1. COLD FRONT - CLOUD STRUCTURE IN SATELLITE IMAGES

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1 1. COLD FRONT - CLOUD STRUCTURE IN SATELLITE IMAGES The satellite image shows a cyclonically curved synoptic scale cloud band usually a few hundred kilometres wide; in the VIS image the cloud band mostly shows white grey shades, demonstrating high albedo; in the IR image the grey shades can vary between white and grey, depending on the existing conceptional sub-model (for instance Ana Cold Front or Kata Cold Front) (see Meteorological physical background) as well as on the stage of development within a single conceptual model; as with the IR image, the grey shades in the WV image also can vary between white and grey, according to classical conceptual model ideas: in the case of an Ana Cold Front the cloud band should be characterized by high pixel values (white), but lower pixel values (grey) in the case of a Kata Cold Front; in reality this differentiation is not so strict: o the typical Ana Cold Front cloudiness can have a structured but often also smoothed appearance; o the typical Kata cold front shows, in the IR and WV images, a structured appearance for the cloud band, sometimes even representing a typical Split Front character (see Cold Front - Split Front ); at the rear part of the cloud band grey shades are darker accompanied by lower pixel values, which often continuously increase northward to the point of the Occlusion; the leading part of the cloud band varies in IR and WV grey shades from light grey to white (see Meteorological physical background); within the cloud band there can be subsynoptic areas forming a continuous substructure; mostly the IR image shows there a vertically and horizontally increased cloud area leading to brighter grey shades. Typical examples are Waves, Front Intensifications by Jet Crossing or merely situations with superimposed PVA maxima (see Key parameters, Wave and Front Intensification By Jet Crossing

2 Figure 1: Typical grey shades from anafronts and katafronts in VIS, IR and WV satellite images Figure 2: Example of a structured Anafront

3 The image shows a typical example of a Cold Front stretching from the western Mediterranean across France and the British Isles. As is very common, the cloud band shows two areas of substructures consisting of colder cloud tops and forming convex cloud bulges. So three conceptual models can be identified within the same cloud band: Cold Front (CF), Wave development (WAVE) and an intensification of frontal cloudiness by crossing of a jet streak (FI BY JET). Figure 1: Example of a well developed Anafront (upper left: VIS, down left: IR, right: WV) This case is an example of a well developed Ana cold front stretching from the southwestern corner of the image (Atlantic approximately 35N/35W) to the West coast of Scotland. The cloud band in the VIS image is characterized by brighter grey shades in the leading and middle part of the band, while the rear part is much more broken with transparent cloudiness (easy to recognize from approximately 40N/30W to approximately 50N/20W). In the IR image this is quite opposite with bright grey shades denoting colder and therefore higher cloud tops from the middle to the rear part. This indicates multi-layered cloudiness with more and higher cloud tops to the rearward edge and fits well with the classical ideas described in the meteorological physical background. These higher cloud tops are even more recognizable in the WV image. The

4 most characteristic features in this channel are the sharp rear cloud edge and the black stripe of dry air parallel to the cloud band (pronounced gradient from white to black). Figure 2: Example of a well developed Katafront (upper left: VIS, down left: IR, right: WV) This case is an example of a well developed Kata Cold Front stretching from the southeastern coast of England across the Baltic Countries, Finland and Russia to the White Sea. The cloud band in the VIS image shows brighter grey shades in the rear part of the band, for instance from north Germany across the Baltic Sea to the south coast of Finland. This is completely opposite to the IR and WV images where the lower grey shades, denoting warmer cloud tops, can be found. Dry air in the WV image overflows the rear part of the cloud band. Such a situation is in accordance with the classical ideas of the Kata Cold Front describing cloud dissolution by advection of dry sinking air (see Meteorological physical background). This is also opposite to the Ana Front model.

5 2. COLD FRONT - METEOROLOGICAL PHYSICAL BACKGROUND If cold and warm air are situated next to each other an inclined boundary (oriented downward from high to low layers) can be found between the two air masses. The main physical process for the development of Cold Fronts is the movement of the colder against the warmer air. As a consequence of this movement, and relative to it, the warm air tends to ascend this air mass boundary while the cold air tends to sink below it. This upward motion often leads to condensation and subsequently the development of clouds and precipitation. Figure 3: Schematic picture of Ana front and Kata front Many conceptual models have been developed to depict fronts; the most classical ones are the for Ana Cold Front and the Kata Cold Front. From the viewpoint of satellite meteorology, the conveyor belt model presents new ideas. Therefore within the following paragraphs these cold front types are described. Ana Cold Front According to classical ideas cold air moves rapidly against warm air, thereby creating convergence at the surface line between the two airmasses. This convergence forces the warm, moist air to ascend on the frontal surface. The cloud band develops, inclined rearward from the surface cold front. Consequently, in this case the main zone of cloudiness and precipitation appear behind the surface front (indicated by the TFP). Satellite images confirm this structure (see Cloud structure in satellite image). Only in the case where there are high upper level winds, does the high cloud extend downstream ahead of the surface front, leading to a TFP maximum within the cloud band. If one describes the situation using conveyor belt theory, the frontal cloud band and precipitation are related to an ascending warm conveyor belt. This conveyor belt has

6 a rearward component relative to the movement of the front. This leads to the same result mentioned above, with the frontal cloud band and precipitation appearing behind the surface front. Parallel to the warm conveyor belt there is a dry stream (dry intrusion). The sharp rear cloud edge of frontal cloudiness marks the transition between the two relative streams and is accompanied by a limiting stream line. As mentioned in literature, real examples of CF do not always show these model characteristics, but sometimes even show parallel or even forward inclined warm conveyor belts. While the rearward component can be explained by ageostrophic wind flow in the planetary boundary layer due to friction, the parallel or even foreward sloping warm conveyor within the middle and higher levels is in accordance with the geostrophic wind relation. Figure 4: The conveyer belt theory for an Ana Cold front Kata Cold Front According to classical ideas, in this type of front the warm air follows processes similar to the Ana Cold Front, however, the ascent of air is restricted by dry descending air originating from behind the front and, consequently, dissipating the higher clouds. In this case, the main zone of cloudiness and precipitation appear in front of the surface front. Satellite images show parts of this process of cloud decay (see Cloud structure in satellite image). It is now generally considered that a Kata Cold Front evolves from an Ana Cold Front. In contrast to the Ana Cold Front, the ascending warm conveyor belt is overrun by dry air, which is transported within the relative stream of the dry intrusion. The air of this intrusion originates from upper levels of the troposphere or even the lower levels of the stratosphere and crosses the Cold Front from behind. Due to this process, the warm conveyor belt acquires a component which is forward inclined relative to the movement of the Cold Front. Therefore, frontal clouds and precipitation tend to lie ahead of the surface front. The cloud tops within the area of the dry sinking

7 tropospheric and/or stratospheric air are lower (warmer) than in the case of an Ana Cold Front. At the leading edge of this dry air, an increase of the cloud tops can be observed (see Cloud structure in satellite image). This area indicates the so-called upper Cold Front. The air mass which is advected by the dry intrusion is colder than the air within the warm conveyor belt. The intrusion cools air above and, later, also ahead of the Cold Front. Furthermore, the air of the upper relative stream is indicated by lower values of equivalent potential temperature. The result of this situation is the development of a conditionally unstable layer close to the leading edge of the frontal cloud band. As a result of ascent, this area is suitable for the development of pronounced instability which is often observable by a change of cloud type from stratiform to cumuliform (see Weather events). Figure 5: The conveyer belt theory for a Kata Cold front Schematic Summary Ana Cold Front Kata Cold Front

8 Figure 6: Schematic structure of Ana and Kata Cold front The schematics above summarise several aspects of conveyor belt theory and compare the different behaviour of Ana and Kata Fronts. The most important feature is always the orientation of the jet streak relative to that of the cloud band, i.e. parallel to an Ana Front but across the cloud band of a Kata Front and descending.

9 Figure 7: 04 October 1995/12 UTC - IR image (Ana Cold Front); relative streams on the isentropic surface of ThetaE = 310K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hpa; lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hpa but corrected with WV imagery, yellow: isobars, magenta: relative streams - system velocity: m/s, white: position of vertical cross section Figure 8: 04 October 1995/12 UTC - WV image (Ana Cold Front); relative streams on the isentropic surface of ThetaE = 318K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hpa; lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hpa but corrected with WV imagery, yellow: isobars, magenta: relative streams - system velocity: m/s The above images show an example of an Ana Cold Front. On the lower isentropic surface (top) a warm conveyor belt can be observed crossing (rearwards) the frontal line approximately northward of the Bay of Biscay. Under its influence is the cloud band in front of the TFP as well as the relevant part behind it stretching from France across south-east England into the North Sea. The second relative stream is a broad one, from the north-western part of the trough behind the front. Frontal cloudiness to the rear of the TFP, from north-west Spain across the Bay of Biscay and Brittany into the English Channel is, increasingly, under its influence. If this situation is compared with the vertical cross section below (upper cross section), it can be seen that the zone of high humidity ahead of the front, at the 310K surface around 800 hpa, represents the warm conveyor belt, while humidity values on this surface, between 800 and approximately 550 hpa, represent the relative stream approaching from the rear. This height also marks the position of the rearward edge of the cloud band, which coincides with a sharp decrease in IR and WV pixel values. On the higher isentropic surface (lower vertical cross section) the relative stream from the rear can be divided into two parts. The one near the anticyclonic side of the jet axis originates from moist regions in the warm sector of the consecutive frontal system and is associated with Cold Front clouds to the rear of the TFP; the other drier relative stream can be found on the cyclonic side of the jet axis. Both are parallel to the frontal cloud band. On this higher surface the warm conveyor belt crosses the TFP much less than on the lower surface.

10 Looking again at the vertical cross section (upper cross section), the humidity maximum in front at the 318K surface between 500 and 400 hpa represents the warm conveyor belt (accompanied by peaks in IR and WV pixel values) while on this isentropic surface further upward, near 350 hpa, cloudiness is associated with the moist part of the relative stream from the rear side (accompanied by a second IR and WV peak in pixel values). The dry part of this stream shows up in the cross section as very low humidity values (around 300 hpa). Figure 9: 04 October 1995/12.00 UTC - Vertical cross section; Ana Cold Front; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values Figure 10: 29 February 1996/06.00 UTC - Vertical cross section; Kata Cold Front; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values The Ana Cold Front (top) shows a backward inclined zone of high humidity from low to high levels while the Kata Cold Front (bottom) shows a similar zone but forward inclined. The driest air, in the case of the Ana Cold Front, lies behind and below the frontal surface, but within and below the frontal surface in the case of the Kata Cold Front.

11 Figure 11: 29 February 1996/06 UTC - IR image (Kata Cold Front); relative streams on the isentropic surface of ThetaE = 286K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hpa; lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hpa but corrected with WV imagery, yellow: isobars, magenta: relative streams - system velocity: m/s, white: position of vertical cross section Figure 12: 29 February 1996/06 UTC - WV image (Kata Cold Front); relative streams on the isentropic surface of ThetaE = 300K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hpa; lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hpa but corrected with WV imagery, yellow: isobares, magenta: relative streams - system velocity: m/s In the case of the Kata Cold Front, the zero line of shear vorticity, marking the jet axis, crosses the cloud band leading less bright (i.e. lower height) frontal clouds at the cyclonic side. This is the main difference from the situation of the Ana Cold Front described before, where the jet axis and frontal cloud band are parallel. On the lower isentropic surface (top image) a warm conveyor belt can be observed which is nearly completely restricted to the area of high cloudiness ahead of the TFP. Behind the TFP, relative stream lines are from the rear. Comparing with the vertical cross section (above, lower cross section) the moist zone ahead of the front, at the 286K surface from the ground up to about 800 hpa, is composed of two relative streams: the warm conveyor belt air mass only exists in the lowest layer (below about 900 hpa) and the air mass of the moist relative stream from behind, dominates above 900 hpa. The second image above shows the situation on a higher isentropic surface with a crossing of the relative stream lines from the north-west over the TFP of the Cold Front. This belongs to the maximum of relative humidity in the vertical cross section (above, lower cross section) ahead of the front at the 300K surface (approximately 500 hpa). Consequently, the high clouds, indicated by the maximum of pixel values, is formed in the moist branch of the relative stream to the rear, while the warm conveyor belt is only associated with a layer of low level cloudiness. The lower cloud tops on the cyclonic side of the jet axis should be the result of the dry part of the relative stream, which is confirmed in the vertical cross section; dry air above 300K associated with the IR peak at about 400 hpa.

12 In this case it is not easy to distinguish between the origins of the two branches of the relative stream from the north-west because the stream lines cross the jet axis to the rear of the trough. One possible explanation is that the system velocity is computed for the Cold Front and does not match that of the subsequent Warm Front system. The following image is the result of a system velocity computation based on propagation of the approaching Warm Front system. The differentiation between the dry and the moist branches in the upstream region is now much clearer. Figure 13: 29 February 1996/06 UTC - IR image (Kata Cold Front); relative streams on the isentropic surface at ThetaE = 300K; frontal lines are in accordance with the maximum of the thermal front parameter (TFP) 500/850 hpa;lines: dashed blue: jet axis in accordance with the zero line of the shear vorticity at 300 hpa but corrected with WV imagery, yellow: isobares, magenta: relative streams - system velocity: m/s, white: position of vertical cross section

13 3. COLD FRONT - KEY PARAMETERS Equivalent thickness Crowding zone of the equivalent thickness: o Ana Cold Front: within the cloud band o Kata Cold Front: behind the cloud band Thermal front parameter (TFP) Maximum of the TFP accompanies the cloud band: o Ana Cold Front: within the leading area of the cloud band o Kata Cold Front: within the rear area of the cloud band Temperature advection (TA) The field of TA shows (weak) WA in front of, and (pronounced) CA behind the surface front. Depending on the conceptual model the zero line of TA should be found within or very close to the cloud band: o Ana Cold Front: cloudiness usually within CA, TA=0 in front of cloud band o Kata Cold Front: cloudiness usually within WA, TA=0 within the cloud band Positive vorticity advection (PVA) in upper levels PVA maxima can be found near the rear edge of the cloud band indicating the propagation of the upper level trough and/or the approach of a jet streak (see Front Intensification By Jet Crossing ). Isotachs at 300 hpa o Ana Cold Front: the jet is situated behind and parallel to the Cold Front cloudiness, the cloud band is on the anticyclonic side of the jet, the jet crosses the frontal system in the area of the point of the Occlusion o Kata Cold Front: the jet crosses the surface cold front at a sharp angle; according to the crossing point the cloud band can be on the anticyclonic or cyclonic side of the jet As already mentioned, there are similarities between Kata Cold Front and Split Front. The main difference can be found in the orientation between jet and front because in the case of a Split front the jet approaches from the rear side nearly at right angles. Shear vorticity at 300 hpa Zero line of the shear vorticity: o Ana Cold Front: the zero line is close to and parallel to the rear cloud edge o Kata Cold Front: the zero line crosses the Cold Front, usually it accompanies the transition of the high cloud tops on the anticyclonic side from the lower cloud tops on the cyclonic side of the zero line; the latter are the result of cloud dissolution within dry air on the cyclonic side see Cloud structure in satellite image).

14 4. COLD FRONT - TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS The isentropes of the equivalent potential temperature show a downward inclined crowding zone, which extends through the whole troposphere, with the warmer air found in front and above, the colder air at the rear and below the crowding zone (see Meteorological physical background). Besides the surface front, often an upper level front, which is situated ahead of the surface front, can be observed. The relative humidity has high values in front of and low values behind the crowding zone. With the help of the models of Ana Cold Front and Kata Cold Front, the field of humidity can be interpreted with the warm ascending air in front of and cold descending air behind the frontal surface. Furthermore, in the case of an Ana Cold Front the field of humidity is characterized by high values (approximately 80%) extending on the warmer side of the crowding zone with a backward inclination from the lower to the higher levels of the troposphere. In contrast to this, the distribution of the relative humidity in the case of a Kata Cold Front shows high values (approximately 80%) in front of the warmer side of the crowding zone from the lower levels with a forward inclination into higher layers of the troposphere, thereby moving away from the highest surface of the crowding zone. The dry cold air (with minimum values of about 20%) extends in the case of an Ana Cold Front from the highest layer of the frontal surface downward below the frontal surface. In contrast to this, in the case of a Kata Cold Front the dry air extends downward below as well as within the frontal surface, thereby leading to cloud dissolution (see Meteorological physical background). The field of temperature advection shows WA in front (ahead) of the crowding zone, indicating warm ascending air of the warm conveyor belt and/or the upper relative stream from behind the front. CA can be found behind (below) the frontal surface. In general, the CA is much more pronounced than the WA. The whole crowding zone from the lower to the upper levels of the troposphere is characterized by positive vorticity advection. In the ideal case the values of the vorticity advection are continously increasing. Therefore its maximum (often situated at 300 hpa) can be found in the upper levels of the troposphere and behind the surface front line. The field of divergence shows a pronounced zone of convergence within and a zone of divergence above the frontal surface. As a consequence of this distribution the field of omega is characterized by negative values (upward motion) within and above the crowding zone. As convergence has its highest values in the lower troposphere the greatest negative values of omega can be found in the mid-levels of the troposphere. Weak positive values (downward motion) can be found within the cold air at the rear side below the crowding zone. The pixel values of the satellite image show in the VIS image high values for both the Ana Cold Front and Kata Cold Front cases. In contrast to this the distribution of the pixel values of the IR and WV images are dependant on the existing cold front type. In

15 the case of an Ana Cold Front the pixel values of the IR and WV images are increasing backwards i.e. behind the TFP, but in the case of a Kata Cold Front the pixel values mostly are increasing forewards i.e. ahead of the TFP (see Cloud structure in satellite image). Figure 14: 26 September 1995/06.00 UTC - Meteosat IR image; Ana Cold Front; position of vertical cross section indicated Figure 15: 29 February 1996/06.00 UTC - Meteosat IR image; Kata Cold Front; position of vertical cross section indicated The first two figures show the main differences in the humidity distribution between Ana and Kata Cold Fronts. The other parameters do not distinguish between the two types of Cold Front and are therefore chosen from the Kata Cold Front type example. All fields are very close to the schematics representing ideal situations with the exception of the isentropes in the Kata Cold Front case which unusually consists of two branches of the frontal zone, one reaching the ground (approximately at 57N/15E) and an upper level front slightly ahead of the surface front reaching down to about 750 hpa.

16 Figure 16: 26 September 1995/06.00 UTC - Vertical cross section; Ana Cold Front; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values Figure 17: 29 February 1996/06.00 UTC - Vertical cross section; Kata Cold Front; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

17 Figure 18: 29 February 1996/06.00 UTC - vertical cross section; Ana and Kata Cold Fronts; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values Figure 19: 29 February 1996/06.00 UTC - vertical cross section; Ana and Kata Cold Fronts; black: isentropes (ThetaE), green thick: vorticity advection - PVA, green thin: vorticity advection - NVA, orange thin: IR pixel values, orange thick: WV pixel values

18 Figure 20: 29 February 1996/06.00 UTC - Vertical cross section; Ana and Kata Cold Fronts; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values, orange thick: WV pixel values 5. COLD FRONT - WEATHER EVENTS Weather events are highly variable and can differ from season to season. Two type of fronts are distinguished: Ana and Kata Cold Fronts Ana Cold Front Parameter Precipitation (incl. thunder) Description Moderate to heavy showery precipitation In winter season snow is possible. Quite often thunderstorms are observed at all seasons. Temperature Falls rapidly after the passage of the front. Over land in wintertime temperature can rise after front passage. Wind(incl. gusts) Around embedded Cbs strong gusts are possible. Veering of the wind at the front passage Other relevant Precipitation in narrow cloud band just ahead of front (line

19 information convection) Enhanced areas with precipitation behind surface front Risk of moderate to severe icing and turbulence. Predestined areas for heavy weather are regions with superimposed PVA, e.g.left exit region of a jet streak Figure 21: 26 September 1995/12.00 UTC - Meteosat VIS image; Ana Cold Front; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm with precipitation, purple: freezing rain, orange: hail, black: no actual precipitation or thunderstorm with precipitation); blue: thermal front parameter 500/850 hpa Kata Cold Front Parameter Precipitation(incl. thunder) Description Precipitation ahead of surface front Precipitation sometimes in narrow bands ahead of surface front Sometimes thunderstorms are observed at all seasons. Temperature Falls rapidly after the passage of the front Over land in wintertime temperature can rise after front passage. Wind (incl. gusts) Other relevant information Around embedded Cbs strong gusts are possible. Veering of the wind at the front passage Risk of moderate to severe icing (risk of freezing rain) Risk of moderate to severe turbulence

20 Figure 22: 29 February 1996/12.00 UTC - Meteosat VIS image; Kata Cold Front; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm with precipitation, purple: freezing rain, orange: hail, black: no actual precipitation or thunderstorm with precipitation); blue: thermal front parameter 500/850 hpa