AC 00-57 Hazardous Mountain Winds And Their Visual Indicators U.S. DEPARTMENT OF TRANSPORTATION Federal Aviation Administration Office of Communications, Navigation, and Surveillance Systems Washington, D.C.
This advisory circular (AC) contains information on hazardous mountain winds and their effects on flight operations near mountainous regions. The primary purpose of this AC is to assist pilots involved in aviation operations to diagnose the potential for severe wind events in the vicinity of mountainous areas and to provide information on preflight planning techniques and in-flight evaluation strategies for avoiding destructive turbulence and loss of aircraft control. Additionally, pilots and others who must deal with weather phenomena in aviation operations also will benefit from the information contained in this AC.
Comments regarding this publication should be directed to the Department of Transportation, Federal Aviation Administration, Flight Standards Service, Technical Programs Division, 800 Independence Avenue, S.W.
Washington, DC 20591.
Pilots can review the photographs and section summaries to learn about and recognize common indicators of wind motion in the atmosphere. The photographs show physical processes and provide visual clues. The summaries cover the technical and “wonder” aspects of why certain things occur - what caused it? How does it affect pre-flight and in-flight decisions? The physical aspects are covered more in-depth through the text.
Acknowledgments Thomas Q. Camey Purdue University, Department of Aviation Technology and Consultant in Aviation Operations and Applied Meteorology A. J. Bedard, Jr. National Oceanic and Atmospheric Administration Environmental Technology Laboratory John M. Brown National Oceanic and Atmospheric Administration Forecast Systems Laboratory John McGinley National Oceanic and Atmospheric Administration Forecast Systems Laboratory Tenny Lindholm National Center for Atmospheric Research Research Applications Program Michael J. Kraus National Oceanic and Atmospheric Administration Forecast Systems Laboratory Cover photo: over Boulder, Colorado, winter 1988, 0 A.J. Bedard, Jr.
FOREWORDv
LIST OF FIGURES/TABLESix
PART I. REVIEW OF METEOROLOGICAL CONCEPTS1
1.0
2.0
3.0
4.0
5.0
INTRODUCTION1
ACClDENT STATISTICS3
THE EFFECTS OF OROGRAPHIC WINDS AND TURBULENCE
ON AVIATION OPERATIONS4
3.1 High-Altitude Operations4
3.2 Takeoff and Landing5
3.3 Lmdevel Mountain Flying5
SOURCES OF MOUNTAIN-INDUCED WIND HAZARDS FOR
AVIATION6
4. I A Review of Key Meteorological Concepts6
4.2 A Review of Static Stability and Stable/Unstable Atmospheric
Stratifications8
4.2.1 Summary Comments on Stability10
4.3 Elementary Theory of Gravity Waves and
Shear-Induced Waves11
4.3.1 Summary Comments on Gravity Waves and
Shear-Induced Waves14
4.4 Breaking Waves and Turbulence14
ATMOSPHERIC DISTURBANCES IN MOUNTAINOUS
AREAS15
5.1 Larger-Scale Hazarcls15
5.1.1 Vertically Propagating Mountain Waves17
5.1.1.1 Forecast and Observed Data21
5.1.1.2 Charts22
5.1.1.3 Other Assistance23
5.1.1.4 Summary Comments on Vertically
Propagating Mountain Waves24
5.1.2 Trapped Lee Waves24
5.1.2.1 Forecast and Observed Data30
5.1.2.2 Summary Comments on Trapped Lee Waves.. .......................................................... .3 I
5.1.3 Persistent Horizontal Roll Vortices (Rotors)32
5.1.3.1 Summary Comments on Horizontal
Roll Vortices34
5.1.4 Kelvin-Helmholtz Waves34
5.1.4.1 Summary Comments on
Kelvin-Helmholtz Waves38
5.2 Smaller-Scale Hazards38
5.2.1 Lee-Side Inversion With Shear Flow (Mountain-Induced
Shear With No Wave Development)38
5.2.1.1 Summary Comments on Lee-Side
Inversions with Shear39
5.2.2 Non-Steady Horizontal Roll Vortices (Moving Horizontal
Vortices)41
5.2.2.1 Summary Comments on Moving Horizontal
Vortices45
5.2.3 Intense Vertical-Axis Vortices46
5.2.3.1 Summary Comments on Vertical-Axis
Mountain Vortices47
5.2.4 Bor as47
5.2.4.1 Summary Comments on Boras48
5.2.5 Other Phenomena48
5.2.5.1 Summary Comments on Other
Phenomena49
PART II. ATLAS OF VISUAL INDICATORS51
6.0 VISUAL INDICATORS OF OROGRAPHIC WIND FIELDS ................................................................................. .5 1
6. I Larger-Scale Features.. ............................................................. 5 1
6.2 Snder-Scale Flows55
6.3 Rotors and Other Turbulent Zones58
6.4 Interpreting Contrails66
6.5 Other Visual Indications of Air Motion
Near Complex Terrain73
vii
PART III. SUMMARY83
7.0 REVIEW OF MAJOR CONCEPTS83
GLOSSARY OF KEY TERMS87
REFERENCES90
VIII
Figure l-l.
Figure 1 - 1. States with general aviation accident rates over 3.0 per 100,000 operations, Fiscal Year 1992.
Table 2-l.
Turbulence-related accidents and incidents occurring in the vicinity of mountains.
Figure 4-1.
Example of a large-scale atmospheric wave pattern as seen on a National Weather Service constant pressure chart (500 mb).
The solid lines are approximately parallel to the wind flow at this level. Rawinsonde observations are plotted. This example happens to be a few hours before a DC-8 experienced engine separation west of Denver, Colorado (see Table 2-l).
Figures 4-Za-c.
Determination of atmospheric stability:
(a) unstable case; (b) neutral case;
(c) stable case.
Figure 4-3.
Oscillations associated with a gravity wave.
Figure 4-4.
Growth and breakdown of waves induced by vertical wind shear in a stable layer of the atmosphere.
Figure 4-Sa-b.
Clouds associated with Kelvin-Helmholtz waves over Laramie, Wyoming (photograph 0, B. Martner).
Figure 5-l.
Schematic of a vertically propagating mountain wave (after Durran and Klemp, 1983).
Figure S-2.
Schematic showing locations of jump and wave breaking region (after Durran and Klemp, 1983).
Figure 5-3.
Aircraft flight tracks and turbulence encounters associated with a wave-induced high-wind event (taken from Lilly, 1978).
Figure 5-4.
Schematic of the strong shear zone associated with a hydraulic jump in a mountain wave.
ix
Figure 5-5. - The Foehn cloud and rotor clouds associated with a jump at the downstream edge of a region of strong downslope winds near Boulder, Colorado (photograph 0, 199 1, R. Holle).
Figure 5-6.
Computer simulation of trapped lee waves behind a 300-meter-high mountain.
Figure S-7.
Lenticular clouds associated with a trapped lee wave (after Durran and Klemp).
Figure S-8.
Clouds associated with a trapped lee wave (photograph 0, 1988, R. Holle).
Figure 5-9.
Conceptual view of a mountain lee wave rotor zone (1993, A.J. Bedard, Jr.).
Figure 5-10.
View from aloft of a wave cloud and associated rotor (photograph 0, NCAR).
Figure 5-11.
Clouds associated with gravity-shear (Kelvin-Helmholtz) waves (photograph 0, 1985, P. Neiman).
Figure 5-12.
Clouds associated with gravity-shear (Kelvin-Helmholtz) waves (phptograph 0, 1990, A.J. Bcdard, Jr.).
Figures 5-13a-c.
A developing upslope flow associated with an approaching cold frontal boundary (photographs 0, 1990, A.J. Bedard, Jr.).
Figure 5-14.
Development of a strong roll vortex associated with a wind surge down the lee slope of a mountain (1993, A.J. Bedard, Jr.).
Figure 5-15.
Strong horizontal vortex (photograph 0, 1984, E. Richter).
Figure 5-16.
Schematic of vertically oriented vortices generated in the lee of an isolated mountain peak.
Figures 6-la-b.
An isolated lenticular cloud near Pikes Peak, Colorado (photograph 0, 1990, R. Holle).
Figure 6-2.
A wave cloud over Laramie, Wyoming (photograph 0, B. Martner).
Figures 6-3a-b.
A wave cloud over Nederland, Colorado (photograph 0, 1993, P. Neiman).
Figure 6-4.
A wave cloud oriented parallel to the upper-level flow looking west from Dillon, Colorado (photograph 0, R. Reinking).
Figures 6-5a-b.
Circular lenticular clouds produced by a pair of eddies in the lee of an isolated mountain peak near Nederland, Colorado (photograph 0, 1990, P. Neiman).
Figure 6-6.
A three-dimensional lenticular cloud that has developed in the strong flow around Mt. McKinley, Alaska (photograph 0, 1981, B. Martner).
Figure 6-7.
A field of circular lenticular clouds that have developed in the complex flow around a number of mountain peaks in Mt. McKinley National Park, Alaska (photograph 0, 1979, B. Martner).
Figure 6-8.
A view of three-dimensional lenticular clouds taken from an aircraft (photograph 0, NCAR).
Figure 6-9.
A view of three-dimensional lenticular clouds north of Boulder, Colorado, taken from an aircraft (photograph 0, 1988, S. Holle).
Figure 6-10.
A lenticular cloud over Boulder, Colorado, at sunset (photograph 0, B. Martner).
Figures 6-lla-b.
Clouds associated with vertically suppressed trapped lee waves at Boulder, Colorado (photograph 0, 1987, P. Neiman).
xi
Figures 6-12a-b.
Vertically enhanced clouds associated with a trapped lee wave at Boulder, Colorado (photograph 0, 1992, P. Neiman).
Figures 6-13a-b.
Rotor cloud near State College, Pennsylvania (photograph 0, 1985, P. Neiman).
Figure 6-14.
Schematic of the flow associated with a rotor zone in the lee of a mountain.
Figures 6-15a-b.
A group of rotor clouds over Boulder, Colorado, one of which has developed a helical structure (photograph 0, 1992, F.M. Ralph).
Figure 6-16.
A small vortex structure formed near a turbulent mixing zone (photograph 0, A.J. Bedard, Jr.).
Figure 6-17.
Turbulent cloud structures near the tropopause over Boulder, Colorado (photograph 0, 1993, P. Neiman).
Figure 6-18.
A linear contrail showing relatively smooth air (photograph 0, 1991, A.J. Bedard, Jr.).
Figure 6-19.
A contrail in the vicinity of mountain wave activity near Boulder, Colorado, showing areas of turbulence (photograph 0, 1988, R. Holle).
Figure 6-20.
A contrail showing a significant area of turbulence aloft (photograph 0, K. Langford).
Figure 6-21.
Contrail with turbulent zone, over Boulder, Colorado (photograph 0, F.M. Ralph).
Figure 6-22.
Contrail associated with lenticular and rotor clouds, showing very turbulent conditions aloft (photograph 0, K. Langford).
Figure 6-23.
Contrail located above rotor and lenticular clouds, indicating smooth conditions aloft (photograph 0, 1988, R. Holle).
xii
Figures 6-24a-b.
Smokestack plumes with an inversion layer in Boulder, Colorado (photograph 0, 1992, P. Neiman).
Figure 6-25.
Blowing snow near mountain peaks, indicating likely wave activity (photograph 0, R. Reinking).
Figure 6-26.
Low-level wind indicators on a lake surface (photograph 0, 1988, A.J. Bedard, Jr.).
Figure 6-27.
Wave cloud occurring above a layer of weak instability (photograph 0, NCAR).
Figure 6-28.
Cap cloud over Mt. Shasta, California, with low-lying weak convection (photograph 0, 1972, R. Reinking).
Figure 6-29.
Banner and cap clouds occurring in the Grand Tetons, Wyoming (photograph 0, B. Martner).
Figures 6-30a-c.
Cap cloud, or cloud associated with a bora (photograph 0, K. Langford).
Figure 6-31.
A Foehn wall near Boulder, Colorado (photograph 0, 1988, R. Holle).
Figure 6-32.
Cap clouds over Owens Valley, California (photograph 0, 1974, R. Reinking).
Figure 6-33.
Banner clouds and blowing snow (photograph 0, 1967, R. Reinking).
Figure 6-34.
Cap cloud over Mt. Rainier, Washington (photograph 0, K. Langford).
Figure 7-1.
Clouds associated with a complicated flow regime in the lee of Long’s Peak, Colorado (photograph 0, A.J. Bedard, Jr.).
1 .O INTRODUCTION Flight in the vicinity of mountainous terrain can be inspiring and immensely enjoyable for both pilots and passengers. However, this aspect of aviation also can present pilots with some of the most challenging and potentially dangerous situations encountered in air operations. Aircraft performance degradation because of high density altitudes, navigation problems associated with en route terrain obstructions, and rapidly changing weather patterns can cause difficulties for pilots of smaller aircraft operating at lower altitudes.
In addition, the crews of high performance turbine equipment must deal with high altitude turbulence as well as reductions in aircraft performance caused by density altitude conditions. All pilots who fly near mountainous terrain must deal with the potential for mountain-induced severe wind events, particularly during takeoff and landing. Although the effects of density altitude and high terrain are of great importance to all pilots who are operating in mountainous areas, our discussion here is limited to the hazardous effects of mountainous weather systems on aircraft operations..
The atmosphere is a fluid in motion. Just as the swiftly flowing water in a stream develops waves and eddies as it passes over and around obstructions, so does the atmosphere contain disturbances that develop as it interacts with mountainous terrain. These atmospheric eddies can range in size from a few centimeters to tens or hundreds of kilometers, and can present the pilot with relatively smooth air, or with turbulence of potentially destructive intensity, and the likelihood of loss of control. The mountain-induced flow fields we will discuss in this AC are frequently accompanied by visual indicators (such as lenticular and rotor clouds or blowing dust).
However, this is not always the case, and extremely severe wind events can occur with little or no visual warning of their presence.
The purpose of this AC is to assist pilots, and others involved in aviation operations, in diagnosing the potential for severe wind events in the vicinity of mountainous areas and to provide information on preflight planning techniques and in-flight evaluation strategies for avoiding destructive turbulence and loss of aircraft control. This AC can be used in several ways. For those readers who wish to obtain a more detailed understanding of the phenomena, the AC