1. Frentes 3ª parte M en C Marcial Orlando Delgado D SENEAM Meteorología Sinóptica y Análisis I Trimestre enero marzo 2010

2. Detached Warm Front - Key Parameters by ZAMG Equivalent thickness: The cloud field of the Detached Warm Front is within the high gradient zone at the eastern branch of a pronounced ridge of the (equivalent) thickness. Absolute topography at 500 hPa: It is very similar to the (equivalent) thickness, therefore the cloudiness of the Detached Warm Front is also within the high gradient zone at the eastern branch of a pronounced ridge of absolute topography. Warm advection (WA): The cloudiness of the Detached Warm Front is superimposed upon a distinct (claro) WA maximum. But often two WA maxima can be observed. In this case the northern maximum is associated with the original Warm Front, and the southern one with the Detached Warm Front.

3. Wind Vectors at 500 hPa: The wind field has, in the area of the cloud field of the Detached Warm Front, a more or less strong southern component, and blows normal to the displacement of the whole frontal system of the ridge and frontal zones. Consequently, cloud elements of the Detached Warm Front are moving quickly southward while the complete cloud configuration is displaced eastward, much more slowly. Shear vorticity at 300 hPa: The zero line coincides with the leading edge of the Warm Front cloud shield. Isotachs at 300 hPa: The leading edge of the Warm Front cloud shield is superimposed upon a jet streak with intensities varying from case to case.

6. 04 January 2005/00.00 UTC - Meteosat 8 IR 10.8 image; blue: geopotential height 500 hPa, green: equivalent thickness 500/850 hPa

8. 04 January 2005/00.00 UTC - Meteosat 8 IR 10.8 image; yellow: isotachs 300 hPa, black: zero line of shear vorticity 300 hPa

9. The parameter distribution is very similar to the ideal situation described above. If the wind vectors at 500 hPa (green arrows) are compared to the relative streams (see Meteorological Physical Background), differences in the directions can be observed at a height close to 500 hPa (296K isentrope). While the absolute winds are coming from a north-westerly direction the relative streams are coming from a more north-easterly direction, which is very close to the orientation of the cloud structure.

10. WARM FRONT Detached Warm Front Warm Front Band Warm Front Shield

11. Detached Warm Front - Typical Appearance In Vertical Cross Sections by ZAMG Vertical cross sections of the Detached Warm Fronts do not differ from the classical band-type Warm Front. As described before, the isentropes of the equivalent potential temperature across the Detached Warm Front show a high gradient zone through the whole troposphere, which is inclined upwards from low to high levels. The colder air can be found in front of and below the high gradient zone, the warmer air being in front, and above The field of humidity shows high values immediately behind and above the frontal surface of the Warm Front. Low values can be found below the high gradient zone of the equivalent potential temperature.

12. Like the distribution of humidity, the field of temperature advection can also be separated into two parts. WA exists above and within the high gradient zone of the Warm Front. The maximum of the WA can be found within the high gradient zone where it often has several maxima from low up to high levels. On the other hand CA can be found below and in front of the high gradient zone. In actual cases the isentropes forming the lower boundary of the frontal surface do not represent the transition from WA to CA, but WA can also mostly be found far below the frontal surface while CA exists only at a larger distance from the surface front. At the leading part of the system, above the frontal surface at approximately 300 hPa, a pronounced isotach maximum can be observed. Well developed fronts are accompanied by a zone of distinct convergence within and divergence above the frontal zone. Consequently, upward vertical motion can be found above the frontal zone, responsible for cloud development. In the ideal case, the satellite radiance values across the Warm Front are characterized by typical distributions. While the IR image shows continuously increasing values of grey shades from the rear to the leading edge across the frontal area, the distribution of the grey shades in the VIS image is reversed (see Cloud structure in satellite image). In contrast, the WV image shows high pixel values within the frontal cloudiness and a pronounced minimum associated with the dry air in front. In reality, these variations of grey shades for Detached Warm Fronts are, by far, not as clear as in the ideal conceptual model.

14. 04 January 2005/00.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated

16. <04 January 2005/00.00 UTC- Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values

18. 04 January 2005/00.00 UTC- Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

20. 04 January 2005/00.00 UTC- Vertical cross section; magenta thin: divergence, magenta thick: convergence, cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion, orange thin: IR pixel values, orange thick: WV pixel values

22. 04 January 2005/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), yellow: isotachs, orange thin: IR pixel values, orange thick: WV pixel values

23. The first cross section shows the high gradient zones of the equivalent potential temperature of a pronounced surface Warm Front with intense WA, especially through the whole troposphere. The IR radiance values are characterized by high pixel values in the centre of the Warm Front cloudiness (pixel values between approximately 190 and 200 units). Lower pixel values, in connection with the dry (anticyclonic) side (and the superimposed jet), can be found in the leading part of the Warm Front (pixel values between approximately 160 and 180 units). The field of relative humidity shows high values within the lower to mid-levels, also immediately behind the surface front (above 80%). The third cross section shows, in the lower and mid- levels, intense upward motion within the high gradient zone which is caused by a zone of convergence in low and mid-levels of the troposphere situated within and below the frontal surface. Divergence can be found above the frontal surface. The fourth cross section is characterized by a pronounced isotach maximum within the leading part of the Detached Warm Front at approximately 200 hPa. This maximum is accompanied by a decrease of the WV pixel values.

24. WARM FRONT BAND by ZAMG Warm Front Bands are accompanied by cloud bands which usually are shorter than Cold Front cloud bands Chapters: Cloud Structure In Satellite Images Meteorological Physical Background Key Parameters Typical Appearance In Vertical Cross Sections Typical Appearance In Vertical Cross Sections Weather Events References

25. Warm Front Band - Cloud Structure In Satellite Images The satellite image shows an anticyclonically curved synoptic scale cloud band which is connected to a Cold Front cloud band. In ideal cases: ◦in the VIS image the grey shades are generally white at the rear edge becoming increasingly grey towards the forward edge; ◦in the IR image the grey shades of the cloud band are grey to white, where the brighter values appear, in the ideal case, towards the forward and downstream cloud edge. In reality: ◦very often no continuous cloud band exists but rather several cloud layers with broken cloudiness, or sometimes even only high cloudiness; ◦in the IR image several white cloud areas are superimposed on grey lower cloud layers (see Meteorological physical background). High, bright WV pixel values can be observed in the area of the frontal cloud band. At the leading edge of the cloud band the WV image shows a sharp gradient from white to black indicating dry air along the cyclonic side of the jet. In contrast to the Warm Front Shield (see Warm Front Shield), the warm sector of the Warm Front Band is usually cloudless, except in winter and spring time when extended fields of fog and low clouds can exist associated with processes in the lowest layers.

26. 15 September 2004/06.00 UTC - Meteosat 8 IR 10.8 image

27. 15 September 2004/06.00 UTC - Meteosat 8 WV 6.2 image

28. This case has an appearance very close to the classical description. The Warm Front cloud band can be observed over the Atlantic (just east of approximately 25W) extending to Northern Ireland. Several features mentioned above can be observed:the gradual increase of cloud top temperatures from the rear to the leading edge (in IR image); the distinct leading boundary of the cloud band (in IR and WV images); the Dark Stripe (in WV image) along the cyclonic side of the above-mentioned cloud edge; the cloudless warm sector.

29. 14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image

30. 14 November 2004/06.00 UTC - Meteosat 8 WV 6.2 image

31. This case shows some deviations from the ideal cases. The cloud band of the Warm Front can be located in the IR and WV images from Iceland across the Atlantic towards Norway. In the IR image it consists of several high, i.e. cold, cloud areas and cloud lines superimposed on dark grey, i.e. warmer, cloud tops. Over Iceland the mountains trigger some lee clouds, embedded by the Warm Front Band. In the WV image a broad white band in the area of the Warm Front indicates high water vapour content bounded by Dark Stripes on the northern as well as the southern edges. The northern stripe represents the dry air along the cyclonic side of the jet. The dark grey area extending over Great Britain is an extensive area of Fog and Stratus.

32. Warm Front Band - Meteorological Physical Background n 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 (see Typical appearance in vertical cross section). This process causes the frontal cloud band, and the associated precipitation, found mainly in front of the surface front (or the TFP) (see Weather events).

33. The idealized structure and physical background of a Warm Front can be explained with the conveyor belt theory: 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.

34. Discussion 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. In the case of a band type the Warm Conveyor Belt can be observed within the warm sector up to the Cold and Warm Front line. If there is high cloudiness in front of and parallel to the Warm Front line, it is situated within an (at least in the area of the fronts) ascending conveyor belt from the rear side of the Cold Front extending from south- west to north-east, the so-called upper relative stream. While the high cloudiness can thus be explained, the Warm Front cloudiness in the lower levels of the troposphere develops, as in the ideal case described above, within the Cold Conveyor Belt.The conveyor belt situation, especially of the Warm Conveyor Belt and the upper relative stream, described above can be found in a thick layer of isentropic surfaces, but there is some tendency for the Warm Conveyor Belt in lower layers to overrun the surface Warm Front to a small extent. The ascending Warm Conveyor Belt in the warm sector is not accompanied by appreciable cloudiness either because of too dry air masses or/and too little lifting. But it can be observed that in the ascending Warm Conveyor Belt cloudiness may develop leading to a second Warm Front type, the Warm Front Shield.

36. 14 November 2004/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

37. 14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 302K - system velocity: 295° 10 m/s, yellow: isobars 300K, position of vertical cross section indicated The 302K isentropic surface chosen for the relative streams in the figure below is very close to the upper boundary of the higher gradient, inclined Warm Front zone.

38. The analysis shows two Conveyor Belts: there is one from eastern directions across The Netherlands, Belgium, France, turning northward in an anticyclonic direction over the Atlantic. This is a typical example of a Warm Conveyor Belt extending across the warm sector to the leading edge of the Cold Front and the rear edge of the Warm Front cloudiness. A second relative stream originates from the cold air mass over the Atlantic behind the Cold Front and approaches the Warm Conveyor Belt over Iceland, extending from there on across the Atlantic (approx. 66N/2W) towards Norway parallel to the Warm Conveyor Belt. Generally speaking, the Warm Conveyor Belt rises from about 800 hPa over the Atlantic up to 400 hPa over Norway. The relative stream from the Atlantic exists in a higher layer, rising from about 600 to 450 hPa. The high cloudiness of the Warm Front exists in this latter relative stream.

40. 15 September 2004/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

41. Warm Front Band - Key Paramete Equivalent thickness: The cloud band of the Warm Front Band is within the higher gradient zone of equivalent thickness. Thermal front parameter (TFP): The TFP can be found close to and parallel to the rear edge of the Warm Front cloud band. This is in contrast to the Ana Cold Front where the TFP accompanies the leading edge of the cloud band. Warm advection (WA): The whole frontal cloudiness lies within a WA maximum, which increases towards the occlusion point. Consequently, the maximum is in front of the frontal line. Shear vorticity at 300 hPa: The zero line coincides with the leading edge of the Warm Front cloud band. Isotachs at 300 hPa: The leading edge of the Warm Front is superimposed upon a jet streak

43. 14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; thermal front parameter (TFP) 500/850 hPa, green: equivalent thickness 500/850 hPa

45. 14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; red: temperature advection 500/1000 hPa, green: equivalent thickness 500/850 hPa

47. 14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; yellow: isotachs 300 hPa, black: zero line of shear vorticity 300 hPa