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L. Baker Perry, Charles E. Konrad, and Thomas W. Schmidlin

Abstract

Northwest flow snow (NWFS) events are common occurrences at higher elevations and on windward slopes in the southern Appalachians. Low temperatures and considerable blowing and drifting of snow, coupled with significant spatial variability of snowfall, substantially increase societal impacts. This paper develops a synoptic classification of NWFS events in the southern Appalachians using 72-h antecedent upstream (backward) air-trajectory analyses. Hourly observations from first-order stations and daily snowfall data from cooperative observer stations are used to define snowfall events. NCEP–NCAR reanalysis data are utilized to identify NWFS events on the basis of 850-hPa northwest flow (270°–360°) at the event maturation hour. The NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory tool is used to calculate 72-h backward air trajectories at the event maturation hour and composite trajectories are mapped in a geographic information systems format. Analyses of vertical soundings are coupled with NCEP–NCAR reanalysis data to determine the synoptic characteristics associated with each trajectory class. Significant variability of trajectories and synoptic patterns is evident from the analyses, resulting in four distinct backward air-trajectory classes. Trajectories with a Great Lakes connection result in higher composite mean and maximum snowfall totals along portions of the higher-elevation windward slopes when compared with other northwest trajectories, but little effect of the Great Lakes is noted at lower elevations and on leeward slopes.

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Heather Guy, Anton Seimon, L. Baker Perry, Bronwen L. Konecky, Maxwell Rado, Marcos Andrade, Mariusz Potocki, and Paul A. Mayewski

Abstract

The tropical Andes of southern Peru and northern Bolivia have several major mountain summits suitable for ice core paleoclimatic investigations. However, incomplete understanding of the controls on the isotopic (δD, δ 18O) composition of precipitation and a paucity of field observations in this region continue to limit ice-core-based paleoclimate reconstructions. This study examines four years of daily observations of δD and δ 18O in precipitation from a citizen scientist network on the northeastern margin of the Altiplano, to identify controls on the subseasonal spatiotemporal variability in δ 18O during the wet season (November–April). These data provide new insights into modern δ 18O variability at high spatial and temporal scales. We identify a regionally coherent subseasonal signal in precipitation δ 18O featuring alternating periods of high and low δ 18O of 9–27-day duration. This signal reflects variability in precipitation delivery driven by synoptic conditions and closely relates to variations in the strength of the South American low-level jet and moisture availability over the study area. The annual layer of snowpack on the Quelccaya Ice Cap observed in the subsequent dry season retains this subseasonal signal, allowing the development of a snow-pit age model based on precipitation δ 18O measurements, and demonstrating how synoptic variability is transmitted from the atmosphere to mountaintop snowpacks along the Altiplano’s eastern margin. This result improves our understanding of the hydrometeorological processes governing δ 18O and δD in tropical Andean precipitation and has implications for improving paleoclimate reconstructions from tropical Andean ice cores and other paleoclimate records.

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Steve Keighton, Douglas K. Miller, David Hotz, Patrick D. Moore, L. Baker Perry, Laurence G. Lee, and Daniel T. Martin

Abstract

In late October 2012, Hurricane Sandy tracked along the eastern U.S. coastline and made landfall over New Jersey after turning sharply northwest and becoming posttropical while interacting with a complex upper-level low pressure system that had brought cold air into the Appalachian region. The cold air, intensified by the extreme low pressure tracking just north of the region, combined with deep moisture and topographically enhanced ascent to produce an unusual and high-impact early season northwest flow snow (NWFS) that has no analog in recent history. This paper investigates the importance of the synoptic-scale pattern, forcing mechanisms, moisture characteristics (content, depth, and likely sources), and low-level winds, as well as the evolution of some of these features compared to more typical NWFS events in the southern Appalachian Mountains. Several other aspects of the Sandy snowfall event are investigated, including low-level stability and mountain wave formation as manifested in vertical profiles and radar observations. The importance to operational forecasters of recognizing and understanding these factors and differences from more common NWFS events is also discussed.

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Jason L. Endries, L. Baker Perry, Sandra E. Yuter, Anton Seimon, Marcos Andrade-Flores, Ronald Winkelmann, Nelson Quispe, Maxwell Rado, Nilton Montoya, Fernando Velarde, and Sandro Arias

Abstract

This study used the first detailed radar measurements of the vertical structure of precipitation obtained in the central Andes of southern Peru and Bolivia to investigate the diurnal cycle and vertical structure of precipitation and melting-layer heights in the tropical Andes. Vertically pointing 24.1-GHz Micro Rain Radars in Cusco, Peru (3350 m MSL, August 2014–February 2015), and La Paz, Bolivia (3440 m MSL, October 2015–February 2017), provided continuous 1-min profiles of reflectivity and Doppler velocity. The time–height data enabled the determination of precipitation timing, melting-layer heights, and the identification of convective and stratiform precipitation features. Rawinsonde data, hourly observations of meteorological variables, and satellite and reanalysis data provided additional insight into the characteristics of these precipitation events. The radar data revealed a diurnal cycle with frequent precipitation and higher rain rates in the afternoon and overnight. Short periods with strong convective cells occurred in several storms. Longer-duration events with stratiform precipitation structures were more common at night than in the afternoon. Backward air trajectories confirmed previous work indicating an Amazon basin origin of storm moisture. For the entire dataset, median melting-layer heights were above the altitude of nearby glacier termini approximately 17% of the time in Cusco and 30% of the time in La Paz, indicating that some precipitation was falling as rain rather than snow on nearby glacier surfaces. During the 2015–16 El Niño, almost half of storms in La Paz had melting layers above 5000 m MSL.

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Steve Keighton, Laurence Lee, Blair Holloway, David Hotz, Steven Zubrick, Jeffrey Hovis, Gary Votaw, L. Baker Perry, Gary Lackmann, Sandra E. Yuter, Charles Konrad, Douglas Miller, and Brian Etherton

Upslope-enhanced snowfall events during periods of northwesterly flow in the southern Appalachians have been recognized as a significant winter forecasting problem for some time. However, only in recent years has this problem received noteworthy attention by both the academic and operational communities. The complex meteorology of these events includes significant topographic influences, as well as a linkage between the upstream Great Lakes and resultant southern Appalachian snowfall. A unique collaborative team has recently formed, working toward the goals of improving the physical understanding of the mechanisms at work in these events and developing more accurate forecasts and more detailed climatologies. The literature shows only limited attention to this problem through the 1990s. However, with modernization of the National Weather Service (NWS) in the mid-1990s came opportunities to bring more attention to new or poorly understood forecast problems. These opportunities included the establishment of new forecast offices, often collocated with universities, the deployment of the Weather Surveillance Radar-1988 Doppler (WSR-88D) network, expansion of the surface observational network in both space and time, improved access to sophisticated numerical models, and growth of the spotter and cooperative observer networks.

A collaborative team, consisting of faculty from five universities and meteorologists from six NWS forecast offices, has established an ongoing, structured dialogue to help advance the understanding and improve the forecasting of these events. The team utilizes a variety of communication strategies to discuss emerging research findings, review recent events, and share data and ideas. The ultimate goal is to continue fostering working relationships among research and operational meteorologists, climatologists, and students, all with a common motivation of continually improving forecasts and understanding of this important phenomenon. This group may serve as a model for other collaborative efforts between the research and operational communities interested in a common forecast problem.

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