Decoding Wind Shear

Pilots often equate wind shear in a preflight briefing with thunderstorms and severe turbulence. But one form of wind shear may occur when the air is glassy smooth — nonconvective low-level wind shear (LLWS).

The forecast for this condition is probably the most misunderstood aviation forecast among pilots. In a terminal aerodrome forecast (TAF), it appears in coded form with a WS code, such as WS020/15045KT. But that’s not necessarily a forecast for turbulence.

This form of wind shear is typically found in the warm sector ahead of the cold front and south of the warm front. It’s also prevalent in the overnight hours during fair weather conditions with clear skies and calm wind near the surface. Though wind seems to be the common element, atmospheric stability is the catalyst behind most nonconvective LLWS occurrences.

Wind shear is a marked change in wind speed or direction over a given distance. Horizontal wind shear means the wind is changing directions as you fly or hover at a particular altitude. If the change in direction or speed occurs over a range of altitudes, it’s vertical wind shear. When wind shear occurs near the surface, it is referred to as low-level wind shear.

We know that wind tends to increase speed with increasing height, but it normally does so gradually. Nonconvective LLWS, or vertical speed shear, occurs when winds are calm at the surface and rapidly increase above the ground.

When winds are expected to increase rapidly within 2,000 ft of an airport’s surface, a nonconvective LLWS forecast will likely be issued in the related TAF. This forecast tells the pilot about the potential for wind speed to increase quickly with height above the ground within a shallow layer. That is, faster air at the top of the shear layer is moving over slower air near its bottom. There also might be an accompanying shift in wind direction with height in this layer, but the speed change is usually what triggers this forecast.

Forecasts for convective and nonconvective LLWS are rather distinct. In a TAF, convective LLWS would reference thunderstorms (TS or VCTS) and contain CB (cumulonimbus) in the cloud group. Also, surface winds are typically forecasted to be strong and gusty. Convective LLWS could occur any time of day, but mostly in the afternoon and early evening when thunderstorms are most prevalent.

Here are examples of convective LLW forecasts:

FM132200 33010G20KT P6SM VCTS SCT015 BKN040CB; FM131600 22013G35KT 3SM TSRA BR BKN035CB; or FM140000 VRB20G55KT 1/2SM +TSRA FG BKN015CB.

Nonconvective LLWS could occur in the warm sector of a low-pressure area, but it also could occur in the presence of a strong nocturnal temperature inversion. Frontal nonconvective LLWS could occur any time and normally has light surface winds and cloudy skies, but surface winds could be strong and gusty when weather is associated with an intense low-pressure area.

Here are examples of TAFs forecasting nonconvective LLWS associated with a frontal system:

FM111600 13010KT 5SM -RA OVC015 WS020/27055KT; FM120100 VRB03KT 4SM BR OVC008 WS015/25045KT; or FM120900 19018G30KT 3SM +SHRA BR OVC005 WS020/17075KT.

Nocturnal nonconvective LLWS occurs overnight or in early morning, often with light surface winds and clear skies. This is a manifestation of radiation cooling and likely occurs in the region under an area of high pressure.

In a TAF forecasting nocturnal nonconvective LLWS, you might see:

FM221100 19004KT P6SM SKC WS015/17040KT; FM230800 VRB03KT P6SM SCT010 WS010/22035KT; or FM230400 00000KT P6SM SKC WS020/23055KT.

For nonconvective LLWS, a TAF will include the code WS immediately after the cloud group.

Take this TAF: FM130300 17005KT P6SM SKC WS020/23055KT. The first element to the right of the WS is height above the airport (in this case 020, or 2,000 ft agl), indicating the top of the wind shear layer. This agl altitude is typically one of three values: 010 for 1,000 ft, 015 for 1,500 ft or 020. The WS layer may extend higher, but the maximum forecasted height is 2,000 ft.

The numbers after the slash are the true wind direction followed by wind speed in knots at the indicated height. This implies the wind is rapidly increasing from the surface through the indicated height, though it says nothing about wind direction throughout this shear layer. This forecast translates into “the wind at 2,000 ft is 230 deg at 55 kt.” But it does not imply there will be turbulence at 2,000 ft agl or below.

The catalyst for development of most instances of nonconvective LLWS is atmospheric stability. Temperature normally decreases with increasing altitude. This change is called lapse rate. A positive lapse rate refers to a temperature decrease with increasing altitude; a temperature increase with altitude is a negative lapse rate (or a temperature inversion).

The larger the rate, the greater atmospheric instability. An unstable environment promotes vertical mixing and a more turbulent air flow potential. A stable atmosphere inhibits vertical mixing and provides for a laminar and nonturbulent flow.

One might suspect vertical speed shear (faster-over-slower air flow) could cause the air to overturn and produce turbulent eddies within this layer. However, about all nonconvective LLWS features a strong surface-based temperature inversion. This stable inversion layer tends to dampen or resist vertical air mixing. Air forced to ascend within this layer will expand, cool and immediately find itself in warmer air aloft. The cool air is forced back to its original altitude. In other words, this air has neutral buoyancy and doesn’t want to rise or sink.

This extreme stability promotes a laminar flow, and the effects of surface friction are no longer “felt” a few hundred feet agl. This allows air just above the treetops to accelerate uninhibited and insulated from surface friction below through the depth of the wind shear layer. (You can think of this as a faster-flowing river of air located just above the surface.) The stronger and deeper the inversion, the less likely there will be any kind of turbulence.

TAFs are one way to identify nonconvective LLWS. However, not all airports are served by a TAF. Meteorologists at the U.S.’s Aviation Weather Center also issue a forecast for widespread nonconvective LLWS that is expected to cover an area of at least 3,000 sq mi.

You’ll see this issued as part of AIRMET Tango, which can be issued for three different reasons: nonconvective moderate turbulence; sustained winds over 30 kt; and nonconvective LLWS below 2,000 ft agl.

It’s unfortunate that this is issued under the auspices of AIRMET Tango, suggesting to the pilot the potential for turbulence.

Nonconvective LLWS Dangers

When the sky is clear and surface winds are light, the nocturnal version of low-level wind shear phenomenon is just as common as low-level thermal turbulence during summer afternoons.

Unless you were fixated on your ground speed approaching an airport late at night or in the early morning, you probably have flown right through it without even noticing.

Just as thermal turbulence isn’t forecasted because it’s so prevalent, nocturnal nonconvective LLWS isn’t usually forecasted either.

Pay close attention when nonconvective LLWS of 50 kt or greater is coupled with the potential for moderate to heavy rain showers or thunderstorms. As the moderate to heavy rain falls through the low-level jet, some of the jet’s momentum gets directed downward toward the surface, creating the potential for wet microbursts or downbursts. In this case, the magnitude of the nonconvective LLWS event and convective outflow can make for a real interesting approach to land. R&WI