Frentes 3ª parte

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 Parametersby 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

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. Detached Warm Front - Typical Appearance In Vertical Cross Sectionsby 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.

11. 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.

13. 04 January 2005/00.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated

15. <04 January 2005/00.00 UTC- Vertical crosssection; black: isentropes (ThetaE), red thick: temperatureadvection - WA, red thin: temperatureadvection - CA, orangethin: IR pixel values, orangethick: WV pixel values

17. 04 January 2005/00.00 UTC- Vertical crosssection; black: isentropes (ThetaE), blue: relativehumidity, orangethin: IR pixel values, orangethick: WV pixel values

19. 04 January 2005/00.00 UTC- Vertical crosssection; magenta thin: divergence, magenta thick: convergence, cyanthick: vertical motion (omega) - upwardmotion, cyanthin: vertical motion (omega) - downwardmotion, orangethin: IR pixel values, orangethick: WV pixel values

21. 04 January 2005/00.00 UTC - Vertical crosssection; black: isentropes (ThetaE), yellow: isotachs, orangethin: IR pixel values, orangethick: WV pixel values

22. 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.