AVIATION WEATHER HAZARDS - The Speed of Sound in the Atmosphere

A J Bedard Jr, National Oceanic and Atmospheric Administration, Boulder, CO, USA

Introduction

A broad range of aviation weather hazards affect operations from takeoff and landing to in-route flight at high altitudes. A goal here is to provide an overview of key meteorological processes disrupting flight, reducing lift, increasing drag, influencing instrument readings, or reducing visibility. These atmospheric factors include gravity currents including thunder-storm gust fronts and sea breeze fronts. Because the motions and decay of aircraft wake vortices are controlled by local winds, turbulence, and stability, these dangerous wake effects are also discussed.

Hazard types covered appear in Table 1, together with brief descriptions of their potential impacts on flight. Figure 1 summarizes some of these meteoro-logical disturbances, indicating typical flow strengths and altitudes affected. Encounters with turbulence aloft can disrupt flight paths and cause injuries to crews and passengers. Such strong encounters can result from organized instabilities of limited duration or extent, such as breaking gravity/shear waves. On the other hand, more random turbulence aloft, when of long duration and covering extended areas, can contribute to structural fatigue and reduce aircraft operating lifetimes.

For selected hazards, atmospheric causal processes are reviewed and key properties such as dimensions

and strengths are summarized. Also, discussions of efforts at hazard prediction, detection, and warning illustrate the progress that has been made in mitigating atmospheric impacts on the aviation system.

Atmospheric Gravity Currents

One important type of atmospheric gravity current is the outflow from a distant thunderstorm downdraft.

For these systems, because the downdraft region is usually quite large in diameter (typically 10 km or more), the wind shears near the downdraft can be relatively weak. However, the wind speed and wind direction change that accompany the leading edge can cause significant relative air speed changes for aircraft.

Table 1 A summary of aviation weather hazards and their areas of impact upon flight operations

Hazard Areas of impact

Atmospheric gravity current wind shears (e.g., thunderstorm gust front and sea breeze front)

$ Relative air speed changes

$ Require runway changes

Microburst wind shears $ Flows can exceed performance capabilities of modern

aircraft

Vertical wind shear $ Deviations from glide slopes

Gravity/shear waves $ Flight disruption and structural fatigue

Icing $ Increased drag and reduced lift

$ Reduced stall angle

$ Flight disruption

Terrain-induced disturbances (e.g., lee waves, rotors, bora) $ Deviations from assigned flight altitudes

$ Structural damage

$ Flows can exceed performance capabilities of modern aircraft

Vicinity of thunderstorms (e.g., hail, funnels, obstacle flows) $ Deviations from flight altitudes

$ Structural damage

Aircraft wake vortices transported to unexpected locations $ Roll moments disrupting flight of following aircraft

Altimeter errors $ Deviations from assigned or expected flight altitudes

Mountain waves

Inversion/shear Density

currents

Downdraft

Free atmospheric shear layers Zonal flows

Dust devil Bora

100 5

Height (km)

Wind speed (m s⁻¹)

Figure 1 Summary of the altitude impact ranges of meteorolog-ical hazards and typmeteorolog-ical wind speed strengths involved.

166 AVIATION WEATHER HAZARDS

Sudden aircraft performance changes caused by the atmosphere (whether increasing or decreasing per-formance) are problematical.Figure 2is a conceptual view of a gust front from a distant thunderstorm crossing an airport. Although at times these bounda-ries are clearly visible because of entrained dust, more often the boundary will occur invisibly in clear air at large distances from the originating thunderstorm.

The speed of motion,c, of a gravity current with no ambient wind can be estimated from the density current equation [1].

c¼Fr DT T gh

!1=2

½1"

Here, Fr is the Froude number (B1), DT is the temperature change in the gust front air relative to the environmental air,Tis the mean temperature,gis local gravity, andhis the height of the outflow boundary.

Corrections for the ambient wind can be made.Table 2 summarizes statistics for gust fronts measured in the Denver, Colorado, region during an intensive field program during the summer of 1982. Data from this and other experiments demonstrate that the density current equation applies quite well. The leading edge is usually accompanied by a temperature drop and pressure rise, unless complicated by the existence of a ground based inversion layer. Figure 3 shows an acoustic sounder display of a gust front propagating on top of a ground-based inversion.

Another important aspect of thunderstorm outflow boundaries is that a preferred region for the initiation of new convection is near the leading edges where

vertical forcing of ambient air occurs. This is especially true where boundaries collide. Microbursts also have a tendency to occur near outflow boundary regions.

Fortunately, Doppler radar can detect these gust front boundaries effectively and provide an invaluable, all-weather resource to guide airport operations when gravity currents are approaching.

Microbursts

Between 1964 and 1985, over 30 commercial aircraft crashes resulted from microbursts. A microburst is defined as a downdraft region with a scale size less than 4 km. The resulting strong outflows usually do not travel radially outward for long distances (410 km), and the durations are short (often less than 10 min). However, in the vicinity of a microburst,

Table 2 Gust front statistics derived from surface meteorological stations in the Denver, Colorado region during the summer of 1982.

Number of events599

Parameter Average Minimum

value

Maximum value

Wind speed change 9.4 m s^!¹ 3.0 17.0 Wind vector change 13.7 m s^!1 5.0 37.0 Temperature change !1.91C !5.1 13.1^a

Pressure change 0.67 hPa !0.4^a 3.3

Rain rate 24 mm h^!1 0 3.0

aThese anomalous readings of temperature increase and pressure decrease resulted from the passage of a gust front on top of and eroding a ground-based inversion.

Inflow

Outflow

20 km Density current

3 km Runway

Thunderstorm

Figure 2 Conceptual view of a gust front from a distant thunderstorm crossing an airport.

AVIATION WEATHER HAZARDS 167

strong winds (450 m s^!¹) and rapid wind direction changes of 1801can occur. When microbursts descend near or on runways, they constitute an extreme flight hazard. Microburst flows are analogous to those produced when squirting a water hose on a flat surface. The downflow jet interacts strongly with the surface, producing strong radially directed flows. The large spatially concentrated horizontal wind vector changes and the downdraft can produce increasing performance/decreasing performance couplets that are difficult to predict and handle. For example, an aircraft flying through a microburst that has impacted the approach end of a runway will first encounter a head wind, increasing performance and causing ex-cursions above the glide slope. As the pilot corrects for this, the aircraft enters the downdraft region, followed by an outflow region, rapidly degrading performance.

Depending upon the timing and relative positions of the aircraft and microburst to the runway, this scenario can be catastrophic. The timing is so critical that even landing differences of several minutes can be important. The statistics of microbursts measured in the vicinity of Denver, Colorado, are presented in Table 3. Microbursts were identified by the winds clearly radiating outward from a center, as distinct from the essentially linear gust front winds. To date, the largest wind speed documented for a microburst was that related to a ‘near miss’ of Air Force One with President Reagan on board when it was on the ground near Washington, DC, on 1 August 1983 (a wind speed surge over 60 m s^!1). The microburst occurred five minutes after the plane landed. Newspaper accounts

said that a secret service officer jumped on top of the president to protect him as the winds buffeted the aircraft.

There are two extreme types of microbursts: ‘dry’

and ‘wet’. Dry microbursts are especially hazardous because the visible virga (raindrops or a snow plume descending from cloud base as in the photograph in Figure 4) related to the microburst initiation process evaporates and becomes invisible as it approaches the surface. Since the downdraft descent time takes about 5 min, it can be difficult to relate an observation of virga to a resulting microburst. Conversely, a wet microburst has a strong rain shaft and is easily seen if not obscured by rain from a surrounding storm (Figure 5).

Doppler radar can detect microbursts once the radial outflow is established by the intense downdraft penetrating to the surface. Also, the concentrated rain shafts for wet microbursts can be detected. The

2100 2200

Height (km)

Time (hours EDT)

Figure 3 Acoustic sounder detection of a gust front propagating on a ground-based inversion.

Table 3 Impacting microburst statistics derived from surface meteorological stations in the Denver, Colorado region during the summer of 1982. Number of events533

Parameter Average Minimum

value

Maximum value

Wind speed change 13.5 m s^!1 2.5 27.5 Wind vector change 20.7 m s^!1 10.0 37.5 Temperature change !1.51C !9.0 15^a

Pressure change 0.66 hPa !1.5^a 2.0

Dew point change !71C 171C

Rain rate 16.4 mm h^!1 0 2.75

168 AVIATION WEATHER HAZARDS

uses of arrays of airport wind sensors, Doppler radars, and improved controller/pilot training (both to recog-nize visual clues and to respond in the best possible way if a microburst is encountered) have helped to reduce microburst-related accidents. Also, the fact that the lapse rate between 500 and 700 hPa is correlated with microburst probability provides forecasting potential for dry microburst likelihood. Dry microbursts are more probable when the lapse rate is481C/km.

Vertical Wind Shear and Gravity/Shear Waves

Whereas the thunderstorm gust front and microburst hazards result primarily from horizontal changes in

wind speed, vertical changes in wind speed and direction can also present a hazard, especially for lower-level flight operations. A ground-based inver-sion is often accompanied by calm winds near the surface and strong winds just above the cooler, stable near-surface air. Aircraft descending or ascending through such layers can encounter strong wind shear-produced performance changes and turbulence, and rapid fluctuations associated with gravity/shear waves. These waves have scale sizes from tens to hundreds of meters, resulting in aircraft interaction times of seconds or less.Figure 6is a conceptual view of such a situation in the vicinity of mountains.Figure 7 is a Doppler lidar display of gravity/shear waves.

Vertical wind shear conditions can be especially

Figure 4 Photograph showing virga descending from cloud base.

Figure 5 Photograph of a ‘wet’ downdraft.

AVIATION WEATHER HAZARDS 169

important for general aviation airports if relative airspeed is suddenly reduced on a low-level approach or during takeoff in the vicinity of terrain. At airports where wind shear above stable air is a frequent problem, boundary layer wind profilers or acoustic sounders can provide valuable real-time monitoring capabilities.

The presence of gravity/shear wave activity often complicates flight through layers of vertical wind shear. A pure shear layer in a neutrally buoyant atmosphere may be modeled as a vortex sheet, highly unstable to disturbances. If wind shear occurs in conjunction with a stable layer, gravity provides a restoring force, and such a system will support wave motion. Hence, the term gravity shear wave. Several questions naturally follow from this situation.

$ Under what conditions will the upper-level wind shear start to erode the stable air and turbulence grow?

$ At what rates do such stable pools of air get removed?

$ What are some examples of situations where these processes are important for flight operations?

The Froude number,Fr, is a measure of the relative importance of inertial and gravity forces. For the situation of flow above a stable layer,Fr is given by eqn [2], whereUis the flow speed above the inversion, gis local gravity,his the height of the inversion, dris the density difference between the two layers, andris the mean density.

Fr¼ U

drgh r

!1=2 ½2"

In field and laboratory experiments, the start of disturbances and waves takes place when the Froude number exceeds about 0.6. Thus, if the height of the inversion and a temperature profile are available, the threshold speedUcan be estimated. Once the erosion process starts, it can often continue at a slow and approximately constant rate. Values of vertical ero-sion rates near 10 cm s^!¹ have been measured near complex terrain. On the positive side, the time scales are of the order of hours for changes, in contrast with minutes for microbursts. Thus, vertical wind profilers for monitoring and knowledge of local climatologies can be quite valuable, particularly for mountain valleys and the lee sides of complex terrain.

Another important dimensionless number is the Richardson number,Ri, which is an important index

Altitude

Wind speed Upper level jet

Height of maximum wind speed

Turbulent air motion

Figure 6 Conceptual view of gravity/shear waves in the vicinity of mountains.

+ + + + + +

2.0

1 11.2 7.5

−3.8 3.8

−7.5

−11.2 0.0

10.0 6.0

2.0

Figure 7 Doppler lidar display of gravity/shear waves in the vicinity of mountains, showing the accompanying wind speed changes. The numbers below the color bar are the radial wind speeds in meters per second. The numbers above the color bar are the distance in kilometers from the lidar. (Courtesy L. Darby, NOAA.)

170 AVIATION WEATHER HAZARDS

for turbulence. This number depends upon the gradi-ents of both temperature and wind speed (eqn [3]).

Ri¼g y

dy dZ

dU dZ

½3"

In eqn [3],gis local gravity,y is the mean potential temperature, dy=dZis the change in potential temper-ature with height, and dU=dZis the change of wind speed with height. The criterion for turbulence, Rio1=4, has been shown to be a valuable index for aircraft turbulence when temperature and wind speed profiles are available to make the comparison between predicted turbulent altitudes and actual turbulence reports from pilots. A challenge is either to predict the temperature and wind speed gradients or to measure them with sufficient accuracy to produce reasonable estimates ofRivalues. Wind profiling radars used in conjunction with the Radio Acoustic Sounding System (RASS) to obtain temperature profiles will be valuable for such applications. Pilot reports as well as the use of visual clues are also an invaluable component for avoiding regions of turbulence aloft.Figure 8depicts gravity/shear waves that are revealed by cloud forma-tions.

Large-amplitude, long-lived gravity waves repre-sent another aspect of the hazard. Such waves have been observed to propagate rapidly (at 35–40 m s^!¹) away from the region of a cyclone where they were generated. The waves in this case traveled through eastern New England in the United States. The high-speed waves were accompanied by precipitation and wind surges. For example, at Boston the wind increased from less than 10 knots to 57 knots over

about 5 min. Such discontinuities can represent sig-nificant hazards, since they are unexpected weather features producing wind vector changes on scales that can affect flight, especially during takeoff and landing.

Icing

Icing potential depends upon the probability of drops of supercooled liquid impacting aircraft surfaces.

Larger droplets are more likely to strike an airfoil, since they do not easily follow the flow streamlines and pass around an obstacle as do smaller droplets. The icing hazard can be insidious because of two factors:

1. Only a small amount of ice deposition can have large, deleterious effects upon lift and drag, thus reducing aircraft performance.

2. Icing and the degradation of performance can increase slowly and imperceptibly, until an emer-gency exists.

Two situations account for most reported in-flight icing encounters. Convective activity involves rela-tively large amounts of supercooled liquid water, and the hazard can extend to higher altitudes with the potential of significant ice accumulation in a short time.

The other frequent situation involves flight through layered cloud decks. Although the supercooled liquid content can be lower than in convective situations, aircraft typically spend more time in these layered clouds, accumulating significant ice on fuselage and control surfaces. Figure 9depicts situations contrib-uting to this form of icing, which is especially

Figure 8 Photograph showing gravity/shear waves visualized by cloud formations.

AVIATION WEATHER HAZARDS 171

important for commuter and general aviation aircraft operating at lower altitudes. The wind shears and turbulence often accompanying such systems can be regions where larger drops are concentrated by mix-ing. Sometimes, by making small altitude changes, a plane can avoid these regions of enhanced icing potential.

Combinations of remote sensors have value for the monitoring and forecasting of icing situations. Polari-metric radars, dual-wavelength radars, wind profilers, RASS, and passive radiometric measurements can, in concert, indicate the locations of supercooled liquid.

Aircraft can also accumulate icing on the ground in

freezing rainstorms. This problem is treated at major airports by application of de-icing fluids before takeoff (seeAircraft Icing).

Terrain-induced Turbulence

Terrain effects can occur at all flight levels, with some disturbances affecting the stratosphere. The flow situations can range from lee waves, bora flows (a form of density current), and rotors, to mechani-cally induced turbulence.Figure 10is a Doppler lidar display showing the roll-up of a vortex sheet in the

SLW

Barrier forcing Convergence

SLW SLW

Cool air

Horizontal winds

Surface

fog SLW

SLW Frontal forcing

Radiative cooling

Shear enhancement of large water droplet sizes

by aggregation

Advection Figure 9 A summary of situations contributing to aircraft icing events. SLW stands for supercooled liquid water.

172 AVIATION WEATHER HAZARDS

Colorado Springs area. At times, organized instabil-ities can occur in the forms of vertical or horizontal axis vortices. These obstacle-involved situations can be exceptionally complex when the terrain flows interact with other meteorological factors (such as lee-side inversions). Since 1964, there have been 15 major accidents and incidents in the vicinity of complex terrain (Table 4).

One study indicated that the general aviation accident rate was 40% higher for US mountain states than for all other continental states, and the rate was

150% higher for a selected group of mountain airports relative to a group of nonmountain airports.

Table 4indicates a pattern of sporadic encounters of aircraft with severe or extreme turbulence in the vicinity of mountains. In many cases, aircraft preced-ing or followpreced-ing the aircraft involved in the event encountered some turbulence, but not the extreme turbulence of the encounters (which often exceeded structural limits). Thus, the regions of severe or extreme turbulence may, at times, be spatially con-centrated and short-lived. This makes predicting the

2.0

Radial velocity 9 m s⁻¹ 2.0

−18.0 −12.0 −12.0 0.0 6.0 12.0 18.0

−11.7 −7.8 −3.9 0.0 3.9 7.8 11.7

6.0 10.0 14.0 18.0

2.0 6.0 10.0 14.0 18.0

+ +

Figure 10 Doppler lidar display showing the rollup of a vortex sheet in the lee of a mountain range. The numbers below the color bar are the radial wind speeds in meters per second. The numbers above the color bar are the distance in kilometers from the lidar. (Courtesy L.

Darby, NOAA.)

Table 4 Turbulence-related accidents and incidents occurring in the vicinity of mountains

Event Date Location Comment

Accident 31 Mar. 1993 Anchorage, Alaska Turbulence, 747 lost engine

Accident 22 Dec. 1992 West of Denver, Colorado Loss of wing section and tail assembly, 2-engine cargo plane, lee waves present

Accident 9 Dec. 1992 West of Denver, Colorado DC-8 cargo plane, loss of engine, lee waves present Unknown cause accident 3 Mar. 1991 Colorado Springs, Colorado 737 crash

Accident 12 Apr. 1990 Vacroy Island, Norway DC-6 Crash

Severe turbulence 24 Mar. 1988 Cimarron, New Mexico 767, 1.7G, Mountain wave

Severe turbulence 22 Jan. 1985 Over Greenland 747,12.7G

Severe turbulence 24 Jan. 1984 West of Boulder, Colorado Saberliner,10.4G to!0.4G Severe turbulence 16 Jul. 1982 Norton, Wyoming DC-10,11.6G to!0.6G

Severe turbulence 3 Nov. 1975 Calgary, Canada DC-10,11.6G

Accident 2 Dec. 1968 Pedro Bay, AK Fairchild F27B, wind rotor suspected

Accident 6 Aug. 1966 Falls City, Nebraska BAC 111, wind rotor suspected Accident 5 Mar. 1966 Near Mt. Fuji, Japan BOAC 707, wind rotor suspected

Accident 1 Mar. 1964 Near Lake Tahoe, Utah Paradise Air Constellation, strong lee wave Accident 10 Jan. 1964 East of Sangre de Cristo

mountains in New Mexico

B52, wind rotor suspected

AVIATION WEATHER HAZARDS 173

time and location of these events more difficult. There is a great need to define the properties of mountain-related hazards, improve short-term forecasting of these events, improve pilot training resources, and develop detection methods.

Major field experiments have addressed this prob-lem, which requires three-dimensional sampling of large volumes of the atmosphere as a function of time, documenting both surface and near-surface effects, as well as upper tropospheric and stratospheric effects.

Both physical scale models and numerical models have

guided the execution of field programs studying mountain flows, which require the commitment of considerable scientific and measurement resources.

Figure 11shows an example of the changes in flows encountered by a research aircraft traversing a lee wave in the Rocky Mountain region of the US. Similar strong changes were also encountered in temperature, pressure, and vertical wind speed, with great differ-ences as a function of horizontal distance and flight level. Doppler lidars have the ability to detect these hazards near the surface in clear air. Figure 12

40 23 6

1813−1820 (GMT)

1828−1834 (GMT)

1849−1857 (GMT) 1730−1737 (GMT)

W E

Horizontal speed (m s⁻¹)

Boulder BAO

0 10 20 30

10 20 30 40 50 0

km miles 45000

40000

30000 35000

20000 25000

10000 5000 15000

150

200

300

400

500 600 700 850

Pressure(hPa)

Height (feet)

Figure 11 Horizontal wind speed changes encountered by a research aircraft in a lee wave situation on 25 January 1984. Data from flight legs at four altitudes are shown.

6.0 + + + +

+ + + +

+ + + + 2.0

6.0

2.0

Elevated rotor Elevated rotor

−10.0 −6.0 −2.0 2.0 6.0 10.0

9.0 6.0

3.0

−3.0 0.0

−6.0

−9.0

−10.0 −6.0 −2.0 2.0 6.0 10.0

9.0 6.0

3.0

−3.0 0.0

−6.0

−9.0

Figure 12 Doppler lidar display showing rotor circulation associated with lee waves. The numbers below the color bar are the radial wind speeds in meters per second. The numbers above the color bar are the distance in kilometers from the lidar. (Courtesy L. Darby, NOAA.) 174 AVIATION WEATHER HAZARDS

In document The Speed of Sound in the Atmosphere (sider 166-178)