Navy Mesoscale Primer:  Gap Winds


Synthetic Aperture Radar image showing gap winds associated
with the Strait of Juan de Fuca and nearby gaps in coastal terrain.
Yellow (red) indicating regions that winds exceed 15 (20) ms-1.
Note that the strongest winds are found at the exit of the Strait.
Picture courtesy of Nathaniel Winstead, Johns Hopkins APL

Introduction


What are gap winds?

    Gap winds are low-level winds that are associated with gaps or low areas in terrain.  Gap winds can range in width from hundreds of feet to hundreds of miles, and in unusual circumstances can be associated with strong winds exceeding 50 knots.  These winds are normally quite shallow, extending hundreds of feet to a few thousand feet above the surface, with large changes of wind (wind shear) at their upper and lateral (side) boundaries.  Gap winds are generally strongest when there is a large difference of pressure across a gap, although there is one class of gap wind that does not depend on a pressure gradient across the gap.

Why are gap winds important for Navy forecasters?

    Navy ship and aircraft operations require accurate information about low-level winds.  Many coastal regions possess substantial terrain, with channels that can produce well-defined gap wind flows.  These gap winds are not only important within the gaps, but well downstream (10-100 miles) as well. Gap winds can reach strengths that can seriously affect Navy operations (20-60 knots), with strong wind shear (change of wind with distance) at the upper and sides of the gap flow.  Such large wind shears can be associated with moderate or even severe aircraft turbulence.  Of course, gap winds can be very important in non-coastal regions as well, with strong winds in and downwind of channels or passes in terrain.  Significant gap flows are found throughout the world, many of them in regions where the Navy has active or potential operations.

What will you learn from this module?

    This module will provide a basic understanding of why gap winds occur, their typical structures, and how gap wind strength and extent are controlled by larger scale (synoptic) conditions.  With the availability of new remote sensing assets and  high resolution numerical modeling, we now have a much better idea about the structures of gap flows and how they develop, and you will become acquainted with these new insights.  It turns out that many of the old ideas, some still found in textbooks, are wrong or incomplete--such as the old funnel  or venturi analogs.  You will learn about a number of important gap flows in coastal regions around the world, with special attention given to a comprehensively documented gap wind cases in the Strait of Juan de Fuca and the Columbia River Gorge.   Basic techniques for evaluating and predicting gap flows will be presented.  We will also review the capabilities and limitations of the current generation of mesoscale models in producing realistic gap winds.  By the end of this module, you should have sufficient background to diagnose and forecast gap flows around the world, and to use this knowledge to understand their implications for operational decisions.
 

Basic Principles


     In some introductory textbooks and in the "common knowledge" of many meteorologists, gap winds are explained by the "funnel" or "venturi" effect, in which the strongest winds occur in constrictions.   Although funnel effects can influence wind speed, the strongest winds are generally not in the narrowest portion of gaps but rather in gap exit regions.  Furthermore,  other mechanisms appear to be far more important in modulating wind speed in and near gaps.

The "Funnel" or "Venturi" Effect:  Only a Part of the Story

       Consider a simple sea-level gap through a mountain range, as shown in the figure below.    Assume that there is a rigid lid below the crest of the mountains...one that prevents any flow from passing over the mountain range.   Thus, any air approaching the mountain barrier can only move across the barrier in one place:  the gap.   In the situation sketched below easterly (from the east) flow approaches  the gap entrance.  Because of conservation of mass, the air must speed up as it flows through the constriction, with the strongest winds at the narrowest point (the amount of fast-moving air moving through the narrow portion of the gap equaling the amount of slower moving air approaching the gap from the east).  As the air moves past the constriction into the gap exit region, the winds slow down as the cross section area of the gap increases.  According to the famous Bernoulli's principle, pressure should be lowest in the constriction, with the air accelerating from high to low pressure east of the constriction and decelerating downstream of the constriction as it traverses from low to high pressure.

    Although physically plausible, the venturi mechanism appears to be at odds with most real gap wind situations, where the strongest winds are generally over the exit region of the gap, not at the narrowest section or constriction.  There a number of reasons for the failure of the funnel model.  First, there is no rigid lid in the real world.  As air approaches a gap the depth of the approaching air (often relatively cold and dense) generally increases due to the blocking effects of the surrounding terrain (see figure in next section).  Such an increasing depth of cold, dense air over and to the east of the barrier contributes to higher pressure near and upwind of the center of the gap, which tends to decelerate the gap flow as it approaches the gap.  As the air moves through the gap exit region, the rapid widening of the gap causes the gap flow to spread out horizontally.  This horizontal divergence of the gap flow causes the height of the dense low-level air to collapse, resulting in lowered pressure and a pressure gradient that accelerates the winds over the exit region.

    Another reason the flow tends to be strongest in the gap exit region is that there is often a synoptic or mesoscale pressure gradient across the gap.   For example, a synoptic high pressure area might be on one side and a low center might be approaching on the other--thus producing a strong pressure gradient across the barrier.  Such pressure gradients result in an acceleration from high to low pressure that continues over the entire gap and thus tend to produce strongest flow over the gap exit.

    Venturi effects are also lessened by the complex three dimensional nature of the air flows in actual gaps.  Often air in a gap does not simply move horizontally from the upstream source region to the exit, but rather air can flow into the gaps from several directions and from several levels along the length of the gap.  Thus, simple mass conservation arguments and the assumptions of simple airflows can be deceptive or wrong.

    Although the venturi effect is often not the dominant mechanism for large gaps (tens or hundreds of km long and several km wide), it can be very important for small scale gaps (on the order of a few km or less in length) where the contributions of the larger scale pressure gradients or changes in the height of near-surface cold air are less important.  Most of us have experienced the funnel effect while hiking though a pass in a mountainous region or felt the winds pick up in the gaps between buildings.  A good example of a significant gap flow associated with a small-scale gap in terrain is the Nuuanu Pali Pass that cuts across the steep, but narrow Koolau Range of eastern Oahu, Hawaii.  As trade winds of 10-20 knots are funneled into the gap, winds frequently exceed 40 knots near the constriction.  Guidebooks warn visitors to hold on to their hats when they visit this windy spot.

The Nuuanu Pali gap looking eastward towards the approaching trade winds.
Hats are not recommended.

The Relationships Between Pressure and Wind With and Without Terrain

       Winds in gaps are typically highly non-geostrophic, so that the flow accelerates down the pressure gradient from high to low pressure.  In contrast, in geostrophic flow the winds are directed parallel to the isobars and thus move perpendicular to the pressure gradient.  This section will review geostrophic and non-geostrophic winds, and will explain why winds tend to be non-geostrophic (or ageostrophic)  in gaps.

    The Simplest Case:  Geostrophic flow

    It is useful to begin by reviewing the relationship of sea level pressure and surface winds when there is no topography and friction (drag).  As described in most basic textbooks, without terrain or friction, surface winds are expected to be geostrophic, with winds paralleling the isobars and the strength of the winds proportional to the pressure gradient (see figure).   When winds are geostrophic there is a balance between the pressure gradient force (directed towards lower pressure) and the Coriolis force, which is directed to the right (left) of the motion in the Northern (Southern) Hemisphere.  In reality, geostrophic flow also demands more than just the absence of friction:  (1) that the pressure and wind fields are not changing rapidly in time and (2) that air trajectories are not highly curved.  In rapidly evolving situations there is not enough time for the winds to adjust to the pressure fields so that geostrophic balance cannot be achieved, while in highly curved flows (and particularly when there are strong winds) another force (the centrifugal force) can become important.  However, since large scale flows are generally slowly evolving and fast highly curved flows are limited, much of the  atmosphere about the surface boundary layer north of 10N is roughly geostrophic.  Over the relatively smooth oceans during neutral or unstable conditions, geostrophic flow is often a good approximation.

The Addition of Surface Friction

    Near the surface, another force--surface drag or friction--is usually important.  Thus, a three way wind balance is established in the lower atmosphere (between the friction, Coriolis, and pressure gradient forces) in which the wind speed is reduced below the geostrophic value and the wind blows at an angle across the isobars (called the cross isobar angle) towards lower pressure.  The influence of surface drag depends on the roughness of the surface and the vertical stability of the air in the lower atmosphere.  Rougher surfaces (such as hills, trees, and tall buildings over land) result in more drag and thus lesser winds and large cross isobar angles, while over smoother surfaces (such as the ocean or bodies of water) the winds are more nearly geostrophic.  Vertical stability can be important as well.  For example, for unstable or less stable situations (such as when cold air passes over warm water) there is enough vertical mixing of fresh, unslowed air from above to allow the surface winds to be more geostrophic, while in stable situations the lack of mixing results in the drag effects being concentrated in a shallow layer near the surface causing far more slowing of the air near the surface.

The Influence of Terrain on Low-Level Winds

    Large topographic barriers (or mountain ranges) have a profound effect on low-level winds and greatly alter the relationship between pressure and wind.  For geostrophic balance to occur, air parcels need sufficient time and space to adjust to the synoptic scale pressure field.  If there is a mountain range in the way and the flow becomes blocked, such an adjustment is impeded.
    Consider the situation shown below in which a large scale (synoptic) pressure gradient is oriented so that the isobars are normal (perpendicular) to the barrier.  The atmosphere is stable so that air tends to move around rather than over mountains.  If we ignore the effects of surface friction (and also assume the flow is relatively slowly varying and that the centrifugal force is not important), then far away from the mountains the flow will be geostrophic, with the winds parallel to the isobars.   Near the mountain barrier such westerly flow is not possible because of the blocking effects of the terrain.  Instead, the flow tends to move parallel to the barrier, going from high to low pressure.  How far away from the barrier are the winds influenced by the terrain?  Typically, this distance (known as the Rossby radius of deformation) is approximately 100 to 200 km.

    It is interesting to also consider the situation when the isobars parallel a mountain barrier, as shown in the second schematic.  In that case, geostrophic winds are possible even near the terrain, because mountain blocking effects don't come into play.  Real-world events are a mixture between the two extreme orientations (pressure gradient perpendicular to or parallel to the mountain barrier).

Mechanisms Associated with Gap Winds In Nearly Level Channels

    For many gaps in mountainous terrain, the center of the gap is within a Rossby radius (100-200 km) of the terrain;  thus, for such gaps the along-gap winds are not in geostrophic balance with the pressure gradient down the gap.  In other words, for most narrow gaps less than 100-200 km wide that are surrounded by terrain the air moves down the gap towards lower pressure.  Thus, if you know how pressure varies along the gap you know the direction of the gap winds and have an idea of the magnitude of the flow.   Stronger pressure gradients along a gap in general produce stronger gap flows.   In such non-geostrophic situations the Coriolis force is not effective in balancing the pressure gradient force and thus winds can accelerate greatly, with only friction (drag) to keep them in check.
 

  The pressure gradients that drive gap winds have two major origins:  (1) pressure gradients associated with synoptic scale  or regional scale features, and (2) pressure gradients in or near the gap associated rapid changes in the depth of cool air at low levels.

    1.  Synoptic scale pressure gradients.

    Pressure gradients across a gap are often associated with synoptic scale features.  For example, if there is an anticyclone on one side of the gap and a low pressure region or cyclone approaches or develops on the other side, a large pressure gradient can build across the gap (see figure).   Generally lesser, but still significant pressure gradients along gaps can be associated with a low or high pressure region approaching or developing on one side of the gap.   No matter what the origin, synoptic scale pressure gradients across the gap tend to produce the strongest winds in the gap exit regions (the low pressure sides), since the winds can accelerate downgradient through the length of the gap.


 

    2.  Pressure Gradients Produced by Changes in the Depth of Cold Air: Hydraulic Effects

    For many gaps, the barrier in which the gap is embedded separates a cool air mass on one side from a warmer air mass on the other.  Surface pressure is higher on the cold air side because the denser cold air produces high pressure at low levels.  In most cases the cold air is relatively shallow (.5-2 km in depth) and overlain by warm air.   As a result, there is an inversion or stable layer at the top of the surface-based cold air.   In general the inversion is positioned below the crest level of the mountains, so that cold air can escape only through the gap (otherwise the cold air would push over and across the barrier).

    After the cold, dense air moves through the gap it spreads out in the gap exit region, where the gap widens rapidly.  Because of conservation of mass, the spreading cold air must become more shallow (see figure below).   Since the surface pressure depends on the depth of the cold, dense air, the rapidly thinning of the cold air layer along the gap exit results in a large drop in pressure along the gap in this region.  In turn, this pressure gradient contributes to an acceleration of the low-level gap flow over the exit region.  Since this shallowing of cold, dense air is similar to the shallowing of a layer of water as in passes out of a reservoir, we term such acceleration a hydraulic effect.

    Thus, both synoptic scale and gap-produced (hydraulic) pressure gradients tend to produce the strongest winds over the exit of a gap, not at constrictions in its mid-section.  In fact, the build up of cold dense air on the windward side of the barrier tends to produce mesoscale ridging (the windward ridge) that tends to slow the air down entering the gap.  As noted above, venturi or funnel effects, whereby winds accelerate in a constriction, are generally not the predominant mechanisms for mesoscale gaps with widths of km to tens of km, but they can be important for smaller gaps where strong flow moves through a constriction.

Gaps With Slopes or in Passes

    Until this point, the tutorial has only reviewed the nature of approximately level gaps in which elevation changes are minor (variations of a few hundred feet at most).  Such level gaps illustrate many of the key elements of gap flow and are representative of a large number of coastal gaps of interest to Navy operations.  There are, of course, higher level gaps--or passes-- in most mountain barriers and many gaps are characterized by substantial height changes.   Under the proper conditions, winds can accelerate down the lee side of a mountain barrier with the strength and location of these downslope winds greatly influenced by gaps or channels in the barrier.  Thus, a forecaster must be aware of some of the complex interplay of gap and mountain effects that often come into play.

   Downslope Wind Acceleration and Gaps

    Under the proper conditions, air can accelerate as it passes over and down a mountain barrier.  The most intense leeside acceleration events, known as downslope windstorms, can produce winds exceeding 100 kt, and can cause major damage, loss of life, and dangerous conditions for aviation.  Downslope windstorms are found throughout the world, with well-known examples including the damaging winds along the eastern slopes of the Colorado Front Range, the bora that descends down the terrain north of the Yugoslavian Adriatic Coast, and the Enumclaw winds of Washington State.

    Most forecasters are familiar with mountain waves that can produce a train of wave clouds downstream of orographic barriers.  Such features are atmospheric gravity waves that form when a relatively strong, stable flow approaches an orographic ridge under conditions in which the wave energy is trapped, so that it is confined to the lower atmosphere.


Trapped Mountain Waves Commonly Associated with a Train of Mountain Lee Wave Clouds

Under the proper conditions mountain lee waves can amplify so that the flow descends and accelerates down the the lee slopes of the barrier.  In general, such downslope acceleration is most evident when strong flow approaches a barrier and either a stable layer (e.g., an inversion or isothermal level) or a critical level (where the wind direction changes 180 degrees in a layer) is near or just above the crest of the barrier.   A closely related features is the bora, in which cold, dense air deepens sufficiently on the windward side of a barrier so that is passes over the crest and then accelerates down the lee slopes.  The detailed physics of downslope windstorms is complex and will be reviewed in more detail in another mesoscale module.


Schematic of a Downslope Windstorm Situation.  Blue lines indicate wind streamlines.




    There is often a profound interaction between gaps and downslope windstorms, with features of both being evident at the same time.   For example, a gap in the barrier can preferentially allow cold air to push across certain sections of a ridge, and then to accelerate down the slopes towards the lowlands.  Such a phenomenon is clearly evident over the Washington Cascades Mountains, where a mesoscale gap area (known as Stampede Gap), facilitates the movement of air across the Cascades.

Topography and important locations in the Washington Cascades with statewide (a) and a blowup view of the central Cascades (b).
Note the lower topography in the mesoscale Stampede Gap region.  Strong winds are found downstream of this gap during easterly flow situations.

When high pressure builds to the east of the Cascades a substantial east-west pressure gradient across the barrier can develop.  In addition, the flow can have a substantial westward component near crest level.  Cold low level air preferences pushes over the mountains in the mesoscale Stampede gap.  As the flow then accelerates down the western slopes of the Cascades it strikes the town of Enumclaw and adjacent communities (figure) with winds that can exceed 100 knots during the worst events.  Easterly winds of 50-70 kts occur annually in this area.  The winds are generally isolated in a swath, about 15 miles wide, that extends towards Puget Sound.   The sharp boundaries of the downslope/gap flow and its ability to maintain integrity dozens of miles (or more) downstream of the gap, have been occurred at locations around the world.


Maximum Wind Gusts during a gap/downslope windstorm downwind of the central
Washington Cascades.  Note how the gap produces a swath of strong winds that extends tens
of miles away from mountains.





An interesting phenomenon can occur when air both passes over a barrier and moves through a gap in its midst.  The sinking (subsiding) air on the lee side of the barrier warms adiabatically as it is compressed by higher pressure, thus causing sea level pressure to fall on the lee side.  On the windward side of the barrier cold air can be "dammed up" and deepen as it blocked by the terrain, producing a mesoscale windward pressure ridge.  Thus, both lee troughing and windward ridging can strengthen the across-barrier pressure gradient and consequently the strength of the gap wind though the terrain.
 

The Winds in the Gap Exit Region:  The Adjustment to the Larger Scale Environment
 
 

Gap Winds and Other Weather Elements

    This tutorial has stressed the importance of gaps in producing strong winds, particularly in the gap exit region.  Although winds are probably the key gap wind feature in most situations, other weather parameters can be significantly affected.  For example, for gaps in coastal terrain the gap wind outflow is often associated with cold, dry air from the interior that can starkly contrast with  more temperate marine air over the offshore waters.  Near the exit regions of such gaps low level air temperatures can be 10-20 F (or more) cooler than in adjacent regions, resulting in gap wind-associated variations in precipitation type during the cool season.   Depending on the temperature structure aloft, the cooler temperatures in and just outside of the gap can produce freezing rain or snow, while outside of gap (on the downwind side) rain would be the rule.

    An excellent example of such a gap wind effect on precipitation type is the Columbia Gorge on the border of Washington and Oregon (see map).   The Cascade Mountains act as a barrier that impedes the westward movement of cold, continental air from the interior, with only the Gorge acting as a near sea level gap across the mountains.  During the winter, cold air from eastern Washington and Oregon moves westward though the Gorge across Portland and the northern Willamette Valley.   When a Pacific weather system, with attendant clouds and precipitation, approaches the coast the gap flow increases as the pressure difference across the Gorge builds.  These systems are often relatively warm with high freezing levels (4-8K ft) that bring rain to most of western Oregon and Washington.  However, in the Gorge the gap flow is often associated with subfreezing air that can extend to a few thousand feet ASL.  As the rain falls into the subfreezing air it is supercooled (and still liquid) and freezes on contact with the ground.  Such freezing rain, known as the "Silver Thaw", can bring treacherous driving conditions and can cause substantial aircraft icing for planes inbound or outbound from Portland.  Similar freezing rain events have been observed in and near coastal gaps at locations around the world.

Rain falling into the cold gap flow of the Columbia Gorge frequently produces major freezing rain events.

The temperature effects associated with gap flow tend to rapidly lessen away from the gap, particularly when the cold, gap flow passes over warm water.  As evidenced by growing cumulus elements, the movement of cold air over warm water produces considerable vertical instability that substantially warms the lower atmosphere within approximately 100 miles or so.

    Outflow through coastal mountains is often relatively dry, since the source region is the continental interior.  Such drying is accentuated if the gap descends from a high interior plateau region.  Low-humidity gap flow air can play an important role in initiating or maintaining snow in marginal situations, since evaporation of precipitation falling from above can be an important cooling mechanism.  During summers, such dry gap flow can be associated with critical fire hazards since both strong winds and low humidities are present.  A well-known example are the strong winds (such as the Santa Anna wind) that descend down the western slopes of the California coastal mountains.  Such descending winds are generally most intense in the gaps, where they can fan wildfires and can make driving large trucks difficult.

Diagnosing and Predicting Gap Wind Flow:  Simple Diagnostic Models

    As noted above, winds in nearly level gaps are closely related to the surface pressure gradient (or pressure difference) across the gap.  Wind direction is relatively easy--the winds will tend to parallel the gap axis and flow from high to low pressure, with the wind speeds roughly proportional to the pressure difference, and the strongest winds in the gap exit region.  Although these ideas are useful, one must go further to quantitatively estimate or predict gap wind speeds.  This section will describe approaches of varying sophistication for estimating gap wind speeds.

Simple Diagnostic Relationships

    The simplest relationship for gap wind flow is a form of the Bernoulli equation, which is derived assuming frictionless, steady-state flow in a constant elevation channel or gap.  This equation relates the acceleration down a gap to the pressure difference across the gap:

Using this equation, one can calculate  wind speed differences across a gap for various pressure differences.  An alternative form of the same equation gives the wind at any point in the gap, if you know the starting wind speed along the gap (u0)and (delta)p (0->x) is the pressure difference from the starting location to the location (x) in question (where x is the distance from the starting point---generally the gap entrance to the location in question):



    The simple Bernoulli equation usually overestimates the wind acceleration in gaps, and thus a better estimate can be derived by adding a term that includes friction or drag.  There are, in fact, two types of drag important in gap flows:  (1) surface friction due to the roughness of surface features and (2) drag due to mixing at the upper boundary of the gap wind flow.  A more complete version of the Bernoulli equation that includes a parameterization of such drag is:


ary layer thickness.

    An illustration of the usefulness of such a simple Bernoulli equation approaches is found in Fig. B.  The red line shows the wind speeds down the Strait of Juan de Fuca measured by the NOAA P3  by Overland and Walter on 21 February 1980.  The blue line presents the wind speed calculating using the observed along-gap pressure gradient and the simple Bernoulli equation without friction.  Clearly, without friction the gap winds are overestimated.  In contrast, using equation 2 with friction (purple line) produces a wind speed estimate that is close to reality.

    In practice, such Bernoulli equations can be used as operational diagnostic or forecasting tools by using observed pressure gradients or by applying pressure gradients produced by larger scale numerical models (e.g., NOGAPS).

High-Resolution Numerical Models

    As will be illustrated below, high-resolution mesoscale models, such as COAMPS or MM5, are capable of realistically diagnosing and predicting gap wind flows-- IF they have sufficient horizontal and vertical resolution AND the large scale conditions are being well handled.    Thus, to evaluate the usefulness of mesoscale model forecasts in a gap flow situation a forecaster must ask several key questions:

1. Is the synoptic scale flow being well forecast?

     Whenever you contemplate using a high-resolution mesoscale model forecast, be it for gap wind flow or anything else, you must evaluate the realism of the model's synoptic scale forecast.  A high-resolution model is like a high powered rifle--when aimed at the right direction it can be very accurate, but when aimed at the wrong direction it is nearly useless.  In fact, worse than useless if the impressive detail instills overconfidence in an incorrect solution.  An accurate synoptic scale forecast provides the model with the correct aim, and with sufficient resolution it may well be able to correctly fill in the local details.   In short, as a first step to using a high-resolution forecast of gap flow one must verify the accuracy of the larger scale forecast, both in terms of structure and timing.  If there is only a timing error, than you can attempt to make the necessary time displacements so that the model's mesoscale detail aids your forecast tasks.

2.  Does the model have sufficient horizontal and vertical resolution?

    Relatively narrow gaps require high resolution to have a chance of reasonably simulating gap flow.   For example, a mesoscale model with 10 km grid spacing is completely inadequate for forecasting the flow in a 10-km gap.  A rough rule of thumb is that it takes at least four grid points to even grossly describe a wave-like disturbance.  Thus, to simulate a 10-km wide gap a forecast model would should have at least 2.5 km grid spacing, with higher resolution being advisable.  Vertical resolution is also important, particularly if one wants to resolve the often sharp upper boundary of the gap flow and the mixing that can occur across this interface.  Previous research simulations suggest that 35-40 levels are typically required for adequate gap flow forecasts, with approximately fifteen of those levels below 850 mb.

    To gain some appreciation for the resolution needs in forecasting gap wind flows,  lets examine the case of the Columbia River Gorge noted above.  The Gorge is approximately 10-km wide.  Consider the results of a MM5 simulation of easterly Gorge flow at 36, 12, 4, 1.33, and .44 km grid  spacing using the domains shown below.  Only the region around the Gorge is displayed to highlight the effects of resolution.  The comparison will be limited to hour 21 of the forecast, a time at which the sustained winds at Troutdale, in the exit region of the Gorge, were sustained at approximately 20 knots with gusts to 30-35 kts.  The winds over the water  in the central Gorge were probably stronger.  The MM5 wind should be compared to the sustained winds, because short-term gustiness is not modeled at the resolutions used in these simulations.


Domain used in an MM5 mesoscale forecast of flow in the Columbia Gorge.  The outer domain has a
36-km horizontal grid spacing, with the inner nests of 12, 4, 1.33, and .44 km resolution.






Using 36-km grid spacing, no real gap exists in the model terrain, but rather a saddle-like feature is apparent (see figures below).  Only a minor acceleration  to 10 kt is apparent and their is no evidence of cold air in the Gorge.
 
 

21-h Forecats of the flow in the Columbia Gorge from the 36-km domain.  Winds are at approximately 30-m ASL, terrain is shown by
the yellow-brown shading, and the temperatures at .31 km with green/blue colors.

At 12-km resolution, the terrain in the Gorge is lower, but is still better described as a saddle rather than a gap.  Isolated pockets of cold air in eastern Washington are now apparent and winds in the Gorge have increased to 15 kt.


21-h Forecats of the flow in the Columbia Gorge from the 12-km domain.  Winds are at approximately 30-m ASL, terrain is shown by
the yellow-brown shading, and the temperatures with green/blue colors.

Using 4-km grid spacing produces obvious benefits:  cold air east of the mountains is far more extensive and pockets of lower temperatures are even apparent in the Gorge.  A gap-like channel through the Cascades is visible, although the sharpness of the Gorge walls is absent.


21-h Forecats of the flow in the Columbia Gorge from the 4-km domain.  Winds are at approximately 30-m ASL, terrain is shown by
the yellow-brown shading, and the temperatures at .31 km with green/blue colors.

A dramatic improvement is seen at 1.33 km grid spacing.  A continuous river of outflow cold air flows from east of the Cascades to Portland is apparent and the Gorge channel is clearly evident, including the steeper slopes on the southern side.  Winds now reach 20 kt in the exit region of the Gorge.


21-h Forecats of the flow in the Columbia Gorge from the 1.33-km domain.  Winds are at approximately 30-m ASL, terrain is shown by
the yellow-brown shading, and the temperatures at .31 km with green/blue colors.

Finally, a smaller domain was run with .44 km horizontal grid spacing realistically defines the Gorge terrain and the extent of the cold air.  Winds have increased to a maximum of approximately 30 kt in the Gorge exit region.

21-h Forecats of the flow in the Columbia Gorge from the .44 km-km domain.  Winds are at approximately 30-m ASL, terrain is shown by
the yellow-brown shading, and the temperatures at .31 km with green/blue colors.





    High-resolution models also appear to be quite capable in handling downslope windstorms and their interactions with gaps in mountain barriers.  As an example, consider a mesoscale forecast using the MM5 of a gap/downslope windstorm that stuck Enumclaw and nearby areas on the western side of the Washington Cascades.  The first figure below shows two domains (9 and 3-km grid spacing), which were nested within a far larger 27-km domain.  Note that there are two passes in the Cascades, with Stampede Gap (the southern one) being the most significant.
 
 


9 and 3-km domains used for the MM5 simulations.  The 9-km terrain is also shown.




Figure (a) below shows the 3-km wind and sea-level pressure fields for a 24-h forecast. Two swaths of strong winds are noticeable, each downwind of a major mesoscale pass in the barrier.  The strongest winds are not within the passes at high elevations, but downwind along the lower slopes as a result of the downslope acceleration of the subsiding flow.   Both the distribution of the winds and their magnitude compare well with surface observations.  A vertical cross section through the Stampede Pass area (Figure b) below, shows potential temperature and wind structures across the barrier.  The strongest wind regions are indicated by shading.  Note that strongest winds occur along the final lee slopes as the air descends rapidly towards sea level.
 
 

   Sea-level pressure and 40-m winds from the 3-km domain for a 24-h forecast from the MM5 (a).  Vertical cross sections of potential temperature and winds through line AA' (b)


    In summary, high resolution numerical weather prediction promises to be a crucial tool for forecasting gap wind flow.   However,  realistic local forecasts only occur when then the synoptic scale flow is well predicted.  The conclusions about the the effects of resolution shown above should be applicable to other mesoscale models (such as COAMPS) and to other regions around the world.
 

Gap Winds Without Pressure Gradient Acceleration:  Wakes Versus Open Regions

    A major aspect of the gap flow discussion and examples provided above is the acceleration of wind within the gap, be it a level gap or one with some slope.  Gap flow acceleration, usually associated with a pressure gradient along the axis of the gap, can produce very strong winds of importance to both maritime and aviation operations.  However, any complete discussion of gap wind flows should include a different type of gap flow, one in which gap accelerations are minor.  Such gap winds are often associated with relatively narrow, but mountainous, island chains with substantial gaps between the islands.  Downstream of the mountains the winds are usually weak for tens to several hundred miles in what is known as a wake.  Essentially, the blocking effects of the mountains and greater surface drag over terrain (compared to water) results in weak winds downstream of the islands.  In contrast, winds remain nearly unchanged and strong in the gap regions (see schematic below).


Two excellent examples of such island-produced gap winds and wakes are found downwind of Japan and the Aleutian Island chain.  The map below shows the terrain of Japan, which includes high mountains, with substantial intervening gaps.
 



Figure (a) below shows surface winds downwind of Japan from the TRMM radar satellite.  Strong winds are found downstream of major gaps in the Japanese terrain, with lesser winds (wakes) downstream of major topography.

    An even more dramatic and instructive example is found in the figure below, which shows northwest winds approaching Alaska's Aleutian chain. To the north of the Aleutians (that is, on the windward side) the winds are relatively uniform and strong in a wide region (yellow/red color).  On the lee (southern) side the wind vary greatly:  very light wind (wakes) downwind of high terrain and strong winds downwind of gap areas.  The wakes extend tens of miles to as much as one hundred miles downstream of the terrain.


SAR (Synthetic Aperture Radar) imagery of wind speed around the Aleutians.
Picture courtesy of Nathaniel Winstead, Johns Hopkins APL


 



Additional Examples of Gap Winds Around the World

In this section we shall review some well documented examples of gap wind flow from around the world.
 

A Detailed Case Study: The Strait of Juan de Fuca of Washington State


    One of the most intensively studied sea-level gaps is Washington State's Strait of Juan de Fuca, which provides a sea level passage from the Pacific Ocean to Puget Sound.  A number of important U.S. Navy facilities are found on the inland side of the Strait, including Whidbey Island Naval Air Station, the Bangor Submarine Base, the Everett Homeport, and the Bremerton docks.  Approximately 100-km long and 20-km wide, the Strait of Juan de Fuca (SJF) is  a low-level gap between the mountains of Vancouver Island (which reach 3000-4000 ft) and the higher Olympic Mountains to the south (Figure 1 below).  The Strait exit region is well known for strong easterly winds:  for example, the study of Reed (1931) noted over 200 occurrences of easterly winds exceeding 36 knots during a 5-year period at Tatoosh Island at the western terminus of the Strait.  With a large pressure gradient associated with inland high pressure and a trough over the offshore waters, easterly winds in the western Strait and immediately downstream can easily reach 50-70 knots.
 
 


Figure 1:  Topographic Map of the area surrounding the Strait of Juan De Fuca


    On 9 December 1995 a special field experiment (COAST) explored the flow in the Strait by flying the NOAA P3 aircraft down the Strait during a moderate easterly wind event.  An analysis using the P3 data and other observational assets is shown in Figure 2 below.  In addition a sea level pressure analysis is shown.   One notes there is a pressure gradient across the gap with higher pressure to the east.  The wind appears to accelerate in the gap to around 35 knots in the exit region.
 
 


Figure 2:  Low level observations and pressure analysis for 1700 UTC 9 December 1995



The NOAA P3 aircraft possesses a tail dual-Doppler radar that enabled the scientists on board to "paint out"  the winds within the Strait.  The results, shown at a number of levels, are presented in Figure 3.  As indicated by the color shading, the strongest winds, exceeding 21 ms-1 or 42 knots) were found in the exit region at low levels ( 100 m).  The gap winds are quite shallow--by 1100 m the easterly flow had greatly weakened.


Winds derived from the Doppler radar observations of the NOAA P3 aircraft at four levels above the Strait of Juan de Fuca of Washington State.
Warmer color indicate stronger winds (ms-1).  Note the strongest winds are found in or downstream of the gap exit region.

Also quite informative is a vertical cross section of the winds down the Strait shown in Fig. 4.  Note how the depth of the easterly flow deepens down the Strait, but then collapses over the eastern exit.  The strongest winds are found near the surface where the easterly had collapsed the most (approximately the 50 km mark westward).



To evaluate how well a high-resolution mesoscale model forecast can duplicate the observed wind field in the Strait, the Penn. State/NCAR mesoscale model versions 5 (MM5) was run down to 1.33-km grid spacing.  An example of the windfield from such a model run valid at 1700 UTC 9 December 1995 is found below.  As in the observations, the model output at 100 and 300 m show the strongest winds in just outside of the western exit region of the Strait, with the easterlies weakening greatly by 1100,
 
 




Central America:  The Tehuantepecer


        Chivela Pass, a gap that has both important atmospheric and oceanographic effects, cuts through the Sierra Madre mountains of Mexico.  This gap--which is approximately 220 km long, 40 km wide, and has a maximum elevation of only 250 m-- provides a path for air from the Bay of Compeche, in the southern Gulf of Mexico, to the Pacific Ocean's Gulf of Tehuantepec (Figure 1 below).
 
 

Regional (a) and close-in (b) view of the terrain surrounding Chivela Pass, Mexico. Figure from Steenburgh et al (1998)



During the winter, when cold, high pressure systems move southward along the eastern slopes of the Rockies and the Sierra Madre Mountains, a large pressure gradient can build across the gap resulting in strong northerly winds (known as Tehuantepecers) immediately downstream of the Pass that can reach 20-40 knots, with gusts exceeding 100 knots in extreme cases.  The figure below illustrates such an evolution, one associated with the "Storm of the Century" cyclone of March 13, 1993.  High pressure moved southward to the east of the southern Rockies and Sierra Madres, producing a large pressure gradient between the Gulf of Mexico and the eastern Pacific (Figure a).  The resulting gap flow across Chivela Pass pushed into the Gulf of Tehuantepec where it spread as it extended southward.  In the satellite picture shown below (Fig. 2b), the leading edge of the gap flow is indicated by a shallow line of convection (a rope cloud).
 
 

Sea Level Isobars and frontal analysis for a Tehuantepecer event on 13 March 1993 (a).
Visible satellite image at the approximately the same time (b)
Note the convective cloud streets over the Gulf of Mexico due to cold air moving over warm water,
and the more stratiform upslope clouds on the windward side of the Sierra Madre Mountains.
On the Pacific side, a semicircle of convective clouds delineate the boundary of the flow that had pushed through Chivela Pass.
Figure from Steenburgh et al (1998)


High resolution mesoscale models appear to be capable of realistically simulating the development of the Tehuantepecer.   3 and 15 hour forecasts of the 13 March 1993 Tehuantepecer were made using the Penn. State/NCAR mesoscale model (MM5) with an inner nest domain of  6.67 km horizontal grid spacing.   As shown in the figures below,  the northerly gap flow was forecast to reach 50 knots after pushing through Chivela Pass into the Pacific Ocean.  Note how the strongest model winds were in the lee (south of the gap) over the ocean with the coldest air coincident with the strongest winds.  The association of higher pressure with the cold temperatures resulted in higher pressure in the center of the gap exit flow that caused the strong winds to spread out in a fan-like pattern.
 
 


Forty meter winds and temperature from the inner (6.67 km) nest of the MM5 run for the 13 March 1993 event.
The simulation is described in more detail in  Steenburgh et al (1998)

    The Tehuantepecer and other larger scale gap flows are often apparent in satellite-based scatterometer winds.  Such winds are produced by relating the wind speed and direction to the amount of microwave radiation scattered off the sea surface.  To see an animation of the scatterometer-sensed winds to the strong Tehuantepecer of October 20-26, 1999 click here.

    Tehuantepecers and other strong, persistent atmospheric gap flows can have a significant influence on the upper coastal ocean.  The strong winds that blow through the mountain gaps of Central America, such as Mexico's Chivela Pass, can produce sustained winds of 30-40 knots for 5-7 days.  Such strong winds result in substantial upper ocean mixing that can bring cooler sub-surface water to the surface, causing sea surface cooling of 4-8C.  The figure shown below shows satellite-based scatterometer winds and  sea surface temperatures during the Tehuantepecer event of 18 February 1997.  Note the coincidence of lower SSTs and gap flows.  The strong, persistent winds from Tehuantepecers can create waves that can propagate southward as swell as far as the Galapagos Island, nearly 1000 miles away.

NRL tutorial

  A detailed tutorial on Tehuantepecers, including the application of TRMM satellite imagery for observing these strong gap winds, is viewed by clicking here.

Tehuantepecer Reference

    Steenburgh, W. J., D. M. Schultz, and B. A. Colle, 1998:  The structure and evolution of gap outflow over the Gulf of Tehuantepec, Mexico.  Monthly Weather Review, 126, 2673-2691

    Schultz, D.M., W.E. Bracken, L.F. Bosart, G.J. Hakim, M.A. Bedrick, M.J. Dickinson, and K.R. Tyle, 1997: The 1993 Superstorm cold surge: Frontal structure, gap flow, and tropical impact. Monthly Weather Review, 125, 5-39.
 

The Strait of Gibraltar


    The Strait of Gibraltar, located at the western entrance to the Mediterranean, is frequently associated with strong gap winds that can produce dangerous seas, especially when they blow against tide and current.  As shown in the figure below, the Gibraltar represents a narrow sea-level passage about 15 km wide and 55 km long that is surrounded by terrain reaching several thousand feet.


Terrain around the Strait of Gibraltar (m)

    The most pronounced gap wind though the Strait is from the east and is known as the Levanter, which can produce winds of 20-40 kt in and to the west of the gap.   The typical synoptic "set-up"  is shown in the the sea-level pressure analysis presented below for 1200 UTC 25 August 1981.  High pressure is found over the eastern Mediterranean, with lower pressure to the west of Gibraltar.  The sinking motions accompanying such anticyclonic conditions often results in the formation of an inversion a few thousand feet above the surface.  Such an inversion provides a vertical stable layer or cap that contains the low-level air and results in greater topographic blocking and stronger gap flow.  A large horizontal gradient exists over the Strait, and winds accelerate downgradient from high to low pressure within the gap.  Under such circumstances, the winds can go from near calm in the eastern Mediterranean (known as the Alboran Basin) to gale force strength on the western side of the Strait.  It is important to stress that the strongest winds are not observed mid-Strait, as might be expected if the funnel mechanism was dominant; rather, the strongest winds are over the western Strait and immediately downwind to the west.  Levanters are most frequent during the warm season from May through October.


Mean Sea Level Pressure (mb) at 1200 UTC 25 August 1981

    An excellent illustration of the distribution of Gibraltar gap winds under easterly conditions is provided by the visible polar orbiting satellite image below.  Taken by the NOAA-6 satellite at 0858 UTC 25 August at a time with sun glint over the Gibraltar region (sun glint is sunlight reflected off a smooth water surface), one notices a wedge shaped region of darkness over and to the west of the Strait.  This darkening is caused by the strong Gibraltar gap winds roughening the surface, greatly lessening the amount of light reflected back to the satellite.  The darkest colors (and thus strongest winds) are found to the west of the Strait, and extend approximately one hundred kilometers to the west.  This image also highlights why Gibraltar possesses such exceptional gap flows:  the terrain surrounding the western Mediterranean forms a topographic bowl with only one exit--the Strait of Gibraltar.
 
 

Visible image from NOAA-6 at 0858 UTC 25 August 1981

Gibraltar References:

    Bendall, A. A., 1982:  Low-level flow through the Strait of Gibraltar.  Meteor. Mag., 111, 149-153
    Dorman, C. E., R. C. Beardsley, and R. Limeburner, 1995:  Winds in the Strait of Gibraltar.  Quart. J. Royal Met. Soc., 121, 1903-1921
    Scorer, R.S., 1952:  Mountain-gap winds; a study of the surface wind in Gibraltar.  Quart. J. Royal Met. Soc., 78, 53-59
 

The Hinlopenstretet Strait near Spitsbergen


    A high-latitude (~80N) example of a gap flow is found in the strait (Hinlopenstretet) between Spitsbergen and Nordaustlandet (see figure below).  In a recent study (Sandvik and Furevik 2002) gap flow through this strait under southeasterly conditions was studied using both satellite imagery (synthetic aperture radar (SAR) imagery that provide surface winds) and high-resolution MM5 simulations.
 
 


Topography near Spitsbergen.  From Sandvik and Furevik (2002)














Using 6-km grid spacing in the model, the trajectories over an 18-h forecast (starting 1200 UTC 14 August 1996), shows the the flow being deflected around the substantial terrain on Spitsbergen and Norauslandet with a large portion of it moving through the Hinlopenstretet Strait (see figure).
 
 


100-m Trajectories for 0 to 18h from a MM5 simulation (6-km nest) initialized
0000 UTC 14 August 1996.  From Sandvik and Furevik (2002)

Another figure shows the wind vectors and wind speeds for the same time.  Upstream (southeast) of the Strait the winds are quite weak, ranging to 4 ms-1 (8 kt).  Wind speeds accelerate in the gap and reach their maximum in the gap exit region, extending downstream tens of km as a relatively narrow swath of strong winds.
 
 

16-m wind vectors and wind speed at  12h into a MM5 simulation (6-km nest) initialized
0000 UTC 14 August 1996.  From Sandvik and Furevik (2002)

Some insight into the nature of the acceleration is provided by a vertical cross section (location shown section B in the figure above) of wind speed along the gap.  Note how the depth of the approaching flow (indicated by the stronger winds) collapses and strengths down the gap, with the strongest winds (15 ms-1, 30 kt) at low levels (approximately 150 m) in the gap exit region.

Vertical cross section along B (see figure above) at 12-h into  a MM5 simulation (6-km nest) initialized
0000 UTC 14 August 1996.  From Sandvik and Furevik (2002)

Hinlopenstretet Strait References

Furevik, B. G. and A. D. Sandvik, 2002:  Case study of a coastal jet at Spitsbergen-comparision of SAR- and model-estimated wind.    Mon. Wea. Rev., 130, 1040-1051
 

Major Facts in Review


    Some key ideas that you should remember:

1.  The winds in mesoscale gaps in terrain generally flow from high to low pressure and thus are highly non-geostrophic.

2.  The most important mechanism in producing strong gap flows are NOT venturi or funnel effects, but rather the the acceleration of air as it moves from high to low pressure.  Thus, the strength of gap flows generally are proportional to the pressure gradient (or difference) across the gap.

3.  The pressure differences across a gap have two main origins:  (1) large scale or synoptic pressure gradients such as when an anticyclone (high) is on one side and and low center approaches the other, and (2) changes in the depth of low-level cold air across the gap.

4.  For narrow island chain a different type of gap flow is possible, with weak winds (wake) downstream of terrain and stronger winds (similar to those upstream of the islands) downstream of the gap.

5.  The strongest gap winds are typically in the gap exit region.

6.  Simple dynamical relationships, such as the balance between the pressure gradient force, drag, and acceleration, are often quite good in relating the gap wind speeds to pressure gradients.

7.  High-resolution numerical models are valuable tools for forecasting gap wind flow, if the synoptic scale forecasts are realistic.