Weather
Warm Front Bands are accompanied by cloud bands which usually are shorter than Cold Front cloud bands.
In the case of a Warm Front, warm moist air moves against colder dry air. At the boundary of these two air masses the warm air tends to slide up over the wedge of colder air. This process causes the frontal cloud band, and the associated precipitation, found mainly in front of the surface front. The idealized structure and physical background of a Warm Front can be explained with the conveyor belt theory as follows: Frontal cloud band and precipitation are in general determined by the ascending Warm Conveyor Belt, which has its greatest upward motion between 700 and 500 hPa. The Warm Conveyor Belt starts behind the frontal surface in the lower levels of the troposphere, crosses the surface front and rises to the upper levels of the troposphere. There the Warm Conveyor Belt turns to the right (anticyclonically) and stops rising, when the relative wind turns to a direction parallel to the front. If there is enough humidity in the atmosphere, the result of this ascending Warm Conveyor Belt is condensation and more and more higher cloudiness. The Cold Conveyor Belt in the lower layers, approaching the Warm Front perpendicularly in a descending motion, turns immediately in front of the surface Warm Front parallel to the surface front line. From there on the Cold Conveyor Belt ascends parallel to the Warm Front below the Warm Conveyor Belt. Due to the evaporation of the precipitation from the Warm Conveyor Belt within the dry air of the Cold Conveyor Belt, the latter quickly becomes moister and saturation may occur with the consequence of a possible merging of the cloud systems of Warm and Cold Conveyor Belt to form a dense nimbostratus. In addition to this idealized structure, the experience from a series of case studies carried out at ZAMG differs somewhat from the one described above, allowing more differentiation.
Middle and high level cloud in a rising warm, moist air stream relative to its system velocity.
The Warm Conveyor Belt is a rising relative stream transporting warm wet air from south-east to northern directions. Warm Conveyor Belts are directly involved in the frontal processes, but typical cloud configurations can also be observed separately from the frontal cloudiness. Different states of the atmosphere are responsible for these two types: 1.) The cloudiness of a front is developed within two relative streams: the Warm Conveyor Belt and a less wet relative stream from the trough area behind. the Warm Conveyor Belt is connected at least with the leading part of the cloudiness, therefore the relative stream lines approach or even overrun the surface front. 2.) In the case of separated Warm Conveyor Belt cloudiness the relative stream lines are in front of the frontal cloud band and the relative stream from behind crosses the frontal line in a forward direction, consequently approaching the stream lines of the Warm Conveyor Belt. A survey by ZAMG has shown that over the past two years over 57% of the Warm Conveyor Belts that occurred happened over North-Africa and the Mediterranean and could be subscribed to the more isolated and separated type of Warm Conveyor Belt. Of the 467 cases examined less than 70 (15%) occurred over western, northern or eastern Europe and were mostly connected to a frontal system.
Cut off low in upper levels without corresponding surface low, accompanied by cloud bands at the boundaries and convective cloud in the low center.
Upper Level Lows are closed cyclonically circulating eddies. As these lows are only upper and mid-tropospheric features, they do not have a corresponding low within the lower levels of the troposphere, at least in the beginning. The development of an Upper Level Low depends on the existence of unstable waves within the general flow in the upper levels of the troposphere. These unstable waves are caused by a phase shift between the temperature wave and the potential wave which can lead to further developments. The temperature wave being situated behind the potential wave. Therefore CA can be found within the area of the upper level trough. During this stage of development the field of the absolute topography is characterized by an increase of the amplitude of the potential wave and sometimes also by a decrease of the wavelength. The same development takes place for the temperature wave. In the northern hemisphere a southward deviation of the isohypses and isotherms of the upper level trough can be observed leading to a deepening of the trough. Finally the trough will start to detach from the meridional stream and the formation of a cut of low (Upper Level Low) can be observed.
Organised cloudiness in the thickness ridge in front of a Cold Front which can be extremely convective in unstable situations.
Generally Thickness Ridge Cloudiness is situated within a rising warm conveyor belt and is generally found within a ridge or a closed maximum of equivalent thickness. Relative streams provide a good explanation of the physical processes involved in the formation of Thickness Ridge Cloudiness. The warm conveyor belt dominates the Thickness Ridge Cloudiness over a thick layer and sometimes influences a part of the frontal cloud band. Besides an upper relative stream/dry intrusion can be seen to the rear of the frontal cloud band. During the summer season there can be strong convective activity within the thickness ridge. As a result of the thermal energy and the high relative humidity within the thickness ridge this convective activity can be very strong.
Low and middle level cloudiness develops at the windward side of mountains if the wind streams almost perpendicularly to the mountain barrier.
Barrage cloudiness is an orographically influenced conceptual model. If there is a stream field orthogonal to a mountain range it is blocked by the barrier, forced to rise over or forced to flow around the barrier according to the extent and height of the mountain ridge. This forced rising of the stream leads to adiabatic cooling and, if there is sufficient humidity, to condensation. Consequently, cloudiness with a high probability for precipitation forms at the windward side of the mountain range. The intensity of the precipitation can be very strong and depends upon the distribution of the wind field with height or the type of Barrage. Different types of Barrage events can be distinguished from the direction of the flow and whether there are separate Barrage Clouds or Barrage within frontal zones. Although the latter is usually accompanied by heavy precipitation, in general, a dominating orthogonal component in the wind fields from 1000 hPa to 500 hPa generates more intensive precipitation.
One typical occurence of Cbs and Mesoscale Convective Systems is within the area of hot air over Spain, in front of an approaching Cold Front.
The Spanish Plume is a geographically specific case of convection, in which intensive Cb and Mesoscale Convective System form over the Spanish plateau and Western Europe. The Spanish Plume is, by definition, an incident where warm air is lifted from the Spanish plateau ahead of an eastward-moving upper-level trough over the Bay of Biscay. The strong heating of air near the surface over the Spanish plateau causes convergence and ascent, enhanced by the dynamical forcing of the eastwards moving upper-level trough. This leads to the formation of a thermal low over the Iberian Peninsula. The thermal low sets up a low-level cyclonic flow over the peninsula, which includes the formation of a convergence line over the plateau. This induces the advection of cooler air and descending motion across the coastal areas of the Iberian Peninsula and Western France from the Bay of Biscay and warm air advection over the interior of the peninsula. Therefore in the area east of the surface trough the advection of warm air enhances the ascents within the warm plume.
Secondary Low Centres are small scale cloud vortices which develop within already mature Occlusion cloud bands.
Secondary Lows appear as a cloud vortex within the frontal cloud band. A Secondary Low often forms within a Neutral Occlusion, that is, the Occlusion is neither warm nor cold. In this type a strong jet runs over the Occlusion point. The low forms usually between the Occlusion point and the midpoint of the occluded Front. Nearly all Secondary Lows form in systems over the sea during the winter season. This follows from the fact that the basic westerly flow is stronger during the winter, and the deepening of a low is easier over the relatively warm water. Although the secondary lows are associated with strong jets, there seems to be no obvious connection to the conceptual model FI by Jet. The processes in the left exit region are involved in less than half of the cases. But if the left exit region is favourably situated in the vicinity of the occluded front, it can enhance the development. In the process of the secondary low development there is temporarily a back bent part in the occluded Front. However the situation differs to the common Back Bent Occlusion.
Unstructured patches of low cloud within a stable stratified troposphere.
Sc clouds are a common cloud type both in northern latitudes as well as mid-latitudes. They are most often observed over cool waters throughout the year but also over land areas, especially in winter. The formation of stratocumulus depends on variable scales of physical factors: Synoptic scale flow and the properties advected by the air streams, e.g. moisture content, have an effect on formation of the cloud layer. On the other hand the development of Sc Sheets is controlled by small-scale factors in the atmosphere, such as turbulent mixing in cloud. Sc Sheet formation is often observed after a cold front passage as well as on the rear side of the low pressure. Cool air flow with reasonable wind shear, stable stratification and a large amount of moisture (not necessarily because of advection of moisture from the sea, but by evaporation of rain and from wet ground) form a suitable condition for Sc formation.
Development of thermally driven circulation between sea and land, often resulting in organized convective cloudiness in the vicinity of coastlines.
The Sea-Breeze is a circulation that develops due to differential heating of air over land and sea. As the sun heats the boundary layer over land, the resulting pressure gradient causes the movement of low-level air from the sea to land (Sea-Breeze) with a return flow aloft. The Sea-Breeze circulation is able to strengthen and/or changing (veering) the surface wind in large scale synoptic onshore circulations. The existence and intensity of the Sea-Breeze depends strongly on seasonal and latitudinal factors and on the time of day. In tropical and subtropical coastal regions the Sea-Breeze is a regular phenomenon throughout the year bringing a pleasant cooling. At higher latitudes it is a common feature during spring and summer only, clear skies, strong insolation and a temperature difference between land and sea at its maximum are relevant and important factors for the Sea-Breeze to develop.
Rapid Cyclogenesis describes a very fast and intensive development of an Occlusion cloud spiral from a low level cloud head. Upper air processes are driving factors in such a development.
Rapid Cyclogenesis (RaCy) describes the development of a cloud head located on the pole ward side of a frontal zone into a cyclonic cloud spiral. The life cycle of this mechanism ranges from 6 to 48 hours. While classical cyclogenesis through Wave development is often slow and the Wave bulge dissolves after some time or produces a spiral after some days, Rapid Cyclogenesis usually leads to a fast development of a cloud spiral. Comparing again Wave development and Rapid Cyclogenesis, a main difference can be found in the orientation of the relative stream forming the cloud head, or cloud bulge. It is similar to a Warm Conveyor Belt in the case of the Wave turning to east/north-eastern direction, but quite opposite in the case of Rapid Cyclogenesis turning to west/south-western directions. While in the case of Wave development stratospheric air (if present at all) does not reach down to levels lower than 300 hPa, in the case of Rapid Cyclogenesis stratospheric air is a key feature and protrudes much further downward (down to approximately 500 hPa or even lower).
Polar Lows look very similar to Commas but usually occur in northern latitudes. They develop under more specific synoptic conditions when compared to the more general features of a Comma.
A Polar Low is a meso-scale cyclone which normally occurs during winter time far north from the main polar front. In a northerly flow cold arctic air pours out over relatively warm seawater. Polar Lows start to develop in small baroclinic zones resulting from old Occlusions or arctic fronts. Polar Lows are closely related to the Conceptual Model of a Comma. Both are mesoscale systems developing in cold polar or arctic air. Whilst both are induced by upper air disturbances such as PVA and Potential Vorticity, there are some distinct differences. Polar Lows only occur during wintertime (October until April) while Commas can exist during the whole year. Weather events associated with Polar Lows are more severe than those accompanying a Comma.
A WCB Occlusion is accompanied by a distinct cloud spiral; clouds of all three frontal parts CF, WF and WCB Occlusion are multilayered.
The Warm Conveyor Belt is a rising relative stream transporting warm wet air from south-east to northern directions. It is strongly involved in the processes and cloud development connected with fronts. A Warm Conveyor Belt is defined as a layer of air which: originates in a relative easterly to south-easterly flow within the lower levels of the troposphere in the ridge area ahead of the front; generally ascends pole ward forming # cloudiness due to the condensation of moist air; after turning to south-eastward directions ascending motion changes to descending motion, where cloud dissolution takes place. Warm Conveyor Belts are directly involved in the frontal processes, but typical cloud configurations can also be observed separately from the frontal cloudiness.