18 Flight Hazards over High Ground

Transcription

1 18 Flight Hazards over High Ground meteorology 18.1 Mountain Effect on Fronts When a warm front passes a mountain range, the air, is lifted over the mountain and will strengthen the formation of cloud and precipitation on the windward side. Sometimes, the retreating cold air becomes trapped between the mountain and the frontal surface resulting in the warm front seeming to halt in front of the mountain, ( Stau in the Alps). On the leeward side the air is being adiabatically heated as it subsides, decreasing any precipitation and occasionally causing cloud dispersal - Föhn-opening, see fig. ME Fig. ME 18.1 Föhn opening Föhn-opening The warm front is arrested on the windward side, whereupon the upper warm front glides away and reforms beyond the mountain. East of the Scandinavian mountains the fronts sometimes remain dissolved all the way to the coast of northern Sweden. Not until well beyond the mountain does the frontal surface resume its original appearance with clouds and rain similar to the conditions prevailing before the mountain crossing. To the observer or meteorologist, it seems that the front has come to a halt and then dissolved. It then suddenly reappears out some distance beyond the mountain, causing forecasting problems. Note Flying weather deteriorates significantly on the windward side of the mountain, and cumulonimbus development is likely. If the air on the leeward side is cold and dense it may persist, so that the warm air glides away as an upper air mass and an upper front. Once again the meteorologist will find difficulties in forecasting, since the change of air mass will not appear in observations from the ground. A cold front lifted over a mountain is always strengthened. In the winter in Northern and Central Europe, the air behind the mountain may be colder at low level than the approaching cold air mass. The cold front then passes the top of the mountain and continues as an upper cold front (e.g. with the warm front). Flight Hazards over High Ground 18-1 Meteorology E5 - Proof 2.indb :21:46

2 meteorology Effect of Mountains on Cold Front A cold front passing over a mountain: Fig ME shows the coldest air on the leeward side and fig. ME 18.3 shows the coldest air on the windward side Flying in Mountainous Areas Flying in the vicinity of mountains provides a new set of challenges to the pilot. He will be faced with the various effects of the mountains impeding the smooth flow of the atmosphere. Cold Colder Fig. ME 18.2 Developing upper cold front Coldest Mountain Waves In the chapter about wind we introduced the typical wind pattern behind a mountain range in a stable atmosphere. There are of course always waves in the atmosphere above a mountain when the air is in motion, but the typical mountain wave disturbance is generated if: The wind blows within 30 of a right angle to the mountain range The wind speed abeam the top of the mountain exceeds 20 kt. The wind speed increases with altitude but direction remains fairly constant The atmosphere is stable (preferably stratified in several thin stable layers) and especially where an inversion is present just above the level of the mountain top Fig. ME 18.3 Strengthened cold front If the coldest air is found behind the cold front, the extra speed given to the cold air downhill may strengthen the cold front some distance behind the mountain, where the warmer air is pushed up, forming a CB cloud. These waves are known as mountain waves, lee waves or standing waves, the latter name because the wave keeps its position in the sky while the air passes through it, see fig. ME Larger wind angles than 30 have much less effect at distance from the mountain, but around the mountain disturbances may still be considerable. We can draw the conclusion that strong winds over a mountain generate both waves aloft, down- and up-draughts on the leeward 18-2 Flight Hazards over High Ground Meteorology E5 - Proof 2.indb :21:46

3 30000 Wind speed Tropopause meteorology Wind CCSL Height (ft) Temp ACSL Lee wave region Stable layer Cap cloud Roll cloud Temperature Main downdraft Main up draft Lower turbulent zone ACSL = Alto Cumulus Standing Lenticular CCSL = Cirro Cumulus Standing Lenticular The Lee wave system. Airflow through the lee waves is indicated by thin solid lines with arrows. Lower turbulent zone is indicated by the darker shading at the bottom of the diagram. Characteristic lenticular ACSL, CCSL, cap, and roll clouds are labelled. Temperature and wind speed soundings taken just upstream of the ridge are shown on the left. Note stable layer just above the mountain top. Fig. ME 18.4 Mountain waves side and the risk of rotors. These features are extremely important from the flightsafety point of view. Cap Cloud Ascending motion on the windward side and the pressure fall at the top and lee side of the mountain intensify condensation and a cloud cap, hiding the top of elevated terrain, appears over the crest of the ridge. On the leeward side the droplets evaporate in the descending air and the temperature increases due to the compression. This is often indicated by a clear line directly downhill known as the föhn gap and the descending heated air is known as a föhn wind. If cap cloud is present, there is a great possibility that other wave disturbances are present as well. Downslope Storm The flow downhill; the föhn effect, is often much stronger than the flow uphill due to Flight Hazards over High Ground 18-3 Meteorology E5 - Proof 2.indb :21:47

4 meteorology the pressure changes around the mountain. Katabatic effect can further accelerate the flow. The area closest to the mountain becomes rather turbulent, but when the air rises up into the first wave the surface wind calms and if a rotor is present the flow can invert over a short distance. There is one more explanation to the föhn wind and this is usually the case when the downhill flow gets really severe. When the air is cold and stable the mountain range might block the low-level flow across the mountain. Wind from higher levels with higher wind speed is then diverted towards the lower levels and creates surface wind gusts of up to about 100 kt. After reaching the surface level the flow rises up into the first wave creating a very strong and extremely turbulent rotor circulation. In arid areas this rotor can be seen as a wall of sand and dust. This type of rotor seems to be situated a bit further downstream compared to the general rotor circulation in the area and it does not follow the undulations of the mountain range. The Chinook east of the Canadian part of Rocky Mountains is an example of this kind of wind Up-and Down-Draughts Mountain waves are characterised by two dimensions, wavelength and amplitude. Wavelength is known to be directly proportional to wind speed and inversely proportional to stability. Typical wavelength values are 2 to 13 NM extending 150 to 300 NM downwind from the mountain crest. The well known meteorologist and glider pilot Tom Bradbury publishes an empirical rule relating the wavelength to wind speed in the layers where the waves are formed: Wavelength (in miles) = 0.17 x wind speed (kt) kt = 1.8 miles 40 kt = 5.2 miles 60 kt = 8.6 miles 80 kt = 12.0 miles Wave amplitude (half the vertical distance from crest to trough) varies with height above the ground but the largest value is usually found at a level of about 5000 ft above the mountain crest, abating with height. Wave amplitude is strongly linked to the size of the mountain range, the slope of the lee side, general stability and wind speed. As a general rule, a shallow layer with great stability (an inversion) and moderate winds, will produce a larger wave amplitude than a deep layer of less stable air (close to the saturated adiabatic lapse rate) and high wind speeds. It is common to generalise the lee slope profiles into three categories. A gradual slope produces a shallow wave, that is a wave with long wavelength and small amplitude A steeper slope results in a shorter wavelength with steep amplitude A cliff on the lee-side can result in a flow separation and an eddy close to the mountain. The eddy then moves away from the mountain and is replaced by a new one Flight Hazards over High Ground Meteorology E5 - Proof 2.indb :21:47

5 Mountain waves create up and down draughts throughout the troposphere but these waves are usually rather smooth with non or only light turbulence. Aircraft flying at high Mach numbers might however overspeed when entering the down draught at the first wave on the lee-side. This might take the crew by surprise. Significant turbulence can also occur. If the air is humid enough, condensation will occur on the rising part of a wave. On the descending part evaporation will occur. The result of this is the lens-shaped cloud known as lenticular or altocumulus lenticularis. These clouds are often easy to see on satellite pictures as wake waves behind mountain ranges or behind a single mountain peak as an interference pattern, see fig. ME Lenticular cloud is often presented in bands stacked in several layers that fade away downwind from the mountain. Since the clouds are constantly fed with new droplets and the time spent inside the cloud is limited before droplets evaporate again, the cloud will consist of supercooled droplets down to very low temperatures (-40 C). This results in significant icing occuring outside the normal temperature range for altocumulus cloud, ( C), but limited vertical and horizontal extents makes the cloud easy to avoid. Lenticular cloud is indicative of up- and down draughts, rotors and high level turbulence in the region. Large sheets of cirrus cloud occur now and then in connection with mountain wave activity. This type of cloud sheet may show no or only small undulations and can after a few hours extend hundreds of miles downwind from the mountain crest Rotor The rotor phenomena can occur in the lower turbulent zone that stretches downwind from the mountain up to a height of about 1 to 2000 ft above the mountain top. It is usually the first rotor situated below and generated by the first wave that is the most intense, with severe to extreme turbulence. The rotor centre is situated at about the same height as the mountain top. meteorology Fig. ME 18.5 MTW over the Alps If a closed circulation is present this will usually show up as a wind reversal at the surface compared to the wind at height. A rotor cloud at the top of the rotor often indicates the presence of a rotor. This looks like either a line of small cumulus close together or sometimes stratus fractus. Flight Hazards over High Ground 18-5 Meteorology E5 - Proof 2.indb :21:47

6 meteorology In arid areas dust might whirl up on the upwind side of the rotor side (closest to the mountain). ROTORS MUST ALWAYS BE AVOIDED Clear Air Turbulence Around Mountains (CAT) or Wave Induced Turbulence (WIT) Conditions in mountainous areas are very difficult to forecast generally, since local effects vary, and mountain waves may even occur behind highlands that may not be expected to be high enough. The mountain waves at altitude also cause turbulence sometimes, mostly in clear air (there may be altocumulus lenticularis) and is therefore called clear air turbulence or CAT. A more correct, but little used, term is wave-induced turbulence or WIT, see fig. ME Under conditions of a stable atmosphere with significant vertical wind shear (>5kt/1000 feet), waves may be rapidly amplified and then subdued like the waves at sea. The official name of this process is shearing gravity waves and will be experienced as very turbulent conditions known as CAT or WIT. In principle it can be said that an up draught is generated by the windward side of the mountain, and a down draught is generated by the lee of the mountain. These up and down draughts will displace the aircraft vertically and cause bumpiness at high speed. This accounts for the common presence of turbulence around the tropopause where there is a stable layer often with a marked vertical wind shear. If a jet core crosses the mountain just below the tropopause, there is a very great risk of CAT about 5000 ft above and below the tropopause. Tropopause Max jet wind CAT CAT Calm Wind shear Fig. ME 18.6 CAT and wave induced turbulence with low level wind shear 18-6 Flight Hazards over High Ground Meteorology E5 - Proof 2.indb :21:47