> **The Nature Of Gap Winds Through The Western Chugach Mountains**

John Papineau

National Weather Service, Anchorage, October 2008

*[Introduction]{.underline}*

The study of gap winds has generated a sizeable body of literature over
the past three decades. The analysis of air movement through and around
a broad spectrum of terrain features ranging from high mountain passes
(Ross & Vosper 2003, Gohm & Mayr 2004), to long river valleys (Flamant
*et al* 2002, Weissmann *et al* 2004, Armi & Mayr 2007, Mayr *et al*
2007), as well as fjords (Finnigan *et al* 1994, Jackson & Styen
1994a,b) and other coastal terrain (Pan & Smith 1999, Colle & Mass
2000), have been conducted under the broad umbrella of 'gap flows'. For
the propose of this paper we shall consider any valley embedded within a
mountain range, whether it transects the entire range or only part of
it, and whether it is at sea-level or at some elevation as a gap.

Observational studies as well as a host of modeling efforts have added
to our collective understanding of gap winds over the past three
decades. What we know is that gap winds can occur as a distinct
meteorological event or in combination with other phenomena such as
downslope windstorms. Gap wind events can be classified into two
fundamental regimes: 1) Those primarily forced by hydrostatic
considerations which manifest itself in the existence of a cross-barrier
pressure gradient; 2) A hybrid event in which the cross-barrier pressure
gradient works in concert with terrain induced mountain waves (boras,
föhns). There is considerable evidence that the latter case results in
the strongest gap winds in many parts of the world.

Observational programs as well as modeling studies have revealed the
following characteristics of gap winds: 1) Air moving through a gap
typically experiences some type of acceleration above that of the inflow
speed. In the case of a long gap (mountain valley or fjord), the
acceleration often occurs near the exit region. In British Colombia's
Howe Sound for example, Finnigan *et al* (1994) found that windspeeds
varied considerably down the long axis of the fjord. The strongest winds
(from subcritical to supercritical) were observed directly downstream of
'control points'; that is, constrictions in the width of the channel
(Jackson & Steyn 1994a,b). 2) There may be areas within the gap where
the localized pressure gradient exceeds the synoptic scale cross-barrier
pressure gradient. 3) During periods of gap winds it is common for the
strongest winds to extend well downstream of the exit region of the gap;
a feature referred to as a gap wind jet. 4) Strong gap wind events are
often but not exclusively associated with the advection of either warm
or cold air; the latter being common at higher elevations and or higher
latitudes during the cooler months of the year.

While the majority of gap wind studies have been conducted in the
context of real terrain, the modeling studies of Zangl (2002) as well as
Gabersek & Durran (2004 a,b) were produced using idealized gap terrain
and homogenous inflow characteristics. The obvious advantage to this
approach is that it allows the modeler to define the input parameters in
hopes that the reduction of complexity leads to a better understanding
of the resulting flow pattern. Zangl for example found that for gaps
embedded within a mountain ridge, both confluence and gravity-wave
interaction with the flow above the gap were important. In the case of a
gap produced by two isolated but neighboring mountains, diffluence in
the entrance region as well as the loss of gravity-wave energy on the
outer flanks of the peaks tends to weaken the flow through the gap. For
relatively 'narrow' gaps, mountain waves generated by the surrounding
terrain can produce wind and pressure perturbations within the gap that
alter the flow. In general, narrow gaps produce stronger winds then wide
gaps with all other parameters held constant. The primary result of
Zangl's study is that low-level pressure difference across a mountain
barrier controls the strength of gap winds. However the pressure
difference is a product of both the synoptic pressure gradient as well
as the mesoscale gradient due to three-dimensional mountain wave
effects.

The modeling results of Gabersek & Durran (2004 a,b) indicate that gap
wind acceleration occurs because of the existence of a temperature
gradient between the entrance and exit regions of any particular gap;
with lower temperatures in the exit region. Many observations of this
gradient however show that the exit region is warmer then entrance
region despite strong accelerations along the length of the gap. This
apparent conundrum is resolved when one considers that in a [mountain
wave regime]{.underline} there is strong descent and hence adiabatic
warming over the exit region. Using mass flux estimations over their
idealized model terrain, Gabersek & Durran concluded that the downward
mass flux near the exit region produced by descending mountain waves is
what actually produces the acceleration within the gap.

We can summarize these previous studies by considering that first order
air flow through a gap is essentially controlled by the cross-barrier
pressure gradient. Windspeeds within the gap however are controlled by
the [local]{.underline} pressure gradient (second order). The local
pressure is often influenced by gap geometry, changes in surface
roughness (friction), and the exchange of mass and momentum from the
adjacent slopes. The speed and direction of winds within and near the
exit region of a gap (including the downstream sector) can be
significantly modified by the presence of leeside mountain waves and
hydraulic jumps. Since it is apparent that gap winds, at least in the
exit region are correlated with the presence of mountain waves, any
adjustment to those mountain waves (stability, wind direction and speed,
cold or warm advection) can also indirectly modify gap winds. This
current study is intended to not only enhance the knowledge base of gap
winds that occur in the western Chugach Mountains but also to verify or
dispute the findings of Zangl and Gabersek & Durran which were as noted,
based on idealized terrain.

*[Turnagain Arm Winds]{.underline}*:

Turnagain Arm is a 65 km long northwest-to-southeast oriented fjord with
a low sill (150-200 m) at the southeastern end (Figure 1). The fjord at
its base is some 7 to 10 km wide but contains numerous side valleys
which make its overall geometry quite complex. The surrounding mountains
range in height from 1000-1500 m. The southeastern entrance is located
adjacent to the community of Whittier (PAWR) located in northwestern
Prince William Sound. The Chugach Mountains which sit astride northern
Prince William Sound generate an effective block to any airmass
attempting to move into Southcentral Alaska. The blocking of an airmass
and the subsequent generation of high pressure along the northern coast
of the Gulf of Alaska is a common occurrence during the cooler months of
the year.

The northwestern exit of the fjord expands into Chickaloon Bay and some
25 km further it merges into northern Cook Inlet. The exit region lies
approximately 20 km southeast of the first order weather station in
Anchorage (PANC). All strong wind events through Turnagain Arm occur
with synoptic east to south flow. On occasions light northwest winds are
generated in the fjord, however, even moderate northwest events are a
rarity.

The typical strong southeast wind event occurs when a surface low
pressure center is located southwest of Cook Inlet, somewhere between
155^o^W and 165^o^W and north of 54^o^N. This pattern generates
southerly flow across the Gulf of Alaska which in turn is blocked by the
substantial height of the Chugach Mountains. It appears that frontal
structure and movement onshore along the Chugach Mountains has minimal
impact on low-level blocking, however, warm or cold air advection can
certainly influence lower tropospheric stability which as will be
demonstrated in this paper, is a contributing factor to mountain wave
modulation. Blocked flow in turn produces high pressure along the Gulf
of Alaska coast; Cordova-to-Whittier mean sea-level pressure (MSLP)
gradients of 8-12 mb are a common occurrence. A portion of this blocked
airmass finds its way through gaps in the terrain into Northern Cook
Inlet where the MSLP is noticeably lower then along the outer coastline.
If a front moves through northern Cook Inlet it can impact gap winds in
the region by modifying (±) the ambient cross-barrier pressure gradient.

There are currently a handful of meteorological platforms located within
Turnagain Arm as noted in Figure 1. Three of them (McHugh Creek, Bird
Point, near Portage Glacier) are owned by the Alaska Department of
Transportation as part of their Road Weather Network (RWIS). These
platforms have were installed in 2005 and record wind direction, speed
and gusts. There are no pressure sensors. The National Weather Service
maintains met stations on the northwest end of Portage Lake (PATO) and
in Whittier (PAWR). These are standard Automated Surface Observing
System (ASOS) equipped platforms. In addition there is an assortment of
citizen weather observers located in Girdwood and in southeast Anchorage
near the exit region of Turnagain Arm. The majority of these sites are
low elevation with the exception of several operated by Aleyska Ski
Resort and the Alaska Railroad. The study of winds in Turnagain Arm is
also of interest because it is the corridor for the Seward Highway; the
only north-south road that connects Anchorage with the communities on
the Kenai Peninsula. During periods of high winds travel along the route
is not recommended for high profile vehicles.

Previous to this work, Turnagain Arm winds have been studied by Yonkers
(1982). Using incidental data noted by helicopter pilots who were
helping to install a natural gas pipeline through the fjord in the early
1960's, Yonkers developed a pressure based equation that estimated the
strength of what he labeled the Turnagain Arm Jet (TAJ). The core of the
wind jet was estimated by pilots to be located at an elevation of 150 m
above the waters surface. Yonker's equation takes into account the
current hour as well as the previous hour's (isallobaric) sea-level
pressure difference between Cordova and Anchorage (distance of 250 km).
This equation has for the past several decades been a good first guess
for operational meteorologists.

Anyone familiar with the area knows that strong TAJ winds continue well
past the exit region of Turnagain Arm, extending at times straight out
into northern Cook Inlet. At other times these winds are deflected
toward the north with the result that south-to-southeast winds on the
order of 10-15 ms^-1^ with higher gusts sweep across west Anchorage. The
controlling factor on the position of the TAJ after it exits the fjord
is the pressure gradient within northern Cook Inlet and the Susitna
Valley. A northerly oriented or neutral gradient will allow the TAJ to
move over west Anchorage while a southerly oriented gradient forces a
deflection down Cook Inlet away from Anchorage. The TAJ can form
independent or in conjunction with Hillside (western slopes of Chugach
Mountains directly east of Anchorage) winds including downslope
windstorms. In fact, during a downslope event southeast winds observed
at PANC may actually be from the TAJ rather than from the Hillside,
depending on the direction of the Cook Inlet pressure gradient and the
presence of hydraulic jumps in the flow aloft. As a rule, light to
moderate Turnagain Arm winds can occur independent of any downslope wind
event. Strong winds through the fjord however are always associated with
downslope windstorms over the western Chugach near Anchorage.

Some general characteristics of strong Turnagain Arm wind events are as
follows: 1) Winds are predominately (80-90% of the time) stronger at
McHugh Creek, near the northwest end of the fjord, compared to the
observations from Bird Point, some 10 km upstream. 2) The gust ratio
(speed of gust divided by speed of sustained winds) at both McHugh and
Bird Point is on the order of 1.3-1.5 while at Portage it typically
ranges from 1.8 to 2.0. 3) Strong winds (TAJ) exiting Turnagain Arm may
extend 20-30 km downstream over Cook Inlet. The position of the TAJ as
noted in the previous paragraph is a function of the northern Cook Inlet
pressure gradient. As a rule-of-thumb, south-to-southeast winds that
originate in Turnagain Arm are only observed at PANC when the
Anchorage-minus-Homer MSLP gradient is less then +2.5 mb. 4) The
correlation between the sustained winds at McHugh Creek and the
Whittier-minus-Anchorage MSLP gradient is on the order of 85%. 5) There
are times where a strong coastal/barrier jet forms across Prince William
Sound which produces strong east winds at Whittier and in Portage
Valley. Depending on the exact nature of the pressure pattern this may
or way not generate moderate to strong winds through the length of
Turnagain Arm. During most of theses 'easterly' events windspeeds at
Portage (PATO) are considerably stronger when compared to those at
Whittier (PAWR). This is due to the fact that the winds reaching Portage
are downsloping from the surrounding terrain to the east of the site,
while the winds measured at Whittier move westward via Passage Canal and
hence are strictly pressure gradient forced.

The top panel of Figure 2 shows a comparison of the McHugh Creek, Bird
Point, and Portage RWIS sustained winds for the March 3-6, 2008 event.
Notice that for the majority of the event the winds are significantly
stronger at McHugh Creek despite the fact that Bird Point extends
further into the center of fjord than McHugh Creek. The exception is for
about an eight hour period on March 5 when the winds at Bird Point are
stronger than at McHugh Creek. Portage winds (this is the RWIS site
about 5 km downstream of PATO) are always weaker during these types of
events. The lower panel shows the sustained winds at McHugh Creek for
the same event with the Whittier-minus-Anchorage MSLP gradient for
comparison. There is a fairly high correlation between the two traces,
although they do deviate at times. Notice that during the period on the
5^th^ when the winds decrease by 30-40%, the pressure gradient drops by
\~10%. However, the winds at Bird Point (Figure 2a) during this same
period have [increased]{.underline}. One suspects therefore that
additional factors also influence the winds within the fjord.

Figure 3a is a scatter diagram of McHugh Creek sustained winds versus
the Whittier-minus-Anchorage MSLP gradient for three strong wind events.
Overall an increase in the pressure gradient leads to stronger winds;
however, two of the events (March 2008, November 2007) display
considerable spread in the data. Note for example that for a 6 mb
pressure gradient, windspeeds can vary from 8-22 ms^-1^, in other words
a given pressure gradient does not produce a unique windspeed at McHugh
Creek. Some of the variability in the data is a result of the fact that
there is considerable hysteresis in the data- for a given pressure
gradient, windspeeds are higher when the gradient is increasing. This is
illustrated in Figure 3b in which in the 3-7 mb MSLP gradient range,
speeds are higher when the 1-hour tendency is increasing compared to
when it is decreasing, as seen by the blue and red lines. This is
suggestive that the isallobaric component of the wind plays a role as
well (Yonkers 1982). Essentially the Whittier-minus-Anchorage MSLP
gradient explains the majority of the Turnagain Arm wind variability
however there are additional factors that must be considered as well in
order to construct a comprehensive conceptual model. In order to explore
the nature of these additional factors, the Weather Research and
Forecast (WRF) model has been implemented.

*[WRF Case Studies]{.underline}*:

Simulation of seven wind events was conducted with the Weather Research
& Forecasting (WRF) Environmental Modeling System configured with an
outer 4-km grid and a nested 1-km grid with one-way nesting from the
outer to the inner domain. Each simulation was initialized with the
North American Regional Reanalysis (NARR) data set using 6 hour boundary
files. Terrain data consisted of the USGS's 30 arc-second data set.
There were 31 vertical grid levels ranging from 20 m to 15,800 m (100
mb), with the upper 5 km of the atmosphere set to dampen vertical
velocities and hence limit unwanted wave reflection off the top of the
model domain. The microphysics routine was based on a five class scheme.
The planetary boundary layer was parameterized using the Yonsei
University scheme, while surface fluxes are based on Monin-Obukhov
similarity theory. In order to make comparisons from one simulation to
another, the grid configuration and model physics were unchanged from
one simulation to the next. The duration of these simulations ranged
from 30 to 60 hours depending on the nature of the event.

The seven events that were simulated are: February 19-20, 2008, January
21-22, 2008, February 3-5, 2003, December 1-3, 1992, October 9-10, 1986,
November 26-27, 1985, and April 1, 1980. These events display both
significant gap and downslope winds as air moved over and down the
western Chugach Mountains. In addition these events occurred when the
nondimensional mountain height Nh~o~/U ranged from 0.6-0.8 (where N is
the Brunt-Väisälä frequency, h~o~ is the average height of the terrain,
and U is the upstream windspeed); well within the mountain wave regime
as noted by Zangl (2002) and Gabersek & Durran (2004a,b).

Figure 4 shows windspeeds through Turnagain Arm on January 21, 2008 at
03Z. Notice in the upper panel the strong acceleration in the exit
region and the jet (\~27 ms^-1^) extending downstream of the fjord with
weak winds on either flank. In the lower panel note the inhomogeniety of
the speeds in the main channel, in both cross-channel as well as
along-channel orientations. Windspeeds in the side valleys are highly
variable as well. There are not enough observations to determine whether
this pattern is representative of the actual winds but the model
simulations are consistent from one event to the next. Comparison of the
model versus observed pressure gradient between Whittier and Anchorage
for the Feb 2008 case (not shown) indicates that the model pressures
tend to run within about 20% of the observed but deviate much larger at
other times. The model is certainly capable of mimicking the
cross-barrier pressure difference and performs reasonably well
considering it is nudged to boundary values only once every six hours.

Analysis of the seven Turnagain Arm simulations provides some general
conclusions:

1\) The strongest winds tend to occur in or directly downstream of the
exit region- hence the largest accelerations occur within the exit
region. 2) There is considerable flow from the side valleys and down off
of the surrounding mountains into the main channel. 3) The strongest
winds occur in conjunction with downslope windstorms over the western
Chugach Mountains; that is when the ridge top winds (\~850 mb) are from
the south-to-southeast. 4) There are preferred areas of wind minima and
maxima within the fjord from one event to the next.

Figure 5 displays a plot of streamlines in the eastern half of Turnagain
Arm for the Jan 21, 2008 event. Notice how the flow moves down off the
surrounding mountains into the main channel and side tributaries. The
flow in any particular valley may also move up slope and exit the valley
as well. Vertical north-to-south cross-sections of winds at the
entrance, middle, and exit region of Turnagain Arm are displayed in
Figure 6. The most obvious feature is that there is considerable
cross-channel variability in windspeeds. Visual inspection of model
output from the seven simulations indicates that there is a correlation
between windspeeds within the fjord and mountain wave activity over the
adjacent mountains. This is illustrated in the following two figures.
Windspeeds at the 950 mb level and potential temperature at 850 mb
(approximately the height of the surrounding mountains) are shown in
Figure 7. The areas labeled "A" and "B" are regions of significant wind
acceleration in both the exit and middle sections of Turnagain Arm. In
addition, although not shown in this figure it is evident that areas of
lower MSLP exist immediately downstream of the zones of acceleration.
The potential temperature pattern shown in Figure 7b is indicative of
mountain waves- the pattern 'spans' Turnagain Arm because the gravity
waves initiated by the terrain are dispersive (Zangle 2002, Gabersek &
Durran 2004a see their figure 3b). The warm areas correspond to
descending portions of the waves and the cooler areas to ascending
portions. This pattern of alternating temperatures over a short distance
then creates via the hydrostatic equation regions within the fjord of
relatively lower and higher pressure. For this particular case
horizontal temperature gradients within the 925-800 mb layer are on the
order of 3^o^C over a distance of10 km in the area labeled "B" in Figure
7, and 6^o^C over 15 km at "A" (the mountain wave has higher amplitude
over "A"). It is important to note however that as the air moving over
the mountains changes speed and direction or if the stability in the
lower half of the troposphere changes, the resulting mountain wave
pattern will probably be altered as well. An additional example of the
correlation between windspeeds within the fjord and mountain wave
activity is shown in Figure 8. In this event model data was extracted
from 21 grid points extending from Whittier (#1) to northern Cook Inlet
(#21) near Fire Island. In the top panel of Figure 8 notice that at this
time, 09Z on April 1, 1980 that the lowest MSLP occurs in the exit
region (grid point 15), which is also the area with the strongest near
surface winds (33 ms^-1^). The third panel shows the vertical velocities
at the 850 mb level while potential temperatures at 750 mb are displayed
in the bottom panel. It is clearly evident that there is a correlation
between lower MSLP and areas of descending warm air aloft.

It should be noted that a large fraction of the mountain wave activity
that develops over the exit region of Turnagain Arm in these simulations
is generated by the flow of air over the higher terrain of the Kenai
Mountains located directly west and east of Hope (either side of the
lower Resurrection Creek drainage). This comes about because most of the
strong wind events occur with southeasterly to southerly flow from the
surface through 800 mb, with south to southwest flow at 700 mb (the
winds typically veer with height). A southeast trajectory is almost
directly parallel to the long-axis of Turnagain Arm and therefore
mountain wave activity over the exit region is limited to lateral
dispersion. A southerly wind trajectory over the Kenai Mountains
produces mountain waves and hydraulic jumps downstream (north) of the
terrain which is directly over the exit region (Gabersek & Durran 2004b
figure 6). As described above, these [temperature gradients and mass
fluxes, although generated above the fjord, influence low-level winds as
well]{.underline}.

In addition to the mountain waves (and/or hydraulic jumps) that form
above the fjord, these features also form over the numerous side valleys
as well. Figure 9 shows an example where easterly flow created a
hydraulic jump to the west of Portage Valley; this is evident by the
presence of a strong potential temperature gradient in this case at 950
mb (Fig 9a). The red dashed line shows where a vertical cross-section
displayed as Figure 9b originates. The hydraulic jump extends down to
the valley floor. Near surface windspeeds (Fig.9c) under the jump
decelerate from a high of 21 ms^-1^ ("A") to

6 ms^-1^ ("B") over a distance of about 5 km. Additional locations where
the speeds have decreased due to hydraulic jumps are labeled "C" through
"E". These locations are favored regions for gap flow windspeed minima
during corresponding periods of strong mountain wave activity.
Essentially there is considerable interaction between pure gap winds and
descending air from above the valley.

Not all moderate wind events however occur during periods of strong
mountain wave activity. If a very strong low-level cross-barrier
pressure gradient exists then moderate winds can occur through Turnagain
Arm without the influence of mountain waves. In this scenario windspeeds
are typically as strong in middle of the fjord as in the exit region,
which differs from the mountain wave regime in which the winds usually
accelerate through the exit region. In addition, in the absence of
mountain waves the winds as they leave the fjord tend to decelerate
quickly, while in the mountain wave regime they often maintain integrity
(as a Turnagain Arm Jet) as far downstream as Fire Island.

Figure 10 displays three examples of 1000 mb windspeeds in the exit
region and downstream. In Figure 10a windspeeds are strongest in the
exit region and slowly diminish over the next 15 km but after which
there is a rapid deceleration west of Fire Island. Also noticed that the
speeds that flank the TAJ are very weak due to the ascending limb of a
hydraulic jump (there is an eddy centered over Birchwood). In Figure 10b
the TAJ maintains its integrity well downstream of the exit region until
it meets the northerly winds moving out of the Susitna Valley (notice
the wind barbs). In Figure 10c the TAJ extends will downstream but is
flanked by stronger winds when compared to Figure 10b, in addition there
is only minor deceleration to the west of Fire Island. The primary
difference between these plots is the nature of the flow above the
boundary layer; in Figure 10a a hydraulic jump extends some considerable
distance to the west of the lee slope. In the remaining two plots the
regime can be characterized as shooting flow; that is, air descends the
lee slope but there is no ascending limb, in other words the 'jump' is
absent.

It should become clear that whether or not southeast winds reach PANC
from Turnagain Arm is primarily a function of the direction and strength
of the Cook Inlet and Susitna Valley pressure gradients; however it also
depends on the strength of the TAJ which in turn is a function of the
mountain wave regime. Figure 11 shows a vertical cross-section of
windspeeds versus height on an east-west transect for the same model
output as those plotted in Figure 10. In these examples the core of the
TAJ descends as it moves to the west. In other events that have been
analyzed the core remained at the same elevation as it extend from the
exit region to Fire Island. Most often the core of strongest winds is
found between 950-900 mb (\~500-1000 m AGL), at this level it often
merges with the downslope winds that are moving off of the lee slopes.

At the start of a wind event it is not uncommon for the evolving jet to
be present above a shallow Cook Inlet surface inversion. More often then
not in these cases there is a weak (\<5 ms^-1^) northerly wind over
northern Cook Inlet extending from the surface through a depth of
several hundred meters. The nature of the TAJ over Chickaloon Bay would
then be to extend towards the west in the layer directly above the
northerly surface winds, but the orientation would typically shift
toward the northwest and possibly north with height (veering).

*[Eagle River and Kink River Valleys]{.underline}*:

Although Turnagain Arm is the main sea-level gap through the western
Chugach Mountains there are of course numerous valleys and gaps in which
the flow of air displays the same characteristics as those previously
discussed in relation to Turnagain Arm. Several examples of low-level
wind speeds through the northwest-southeast oriented Eagle River Valley
(\~35 km in length) are shown in Figure 12. The upper, middle and lower
portions of the valley are labeled as "U", "M", "L" respectfully. In
Figure 12a the main point of interest is the zone of deceleration from
24 ms^-1^ to

3 ms^-1^ which occurs between 149.4^o^W and 149.5^o^W. In the lower
valley a part of the deceleration can be attributed to northerly flow
cross Kink Arm, however the deceleration centered at 149.4^o^W is due
the cooling resulting from a mid-tropospheric mountain wave. This
cooling generates a low-level geopotential height and MSLP anomaly (not
shown). In particular the MSLP gradient increases by

2.0 mb over a distance of 8 km centered along 149.4^o^W. The windspeed
minima highlighted as "A" and "B" in Figure 12b are due to lower
tropospheric (900-700 mb) mountain waves that are generated as the air
moves off of the higher terrain to the south. The low-level outflow from
the Eagle River Valley is as one would expect a function of the Cook
Inlet pressure gradient; in both plots of Figure 12 the reverse MSLP
gradient diminishes the flow in the exit region. In this particular
scenario (weak northerly flow over Cook Inlet) the outflow takes on a
distinct jet structure between 950-700 mb as it moves out over Kink Arm.
When the northern Cook Inlet pressure gradient is weak, the outflow from
Eagle River (and other west side valleys) tends to blend in with the
flow off of the surrounding terrain.

Air flow through the 'L' shaped Kink River Valley is highly variable
from one location to the next as displayed in Figure 13. The upper
valley which is oriented NE-SW tends to show marked windspeed gradients
from the east to west side during moderate to strong SE wind events.
Figure 13a shows the streamline pattern at 1000 mb for the January 21,
2008 event with the associated windspeeds shown as Figure 13b. Notice
the convergence line in the upper valley and the eddy in the 'bend'
region. The "A" marks the location of a hydraulic jump, above the slopes
to the east the flow is descending, while over the center of the valley
and the steep terrain to the west, the air is ascending. The result is
higher pressure over the western slopes ("B") which produces blocked
flow and the subsequent minimum windspeeds in this area. There is an
eddy structure to the flow on either side of "C" in the top panel, which
corresponds to weak winds as seen in the lower panel. There is a
low-level speed maximum at "D" which can be attributed to mid-level
mountain wave that was generated by the peaks to the south ("E"). This
wave produced a 2.5 mb MSLP gradient over a 7 km distance as depicted by
the bold yellow line in Figure 14a. Vertical velocities at 800 mb level
are shown in Figure 14b. Note the juxtaposition of MSLP gradient with
the elevated wave.

*[Discussion]{.underline}:*

There are three components that force/drive the flow through Turnagain
Arm and adjacent valleys: 1) Cross-barrier pressure gradient (synoptic
or mesoscale pressure gradient). 2) Mass flux into or out of the valley.
This can operate laterally as well as vertically. Diffluence in the flow
downstream of the exit region produces deceleration. Confluence,
including mass inflow from aloft, causes the flow to acceleration or
maintain its speed against opposing forces (friction). 3) Localized
pressure gradient (and/or height gradient) is modified by mountain wave
activity aloft. Warmer air aloft in the descending portion of a mountain
wave can induce via the hydrostatic equation a reduction in surface
pressure (or lower geopotential heights). The first component operates
independently of any mountain wave activity and is the easiest to
monitor. The second and third components however are forced by air
flowing over irregular terrain in a stratified atmosphere. They can
operate in conjunction with one another or separately.

Temporal changes in Turnagain Arm winds, that is, changes in the
windspeed within the fjord, outflow distance and direction, as well as
the duration of an event are a function of changes in the aforementioned
forcings. For example, a change in the cross-barrier pressure gradient
will have some impact on wind direction and speed, however, more subtle
changes that can occur in lower tropospheric conditions, such as a
change in stability (warm or cold air advection for example), as well as
in the veering or backing of the mountain level winds, can also impact
the character of the winds within Turnagain Arm via the dynamic
connection with mountain waves.

It should be noted that windspeeds in the exit region of valleys
embedded in the western Chugach Mountains are significantly weaker and
do not typically extend as far downstream as those exiting Turnagain Arm
as seen in Figure 15. Note that the outflow from all valleys except
Turnagain Arm and Kink River are weak due to the low-level northerly
winds emanating out of the Susitna Valley. A few hours later (not shown)
as the northerlies weakened, the flow out of these smaller valleys
extended out across Kink Arm. It should also be noted that the winds
exiting these valleys are at times indistinguishable from the flow
descending from the adjacent slopes of the Chugach Mountains. Figure 16
shows an example of windspeeds through a north-south cross-section of
Eagle River Valley and Peters Creek drainage. The strong flow over the
mountains merges with the jet over Eagle River, with strong winds
descending well into the lower reaches of the valley. During strong SE
wind events as is the case in Figure 16, it is difficult or impossible
to distinguish between 'pure' gap winds and downslope winds for the
valleys of the western Chugach Mountains.

Since one of the objective of this paper was to evaluate the results of
Zangl and Gabersek & Durran since their studies were conducted using
idealized terrain. It has been shown in this paper that mountain waves
play an important role in the generation of strong winds through gaps in
the western Chugach Mountains. To what degree mass influx and localized
hydrostatic pressure anomalies due to wave motion over a gap, appears to
vary in time and space. This of course makes it difficult to assign
specific values to their importance; nevertheless agreement with the
findings of the two previously noted studies is high.

*[References]{.underline}:*

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