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Atmospheric Rivers in Proximity to Western North Pacific Tropical Cyclones in October, 2010 – the Development and Evolution of 2 Atmospheric Rivers

This study investigates the evolution of 2 atmospheric rivers (ARs) that are zonally elongated that produced more than 200 mm of precipitation over mountainous regions of Northern California in late October 2010. Atmospheric Rivers are indicted to develop within high-CAPE environments that are characterised by troposphere-deep ascent as water vapour was transported from western North Pacific tropical cyclones (TCs) towards the equatorwards region of entrance of a North pacific jet stream that is intensifying, by synoptic scale analysis and analysis of air parcel trajectory. Water vapour was transported from extratropical and subtropical regions over the North Pacific to subsequently maintain the same atmospheric rivers in an environment that is characterised by quasi geostrophic forcing of ascent and strong frontogenesis along the anticyclonic sheer side of a North Pacific Jet Stream that was intense and extended zonally. It is illustrated by an atmospheric water budget that decreases in integrated water vapour (IWV) by precipitation are offset largely by the horizontal aggregation of water vapour along the corridors of the atmospheric rivers via integrated water vapour flux convergence in the presence of frontogenesis, though the atmospheric rivers develop in conjunction with the transport of water vapour from regions that are near tropical cyclones and in the presence of tropospheric deep ascent. The frameworks that are used for investigation of predecessor rain events (PREs) ahead of tropical cyclones and of interactions between recurving cyclones and the North Pacific jet stream are illustrated by also utilising many processes that a dynamically similar to the development and evolution of atmospheric rivers. Similarities include the transport of water vapour directly from a tropical cyclone, tropospheric deep ascent in a high CAPE environment beneath the equatorwards entrance region of an upper tropospheric jet streak that is intensifying, interaction between diabatic outflow and upper tropospheric jet streak, and strong frontogenesis.

The majority of water vapour transported in the atmosphere is focused in regions of vertically integrated water vapour that are thousands of kilometres long and about 500 km wide that are frequently referred to as atmospheric rivers (ARs., Zhu & Newell, 1998 Ralph et al., 2004; Neiman et al., 2008a,b, among others) or moisture conveyor belts (e.g., Bao et al., 2006; Knippertz & Martin, 2007). The warm sector of transient maritime extratropical cyclones is where atmospheric rivers are often found and can represent regions of concentrated integrated water vapour transport along a precold-frontal low level jet (e.g., Ralph et al., 2004) or regions of integrated vapour transport into a narrow substructure of a broad ascending “warm conveyor belt” (e.g., Carlson, 1980; Browning, 1990; Carlson, 1998; Eckhardt et al., 2004). The transport of tropical water vapour into the extratropics (i.e., export of tropical moisture; Knippertz & Wernli, 2010;  Knippertz et al., 2013) or result in the lateral transport of subtropical and extratropical water vapour  (e.g., Bao et al., 2006; Neiman et al. 2013), can result from the concentrated regions of integrated water vapour and Integrated vapour transport associated with atmospheric rivers. It is indicated that once in the extratropics (Zhu & Newell, 1998) integrated vapour transport is confined predominantly to atmospheric rivers and that about 90% of the horizontal integrated vapour transport occurs within only 10% of the hemi-circumference.

The investigation of the evolution of 2 atmospheric rivers that are elongated zonally over the North Pacific basin in October 2010 is the objective of the current study. These 2 atmospheric rivers developed close to 3 tropical cyclones, Tropical cyclones Megi, Chaba, and 17W, in the western and central North Pacific and contained contiguous regions where the integrated water vapour values were high as they crossed the North Pacific between the 20^th and 24^th October 2010. The 2 atmospheric rivers in this study ultimately merged over the central and eastern North Pacific, making landfall over North California on 24 October 2010. Totals of precipitation over 48 hours of more than  200 mm was associated with the landfall of the merged atmospheric river which ended at 1200 UTC 25 October 2010 over coastal and interior mountainous regions of Northern California. The locations where the precipitation totals were highest are consistent with the orogenic enhanced precipitation that is commonly associated with landfalling atmospheric rivers (e.g., Ralph et al., 2003, 2004, 2005, 2006; Neiman et al., 2008a, b, 2011; Stohl et al., 2008; Smith et al., 2010; Ralph & Dettinger, 2012). Four Trees (FOR), California, at elevation of 1450 m MSL, received the highest event total of 333 mm for the 72 hour period which ended at 1200 UTC 25 October 2010. It is indicated by this total precipitation that the event was classified as a ´R-Cat 2” case by the use of the extreme case scaling criteria from Ralph & Dettinger (2012). It is indicted by a climatological analysis of 48-hour area-averaged precipitation totals over Northern California that the landfall of the merged atmospheric river was associated with the largest 48-h October precipitation event between 1970 and 2010.

According to Cordeira et al. the development of the 2 atmospheric rivers in the study in proximity to tropical cyclones Megi, Chaba, and 17W, and the subsequent landfall of the merged atmospheric river over Northern California suggests that tropical cyclones in the western North Pacific may influence the occurrence of autumnal extreme precipitation events associated with landfalling atmospheric rivers along the west coast of North America.

Cordeira et al. suggest this study is applicable to another strong atmospheric river, that was orientated zonally, that impacted California in October 2009 (Ralph & Dettinger, 2011), which also produced a precipitation magnitude of R-Cat 2. Tropical cyclones in the western North Pacific have been demonstrated by a majority of observational and modelling studies to date to be capable of influencing sensible weather far downstream by the evolution of the large-scale flow. It is emphasised by these studies that the interaction between a tropical cyclone and the extratropical waveguide during recurvature and extratropical transition that can result in a response in the form of a large scale flow associated with Rossby wave train amplification and dispersion (e.g., Harr & Elsberry, 2000; Klein et al., 2002; Atallah & Bosart, 2003; McTaggart-Cowan et al., 2007a, b; Riemer et al., 2008; Harr & Dea, 2009; Riemer & Jones 2010; Archambault, 2011; Archambault et al., 2013). It has also been demonstrated by recent studies that extreme precipitation events in the form of predecessor rain events (PREs) far downstream of tropical cyclones in association with the transport of water vapour over large horizontal distances can be influenced by tropical cyclones.

Whether the physical processes contributing to the development of predecessor rain events and whether or not the extreme precipitation event over Northern California can be considered a predecessor, is explored by this study. These mesoscale rainstorms are associated with rainfall rates of ≥100 mm/day and typically occur about 1,000 km downstream of tropical cyclones. It is indicated by composite analyses and case studies and predecessor rainfall events of the US that predecessor rain events develop when a broad plume of atmospheric perceptible water (i.e., integrated water vapour) that emanates from a tropical cyclone along a low-let jet that intersects a baroclinic zone, that is quasi-stationary, and is forced to ascend in conjunction with a thermally direct lower tropospheric frontogenetic circulation in the equatorwards entrance region of an upper tropospheric jet streak that is quasi-stationary and intensifying (Bosart & Carr, 1978; Bosart & Dean, 1991; Cote, 2007; Galamaeau et al., 2010; Moore, 2010; Schumacher et al., 2011; Bosart et al., 2012; Moore et al., 2013). Predecessor rain events have also been shown to occur over eastern Asia ahead of tropical cyclones in the North Pacific (e.g., Wang et al., 2009; Byun & Lee, 2012). The horizontal transport of water vapour from tropical cyclones is suggested by caparisons among past studies of atmospheric rivers, that includes the moisture conveyor belts and exports of tropical moisture, and predecessor rain events, that the export of moisture could lead to the formation of atmospheric rivers (e.g., Figs, 3 and 6 of Stohl et al., 2008, and Figs. 7a, b of Knippertz et al., 2013). The formation of such an atmospheric river would, however, need to overcome the potential deleterious effect of the removal of integrated water vapour via processes of precipitation in the presence of saturated, troposphere-deep upright ascent in the case of a predecessor rain event or ascent of a sloped air parcel in the case of a warm conveyor belt. Cordeira et al. hypothesise that given the longevity of such an atmospheric river the deleterious effect of precipitation on the integrated water vapour flux tendency within the atmospheric rivers is likely to be offset by water vapour flux convergence within regions of frontogenesis in the atmospheric river regions as was suggested (Bao et al., 2006; Neiman et al., 2013). The hypothesis that was proposed for the formation and evolution of the 2 atmospheric rivers that were zonally elongated in the present study was explored by traditional synoptic scale analysis, an atmospheric river water budget analysis, and a Lagrangian trajectory analysis.

Implications of results

It was demonstrated by the results of this study that formation of atmospheric rivers of the western North Pacific can occur in association with water vapour transport polewards from tropical cyclones. Research that describes the formation of atmospheric river-like features downstream of upper tropospheric subtropical troughs at low latitude is also complemented by the processes that are described for the formation of atmospheric rivers in this study (e.g., McGuirk et al., 1987; Waugh & Funatsu, 2003; Knippertz, 2007; Ralph et al., 2011). Atmospheric river-like features are indicated by the results of this study to may also form when there is no subtropical trough upstream, though given sufficient polewards transport of water vapour into the midlatitudes on the east and polewards side of a tropical cyclone. It is also suggested by analysis of the trajectory of air parcels that the 2 atmospheric rivers in this study integrated vapour transport corridors that are likely to constitute a portion of the warm ascending conveyor belt as in Eckhardt et al., (2004).

An atmospheric water vapour budget was used in this study to demonstrate that when integrated water vapour decreases via precipitation processes in the presence of troposphere-deep upwards vertical motion during the development phase of an atmospheric river can subsequently be offset when integrated water vapour increases via integrated vapour transport convergence and evaporation during the evolution phase of an atmospheric river. In this study the subsequent evolution of atmospheric rivers that are zonally elongated occurs in association with a transition from a tropical environment to an extratropical environment that is characterised by QG forcing for ascent and a strong ageostrophic circulation that is thermally direct in the region of entrance of an upper tropospheric jet streak that is intensifying and zonally elongated. The extratropical environment and dynamical processes that are related to it act to sustain large values of integrated water vapour in conjunction with horizontal aggregation of subtropical and extratropical water vapour along the atmospheric river corridors via frontogenesis. In this study the dynamical processes associated with the integrated vapour transport convergence are similar to results that have been documented (Bao et al., 2006 Neiman et al., 2013), where it was shown that some significant atmospheric rivers strengthen markedly when in the presence of frontogenesis and when there is no direct water vapour transport from the tropics.

The atmospheric river evolution occurs equatorwards of a frontal low that propagated rapidly across the North Pacific with characteristics that were similar to those of a diabatic Rossby wave. The characteristics of a Rossby wave are a lower-troposphere potential vorticity maximum that propagates typically eastwards due to the diabatic potential vortocity tendencies that are induced by the release of latent heat in the presence of saturated ascent along a strong lower troposphere baroclinic zone. Strong lower tropospheric frontogenesis and large integrated water vapour values characterise an atmospheric river that is zonally elongated, may be particularly conducive to the development and propagation to the east of diabatic Rossby waves.  Though not discussed in this study, it is illustrated (Lackmann, 2002) that the release of heat is an important contributor to the development and eastwards propagation of elongated lower tropospheric potential vorticity bands that accompany regions where there is water vapour transport occurring in the warm sector of some cyclones. Connections have not specifically been drawn between atmospheric rivers that are zonally elongated and AR diabatic Rossby waves in previous studies of diabatic Rossby waves; several studies have, however, identified tropical source regions of moisture and air parcel trajectories that are consistent with the warm conveyor belt region of cyclones that are intensifying as ingredients in at least some life cycles of diabatic Rossby waves (e.g., Wernli et al., 2002; Moore et al., 2008; Cordeira & Bosart, 2011).

A noteworthy advance in the understanding of the connection between tropical processes and atmospheric rivers in middle latitudes is represented by this study. This study shows an indirect effect of the tropics on a midlatitude atmospheric river via dynamical processes that are associated with the interaction of tropical cyclone and predecessor rain event-like diabatic outflow with the North Pacific jet stream. The fact that atmospheric rivers are features that are highly dynamic for which continuous regeneration of integrated water vapour through evaporation and water vapour flux convergence act to maintain the integrated water vapour, that is distinctive, against strong water vapour loss through precipitation.

The landfalling aspect of this event will be explored in future work, which includes evaluation of the role of the merger of the 2 atmospheric rivers that were initially separate atmospheric rivers into a single strong atmospheric river at landfall. According to Cordeira et al. it is likely that both the intensity and duration of the event was contributed to by the merging atmospheric rivers. These aspects are to be compared with findings of a climatological study of the duration of atmospheric rivers (Ralph et al., 2013), and of the role played by the Sierra barrier jet (SBJ) in deflecting a landfalling atmospheric river upwards over the Sierra barrier jet (Kingsmill et al., 2013). Also, as there can be significant uncertainties in the representation of key atmospheric processes in datasets of reanalysis, they have aimed future work at further investigations of the terms in the atmospheric water vapour budget from both a numerical modelling perspective and from aircraft and from observations of remote sensing. It would need to be recognised by such observations that the magnitudes of the rate of precipitation, rate of evaporation, and the tendencies of the integrated water vapour flux convergence to vary grealty from one region to another during an event, and would need to span the complete life cycle of an atmospheric river (i.e., from the “development” phase to the “evolution” phase).  

Cordeira, J. M., et al. (2013). "The Development and Evolution of Two Atmospheric Rivers in Proximity to Western North Pacific Tropical Cyclones in October 2010." Monthly Weather Review 141(12): 4234-4255.