Abstract

Several recent landfalling tropical cyclones (e.g., Dennis, Floyd, and Irene 1999) have highlighted a need for a refinement in the forecasting paradigms and techniques in the area of quantitative precipitation forecasting. Floyd proved to be a particularly challenging forecast problem as it was accompanied by catastrophic flooding over large regions of the East Coast, in spite of its relatively quick northward movement. The extent and intensity of the precipitation distribution was strongly modulated by the storm's interaction with a midlatitude trough. In an attempt to better understand and quantify the relevant dynamics during this interaction, potential vorticity (PV) and quasigeostrophic perspectives are utilized.

As Floyd approached the East Coast, precipitation shifted to the left of the storm track due to the presence of a deep midlatitude trough in the Ohio valley. The juxtaposition of a cold-core PV anomaly associated with the midlatitude trough and a warm-core PV anomaly associated with Floyd created a strong and tropospheric-deep baroclinic zone along the eastern seaboard. This baroclinic zone provided a region favorable for deep isentropic ascent as the circulation of Floyd approached, resulting in prolific precipitation production. The latent heat release associated with this precipitation in turn acted to enhance outflow ridging north of Floyd, which was underpredicted by current numerical models. The enhanced outflow ridge resulted in enhanced jet-streak dynamics and a restructuring of the tilt of the midlatitude trough in a manner favorable for excessive precipitation production. Furthermore, the uplifting of the dynamic tropopause in southwesterly flow ahead of Floyd in response to ascent and differential diabatic heating resulted in a tropopause fold, a feature usually associated with upper-level fronts and differential subsidence in northwesterly flow.

Abstract

Tropical cyclones in the western North Atlantic basin are a persistent threat to human interests along the east coast of North America. Occurring mainly during the late summer and early autumn, these storms often cause strong winds and extreme rainfall and can have a large impact on the weather of eastern Canada. From 1979 to 2005, 40 named (by the National Hurricane Center) tropical cyclones tracked over eastern Canada. Based on the time tendency of the low-level (850–700 hPa) vorticity, the storms are partitioned into two groups: “intensifying” and “decaying.” The 16 intensifying and 12 decaying cases are then analyzed using data from both the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR) and the NCEP global reanalysis. Composite dynamical structures are presented for both partitioned groups, utilizing both quasigeostrophic (QG) and potential vorticity (PV) perspectives. It is found that the proximity to the tropical cyclone and subsequent negative tilt (or lack thereof) of a precursor trough over the Great Lakes region is crucial to whether a storm “intensifies” or “decays.” Heavy precipitation is often the main concern when tropical cyclones move northward into the midlatitudes. Therefore, analyses of storm-relative precipitation distributions show that storms intensifying (decaying) as they move into the midlatitudes often exhibit a counterclockwise (clockwise) rotation of precipitation around the storm center.

Abstract

Available potential energy (APE), a measure of the energy available for conversion to kinetic energy, has been previously applied to examine changes in baroclinic instability and seasonal changes in the general circulation. Here, pathways in which the troposphere can build the reservoir of zonal available potential energy A _Z on synoptic (3–10 day) time scales are explored. A climatology of A _Z and its generation G _Z and conversion terms are calculated from the National Centers for Environmental Prediction–Department of Energy Reanalysis 2 dataset from 1979 to 2011 for 20°–85°N. A standardized anomaly-based identification technique identifies 183 A _Z buildup events, which are grouped into two event types based upon their final A _Z standardized anomaly (σ) value: 1) buildup anomalous (BA) events, which exceed 1.5σ, and 2) buildup neutral (BN) events, which do not exceed 1.5σ. Increases in G _Z and reductions in baroclinic conversion C _A , source and sink terms for A _Z , are shown to equally contribute toward increasing A _Z in most seasons. A synoptic analysis of composited mass fields for winter BA events (n = 18 events) and winter BN events (n = 28 events) is performed to identify contributions to anomalously low C _A and high G _Z . A process of high-latitude cooling near 160°E–120°W is found for both composite event types. The cooling processes are characterized by a period of poleward moisture flux and ascent followed by an isolation of the Arctic from the midlatitude flow, resulting in enhanced G _Z . Negative anomalies in C _A are also diagnosed, which generally occur in regions with northerly dynamic tropopause wind anomalies and neutral to positive thickness anomalies.

Abstract

The 19–21 June 2013 Alberta flood was the costliest (CAD $6 billion) natural disaster in Canadian history. The flood was caused by a combination of above-normal spring snowmelt in the Canadian Rockies, large antecedent precipitation, and an extreme rainfall event on 19–21 June that produced rainfall totals of 76 mm in Calgary and 91 mm in the foothills. As is typical of flash floods along the Front Range of the Rocky Mountains, rapidly rising streamflow proceeded to move downhill (eastward) into Calgary.

A meteorological analysis traces an antecedent Rossby wave train across the North Pacific Ocean, starting with intense baroclinic development over East Asia on 11 June. Subsequently, downstream Rossby wave development occurred across the North Pacific; a 1032-hPa subtropical anticyclone located northeast of Hawaii initiated a southerly atmospheric river into Alaska, which contributed to the development of a cutoff anticyclone over Alaska and a Rex block (ridge to the north, cyclone to the south) in the northeastern North Pacific. Upon breakdown of the Rex block, lee cyclogenesis occurred in Montana and strong easterly upslope flow was initiated in southern Alberta.

The extreme rainfall event was produced in association with a combination of quasigeostrophically and orographically forced ascent, which acted to release conditional and convective instability. As in past Front Range flash floods, moisture flux convergence and positive θ _e advection were collocated with the heavy rainfall. Backward trajectories show that air parcels originated in the northern U.S. plains, suggesting that evapotranspiration from the local land surface may have acted as a moisture source.

Abstract

Rossby wave breaking (RWB) events are a common feature on the dynamic tropopause and act to modulate synoptic-scale jet dynamics. These events are characterized on the dynamic tropopause by an irreversible overturning of isentropes and are coupled to troposphere-deep vertical motions and geopotential height anomalies. Prior climatologies have focused on the poleward streamer, the equatorward streamer, or the reversal in potential temperature gradient between the streamers, resulting in differences in the frequencies of RWB. Here, a new approach toward cataloging these events that captures both streamers is applied to the National Centers for Environmental Prediction Reanalysis-2 dataset for 1979–2011. Anticyclonic RWB (AWB) events are found to be nearly twice as frequent as cyclonic RWB (CWB) events. Seasonal decompositions of the annual mean find AWB to be most common in summer (40% occurrence), which is likely due to the Asian monsoon, while CWB is most frequent in winter (22.5%) and is likely due to the equatorward shift in mean baroclinicity. Trends in RWB from 1980 to 2010 illustrate a westward shift in North Pacific AWB during winter and summer (up to 0.4% yr⁻¹), while CWB in the North Pacific increases in winter and spring (up to 0.2% yr⁻¹). These changes are hypothesized to be associated with localized changes in the two-way interaction between the jet and RWB. The interannual variability of AWB and CWB is also explored, and a notable modality to the frequency of RWB is found that may be attributable to known low-frequency modes of variability including the Arctic Oscillation, the North Atlantic Oscillation, and the Pacific–North American pattern.

Abstract

Zonal available potential energy A _Z measures the magnitude of meridional temperature gradients and static stability of a domain. Here, the role of Northern Hemisphere dynamic tropopause (2.0-PVU surface) Rossby wave breaking (RWB) in supporting an environment facilitating buildups of A _Z on synoptic time scales (3–10 days) is examined. RWB occurs when the phase speed of a Rossby wave slows to the advective speed of the atmosphere, resulting in a cyclonic or anticyclonic RWB event (CWB and AWB, respectively). These events have robust dynamic and thermodynamic feedbacks through the depth of the troposphere that can modulate A _Z . Significant synoptic-scale buildups in A _Z and RWB events are identified from the National Centers for Environmental Prediction Reanalysis-2 dataset from 1979 to 2011 for 20°–85°N. Anomalies in AWB and CWB are assessed seasonally for buildup periods of A _Z . Positive anomalies in AWB and negative anomalies in CWB are found for most A _Z buildup periods in the North Pacific and North Atlantic basins and attributed to localized poleward shifts in the jet stream. Less frequent west–east dipoles in wave breaking anomalies for each basin are attributed to elongated and contracted regional jet exit regions. Finally, an analysis of long-duration AWB events for winter A _Z buildup periods to an anomalously high A _Z state is performed using a quasi-Lagrangian grid-shifting technique. North Pacific AWB events are shown to diabatically intensify the North Pacific jet exit region (increasing Northern Hemisphere A _Z ) through latent heating equatorward of the jet exit and radiative and evaporative cooling poleward of the jet exit.

Abstract

Though prolonged freezing rain events are rare, they can result in substantial damage when they occur. While freezing rain occurs less frequently in the south-central United States than in some regions of North America, a large number of extremely long-duration events lasting at least 18 h have been observed there. We explore the key synoptic–dynamic conditions that lead to these extreme events through a comparison with less severe short-duration events. We produce synoptic–dynamic composites and 7-day backward trajectories for parcels ending in the warm and cold layers for each event category. The extremely long-duration events are preferentially associated with a deeper and more stationary 500-hPa longwave trough centered over the southwestern United States at event onset. This trough supports sustained flow of warm, moist air from within the planetary boundary layer over the Gulf of Mexico northward into the warm layer. The short-duration cases are instead characterized by a more transient upper-level trough axis centered over the south-central U.S. region at onset. Following event onset, rapid passage of the trough leads to quasigeostrophic forcing for descent and the advection of cold, dry air that erodes the warm layer and ends precipitation. While trajectories ending in the cold layer are very similar between the two categories, those ending in the warm layer have a longer history over the Gulf of Mexico in the extreme cases compared with the short-duration ones, resulting in warmer and moister onset warm layers.

Abstract

Freezing rain can cause severe impacts, particularly when it persists for many hours. In this paper, we present the climatology of long-duration (6 or more hours) freezing rain events in the United States and Canada from 1979 to 2016. We identify three focus regions from this climatology and examine the archetypal thermodynamic evolution of events in each region using surface and radiosonde observations. Long-duration events occur most frequently in the northeastern United States and southeastern Canada, where freezing rain typically begins as lower-tropospheric warm-air advection develops the warm layer aloft. This warm-air advection and the latent heat of fusion released when rain freezes at the surface erode the cold layer, and freezing rain transitions to rain once the surface temperature reaches 0°C. In the southeastern United States, a larger percentage of events are of long duration than elsewhere in North America. Weak surface cold-air advection and evaporative cooling in the particularly dry onset cold layers there prevent surface temperatures from rising substantially during events. Finally, the south-central United States has a regional maximum in the occurrence of the top 1% of events by duration (18 or more hours), despite the relative rarity of freezing rain there. These events are associated with particularly warm/deep onset warm layers, with persistent low-level cold-air advection maintaining the cold layer. The thermodynamic evolutions we have identified highlight characteristics that are key to supporting persistent freezing rain in each region and may warrant particular attention from forecasters tasked with predicting these events.

Abstract

Quantitative precipitation forecasting (QPF) continues to be a significant challenge in operational forecasting, particularly in regions susceptible to extreme precipitation events. St. John’s, Newfoundland, Canada (CYYT), is affected frequently by such events, particularly in the cool season (October–April).

The 50 median events in the extreme (>33.78 mm during a 48-h period) precipitation event category are selected for further analysis. A manual synoptic typing is performed on these 50 events, using two separate methodologies to partition events. The first method utilizes a Lagrangian backward air parcel trajectory analysis and the second method utilizes the evolution of dynamically relevant variables, including 1000–700-hPa horizontal temperature advection, 1000–700-hPa (vector) geostrophic frontogenesis, and 700–400-hPa absolute vorticity advection.

Utilizing the first partitioning method, it is found that south cases are characterized by a strong anticyclone downstream of St. John’s, southwest events are synoptically similar to the overall extreme composite and are marked by a strong cyclone that develops in the Gulf of Mexico, while west events are characterized by a weak Alberta clipper system that intensifies rapidly upon reaching the Atlantic Ocean. The second partitioning method suggests that while cyclone events are dominated by the presence of a rapidly developing cyclone moving northeastward toward St. John’s, frontal events are characterized by the presence of a strong downstream anticyclone and deformation zone at St. John’s.

It is the hope of the authors that the unique methodology and results of the synoptic typing in this paper will aid forecasters in identifying certain characteristics of future precipitation events at St. John’s and similar stations.

Abstract

Significant cool season precipitation along the western coast of North America is often associated with intense water vapor transport (IWVT) from the Pacific Ocean during favorable synoptic-scale flow regimes. These relatively narrow and intense regions of water vapor transport can originate in either the tropical or subtropical oceans, and sometimes have been referred to as Pineapple Express events in previous literature when originating near Hawaii. However, the focus of this paper will be on diagnosing the synoptic-scale signatures of all significant water vapor transport events associated with poleward moisture transport impacting the western coast of Canada, regardless of the exact points of origin of the associated atmospheric river. A trajectory analysis is used to partition the events as a means of creating coherent and meaningful synoptic-scale composites. The results indicate that these IWVT events can be clustered by the general area of origin of the majority of the saturated parcels impacting British Columbia and the Yukon Territories. IWVT events associated with more zonal trajectories are characterized by a strong and mature Aleutian low, whereas IWVT events associated with more meridional trajectories are often characterized by an anticyclone situated along the California or Oregon coastline, and a relatively mature poleward-traveling cyclone, commonly originating in the central North Pacific.