ESDEarth System DynamicsESDEarth Syst. Dynam.2190-4987Copernicus PublicationsGöttingen, Germany10.5194/esd-8-1247-2017Evaluation of the moisture sources in two extreme landfalling atmospheric river events using an Eulerian WRF tracers toolEiras-BarcaJorgejorge.eiras@usc.esjorge.eiras.b@gmail.comhttps://orcid.org/0000-0003-4401-5944DominguezFrancinaHuHuancuiGaraboa-PazDanielMiguez-MachoGonzalohttps://orcid.org/0000-0002-4259-7883Non-Linear Physics Group, Universidade de Santiago de Compostela, Galicia, SpainDepartment of Atmospheric Sciences, University of Illinois at Urbana–Champaign, Urbana–Champaign, IL, USAJorge Eiras-Barca (jorge.eiras@usc.es, jorge.eiras.b@gmail.com)22December2017841247126116June201727June201723October201731October2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://esd.copernicus.org/articles/8/1247/2017/esd-8-1247-2017.htmlThe full text article is available as a PDF file from https://esd.copernicus.org/articles/8/1247/2017/esd-8-1247-2017.pdf
A new 3-D tracer tool is coupled to the WRF model to analyze the origin of
the moisture in two extreme atmospheric river (AR) events: the so-called
“Great Coastal Gale of 2007” in the
Pacific Ocean and the “Great Storm of
1987” in the North Atlantic. Results show that
between 80 and 90 % of moisture advected by the ARs, and a high
percentage of the total precipitation produced by the systems have a
tropical origin. The tropical contribution to precipitation is in general
above 50 % and largely exceeds this value in the most affected areas.
Local convergence transport is responsible for the remaining moisture and
precipitation. The ratio of tropical moisture to total moisture is maximized
as the cold front arrives on land. Vertical cross sections of the moisture
content suggest that the maximum in tropical humidity does not necessarily
coincide with the low-level jet (LLJ) of the extratropical cyclone. Instead,
the amount of tropical humidity is maximized in the lowest atmospheric level
in southern latitudes and can be located above, below or ahead of the LLJ
in northern latitudes in both analyzed cases.
Introduction
Atmospheric rivers (hereafter, ARs) are long and narrow structures in the
lower troposphere that carry large amounts of water vapor .
have estimated that ARs have a median length of about
3600 km, a median length / width ratio of about 7 and a mean integrated vapor
transport (IVT) of 370 kg m-1 s-1. ARs are associated with the
pre-cold frontal region and the warm conveyor belt (WCB) of extratropical
cyclones . The maximum moisture flux often occurs within
the low-level jet (LLJ) located at around 1 km height along the cold frontal
boundary, and for this reason ARs and the LLJ are sometimes identified with each
other . However, to account for most of the high
IVT and moisture content defining an AR, usually a wider region encompassing
the layers below 2.5 km ahead of the cold front must be considered
. On occasion, the relationship between ARs and
cyclones has been shown to be complex, with documented cases in which multiple
cyclones are associated with a single AR .
ARs supply moisture to the WCB of the cyclones and are therefore considered
as one of the potential precursors of extreme precipitation, particularly
when landfall occurs e.g.,. The relationship between ARs
and flood events has been extensively analyzed for the US West
Coast region
e.g.,,
Europe
and other regions worldwide e.g.,. It is
important to better understand the physical mechanisms leading to extreme
flooding associated with ARs, considering their impacts on human and natural
systems and the mounting evidence that ARs are projected to become more
frequent and intense in the future
e.g.,.
Between three and five ARs can be found per hemisphere at any given time
, accounting for approximately 84 % of the meridional
IVT for the Northern Hemisphere and about 88 % in the Southern Hemisphere
. Since these structures can transport an amount of
precipitable water equivalent to several times the discharge of the
Mississippi River , ARs have been identified as a primary
feature of the global water cycle.
There are several proposed methods of AR detection, most of which are based
on thresholds of integrated water vapor (IWV) and/or IVT, shape criteria,
and from satellite or reanalysis data
e.g.,.
developed a global detection method using filters of
intensity, direction, geometry and coherence of the structures. More
recently, proposed a combined IVT and IWV
variable-threshold detection algorithm, which operates both in summer and
winter months. These objective detection criteria have shown that AR
structures of IWV and IVT can extend from the tropics into midlatitudes;
however, they do not provide information about the source and sink regions of
AR water vapor.
Tropical moisture exports have been identified as a primary source of
moisture for ARs in Europe and the US West Coast
. AR structures link remote sources of moisture from
the tropics to midlatitudes through long corridors of advection
e.g.,.
These studies have primarily used backward Lagrangian tools to evaluate the
source–sink regions. However, moisture from midlatitudes (local sources) has
also been identified as an important source of water vapor convergence in AR
events . used the FLEXible PARTicle
dispersion model (FLEXPART) to show that both tropical and local
sources of moisture are present in AR landfall events for different European
latitudes.
Some authors argue that local sources are primarily responsible for the high
water vapor content within the AR core
. By calculating the water vapor budget
of 200 extratropical cyclones, conclude that tropical
moisture reaching the extratropics only contributes to mid-level moisture,
above the boundary layer. Following this perspective, ARs can be thought of
as the footprints left behind the cyclone pathway, and not as a conduit for
meridional transport of water vapor and latent heat. One possible explanation
for the lack of agreement may be the sensitivity to the physics and
parametrization schemes used in the latter analysis . The
current understanding is that tropical moisture exports can provide a
significant amount of moisture to ARs. Most of them also incorporate
midlatitude sources of vapor along their path .
Integrated vapor transport (IVT, vectors, kg m-1 s-1),
sea level pressure (isobars, hPa) and integrated water vapor (IWV,
background, kg m-2) fields for both the Great Coastal Gale of
2007 (a–d) and the Great Storm of 1987 (e–h) events
throughout a 4-day time frame. Source: ERA-Interim.
Considering that there is still an important discussion related to the origin
of moisture in ARs, in this paper, we use a new forward moisture tracer
tool coupled to the Weather Research and Forecast (WRF) model
to evaluate the
moisture sources of two particularly extreme AR case studies, as well as the
transport mechanism of this humidity. The selected cases were associated
with extreme precipitation and flooding that led to significant socioeconomic
impacts. The first selected AR developed over the Pacific Ocean and affected
the western coast of North America, whereas the second AR developed over the
Atlantic Ocean and impacted the western coast of the Iberian Peninsula. The
tracer tool allows us to track the tropical moisture associated with these
two events and evaluate the relative contribution of this tropical moisture
to total moisture and precipitation. In addition, the WRF tracer tool also
provides information about the vertical distribution of tropical moisture, as
well as the position of the maximum of moisture with regard to the LLJ.
WRF output total precipitation for the Great Coastal
Gale of 2007 (a) and the Great Storm of 1987 (c) against observations
from LIVNEH (b) and IBERIA02 (d) for the same 24 h
period.
Data and methodsData
Both events were very intense in terms of IVT and IWV and are well detected
by different methods . The first
AR occurred on December 2007, affecting mostly the Pacific Northwest region
of the United States (Fig. a–d). Locally known as the
“Great Coastal Gale of 2007”, this event primarily impacted the western
state of Washington, and the associated flooding resulted in approximately
USD 680 million direct losses from three severely impacted counties in the
state and 11 fatalities .
Formed from the remnants of the two typhoons Hagibis and Mitag, the event produced hurricane force winds . The rapid and explosive
development of the cyclone (the central pressure fell more than 24⋅sinφ/sin (60∘) hPa in 24 h, where φ is the given
latitude) is shown in Figs. a–d and
in Appendix A. The selected AR case was the third and most intense of a series of
three storms and led to extreme precipitation . The
landfalling event and the resulting precipitation is shown in
Figs. d and d, respectively.
The episode developed from the merging of a cold front, already undergoing wave
development, and a faster moving low pressure system catching up from behind.
Both systems originated from typhoons and already had a high water vapor
content. The interaction between the two resulted in an instant occlusion-like
mechanism that led to the rapid deepening of the combined cyclone over the
Pacific.
Figure h shows the explosive cyclogenesis for the European
event. In this case, there is also a complex development process, with the
interaction between the remnants of a tropical system with high water vapor
content and a wave on the long frontal boundary across the North Atlantic as
a
precursor of the explosive cyclogenesis occurring northwest of the Iberian
Peninsula (see Fig. A2 in Appendix A).
The water vapor signature of the second case, which developed during October 1987,
extended from the western tropical Atlantic Ocean to the Iberian
Peninsula and British Isles. This event is the well-known “Great Storm of
1987”, with reported losses of millions of pounds and 18 fatalities over the
British Isles e.g.,. For this case,
showed that two-thirds of the central pressure falling could be ascribed to
latent heat release, which suggests that the AR played a key role in the fast
deepening of the cyclone (35 hPa of pressure drop in 24 h)
(Fig. f–h). Figure A1 in Appendix A shows how the
cooperative linkage between a trough in the tropopause and low-level
baroclinicity contributed to the rapid growth of the system as well
. The resulting precipitation (Fig. b)
was reported at above 100 mm throughout the Spanish region of Galicia, shown in
Fig. b.
The Eulerian tracer tool
The Eulerian tracer model is based on coupling a moisture tagging technique
with the WRF meteorological model. The strategy
consists in replicating the prognostic equations for the different
moisture species with equations for moisture tracers. A moisture tracer in
this context is defined as moisture originating from a predetermined source.
The set of equations for tracers is solved coupled to the model's governing
equations, meaning that tracers undergo turbulent diffusion with the same
eddy diffusivities as their full moisture counterparts, and that convection
and microphysics processes for tracers mimic those for full moisture, with
the assumption that phase changes among the different tracer species occur in
amounts proportional to the tracer fraction in the species undergoing the
change. The tracer tool running coupled to the model can separate moisture
from different sources with a very small error (less than 1 % in
traceability). Thus, the tracer tool is very accurate in the model world,
while the uncertainty in the real word is due to the WRF model error. A more
in-depth description of the Eulerian tracer tool and validation results can
be found in .
The Eulerian tracer tool operates as follows: a wide region in the domain
covering the tropical latitudes is set up as a three-dimensional tracer mask.
All the water vapor in this three-dimensional volume (including the water
vapor evaporated and advected into the mask region) is tracked in space and
time. Notably, while previous moisture tracer configurations in WRF
focused on tracking water that evaporated from a two-dimensional region at
the surface e.g.,, in this study all
the moisture in a three-dimensional volume (including the water vapor
evaporated and advected into the masked region) is tracked in space and time.
For details, see . Figure shows
the masks labeled in red for the Pacific (panel a) and Atlantic (panel b)
simulations. Once the simulation starts, the model tracks the humidity
originating from within the mask at any time, and the quantity of this
moisture is known in relation to the total moisture content at each point of
the domain, throughout the entire simulation. Similar to the rest of the
moisture, the “tagged” water vapor can change phase, and the fraction of
the condensed phase to the total condensate is also reported in the model
output.
Domains of the WRF simulation (blue) for the Great Coastal
Gale of 2007 (a) and the Great Storm of 1987 (b). Areas highlighted
in red correspond to the masked region where the moisture is initially
labeled as tracer.
Absolute value of IVT in kilograms per meter per second for the Pacific
event from WRF (a) and MERRA (b) as well as for the
Atlantic event from WRF (c) and MERRA (d).
(a) Total water vapor mixing ratio in grams per kilogram at
3 December 2007 12:00 UTC for the Pacific domain. (b) Tracers water
vapor mixing ratio in grams per kilogram at the same time and domain.
(c) Vertical cross sections of (a). (d) Vertical
cross sections of (b).
WRF simulations setup
We use WRF 3.4.1 to simulate
these two events. For the Pacific case, the WRF horizontal resolution is
15 km and the vertical column is divided into 40 levels. For the Atlantic
simulation, grid spacing is 20 km in the horizontal and there are 50
vertical levels. Both simulations cover a period of 10 days, from
26 November 2007 in the Pacific case and from 8 October 1987 in the Atlantic
simulation. The moisture tracer tool is implemented in the Yonssei
University (YSU) planetary boundary layer parameterization
, the Kain–Fritsch convection scheme
and the WRF Single-Moment 6-Class Microphysics Scheme
(WSM6) , which are the parametrizations employed in the
simulations. In addition, the Rapid Radiative Transfer Model (RRTM)
and Dudhia schemes were used for long-wave and shortwave radiation, respectively. Spectral nudging of waves above
the boundary layer, longer than 1000 km, with a relaxation timescale of 1 h, is applied to avoid distortion of the large-scale circulation within
the regional model domain due to the interaction between the model's solution
and the lateral boundary conditions
. Further descriptions about WRF can
be found in or . Finally, considering that the
ECMWF reanalysis (ERA-Interim) has been shown to be a reliable tool in the
analysis of ARs , the dataset provides lateral boundary and
initial conditions for the runs.
Since spectral nudging has been used in the simulations, the large-scale
circulation in the model closely follows ERA-Interim and no further
validation is required . Water vapor is not nudged to
ensure the mass conservation needed for the traceability of humidity from
different sources. Given that the subject of this study is moisture transport
and precipitation, we focus validations on these two variables.
Finally, the integrated column of water vapor, and the integrated column
of water vapor tracers (IWVTR) can both be calculated
from the WRF simulations using Eqs. (2) and (3), respectively, where q is
the specific humidity, g is gravity, u and v represent the wind
fields, sfc is the land surface, and l is the highest model level, well
above the tropopause. The conversion between specific humidity (q) and
mixing ratio (w) has been performed using Eq. (4).
IVT=1g∫sfclqudpIWV=1g∫sfclqdp
Same as Fig. but for the European domain in the
Great Storm of 1987 (15 October 1987 at 12:00 UTC).
IWVTR=1g∫sfclqTRdpq=ww+1,withw≪1⇒q≈wu=(u,v)
Results and discussion
Figure shows the comparison between WRF-simulated and
observed precipitation. Observations are from the dataset
for the Pacific simulation and from the Iberia02 precipitation dataset in the
case of the Atlantic simulation. The latter dataset is a combination of
Spain02 and Portugal02 , both of
which include a high density of good-quality stations .
Further comparison of the simulations with observations, against IWV (IVT, Eq. 1) from NASA's Modern-Era Retrospective Analysis for
Research and Applications (MERRA) is provided in
Fig. .
Whereas the simulated IVT field is realistic when compared to observations,
WRF tends to overestimate precipitation. The overestimation is particularly
high in the mountains of Oregon and Washington for the 2007 event. However,
despite the fact that precipitation is arguably the most difficult
variable to simulate in a numerical model
e.g.,, the spatial pattern of precipitation
is realistically represented.
(a) Ratio of tagged water vapor to total water vapor for
the Pacific event. (b) Same as (a) but for precipitation.
Panels (c, d) are equivalent to (a, b), respectively, but for the
Atlantic event.
Figure shows the three-dimensional distribution of water
vapor mixing ratio (panel a), and tracer water vapor mixing ratio (panel b)
for the event in the Pacific that made landfall along the US West Coast.
While the former accounts for the total amount of moisture, the latter shows
only the moisture originating from tropical latitudes, labeled with the 3-D
mask depicted in Fig. . The simulation was started 8 days
before the time shown in Fig. . Panels (c) and (d) show a
snapshot of the water vapor mixing ratio and the tracer water vapor mixing
ratio in the form of a series of cross-section slices that allow the
visualization of the vertical distribution of moisture. The images suggest
that the vast majority of the moisture contained in the pre-frontal region
has its origin in the tropical regions. This is especially true at lower
latitudes. The maximum content of tropical moisture remains mostly in the
lower levels. The high moisture values behind the front and at the leading
edge of the AR structure, where the WCB is located, are not related to
tropical advection and are thus generated by convergence of moisture from
local sources occurring along the frontal region. Slightly different
conclusions are obtained for the Atlantic case study shown in
Fig. . For the sake of simplicity, only the eastern
longitudes of the domain of simulation are shown. Even though the tropical
moisture still remains in the lowest levels of the troposphere, its
contribution to the total is less significant than for the Pacific case
study. The reason is shown in Fig. e–h, indicating that most
of the connection of the WCB with tropical regions is through a much longer
path, due to the blocking position of the Azores High.
Transversal cross sections along the central axis of the atmospheric
river at latitudes 42.0 (transversal cross section P1), 37.4 (transversal
cross section P2) and 30.6 (transversal cross section P3). The plots show the
tracers' water vapor mixing ratio in grams per kilogram together with the wind
module in meters per second. The estimated position of the LLJ is shown
in the figures as well.
Figure a shows the percentage of IWV that comes from the
tropics (IWVTR/IWV) for the Pacific event. In a
region extending from the tropics to the coast of
Washington state,
tropical moisture accounts for about 80–90 % of the precipitable water
and locally exceeding this contribution. These high percentages extend inland
along the northwestern US coastal regions. Likewise, Fig. b
shows the 24 h accumulated percentage of precipitation that is composed of
condensed tropical water vapor (PrecTR/Prec).
For clarity, we only plot the region where precipitation exceeded
3 mm day-1. Precipitation of tropical origin
accounts for 70 to 90 % of total precipitation from northern California
to southern Oregon, and the ratio decreases at higher latitudes.
Interestingly, tropical moisture is funneled by local topography, and it
contributes to about 70 to 80 % of precipitation west of the Cascade
Range. In the Atlantic case, we also see a clear plume where tropical water
vapor accounts for more than 80 % of precipitable water; however, the
percentage decreases to around 70 % close to the center of the system
and just before arriving on the Iberian coast (Fig. c–d).
In this case, cyclogenesis occurs just off the coast of Galicia, on the
northwestern tip of the Iberian Peninsula, and thus the enhanced convergence
of the existent local moisture feeds the AR and is involved in the heavy
precipitation, which consequently is only between 60 and 80 % of tropical
origin. The high ratios observed in mountainous ranges such as the Pyrenees
and the Rockies, far ahead of the cold front and the AR associated with the
systems, are very likely due to the slantwise lift of tropical moisture in
mid-levels along the warm frontal boundary.
Figure plots a range of transversal cross sections showing
the vertical distribution of the tracer water vapor mixing ratio through the
central axis of the AR, as well as wind speed. From these results, there is
evidence that the maximum of tropical moisture does not necessarily coincide
with the LLJ, which is the maximum in wind speed at lower
levels. More precisely, at the root of the AR, at subtropical latitudes
(Fig. d), most of the tropical moisture remains close to
the surface and below the LLJ, which can be identified at a height of
1 km. As the central axis of the LLJ goes upward with latitude, tropical
moisture tends to ascend in the vertical column, but the maximum of moisture
can be located in front of or behind the LLJ, as well as remaining near the
surface levels. In the leading part of the AR, the interaction of the
humidity with the topography of the Pacific coast of North America makes the
situation more difficult to analyze. Very likely, the complex formation
process of the cyclone, from the interaction of two preexistent frontal
systems, loaded with tropical moisture, adds complication to the
thermodynamic structure and moisture distribution of the resulting front.
Notwithstanding, this AR event is a particularly well defined case from the
perspective of vertically integrated quantities such as IVT and IWV.
An analogous plot for the Atlantic case is presented in Appendix A (Fig. A2).
The results are similar to the Pacific case, with no clear one-to-one
connection between the LLJ and the maximum in tropical humidity. We note that
no general conclusion can be obtained from particular case studies, but
these results suggest that the perception that ARs are clearly associated
with the LLJ of extratropical cyclones should be reviewed.
Figure a shows the area-averaged total precipitation
(black circles) and the ratio between tracer precipitation and total
precipitation (red crosses) throughout the region highlighted in
Fig. b for the Pacific event. As expected, the plot
shows that the maximum in tropical precipitation is observed during the
landfall of the cold front and the AR, which closely coincides with the
maximum in total precipitation. The secondary maximum in the total
precipitation observed 1 day before the landfall of the AR event is due to
the landfall of the warm front associated with the cyclone (see Fig. A3). The
convergence of local moisture would be the dynamical source for precipitation
in the warm front.
Evolution over time of the tropical precipitation (red crosses)
and total precipitation (black circles) during the Pacific
event (a). Data represent the spatial integration of both variables
through the region highlighted in red in (b).
Conclusions
A new 3-D Eulerian forward water vapor tracer tool implemented in the WRF
model has been used to analyze two important AR events. The first event
developed over the Pacific Ocean and corresponds to the Great Coastal Gale of
2007 on the US Pacific West Coast, which resulted in an estimated
USD 678 million in direct economic damages . The
Atlantic event corresponds to an atmospheric river event in October 1987
that resulted in record winds of 100 km h-1 and daily precipitation of
over 100 mm day-1 in Galicia (in the northwest of Spain) and
Portugal. In an effort to understand the origin of moisture for these two AR
events, we use 3-D water vapor tracers to quantify the percentage of total
precipitable water and precipitation that originates from the tropics.
Results show that most of the moisture within and surrounding the two
atmospheric rivers had its origin in the tropical regions that we labeled
with the 3-D tracer mask. Consequently, most of the precipitation that fell
during these two events was composed of condensed tropical water vapor. The
Pacific event shows a more intense connection with tropical regions than the
Atlantic case. As a result, the percentage of tropical precipitation for this
event over North America is higher and peaks around 85 %. Nevertheless,
for the Atlantic event, still more than 60 % of the resulting
precipitation is of tropical origin.
The two selected case studies have been chosen due to the associated
severity of flooding and socioeconomic damages. Both correspond to a strong
AR feeding the system of a very intense extratropical storm. The conclusions
drawn from these two AR events are thus not necessarily representative of the
bulk of Pacific or Atlantic AR events. However, the results highlight the
importance of tropical moisture for the two case studies. We also find
evidence that convergence of local moisture also contributes to total
precipitable water, especially in the post-frontal region, the leading edge
of the AR and in the far northern latitudes where the tropical link has
weakened. It is well known that in a mature system, when baroclinic
structures are well differentiated, the stored water vapor tends to be
constant (e.g., ), and since the fate of tropical
moisture is to precipitate, local convergence should keep the balance by
lateral inflow.
Based on these results, we hypothesize that the highest amounts of
precipitable water can only be attained in a system in which a clear tropical
source of moisture is sustained until landfall. Strong ARs with a direct
link to tropical latitudes should be expected to result in more precipitation
than those with local convergence as a primary feeding mechanism. It is our
aim for the future to extend this work by including more cases.
Finally, our findings suggest that the maximum of tropical moisture does not
necessarily coincide with the LLJ of either extratropical cyclone analyzed.
Instead, this maximum is located near surface levels at lower latitudes to
gradually ascend in northern latitudes, but still remaining below 2 km,
mostly within the boundary layer, in contrast with findings in other studies
. The maximum of tropical moisture may be situated below and
toward the back, or ahead of the LLJ, which is located along the cold front.
Both events are clear examples of ARs from the point of view of vertically
integrated variables, such as IWV and IVT used in most detection algorithms;
however, the vertical distribution of moisture of tropical origin reflects
the complex processes leading to precipitation. The new 3-D tracer tool will
allow us to delve into these processes and explore the role of tropical
moisture exports in the initiation and intensification of AR events.
No public data are derived from this research. For further
information on the WRF tracer tool, please contact the corresponding
author.
Supplementary figures
Front maps for the Pacific Great Coastal Gale of 2007 event.
Showing the 500 hPa geopotential field together with sea level
pressure (a–c) and 850 hPa
temperature (d–f) in the Great Storm of 1987 event from 14 to
16 October. The figure highlights the cooperative linkage between a trough
and low-level baroclinicity in the rapid development of the cyclonic system.
Same as Fig. but for the Atlantic case. The
corresponding latitudes are 41.8∘ for transversal cross section A1
and 38.0∘ for transversal cross section A2.
The Supplement related to this article is available online at https://doi.org/10.5194/esd-8-1247-2017-supplement.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “The 8th EGU Leonardo
Conference: From evaporation to precipitation: the atmospheric moisture
transport”. It is a result of the 8th EGU Leonardo Conference, Ourense,
Spain, 25–27 October 2016.
Acknowledgements
This work has been founded by the Ministerio de Economía y Competitivad
(CGL2013-45932-R) from the Spanish Government and its mobility grants for
pre-doc researchers. Jorge Eiras-Barca would like to express his gratitude to
the Department of Atmospheric Sciences of the University of Illinois at
Urbana–Champaign for the kind support in this project. Funding for Dominguez
and Hu comes from NASA
grant NNX14AD77G. Edited by: Diego G. Miralles
Reviewed by: Brianna Pagán, Ruud van der Ent, and Lan
Wang-Erlandsson
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