Weather
Mid-level cloud streets and/or high level cloud sheets extending from the mountain ridges to the lee side. They develop when the wind stream is almost perpendicular to the mountain barrier.
There are two possibilities for air to flow past a mountain ridge. Either the air parcels go around the mountain or it is forced to rise over the mountain: Which of these cases occurs, is dependent on several parameters: The static stability, the height of the mountain or mountain ridge and the wind component perpendicular to the mountain. The term combining these parameters gives an idea whether there is a flow across the mountain or not: If U is small the mountain is difficult to cross; also if N increases the mountain will be more difficult to cross. But even in the case of air partially streaming around the mountain, those streamlines approaching a greater height will cross the mountains. When the wind has a component perpendicular to a mountain chain the air accumulation due to deceleration of wind speed by the obstacle on the upwind side creates high pressure. Part of the air is deflected upward giving rise to mountain waves. According to the theory of internal gravity waves, an air parcel within a stable stratified atmosphere will start to oscillate, as long as the waves are damped by friction. Waves can only occur in a stable atmosphere. An air parcel which is removed from its original place cannot oscillate if there is no returning force. Lee Cloudiness will form where there is a sufficient supply of humidity near the wave crests and where there is upward motion.
Upper Waves are cloud bulges at the rear edge of Cold Front cloud bands. They are associated with upper level processes and do not develop.
The term upper level refers to heights that are below the tropopause, but generally above 500 hPa. Wave developments in general take place within a Cold Front. According to the polar front theory a low pressure area in the lower levels of the troposphere can develop if a small-scale disturbance is superimposed on the stream field, causing an ageostrophic circulation within the baroclinic zone of the frontal cloud band. A Wave bulge is an indication of such a process. This circulation causes a fall in surface pressure, convergence, a maximum in temperature advection and an increase in cyclonic vorticity within the lower levels of the troposphere. However, in the physical background of an Upper Wave some processes can be distinguished from classical theory. The observed cloud bulge of an Upper Wave is a sign of a disturbance in the upper stream, but there is no corresponding and deepening pressure minimum at the surface. The dominating driving mechanisms in the upper levels can also be seen from a sharp and pronounced trough at 300 hPa leading to an increase of curvature vorticity. An additional reason for the release of relative vorticity is the right entrance region of the associated jet streak, supporting ageostrophic circulation at upper levels within the stream field.
Development of thicker and mostly more convective cloud within a frontal cloud band in the left exit region of a jet streak during frontal crossing.
If a jet streak approaches and crosses a frontal zone at a large acute angle, horizontal and vertical increase of frontal cloudiness in the left exit region of this crossing jet streak can be observed. These cloud phenomena are produced by the interaction between front and jet streak. The intensification of cloudiness within the left exit region is caused by the following processes: the frontal circulation (for instance an Ana Cold Front) causes lifting processes within the front; the cross circulation in the left exit region of a jet streak causes an intensification of the temperature gradient (frontogenesis); a PVA maximum in the left exit region contributes to upward motion.
Jet Fibres are long and narrow bands of high clouds caused by the Jet streak. They usually form directly attached to Cold Fronts or Warm Front Shields.
Jet Fibres, also known as cirrus streaks, are elongated patches of cirrus clouds, which move rapidly near the jet axis. There is no single theory explaining the formation of the Jet Fibres, but there are several mechanisms probably acting in combination. Generally, there are two necessary conditions needed for clouds to develop: sufficient humidity content and upward motion. In general, bands of cirrus clouds tend to form or persist on the anticyclonic side of the jet. The reason for this is that jet streams are associated with the strong thermal contrast across frontal zones and the layered clouds form in the warm moist air of the Warm Conveyor Belt with cloud edge lying along the jet core. This synoptic scale upward motion would bring a lot of humid air to upper levels and form a broad band of clouds. In order to produce a cloud structure of relatively small dimensions, such as Jet Fibres, there must be an additional smaller scale mechanism. Since a jet is necessarily involved in the formation of Jet Fibres, ascent may also occur because of the circulation in the cells around the entrance and the exit regions of the jet streak. Taking into account the vertical wind shear, directly above the jet core there is also upward motion on a smaller (turbulence) scale
During an Instant Occlusion process, a Comma approaches and merges with a frontal cloud band. As a consequence, an Occlusion-like form of a cloud spiral develops.
The name Instant Occlusion results from the resemblance of the mature stage of development of the cloud system with that of an Occlusion. But the meteorological background of the development is completely different from an Occlusion process where warm air is lifted from the surface. The process of the Instant Occlusion is characterized by the mergence of a distinct cold air, Comma-like cloud feature with a cloud band associated with the polar front (Cold Front). The adjective instant represents the fact that the Occlusion appearance is assumed suddenly without the usual development at a front. Consequently three different systems are involved in the process: comma, cold front and wave.
Orography has a strong influence on the propagation of frontal cloud bands; some parts can be retarded which might result in a delay or a decoupling of clouds at different heights. Also, fronts can be rapidly intensified or rapidly shifted.
Whilst passing a mountainous area a front can be modified by orography. Front delay is one possibility, decoupling of clouds at different heights is another. Two phenomena cannot be completely distinguished and are sometimes even difficult to detect because they occur in the same front. Therefore, these two modifications will be treated as one CM in the Manual. Front Delay occurs in cases where there is a long frontal cloud band which is orientated normal or almost normal to a major mountain chain. In the Alpine region this is the common north-south and northeast-southwest orientation of fronts. Moreover, it is observed that the deformation occurs more often if the front approaches the Alps from the west than if it comes from the northwest. If air is being advected towards an obstacle there are two possibilities: either going around or going above. Which possibility dominates depends on wind speed and stratification: low wind speeds inhibit ascent, as does stable stratification. Clouds are displaced slower in the mountainous region then the surrounding regions. As this phenomenon only lasts several hours, it is more important for nowcasting than for forecasting.
Foehn is a wind system created by the impact of orographic obstacles like mountains on the flow, which often shows a typical cloud structure.
The term Foehn in a general sense refers to a typical wind phenomenon in mountainous areas. At least in the Alpine area, there are cases where such a wind occurs but no or almost no cloudiness develops. Since this manual deals with satellite meteorology, only Foehn cases with cloud features are treated. The basic condition leading to a Foehn process is a flow perpendicular to a mountain range. The basic synoptic-scale precondition is an intensive low to the W, NW of this mountain chain for such a perpendicular stream. Reaching the mountain chains the air is forced to rise. Initially the cooling of the air mass is dry-adiabatic (1C/100 m) until saturation is reached. Condensation begins and from then on the cooling is at the moist adiabatic lapse rate which is 0.65C/100 m. At the height of the mountain tops the air mass will have lost much of its humidity content and can now be regarded as relatively dry. The sinking on the other side of the mountains and associated warming therefore takes place with the dry-adiabatic lapse rate. Consequently the air warms up during descent at a rate higher than the cooling during ascent and reaches the plains as a very dry, very warm and very strong wind - the Foehn.
Smooth, very low cloud sheets with warm tops called Stratus unless reaching the ground in which case they are Fog.
Fog and Stratus are seen in satellite images as low-level cloud sheets with a variable horizontal extent. Small local patches of fog are difficult to distinguish in satellite images (size in the order of a few kilometres). However, large areas of fog and stratus can cover areas over hundreds or thousands of square kilometres. Fog (or low Stratus) is formed, when moist air near ground level starts to condensate. This condensation can be produced in the atmosphere by three mechanisms: by ascent and resultant cooling of an air parcel, by radiative heat loss and by the mixing of two parcels of slightly unsaturated air initially having different temperatures. The necessary condition for both the advection and radiation St/Fog formation is a sufficient moisture content in the lowest layers of the atmosphere.
Mesoscale areas of cloud dissolution within frontal cloud bands driven by sinking cold air on the anticyclonic side of a jet streak.
The conceptual model Front Decay describes a mesoscale dissipation of clouds within a Cold Front. Here parts of the frontal clouds disappear due to several reasons. In general, the conveyor belt concept provides an excellent explanation for the decay of cloudiness within a frontal cloud band. A distinction can be made between cases with rising and sinking warm conveyor belts. In the case of a rising warm conveyor belt any gap within the frontal cloud band will probably close again, while in case of a sinking warm conveyor belt a complete dissipation of cloud in the IR imagery is probable. Front decay can also be caused by other processes such as deformation (elongation) of the frontal cloud band. Often Front Decay happens during the development of the CM of Rapid Cyclogenesis.
Enhanced Cumulus areas (ECs) consist of a cluster of thicker and larger cloud cells within the usual cold air cloudiness behind frontal cloud bands.
Enhanced Cumulus (EC) areas consist of clusters of convective cells which are thicker and mostly larger than the surrounding cloud cells. As such they are a special form of Cumulonimbus (Cb) Clusters. Enhanced Cumulus areas (ECs) are mesoscale cloud phenomena, which develop within the cold air mass behind a Cold Front in the area of an upper level trough. They do not develop at the boundary of two air masses like Cold or Warm Fronts. In some cases ECs are embedded within cellular cloudiness with tops reaching only up to the lower levels of the troposphere. Although ECs develop in cold air, temperature advection is relatively small.
Cloud bands or lines which form within a deformation field and stretch or dissolve under the influence of elongation.
Deformation zones are generated by opposing flows, where cloud systems of two synoptic or meso-scale cloud systems are brought closer to each other. A deformation zone needs a cyclonic and an anticyclonic circulation to form. As anticyclonic circulations tend to be rather dry, characteristic patterns for deformation zones can be best seen in water vapour image loops. After the deformation stage the cloudiness dissipates or merges with other cloudiness. The deforming band elongates in the direction of the upper level flow. Often there is sinking motion on the poleward side of the cloud band. This sinking air can be seen spreading in both directions with the upper level wind field.
Rather narrow dark stripes in WV images, representing dry sinking air along the cyclonic side of a jet stream.
One of the main features in WV images are rather narrow Dark Stripes lying along the cyclonic side of bright bands and fibres. Typically they occur on the cyclonic side of a jet axis, to the rear of Cold Fronts and Occlusions, and along the leading edge of Warm Fronts. The Dark Stripe represents sinking dry air which can be of stratospheric origin. As such, the stripe can be used as an indication of cloud dissipation from above or to indicate possible unstable development at the boundary between dry and moist air. Dark Stripes in WV images are especially useful if they are combined with relative streams where they can frequently be found associated with dry intrusions as well as with isentropic potential vorticity.