Navy Mesoscale Primer:  Gap Winds


Synthetic Aperature 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 exeed 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 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 the gap in question.

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 low spots 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  low spots 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 very 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, whereby the strongest winds occur in constrictions.   As will be shown below, 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 situaton 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 small area 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 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 upstream of the constriction and decelerating downstream of the constriction as it traverses from low to high pressure.

    Although physicially 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.  Such an increasing depth of cold, dense air over and to the east of the barrier contributes to higher pressure near the center of the gap, which tends to decelerate the gap flow as it moves from the entrance to the central portion of the gap (see figure below).  As the air moves towards the gap exit region, the rapid widening of the gap causes the gap flow to spread out horizontally.  This divergence of the gap flows causes the height of the dense low-level air to collapse, resulting in a pressure gradient that accelerates the winds over the exit region.

    Another reason the flow tend 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 region and thus tend to produce strongest flow in the gap exit region.

    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 can from one source region (at low levels upstream of the gap), but rather can flow into the gaps from several directions and from several levels along the length of the gap.  Thus, simple mass conservation arguments can be deceptive at best.

    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 imporant for small scale gaps (on the order of a few km or less), where the contributions of the larger scale pressure gradients or changes in the height of the cold air are far 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 between gaps in buildings.
 

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 parallel the isobars and thus move perpendicular to the pressure gradient.  This section will review geostrophic and non-geostrophic winds, and will why winds tend to be non-geostrophic in gaps.

    The Simplest Case:  Goestrophic 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 the surface winds are expected to be geostrophic, with winds parallelling 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 other conditions:  that the pressure and wind fields are not changing rapidly in time and that the air trajectories are not highly curved.  In rapdily evolving situations there is not enough time for the winds to adjust to the pressure fields so that geostrophic balance can be achieved, while in highly curved flows 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-based 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, can be important.  Thus, a three way wind balance is established in the lower atmosphere in which the wind speed is lessened below the geostrophic value and the wind direction is altered so that that the wind blows at an angle (called the cross isobar angle) across the isobars 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 angle, 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 enought vertical mixing of fresh 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 resulting in far more slowing of the air near the surface(see figure).
 

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 figure below.  In that case, geostrophic winds are possible even near the terrain, because the 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 downthe gap.  In other words, for most narrow gaps 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 and idea of the magnitude of the flow.   Stronger pressure gradients along a gap in general produce strong 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 the winds 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 gap flows the barrier in which the gap is embedded separates a cool air mass on one side by 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 (1-2 km in depth) and overlain by warm air above.   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).   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 change in pressure (or pressure gradient) 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.  Consistent with these mechanisms, the vast majority of observed gap flows are found over the exit region.  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.

Diagnosing and Predicting Gap Wind Flow:  Simple and Complex Numerical Models

    As noted above, winds in nearly level gaps in terrain are closely related to the surface pressure gradient or difference along the gap.  Wind direction is relatively easy--the winds will 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 generally correct, one must go farther to secure a quantitative estimate or prediction of 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 Bernouli 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:

                    Put equation here

where u1(u2) is the entrance (exit) wind velocity along the gap, Dp is the pressure difference along the channel(p2-p1), and r is air density (Figure).  Using this equation, one can callculate  wind speed differences across a gap for various pressure differences (see Table 1).

    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 Bernouilli equation that includes a parameterization of such drag is:

insert equation.

where,.........  Table 1 above shows the values of the jet exit wind speeds using a reasonable friction coefficient K;  exit wind speeds are reduced by 15-40% due to frictional effects.

    An illustration of the usefulness of such simple Bernoulli equation approaches is found in Fig. B.  The solid line shows the wind speeds down the Strait of Juan de Fuca measures from the NOAA P3  by Overland and Walter on 21 February 1980.  The dashed 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 (dotted line) produces a wind speed estiamte that is close to reality.

    In practice, such Bernoulli equations can be used as operational diagnositc 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.   To evaluate the usefulness of mesoscale model forecast several elements a forecaster must ask several key questions:

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

    Relatively narrow gaps require high resolution to have any chance of reasonably simulating gap flow.   For example, a mesoscale model with a relatively high resolution of 10 km is completely inadequate for forecasting 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 even better.  Vertical resolution is also important, particularly if one wants to resolve the often sharp upper boundary of the gap flow and the mixing 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.

2. 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 fine rifle--when aimed at the right direction it can be very accurate, but when aimed at the wrong direction it is nearly useless.  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 structures and timing.
 

diurnal effects

In Summary
 

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 analsys 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, exeeding 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.

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 verions 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



 
 


 


 

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 posseses such exceptional gap flows:  the terrain surrounding the western Mediterrean 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
 
 
 

Evaluation of High-Resolution Forecast Models to Gap Winds 



 

Forecasting Gap Winds:  A Suggested Approach