precipitation formation in commonly seeded clouds.

  PublisherDOI

• A Case Study of Cold-season Emergent Orographic Convection and its Impact on Precipitation. Part 2: High-resolution LES Analysis of Convective Cell Evolution and Precipitation Processes

  Monthly Weather Review · 2026-05-12

  articleSenior author

  Abstract Part 1 of this study demonstrated how terrain-induced gravity waves triggered elevated convection, with tops up to 6 – 7 km above sea level, in a potentially unstable layer during a winter storm event over the Idaho Central Mountains on 7 February 2017. Herein, this case is explored further with a Large Eddy Simulation (LES) at 100 m grid spacing, to examine the detailed structure and evolution of convective cells emergent from shallow stratiform clouds, their interaction with complex terrain, and the resulting precipitation processes. The 100 m LES produced fine-scale precipitation structures similar in depth and width to radar observations, with vertical velocity distributions and cloud microphysical properties matching airborne observations. The 100 m LES confirmed the role of vertically propagating gravity waves over the highest terrain ridges in providing the initial lift necessary to release potential instability. Unlike coarser-resolution simulations, the 100 m LES produced clusters of convective towers, ~2 km wide, roughly matching observations, although they were more regularly spaced than observed. Co-spectral analysis of these towers confirms their convective nature. The small-scale convective updrafts, locally exceeding 2 m s −1 , and mostly within the -10 to -20°C temperature zone, enabled snow particles to grow rapidly through depositional growth and riming, and a significant fraction of the simulated precipitation fell as graupel, according to the LES model. Precipitation from this emergent convection occurred primarily in the lee of the main terrain ridge, on account of the strong flow above mountain top level. Cumulatively, the LES produced 18% more precipitation than non-LES models in this case.

  PublisherDOI

• Observed and Simulated Dynamic Responses to Glaciogenic Seeding in Wintertime Mixed-Phase Clouds

  2026-03-13

  articleOpen access

  Observations from recent field campaigns investigating glaciogenic cloud seeding demonstrate the process of silver iodide (AgI) dispersion through ice nucleation, crystal growth, then enhanced snowfall at the surface. These observations, combined with numerical simulations, were used to quantify seeding’s impact on enhancing precipitation in targeted regions. With the microphysical chain of events established, fundamental knowledge gaps remain on the mechanisms by which seeding modifies the cloud dynamics, structure, and precipitation enhancement. This study presents the first direct observational evidence that glaciogenic seeding generates buoyant forces in wintertime orographic clouds that elevate cloud tops and secondary circulations that alter the cloud structure. In this study, we analyze dynamic responses induced from seeding in the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE) and CLOUDLAB field campaigns. The SNOWIE cases occurred in the Payette mountains in presence of widespread supercooled liquid conditions and low natural ice number concentrations. Ground-based X-band radars tracked the development and evolution of cloud and precipitation from five seeding legs. Distinct cells, directly attributable to airborne seeding, developed from smaller weaker echoes (10 dBZ) at the natural cloud top and rapidly intensified to produce precipitation with echoes >30 dBZ. The key observed processes were dynamic responses induced by the latent heat released from seeding that led to enhancing cloud top by 350 m compared to the natural cloud. An airborne W-band Dual-Doppler cross-section illustrates the detailed dynamic structure for one cell consisting of a central updraft, divergence near cloud top, and toroidal circulations along its periphery in an observed moist-neutral environment. In situ measurements show distinct microphysical regimes in the elevated cloud top, with seeding generated ice number concentrations up to 580 L-1. A WRF-WxMod ensemble shows the evolution of dynamic responses, the microphysical characteristics, and precipitation enhancement up to 200 km downwind of release. We combine these results with preliminary observations from the 2025-2026 CLOUDLAB field campaign that further investigate the roles each step in a dynamic response has on seeded cloud microphysical properties. We show the evolution of seeded cloud from Ka-band cloud radars, combined with in-situ measurements from a holographic imager, to show dynamic response impact on microphysical structure and cloud properties.

  PublisherDOI

• Dominant Modes of Terrain-Tied Vertical Motion Variability over the Payette River Basin of Idaho: Results from SNOWIE

  Journal of Applied Meteorology and Climatology · 2026-01-27

  article

  Abstract Precipitation enhancement over complex terrain is predominantly driven by quasi-stationary, terrain-tied vertical motions, making their variability a critical factor in shaping precipitation distributions and accumulation. This study quantifies the dominant modes of terrain-tied vertical motion variability over the Payette River basin of Idaho. Principal component analysis is applied to a seasonal simulation spanning November 2016–April 2017, which encompassed the Seeded and Natural Orographic Wintertime Clouds: the Idaho Experiment (SNOWIE) field campaign (January–March 2017). The first mode, accounting for more than 20% of the variance in vertical motion, captures ridge-tied updrafts and represents the primary pattern of terrain-induced ascent. The second mode (8%) reflects how synoptic-scale variations modulate updraft orientation, distinguishing between north–south and east–west ridgelines. The third mode (6%) isolates variability in updraft width and magnitude. These three dominant modes of variability, which explain over one-third of the vertical velocity variance in the seasonal simulation, strongly influence the distribution of supercooled liquid water (SLW) and precipitation over the terrain. Results show that the dominant modes of vertical motion variability were consistent with patterns commonly observed during SNOWIE research flights. Additionally, we quantified vertical motion, SLW, and precipitation means as a function of phase space between the modes, demonstrating that enhanced SLW and precipitation occurred when quasi-stationary waves were present over the terrain.

  PublisherDOI

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

  2025-03-15

  preprintOpen access

  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

• Mesoscale and Microphysical Characteristics of Elevated Convection and Banded Precipitation over an Arctic Cold Front: A Case Study from IMPACTS

  Journal of the Atmospheric Sciences · 2025-04-23 · 1 citations

  articleOpen access

  Abstract The mesoscale and microphysical structure of a cloud system associated with an Arctic front is analyzed using data from two research aircraft, two WSR-88D radars, the HYSPLIT model, and initialization fields from the RAP model. The flights, conducted during the NASA Investigation of Microphysics and Precipitation in Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign, collected in situ and remote sensing data as the cloud system moved across Illinois. The system developed within an air mass that, based on back trajectory analysis, originated over the subtropical eastern Pacific before being lifted over the Arctic front. This led to a region of potential instability extending upward over the frontal zone. The ascending flow triggered the release of the instability that manifested as elevated convection in the storm’s southern sector. In the convective region, supercooled water was found in cloud towers, leading to saturated conditions that supported growth of a range of particle habits and growth by riming. Within this region, and in shallower clouds between convective towers, needle particle habits, supercooled water, and high ice particle concentrations implied active secondary ice processes. Two snowbands formed north of the convective region, with radar evidence suggesting that precipitation within these bands originated in cloud towers at altitudes of 4–6 km in a near-neutral to weakly unstable region. Water saturated conditions, evidenced by supercooled water at the sampling level, permitted the growth of a range of particle habits. Despite ice particle concentrations < 15 L −1 within the bands, some aggregated particles exceeding a centimeter in maximum dimension were observed at −5°C, likely contributing to the 21–27 dB Z e reflectivity characteristic of the bands.

  PublisherOA PDFDOI

• Manifestation of Elevated Convection within Wintertime Extratropical Cyclones during IMPACTS. Part II: Hydrometeor Vertical Motions within and Outside of Elevated Potentially Unstable Layers

  Journal of the Atmospheric Sciences · 2025-04-03 · 1 citations

  article

  Abstract This study uses airborne, vertical W-band radial velocity ( V r ) radar data from seven ER-2 flights during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign together with High-Resolution Rapid Refresh (HRRR) model initialization data to investigate hydrometeor vertical motions within elevated potentially unstable and stable layers in winter extratropical cyclones. Cohen’s D test ( C d ) is used to evaluate how distributions of V r vary with cyclone type and intensity, within stable and unstable layers, and with characteristics of elevated potential instability (EPI) described in Part I. In general, hydrometeor vertical motions rarely exceeded 2 m s −1 within stable and EPI layers. The V r distributions, including stable and EPI layers, varied more by cyclone intensity than cyclone type. The V r distributions shifted toward positive values and broadened in stronger cyclones. Surprisingly, V r distributions were similar in stable and EPI layers ( C d = 0.15). The hydrometeor vertical motions in stable layers were associated with orographically induced gravity waves, shear-induced turbulence, and cloud-top generating cells. In general, the distance from the low pressure center, region within the comma head, depth of EPI layers, and number of EPI layers had little influence on the V r distributions. The V r distributions varied most by base height of the EPI layer ( C d = 0.28–0.68) followed by EPI magnitude ( C d = 0.37–0.66) where the higher the layer base, the more positive the V r mode. The stronger the instability, the more negative the V r mode, likely due to riming within elevated convection. Significance Statement Snowfall within winter storms can disrupt everyday life, impacting transportation, schools, businesses, and power. Snowfall arises from different circulations in the atmosphere ranging from broadscale ascent of air to small-scale buoyant circulations associated with elevated potential instability (EPI) above frontal zones, gravity waves, and shear turbulence. This paper examines the magnitude of hydrometeor vertical motions ( V r ) within the comma head region of winter cyclones based on vertically pointing radar data collected during 2 years of flights during the NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaigns as it relates to EPI characteristics, cyclone type, and cyclone intensity. In general, V r rarely exceeded 2 m s −1 within stable and EPI layers. The V r distributions, including stable and EPI layers, varied more by cyclone intensity than cyclone type. The V r distributions shifted toward positive values and broadened in stronger cyclones. Enhanced V r in stable layers was associated with orographically induced gravity waves, shear-induced turbulence, and cloud-top generating cells. In general, the distance from the low pressure center, region within the comma head, depth of EPI layers, and number of EPI layers had little influence on the V r distributions.

  PublisherDOI

• The Impact of Cold Pool Strength on the Propagation Mechanisms of a Nocturnal Mesoscale Convective System

  Monthly Weather Review · 2025-08-01

  article

  Abstract In the first part of this study, a Weather Research and Forecasting (WRF) Model simulation of the 20 June 2015 Plains Elevated Convection at Night (PECAN) mesoscale convective system (MCS) was analyzed using a novel strategy for analyzing air parcel trajectories to show that the storm transitioned from surface-based to elevated as a response to the evolving nighttime environment, despite the surface cold pool remaining strong even after the system became elevated. To better understand the role of the cold pool in the propagation of an elevated MCS, three additional WRF simulations were carried out in which the magnitude of latent cooling due to evaporation was either doubled, halved, or removed entirely, effectively controlling the strength of the cold pool. The novel trajectory analysis developed in the first part of this study was then repeated for the additional simulations. It was found that in an environment where both surface-based and elevated instability were present, a stronger cold pool led to less surface-based convection, while a weaker cold pool led to more surface-based convection. During the second half of the simulation, when only elevated instability existed, all simulated storms remained elevated, but the mechanism by which the elevated convection propagated differed. A strong cold pool led to a bore-like feature developing in response to the forcing of the cold pool, which displaced the stable boundary layer and forced elevated, unstable air upward to its level of free convection. A weaker cold pool led to waves developing atop the stable boundary layer and propagating ahead of the surface outflow and initiating new convective updrafts. Significance Statement The mechanisms by which summertime nocturnal thunderstorms can remain long-lived are not as well understood compared to daytime thunderstorms, making them difficult to forecast. While significant progress has been made toward understanding these nocturnal storms, there are still questions about how they develop and are maintained. This paper investigates how a nocturnal storm’s ability to ingest air from different sources changes as a response to the strength of its cold pool. During periods that there were energy sources available near the surface and aloft, a stronger cold pool made the storm less likely to be fueled by surface air, while a weaker cold pool led to the storm being more likely to be fueled by surface air. These results counter conventional understanding of how these storms remain long-lived, and explanations for such counterintuitive behavior are given.

  PublisherDOI

• A Case Study of Cold-Season Emergent Orographic Convection and Its Impact on Precipitation. Part I: Mesoscale Analysis

  Monthly Weather Review · 2025-07-31 · 1 citations

  articleOpen access

  Abstract It is not uncommon for layers within the warm conveyor belt in a frontal system to become potentially unstable, releasing elevated convection. The present study examines this destabilization process over complex terrain, and resulting precipitation, with a focus on the surface coupling, orographic ascent, and the initiation and evolution of convective cells. This study uses detailed observations combined with numerical modeling of a baroclinic system passing over the Central Idaho Mountains in the United States on 7 February 2017. The data were collected as part of the Seeded and Natural Orographic Wintertime Clouds: the Idaho Experiment (SNOWIE). Specifically, observations from a ground-based scanning X-band radar and an airborne profiling Doppler W-band radar along ∼100-km-long flight tracks aligned with the wind describe the development and evolution of convective cells above shallow stratiform orographic clouds. Convection-permitting numerical simulations of this event, with an inner domain grid resolution of 0.9 km, capture the emergence and vertical structure of the convective cells. Therefore, they are used to describe the advection of warm, moist air over a retreating warm front, cold-air pooling within the Snake River basin and adjacent valleys, destabilization in a moist layer above this shallow stable layer, and instability release in orographic gravity wave updrafts. In this case, the convective cells topped out near 6 km MSL, and the resulting precipitation fell mostly leeward of the ridge where convection was triggered, on account of strong cross-barrier flow. Sequential convection initiation over terrain ridges and rapid downwind transport led to banded precipitation structures. Significance Statement In winter storms, most precipitation falls on the upwind side of the main mountain crest, especially when there is deep subsident flow in the lee side. This is evident simply from the vegetation on opposite sides of the Cascade Mountains in the United States, for instance. This paper examines a case with significant leeward precipitation. Radar observations from aboard an aircraft and from a scanning radar located on a mountaintop, combined with numerical simulations, show that the cumulus clouds grew over terrain ridges and enhanced precipitation over and downwind of the mountains, notwithstanding the tiny amount of potential energy available for convection. Key ingredients are upstream stable near-surface conditions, an elevated layer of potential instability, and strong cross-barrier flow.

  PublisherOA PDFDOI

Recent grants

• Precipitation Studies in Trade Wind Clouds - The Rain In Cumulus over the Ocean (RICO) Experiment

  NSF · $1.5M · 2004–2010

• Scientific Program Overview (SPO): Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES)

  NSF · $53k · 2016–2018

• Collaborative Research: Further Investigations from the Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment (SNOWIE)

  NSF · $372k · 2020–2024

• Bow Echoes and Mesoscale Gravity Waves - The Role of Microphysical Processes

  NSF · $746k · 2004–2009

• Collaborative Research: Impacts of Microphysical, Thermodynamic, and Dynamical Processes on Nocturnal and Oceanic Convective Systems via Analyses from PECAN and HAIC/HIWC

  NSF · $350k · 2019–2023

Frequent coauthors

• Brian F. Jewett

  University of Illinois Urbana-Champaign

  102 shared

• Greg M. McFarquhar

  96 shared

• Mohan K. Ramamurthy

  Tamil Nadu Dr. M.G.R. Medical University

  95 shared

• Harry T. Ochs

  Atmospheric and Space Technology Research Associates (United States)

  86 shared

• Kenneth V. Beard

  Atmospheric and Space Technology Research Associates (United States)

  70 shared

• D. C. Rogers

  68 shared

• Donald H. Lenschow

  NSF National Center for Atmospheric Research

  66 shared

• David A. R. Kristovich

  Illinois Archaeological Survey

  65 shared

Education

• Ph.D., Atmospheric Science

  Colorado State University

  1985

• M.S., Atmospheric Science

  Colorado State University

  1981

• B.S., Physics

  Pennsylvania State University

  1978

• B.A., English

  Pennsylvania State University

  1973