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Abstract The Winter Precipitation Type Research Multiscale Experiment (WINTRE-MIX) was conducted during February–March 2022 to observe multiscale processes impacting the variability and predictability of precipitation type and amount under near-freezing conditions over the Saint Lawrence River valley. Intensive observation period (IOP) 4 of the campaign occurred 17–18 February 2022 in association with an upper-level trough positioned over the north-central United States and a surface cyclone that traversed the study domain along a frontal boundary that extended northeast of the cyclone. The timing of precipitation-type transitions during the event was consistently too slow within operational forecast models at 2–5-day lead times. Consequently, this study aims to understand how forecast model representations of dynamical and thermodynamical processes on the synoptic scale to mesoscale may have influenced the predictability of precipitation type during IOP4. To do so, an ensemble of operational forecasts from the Global Ensemble Forecast System initialized 5 days prior to IOP4 was divided into three clusters according to the strength and position of the frontal zone over the Saint Lawrence River Valley during the event. Ensemble sensitivity analyses and spatial composites suggest that differences in the position of the frontal zone between clusters are dynamically linked to the differences in the structure of the associated upstream upper-level trough at prior forecast lead times. A diagnosis of the divergent circulation prior to the event suggests that feedback mechanisms between the surface cyclone, its attendant frontal boundaries, and the upper-level flow pattern help to further explain differences in the frontal zone between clusters. Significance StatementMixed-phase precipitation events, which can produce rain, freezing rain, ice pellets, and snow, are difficult to accurately forecast. This study investigates the large-scale processes influencing our ability to accurately forecast the precipitation type and amount during one of these events that was observed by a field campaign in February 2022. In forecasts initialized 5 days prior to the event, differences in the forecast upper-level atmospheric conditions led to differences in the forecast interactions between the upper-level flow and a low pressure system at the surface. As a result, there was large uncertainty in the predicted position of a surface front associated with the low pressure system and the precipitation-type distribution during the event. 

Abstract During near-0°C surface conditions, diverse precipitation types (p-types) are possible, including rain, drizzle, freezing rain, freezing drizzle, ice pellets, wet snow, snow, and snow pellets. Near-0°C precipitation affects wide swaths of the United States and Canada, impacting aviation, road transportation, power generation and distribution, winter recreation, ecology, and hydrology. Fundamental challenges remain in observing, diagnosing, simulating, and forecasting near-0°C p-types, particularly during transitions and within complex terrain. Motivated by these challenges, the field phase of the Winter Precipitation Type Research Multi-scale Experiment (WINTRE-MIX) was conducted from 1 February – 15 March 2022 to better understand how multiscale processes influence the variability and predictability of p-type and amount under near-0°C surface conditions. WINTRE-MIX took place near the US / Canadian border, in northern New York and southern Quebec, a region with plentiful near-0°C precipitation influenced by terrain. During WINTRE-MIX, existing advanced mesonets in New York and Quebec were complemented by deployment of: (1) surface instruments, (2) the National Research Council Convair-580 research aircraft with W- and X-band Doppler radars and in situ cloud and aerosol instrumentation, (3) two X-band dual-polarization Doppler radars and a C-band dual-polarization Doppler radar from University of Illinois, and (4) teams collecting manual hydrometeor observations and radiosonde measurements. Eleven intensive observing periods (IOPs) were coordinated. Analysis of these WINTRE-MIX IOPs is illuminating how synoptic dynamics, mesoscale dynamics, and microscale processes combine to determine p-type and its predictability under near-0°C conditions. WINTRE-MIX research will contribute to improving nowcasts and forecasts of near-0°C precipitation through evaluation and refinement of observational diagnostics and numerical forecast models. 

Abstract. Atmospheric rivers (ARs) transport large amounts of moisture from the mid- to high-latitudes and they are a primary driver of the most extremesnowfall events, along with surface melting, in Antarctica. In this study, we characterize the climatology and surface impacts of ARs on WestAntarctica, focusing on the Amundsen Sea Embayment and Marie Byrd Land. First, we develop a climatology of ARs in this region, using anAntarctic-specific AR detection tool combined with theModern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) and the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) atmospheric reanalyses. We find that while ARs are infrequent (occurring 3 % of the time), they cause intense precipitation in short periods of time and account for 11 % of the annual surface accumulation. They are driven by the coupling of a blocking high over the Antarctic Peninsula with a low-pressure system known as the Amundsen Sea Low. Next, we use observations from automatic weather stations on Thwaites Eastern Ice Shelf with the firn model SNOWPACK and interferometric reflectometry (IR) to examine a case study of three ARs that made landfall in rapid succession from 2 to 8 February 2020, known as an AR family event. While accumulation dominates the surface impacts of the event on Thwaites Eastern Ice Shelf (> 100 kg m−2 or millimeters water equivalent), we find small amounts of surface melt as well (< 5 kg m−2). The results presented here enable us to quantify the past impacts of ARs on West Antarctica's surface mass balance (SMB) and characterize their interannual variability and trends, enabling a better assessment of future AR-driven changes in the SMB. 

Abstract A polar–subtropical jet superposition is preceded by the development of a polar cyclonic potential vorticity (PV) anomaly at high latitudes and a tropical anticyclonic PV anomaly at subtropical latitudes. A confluent large-scale flow pattern can lead to the juxtaposition of these respective PV anomalies at middle latitudes, resulting in the addition of the nondivergent circulations induced by each PV anomaly and an increase in upper-tropospheric wind speeds at the location of jet superposition. Once these PV anomalies become juxtaposed, vertical motion within the near-jet environment facilitates the advection and diabatic redistribution of tropopause-level PV, and the subsequent formation of the steep, single-step tropopause structure that characterizes a jet superposition. Given the importance of vertical motion during the formation of jet superpositions, this study adopts a quasigeostrophic (QG) diagnostic approach to quantify the production of vertical motion during three types of jet superposition events: polar dominant, eastern subtropical dominant, and western subtropical dominant. The diagnosis reveals that the geostrophic wind induced by polar cyclonic QGPV anomalies is predominantly responsible for QG vertical motion in the vicinity of jet superpositions. The QG vertical motion diagnosed from the along-isotherm component of the Q vector, which represents the vertical motion associated with synoptic-scale waves, is dominant within the near-jet environment. The QG vertical motion diagnosed from the across-isotherm component of the Q vector, which represents the vertical motion associated with frontal circulations in the vicinity of the jet, is subordinate within the near-jet environment, but is relatively more important during eastern subtropical dominant events compared to polar dominant and western subtropical dominant events. 

Abstract Anomalously strong North Atlantic jets, defined in this study as jets with wind speeds exceeding 100 m·s⁻¹, are notable due to their potential to induce high‐impact weather. This study examines the kinematic processes that contribute to the intensification of anomalously strong North Atlantic jets, as well as the variability in those processes across a large number of events. Anomalously strong jets are objectively identified during September–May 1979–2018 within the Climate Forecast System Reanalysis and composited to reveal the synoptic‐scale flow evolution associated with jet intensification. The analysis demonstrates that anomalously strong North Atlantic jets are most frequent during the winter compared with the fall and spring, and that their development is preceded by low‐level warm‐air advection, poleward moisture advection, and moist ascent within the warm conveyor belt of a surface cyclone beneath the equatorward jet‐entrance region. A diagnosis of the irrotational and nondivergent components of the ageostrophic wind within the near‐jet environment reveals that both wind components facilitate jet intensification via their nonnegligible contributions to negative potential vorticity (PV) advection and PV frontogenesis in the vicinity of the dynamic tropopause. Weather Research and Forecasting (WRF) model simulations of a jet event from December 2013 with and without latent heating further suggest that the ageostrophic wind field within the near‐jet environment is substantially modulated by latent heating. The foregoing results indicate that a diagnosis of jet intensification during anomalously strong jet events is dependent on an accurate representation of the cumulative effects of latent heating within the near‐jet environment. 

Abstract A polar–subtropical jet superposition represents a dynamical and thermodynamic environment conducive to the production of high-impact weather. Prior work indicates that the synoptic-scale environments that support the development of North American jet superpositions vary depending on the case under consideration. This variability motivates an analysis of the range of synoptic–dynamic mechanisms that operate within a double-jet environment to produce North American jet superpositions. This study identifies North American jet superposition events during November–March 1979–2010 and subsequently classifies those events into three characteristic event types. “Polar dominant” events are those during which only the polar jet is characterized by a substantial excursion from its climatological latitude band, “subtropical dominant” events are those during which only the subtropical jet is characterized by a substantial excursion from its climatological latitude band, and “hybrid” events are those characterized by a mutual excursion of both jets from their respective climatological latitude bands. The analysis indicates that North American jet superposition events occur most often during November and December, and subtropical dominant events are the most frequent event type for all months considered. Composite analyses constructed for each event type reveal the consistent role that descent plays in restructuring the tropopause beneath the jet-entrance region prior to jet superposition. The composite analyses further show that surface cyclogenesis and widespread precipitation lead the development of subtropical dominant events and contribute to jet superposition via their associated divergent circulations and diabatic heating, whereas surface cyclogenesis and widespread precipitation tend to peak at the time of superposition and well downstream of polar dominant events.