Heather M. Archambault

Influence of Large-Scale Flow Regimes on Cool-Season Precipitation in the Northeastern United States

The influence of large-scale flow regimes on cool-season (November–April) northeastern U.S. (Northeast) precipitation is investigated for the period 1948–2003 from statistical and synoptic perspectives. These perspectives are addressed through (i) a statistical analysis of cool-season Northeast precipitation associated with the North Atlantic Oscillation (NAO) and Pacific–North American (PNA) regimes (one standard deviation or greater NAO or PNA daily index anomalies persisting several days), and (ii) a composite analysis of the synoptic signatures of major (two standard deviation) 24-h cool-season Northeast precipitation events occurring during NAO and PNA regimes. The statistical analysis reveals that negative PNA regimes are associated with above-average cool-season Northeast precipitation and an above-average frequency of light and moderate precipitation events, whereas the opposite associations are true for positive PNA regimes. In comparison with PNA regimes, NAO regimes are found to have relatively little influence on the amount and frequency of cool-season Northeast precipitation. The composite analysis indicates that a surface cyclone flanked by an upstream trough over the Ohio Valley and downstream ridge over eastern Canada and upper- and lower-level jets in the vicinity of the Northeast are characteristic signatures of major cool-season Northeast precipitation events occurring during NAO and PNA regimes. Negative NAO and positive PNA precipitation events, however, are associated with a more amplified trough–ridge pattern and greater implied Atlantic moisture transport by a low-level jet into the Northeast than positive NAO and negative PNA precipitation events. Furthermore, a signature of lateral upper-level jet coupling is noted only during positive and negative PNA precipitation events.

Relationships between Large-Scale Regime Transitions and Major Cool-Season Precipitation Events in the Northeastern United States

Abstract This observational study investigates statistical and synoptic–dynamic relationships between regime transitions, defined as a North Atlantic Oscillation (NAO) or Pacific–North American pattern (PNA) index change from at least a 1 standard deviation anomaly to at least a 1 standard deviation anomaly of opposite sign within 7 days, and cool-season (November–April) northeastern U.S. (NE) precipitation. A statistical analysis is performed of daily cool-season NE precipitation during all NAO and PNA transitions for 1948–2003, and a composite analysis and case study of a major cool-season NE precipitation event occurring during a positive-to-negative NAO transition are conducted. Datasets used are the 0.25° NCEP Unified Precipitation Dataset, the 2.5° NCEP–NCAR reanalysis, and the 1.125° 40-yr ECMWF Re-Analysis (ERA-40). Results of the statistical analysis suggest that cool-season NE precipitation tends to be enhanced during positive-to-negative NAO and negative-to-positive PNA transitions, and suppres...

A Composite Perspective of the Extratropical Flow Response to Recurving Western North Pacific Tropical Cyclones

AbstractThis study investigates the composite extratropical flow response to recurving western North Pacific tropical cyclones (WNP TCs), and the dependence of this response on the strength of the TC–extratropical flow interaction as defined by the negative potential vorticity advection (PV) by the irrotational wind associated with the TC. The 2.5° NCEP–NCAR reanalysis is used to construct composite analyses of all 1979–2009 recurving WNP TCs and of subsets that undergo strong and weak TC–extratropical flow interactions.Findings indicate that recurving WNP TCs are associated with the amplification of a preexisting Rossby wave train (RWT) that disperses downstream and modifies the large-scale flow pattern over North America. This RWT affects approximately 240° of longitude and persists for approximately 10 days. Recurving TCs associated with strong TC–extratropical flow interactions are associated with a stronger extratropical flow response than those associated with weak TC–extratropical flow interactions...

An analysis of multiple predecessor rain events ahead of tropical cyclones Ike and Lowell: 10–15 September 2008

AbstractAn analysis of three predecessor rain events (PREs) that occurred ahead of North Atlantic tropical cyclone (TC) Ike and east Pacific TC Lowell during 10–15 September 2008 is presented. The three PREs produced all-time daily record rainfall at many locations, including Lubbock, Texas (189.5 mm); Wichita, Kansas (262 mm); and Chicago–O’Hare, Illinois (169 mm), on 11–13 September, respectively.PRE 1 organized over Texas on 10 September with moisture from a stalled frontal boundary and the Bay of Campeche, and matured with moisture from TC Lowell. PRE 2 organized over the Texas Panhandle on 11 September with moisture from the Bay of Campeche, and developed and matured over Kansas and Missouri with moisture from TC Lowell. PRE 3 developed over Texas on 11 September, merged with and absorbed PRE 2 over Kansas and Missouri, and matured as it ingested moisture from TC Ike. All three PREs matured in the equatorward entrance region of an intensifying subtropical jet stream (STJ).Heavy rainfall with the thre...

The Key Role of Diabatic Outflow in Amplifying the Midlatitude Flow: A Representative Case Study of Weather Systems Surrounding Western North Pacific Extratropical Transition

AbstractRecurving tropical cyclones (TCs) undergoing extratropical transition (ET) may substantially modify the large-scale midlatitude flow pattern. This study highlights the role of diabatic outflow in midlatitude flow amplification within the context of a review of the physical and dynamical processes involved in ET. Composite fields of 12 western North Pacific ET cases are used as initial and boundary conditions for high-resolution numerical simulations of the North Pacific–North American sector with and without the TC present. It is demonstrated that a three-stage sequence of diabatic outflow associated with different weather systems is involved in triggering a highly amplified midlatitude flow pattern: 1) preconditioning by a predecessor rain event (PRE), 2) TC–extratropical flow interaction, and 3) downstream flow amplification by a downstream warm conveyor belt (WCB). An ensemble of perturbed simulations demonstrates the robustness of these stages. Beyond earlier studies investigating PREs, recurv...

THE PRE-DEPRESSION INVESTIGATION OF CLOUD-SYSTEMS IN THE TROPICS (PREDICT) FIELD CAMPAIGN Perspectives of Early Career Scientists

The Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) field experiment successfully gathered data from four developing and four decaying/nondeveloping tropical disturbances over the tropical North Atlantic basin between 15 August and 30 September 2010. The invaluable roles played by early career scientists (ECSs) throughout the campaign helped make possible the successful execution of the field programs mission to investigate tropical cyclone formation. ECSs provided critical meteorological information— often obtained from novel ECS-created products—during daily weather briefings that were used by the principal investigators in making mission planning decisions. Once a Gulfstream V (G-V) flight mission was underway, ECSs provided nowcasting support, relaying information that helped the mission scientists to steer clear of potential areas of turbulence aloft. Data from these missions, including dropsonde and GPS water vapor profiler data, were continually obtained, processed, and qual...

The Extratropical Transition of Tropical Cyclones. Part I: Cyclone Evolution and Direct Impacts

AbstractExtratropical transition (ET) is the process by which a tropical cyclone, upon encountering a baroclinic environment and reduced sea surface temperature at higher latitudes, transforms into an extratropical cyclone. This process is influenced by, and influences, phenomena from the tropics to the midlatitudes and from the meso- to the planetary scales to extents that vary between individual events. Motivated in part by recent high-impact and/or extensively observed events such as North Atlantic Hurricane Sandy in 2012 and western North Pacific Typhoon Sinlaku in 2008, this review details advances in understanding and predicting ET since the publication of an earlier review in 2003. Methods for diagnosing ET in reanalysis, observational, and model-forecast datasets are discussed. New climatologies for the eastern North Pacific and southwest Indian Oceans are presented alongside updates to western North Pacific and North Atlantic Ocean climatologies. Advances in understanding and, in some cases, mode...

A Comparison of South American and African Preferential Pathways for Extreme Cold Events

AbstractIn the Southern Hemisphere, a relatively well-known preferential pathway along which cold air surges equatorward is situated to the east of the Andes Mountains. In this study, a second preferred pathway is identified to the east of the African Highlands, with additional minor pathways identified east of the Brazilian Highlands and Madagascar. The primary objective of this study is to compare climatological and synoptic characteristics of extreme cold events (ECEs) along the Andes and African Highlands pathways. ECEs are defined as the top 1% coldest 925-hPa temperatures within the Andes and the African Highlands pathways using the 1977–2001 subset of the 2.5° × 2.5° 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). ECEs within the Andes and African Highlands pathways are associated with dynamically forced anticyclogenesis and have low-level characteristics that vary substantially. Along the Andes pathway, ECEs feature 925-hPa temperatures as much as 17°C below ...

Interactions of North Pacific Tropical, Midlatitude, and Polar Disturbances Resulting in Linked Extreme Weather Events over North America in October 2007

AbstractThis study uses observations and model reanalyses to examine the multiscale processes associated with four high-impact extreme weather events (EWEs) over North America during late October 2007. The EWEs consisted of wind-driven wildfires in California, prolonged anomalous cold conditions in Mexico linked to two cold surges, heavy rainfall in the eastern United States, and severe flood-producing heavy rainfall in southern Mexico. The EWEs involved a pronounced large-scale flow reconfiguration across the North Pacific and North America in conjunction with the formation of a high-amplitude Rossby wave train. The flow reconfiguration involved perturbations to the North Pacific jet stream linked to polar, midlatitude, and tropical disturbances, including three tropopause-level polar disturbances originating over northeastern Asia, transient extratropical cyclones, a diabatic Rossby vortex, and western North Pacific Tropical Cyclone Kajiki. Eulerian and Lagrangian diagnostics indicate that ridge amplifi...