About

The faculty, staff and students in the Department of Atmospheric and Climate Science at the University of Washington are engaged in the study of a broad range of atmospheric phenomena and processes, using methods ranging from mathematical analysis to field experimentation. Research projects range in size from small studies involving individual scientists to large national and international programs involving teams of scientists. Research groups in the department are concerned with Atmospheric Chemistry, Atmospheric Dynamics, Boundary Layer Processes, Cloud and Aerosol Research, Glaciology and Planetary Atmospheres, Cloud Dynamics, Precipitation Processes, Storms, Weather Analysis and Forecasting, Climate, Global change, Airflow over mountains, and other topics. Some groups maintain special research facilities for the use of their students. In some of these activities, there is close cooperation with the University of Alaska Fairbanks, Oregon State University and the National Oceanic and Atmospheric Administration (NOAA) Regional Center through the Cooperative Institute for Climate, Ocean and Ecosystem Studies. Faculty members often have interests in more than one area, and research projects frequently involve questions of broad scope which do not fall neatly into a single category. This is particularly true of research projects directed toward understanding the chemical and physical modification of the environment by human activities.

Research topics

• Climatology
• Geology
• Geography
• Meteorology
• Environmental science
• Political Science
• Computer Science
• Engineering
• Atmospheric sciences

Selected publications

• 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

• Manifestation of Elevated Convection within Wintertime Extratropical Cyclones during IMPACTS. Part I: Analysis of Elevated Potential Instability

  Journal of the Atmospheric Sciences · 2025-04-16 · 1 citations

  articleOpen accessSenior author

  Abstract Elevated potential instability (EPI) often occurs in the comma head of wintertime extratropical cyclones as air within the storm’s dry slot moves above a warm or occluded frontal zone. Lifting of EPI layers may result in elevated convection, enhanced snowfall, and thundersnow. High-Resolution Rapid Refresh (HRRR) initialization values of equivalent potential temperature θ e are used to analyze EPI characteristics along tracks of the NASA Earth Resources-2 (ER-2) aircraft within the comma head of 14 cyclones sampled during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign. EPI was found in 53% of HRRR 1-km-wide vertical columns along ER-2 flight legs, typically 210–410 km from the low pressure center within the northern comma head region, but 0–400 km from the low in the western region of strong cyclones [sea level pressure gradients > 4 hPa (100 km) −1 ]. The highest and lowest frequencies of EPI were in Miller type B (MB) and Great Plains cyclones, respectively. In 67% of 1-km-long columns exhibiting EPI, EPI occurred within a single layer. The median base of the potentially unstable layers (Δ θ e /Δ z < 0) was 4.2 km, the median depth was 0.59 km, and the median equilibrium level was 0.94 km above the base. MBs had more, shallower EPI layers compared to fewer, deeper layers in Miller type A and Gulf Coast cyclones. The median value of Δ θ e /Δ z within potentially unstable layers was −0.6 K km −1 , the 95th percentile was −2.8 K km −1 , and the greatest values of EPI occurred in MBs and cyclones with sea level pressure gradients < 2 hPa (100 km) −1 . Significance Statement Snowfall associated with wintertime extratropical cyclones can cause large disruptions to everyday life, impacting transportation, schools, businesses, and power. Snowfall arises from different circulations in the atmosphere ranging from broad-scale ascent of air to small-scale buoyant circulations associated with elevated potential instability (EPI) above frontal zones. This paper examines the distribution, frequency, and intensity of EPI within storms sampled during the 3-yr NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign and the relationship of the instability to cyclone type and intensity. We address two questions: 1) How common is EPI and where does it develop within the comma head region of cyclones? Does the occurrence of EPI vary based on cyclone type and strength? And 2) what are the characteristics of EPI (number of layers, depth of layers, and strength of EPI)? How do these characteristics vary based on cyclone type and strength? Detailed, quantitative answers to these questions are provided.

  PublisherOA PDFDOI

• Influence of Cloud Microphysical Properties on Airborne Lidar Measurements: Results From the IMPACTS Field Campaign

  Journal of Geophysical Research Atmospheres · 2025-11-26 · 1 citations

  articleOpen accessSenior author

  Abstract Ice‐ and mixed‐phase clouds play an important role in the global radiative budget and hydrological cycle, yet the complexity of ice crystal shapes and the presence of supercooled liquid water (SCLW) present challenges for retrieving cloud properties from airborne and spaceborne remote sensing instruments. Airborne lidar measurements of the backscatter coefficient ( β ), color ratio ( χ ), and volume depolarization ratio ( δ ) provide additional spatial context for the cloud phase and to some extent the particle shapes both vertically and horizontally through clouds. Coordination between a NASA P‐3 and ER‐2 aircraft during the Investigation of Microphysics and Precipitation for Atlantic Coast‐Threatening Snowstorms field campaign provided 3.3 hr of Cloud Physics Lidar observations from 16 events to be spatially related to the particle size and morphological properties from the Cloud Particle Imager, in addition to temperature and SCLW measurements obtained by the P‐3 aircraft. After the lidar data were matched to the P‐3 location, in situ microphysics data were mapped in β‐δ and triple‐wavelength frameworks involving χ . Compared to SCLW regions, regions dominated by ice typically exhibited lower β (<10 −2 km −1 sr −1 ), lower 532/355‐nm χ (<0.3), and greater δ (>0.2) and were associated with particle sizes that were on average 105% larger and area ratios that were 40% lower. The relationships between cloud properties and lidar measurements established in this study have implications for future cloud phase and particle habit algorithms using airborne and spaceborne lidar data.

  PublisherDOI

• Environmental Conditions Leading to Observed Convective Organization in Central Argentina

  2025-04-21

  preprintOpen accessSenior author

  Deep convection frequently forms along the Sierras de Crdoba (SDC) mountain range, downstream of the Andes, and grows rapidly upscale, spatially aggregating into larger systems.The 2018-19 Remote Sensing of Electrification, Lightning, And Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) field campaign collected detailed observations of convective systems and their environments, including periods of upscale growth near the SDC.In this study, we analyze two intensive observational periods (IOPs) where storms grew upscale with different rates and degrees of upscale growth.On 13-14 December 2018, strong synoptic forcing led to a Northwestern Argentina low with a cold front and an elevated South American Low-level Jet (SALLJ).Convection grew very rapidly upscale overnight behind the front where the northerly winds from the SALLJ encountered the front, leading to initially elevated convection.Strong deeplayer (0-6 km) wind shear was observed with a large front-parallel component and 2-6 km shear, which includes SALLJ peak winds, was oriented even more parallel to the low-level forcing produced by the front, favoring upscale growth.In contrast, weak synoptic forcing led to afternoon convection focused over the SDC on 5 December 2018.Convection grew upscale near the SDC but grew more slowly, had a lesser degree of organization, and was more spatially limited than on 13-14 December.These IOPs highlight how the alignment of favorable synoptic and localized environments connected to the SDC impacts upscale growth and emphasize the importance of accounting for varying SALLJ heights when choosing vertical layers over which to calculate relevant environmental parameters.

  PublisherOA PDFDOI

• 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 accessSenior author

  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

• Environmental Influences on Deep Convective Upscale Growth Rate in Central Argentina from a Convection-Permitting Simulation

  2025-05-12

  preprintOpen access

  This study uses a convection-permitting model simulation to describe the environmental conditions under which convective upscale growth occurs in central Argentina, particularly examining environmental parameters when deep convection initially forms that could differentiate the rate of initial upscale growth. Simulated MCSs are separated into slow and rapid growth by the rate of spatial growth from convective initiation until reaching the mesoscale convective system (MCS) scale. A low-level jet (LLJ) is found more frequently near the deep convection that experiences rapid growth to an MCS, but its presence alone is not predictive of rapid growth. Using spatially-averaged parameters, we find that rapid growth to MCS also occurs in environments that are significantly more thermodynamically favorable with greater low-level moisture and instability. Fewer significant differences are found in the kinematic environment with only the 0-2 km vertical wind shear magnitude being significantly larger for rapid growth MCSs compared to slow growth MCSs, potentially related to LLJs often peaking near this height. When focusing only on MCSs with the slowest and fastest growth rates, elevated-layer shear is significantly smaller for very rapid growth MCSs, suggesting elevated-layer shear may help discriminate between the upper and lower bounds of growth rate. Finally, when upscale growth occurs near the Sierras de Córdoba (SDC) with a LLJ present, rapid growth is also supported by favorable wind shear orientation. However, this does not hold for upscale growth occurring away from the SDC, highlighting the importance of interpreting shear direction relative to the orientation of features initiating deep convection.

  PublisherOA PDFDOI

• Ambient and intrinsic dependencies of evolving ice-phase particles within a decaying winter storm during IMPACTS

  Atmospheric chemistry and physics · 2025-07-29

  articleOpen accessCorresponding

  Abstract. Mesoscale bands develop within winter cyclones as concentrated regions of locally enhanced radar reflectivity, often corresponding to intensified precipitation rates lasting several hours. Surface precipitation characteristics are governed by the microphysical properties of the ice-phase particles aloft, yet their unique microphysical evolutionary pathways and ambient environmental dependencies in banded regions remain poorly understood, in part due to a paucity of observations within clouds. Addressing this need, the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms measured properties of winter cyclones from airborne in situ and remote sensing platforms. Observations collected within a banded region of a decaying-stage northeast United States cyclone revealed a microphysical pathway characterized by precipitation fallout from a weak generating cell layer through an ∼ 2 km deep subsaturated downdraft region. Sublimation was a dominant evolutionary process, resulting in a > 70 % reduction in the initial characteristic ice water content (IWC). This vertical evolution was reproduced by a one-dimensional (1D) particle-based model simulation constrained by observations, conveying accuracy in the process representation. Four sensitivity simulations assessed evolutionary dependencies based on observationally informed perturbations of the ambient relative humidity, RH, and vertical air motion, w. Perturbations of ∼ 2 % RH significantly varied the resultant characteristic IWC loss, by as much as 29 %, whereas comparable perturbations of w had negligible effects. Intrinsic particle evolution during sublimation demonstrated a notable imprint on vertical profiles of radar reflectivity, but the Doppler velocity was more strongly governed by the ambient w profile. These findings contextualize radar-based discrimination of sublimation from other ice-phase processes, including riming and aggregation.

  PublisherOA PDFDOI

• Unveiling In-situ Observations of Ice Crystal Chain Aggregates in Winter Storms

  2025-08-01

  preprintOpen accessSenior author

  Ice crystal chain aggregates—linear structures of conjoined monomer crystals—have previously been observed in strongly electrified deep convection, likely forming via electric-field-enhanced aggregation. The NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign conducted aircraft-based sampling of cold-season storms using state-of-the-art cloud physics probes. These measurements reveal chain aggregates in weakly electrified winter storms. Chain aggregates were identified in 28 of 34 research flights from 2020 to 2023 (~10% of total P-3 flight time), spanning –38 to +2.5°C and 1.5 to 9.7 km. High-resolution CPI and PHIPS imagery revealed sublimation signatures on chain aggregates, consistent across instruments. The widespread occurrence of chain aggregates in weakly electrified clouds suggests alternative formation pathways. Additional machine learning–based classification is underway to contextualize in-situ observations using available, collocated remote sensing data.

  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

  articleSenior author

  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 3D Structure of a Shallow Generating Cell Driven Snowstorm over the Midwest and Its Microphysical Characteristics Determined Using Particle Aspect Ratio: A Case Study from IMPACTS

  Journal of the Atmospheric Sciences · 2025-09-09

  article

  Abstract Microphysical measurements within winter storms are commonly analyzed using two-dimensional radar cross sections from airborne vertically pointing radars or ground-based scanning radars. While these radars offer valuable insights, they provide limited insights into the storm’s microphysical characteristics within the context of the storm’s three-dimensional structure. To address this limitation, this analysis uses conically scanning X-band radar data to investigate the three-dimensional structure of a shallow generating cell (GC) driven snowstorm (<6 km deep) sampled over central Illinois and Indiana on 25 February 2020 during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. The observed microphysical properties and reflectivity structures along the nadir-pointing radar cross section represent the superposition of 3D trajectories of fall streaks originating in GCs upwind of the aircraft’s flight track. GCs formed in a potentially unstable layer near cloud top based on HRRR analysis, where supercooled water formed and created a droplet-rich environment for ice crystal nucleation, growth, and fallout. In situ microphysics measurements beneath cloud top allowed for the assessment of particle aspect ratios within and outside of GC fall streaks. When sampled 2–3 km below cloud top, fall streaks typically contained larger ice crystals and aggregates with higher aspect ratios compared to the surrounding cloud, and increased reflectivity in nadir and plan-view scans. The southern end of the storm lacked GCs, was supercooled, and contained smaller, low-aspect-ratio ice crystals in high concentrations. Significance Statement Microphysical characteristics of winter storms are typically analyzed using ground-based plan-position indicator scans from operational radars or two-dimensional radar cross sections from research airborne vertically pointing radars or ground-based scanning radars. While informative, these approaches offer limited insight into a storm’s observed microphysical properties within the context of its three-dimensional structure. To address this limitation, this study examines the three-dimensional structure of a shallow snowstorm driven by cloud-top generating cells using airborne scanning radar data. The analysis reveals that generating cell fall streaks, not evident in the vertical two-dimensional cross section, precipitate through the cross section impacting the observed microphysical characteristics.

  PublisherDOI

Recent grants

• Climatology of Forecast Skill and Major Forecast Failures over North America and Adjacent Waters

  NSF · $357k · 2005–2009

• Collaborative Research: Radar Observations of Convective Lifecycle near Argentina's Sierras de Cordoba

  NSF · $836k · 2017–2022

• West Coast Precipitation Processes as Modulated by Storm Structure and Terrain

  NSF · $620k · 2017–2021

Frequent coauthors

• David S. Battisti

  University of Washington

  25 shared

• Eliza Dawson

  Stanford University

  25 shared

• Andy Rhines

  Palo Alto University

  25 shared

• Aaron Donohoe

  University of Washington

  25 shared

• Robert A. Houze

  University of Washington

  21 shared

• Angela K. Rowe

  17 shared

• Joseph A. Finlon

  Earth System Science Interdisciplinary Center

  12 shared

• Joseph P. Zagrodnik

  Washington State University

  11 shared

Education

• PhD, Atmospheric Sciences

  University of Washington

  1988

Awards & honors

• Atmospheric Sciences Department Teaching Award (1999, 2004,…
• NASA PMM Science Team Award (2016)