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Australia: The Land Where Time Began |
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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 20th and 24th 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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |