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Appearance in Satellite Data

An Upper Wave is a smooth Wave feature accompanied by a rather flat convex bulge of cloudiness on the rearward side of a frontal cloud band.

 

Appearance in METEOSAT 8 imagery

  • IR 10.8 imagery
    • Convex cloud bulge at the rear of a frontal cloud band, often large elongation of cloud bulge
    • Grey to white due to high level cloudiness, Wave bulge appears brighter than the surrounding frontal clouds
    • Sharp edge to the cold air behind the front
    • Fibres indicating strong upper winds or jet streak
  • WV 6.2 imagery:
    • Bright grey or white due to height of cloud bulge, which appears more enhanced (brighter) in relation to surrounding frontal cloud band
    • Sharp edge to the black dry zone to the rear of the front
    • WV dark zone at rear indicating sinking of dry air from the stratosphere
  • VIS 0.6 imagery:
    • White due to thickness of cloud bulge
    • Fibre - like character due to high clouds

At first sight the upper wave seems to look similar to a classical wave but more detailed study shows differences (see Wave ):

  • Often the Upper Wave bulge as seen in satellite imagery is less pronounced than the "classical" Wave
  • There is generally less amplification than with a "classical" Wave
  • An Upper Wave propagates rapidly along the rear side of the frontal cloudband
  • In an Upper Wave there is often a fibre cloud indicating strong upper level winds (schematic below)

The set of images below shows an Upper Wave over the Atlantic on 16 March 2005 at 18.00 UTC. In the IR 10.8 image (below left) a well developed Upper Wave bulge can be seen with multilayered bright white clouds. At its edge a fibre cloud indicates strong upper winds. The WV 6.2 image (below right) also displays a white enhanced Upper Wave bulge. There is a distinct black zone behind the Wave indicating sinking of dry air.

16 March 2005/18.00 UTC - Meteosat 8 IR 10.8 image
16 March 2005/18.00 UTC - Meteosat 8 WV 6.2 image
16 March 2005/16.00 UTC - Meteosat 8 VIS 0.6 image

In the visible image, above left, the Wave bulge has fibrous characteristics. Additionally, the cloud thickness and features are well indicated.

A second case of an Upper Wave is shown below (22 August 2005 at 12.00 UTC). The cloud bulge extends over Eastern England. The presence of the jet can be seen very well in both the IR 10.8 as well as the WV 6.2 (below left and right). The Wave feature is also much distinct, whereas the VIS 0.6 image (2nd row below left) clearly shows the fibrous clouds within the enhanced Wave bulge of upper level cloudiness.

22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image
22 August 2005/12.00 UTC - Meteosat 8 WV 6.2 image
22 August 2005/12.00 UTC - Meteosat 8 VIS 0.6 image

The presence of the jet and its accompanying deepening of stratospheric air to the rear of the Wave can also wel been in the Airmass-RGB as the darker area over Southwestern England. The Airmass-RGB is an artificial composite consisting of the WV 6.2 - WV 7.3 brightness temperature difference (BTD), on red, the IR 9.7 - IR 10.8 BTD on green and the WV 6.2 channel on blue. Typical features seen on the Airmass RGBs include dry intrusions, deformation zones and jet streaks. What the other colours in this RGB represent can be found out when clicking here .

A second RGB consists of NIR 1.6, VIS 0.8 and VIS 0.6. In this so-called natural colour RGB the fibrous character of the upper Wave is well observed.

22 August 2005/12.00 UTC - Meteosat 8 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i)
22 August 2005/12.00 UTC - Meteosat 8 RGB image (NIR1.6, VIS0.8 and VIS0.6)

The Upper Wave bulge usually propagates very quickly downstream along the rearward edge of the frontal cloud band, steered by upper level winds. During this propagation it generally moves "through" the jet streak (maximum of wind speed), the cloud bulge becomes less pronounced and finally vanishes within the Occlusion point of the system, or within the anticyclonic curvature of the Warm Front Band.

The loop below shows the fast propagation of the Upper Wave bulge along the frontal cloud band (22 August 2005 06.00 UTC to 23 August 2005 00.00 UTC). During the whole life cycle of the Upper Wave, there is no further development, such as amplification or enhancement of the cloud bulge. The amplitude of the upper wave even becomes less due to an oncoming depression moving in between Iceland and Scotland.

Loop: 22 August 06.00 - 00.00 UTC hourly image loop
22 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image

 

Distinction between Wave and Upper Wave according to appearance in satellite images

A Wave and an Upper Wave often look similar in satellite imagery. In order to distinguish between them one can highlight and recognise a number of differences.

Upper Wave Wave
More elongated flat cloud bulge Distinct, more amplified cloud bulge
Sharp edge to the rear; also fibre cloud Less fibre cloud
Multilayered or high non convective cloudiness Multilayered cloudiness, convective cells embedded
Distinct, relatively fast propagation along the rear of the frontal cloud band Slow propagation, possible further cyclogenesis


22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image
04 July 2005/12.00 UTC - Meteosat 8 IR 10.8 image
22 August 2005/12.00 UTC - Meteosat 8 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i)
04 July 2005/12.00 UTC - Meteosat 8 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i)

Both waves are marked by deepening of stratospheric air behind the cloudbulge. A feature both common for the Waves, however it is more pronounced and better observed in the case of the Wave (above right). This has to do with the fact that normally no further development is expected within an upper wave in contrast to the wave (further development into Occlusion). Other differences in appearance are the clouds itself. Whereas the Wave consist of thick clouds which appear as firm structured and white in the RGB, the upper wave is more translucent and consistent of high fibrous clouds appearing more bluish (due to the cold polar air mass origin). More on the different colours observed in the Airmass RGB can be found out here

 

Meteorological Physical Background

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.
    • The dark zone in WV 6.2 imagery to the rear of the Wave bulge is a clear sign of significant tropopause folding to the rear of the cloud band, also reflected in high values of potential vorticity lower down in the troposphere.

Seen in terms of conveyor belts, the dominating mechanism of the "classic" Wave is a Warm Conveyor Belt superimposed on the Wave bulge. In contrast, the cloud bulge of an Upper Wave usually lies completely within cold advection at lower levels, caused by the intrusion of cold air within this layer. At upper levels (above 500 hPa) regions of more or less warm advection can be seen, associated with ageostrophic circulation. At 300 hPa and higher strong warm advection appears associated with the bulge of the Upper Wave. This maximum can be related to the stable air from the tropopause which is sinking to the rear of the Wave point.

The mechanisms described above are well indicated in relative streams:

  • An upper relative stream, which is sinking behind the Wave bulge and rising along the Upper Wave.
  • A rising upper relative stream dominates the Wave bulge at upper levels. The Warm Conveyor Belt is not involved in the process and the limiting streamline appears within the frontal cloud band, ahead of the Wave feature.

The case of 22 August 2005 at 12.00 UTC shows such a distribution of relative streams. In the image below left the Cold Front can be seen in the vertical cross section. The relative streams on the 310K isentrope displays sinking motion to the rear, and rising above the Upper Wave feature (below upper right).

22 August 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values
22 August 2005/12.00 UTC - Meteosat IR 10.8 image; magenta: relative streams 310K - system velocity: 222° 2 m/s, yellow: isobars 310K, position of vertical cross section indicated
22 August 2005/12.00 UTC - Meteosat IR 10.8 image; magenta: relative streams 316K - system velocity: 222° 2 m/s, yellow: isobars 316K
22 August 2005/12.00 UTC - Meteosat IR 10.8 image; magenta: relative streams 324K - system velocity: 222° 2 m/s, yellow: isobars 324K

At 316K, representing levels around 500 hPa, the sinking to the rear is still visible and the limiting streamline appears at the front edge of the cloud band (above left). At 324K an upper relative stream over the Wave bulge is still gently rising and the Warm Conveyor Belt can be seen ahead (to the east) of the cloud band and not part of the circulation of the Upper Wave.

 

Distinction between Wave and Upper Wave according to the physical background


Upper Wave Wave
Moderate rising motion - mainly at upper levels Distinct rising motion and ageostrophic circulation within the cloud band
Narrow and pronounced trough at upper levels to the rear of the frontal cloud band Significant surface trough or separate low; upper level trough to the rear of the frontal cloud band
Cold advection at lower levels due to domination of the relative streams from behind the Cold Front Dominating Warm Conveyor Belt leads to maximum of temperature advection (WA) at lower levels
Dominating upper relative stream; rising over Wave bulge, sinking behind Dominating rising Warm Conveyor Belt over the Wave bulge at middle to upper levels
Sinking of potential vorticity indicates tropopause folding, no PV >=1 units at middle and low levels of the troposphere Sinking of potential vorticity indicates tropopause folding, leading to PV > 1 units at middle and low levels causing stronger circulation at lower and middle levels.

The IR10.8 images below show the differences described above:
Below left is the case of an Upper Wave with relative streams at upper levels, below right is the case of a "classical" Wave. The case of the 16 April 2005 displays a pronounced Wave bulge over France. The relative streams at upper levels are clearly dominated by a Warm Conveyor Belt, which is contrary to the upper relative streams associated with an Upper Wave.

22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 322K - system velocity: 222° 2 m/s, yellow: isobars 316K
16 April 2005/00.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 304K - system velocity: 181° 13 m/s, yellow: isobars 304K

 

Key Parameters

  • Height contours at 500 and 1000 hPa:
    • Surface and mid level height contours: similar appearance to a CF (see Cold Front ), no separate pressure minimum or deepening trough, normally associated with a "classical" Wave (see Wave )
  • Height contours at 300 hPa:
    • Significantly sharp and deepening trough to the rear of the cloud band
  • Vorticity Advection at 300 hPa:
    • Distinct maximum caused by the advection of shear and curvature vorticity
    • Also resulting from sharp trough to the rear
    • Distinct advection of curvature vorticity through the stream field
  • Shear and Curvature Vorticity at 300 hPa:
    • Isolines of Curvature Vorticity cross the cloud band at the Wave point at an acute or nearly right angle
    • Shear vorticity: as in Cold Fronts, associated with a strong jet
  • Isotachs and Shear - Zero - Line at 300 hPa:
    • Position of the Wave bulge relative to the Isotachs depends on the stage in its life cycle: during the initial stage the Upper Wave forms at the entrance region of the jet, later the Upper Wave bulge propagates towards the exit region
    • The majority of Upper Waves studied show an extension of the cloud bulge into the side of cyclonic shear
  • Potential Vorticity:
    • Tropopause folding to the rear of the cloud band upwind of the Wave bulge (PV anomaly)
    • PV=1 has protruded downward to about 500 to 400 hPa during the initial stage
    • PV anomaly slightly weakening during life cycle (no further development of Upper Wave)
  • Temperature Advection at 700 hPa:
    • Cold Advection is associated with the Upper Wave during the whole life cycle

Height contours at 1000 and 500 hPa

22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, magenta: height contours 1000 hPa

 

Height contours at 300 hPa and PVA 300 hPa

22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; green: height contours 300 hPa, red: positive vorticity advection (PVA) 300 hPa

 

Shear and Curvature Vorticity at 300 hPa

22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; blue: shear vorticity 300 hPa, brown: curvature vorticity 300 hPa

 

Isotachs and Shear - Zero - Line at 300 hPa

22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; black: zero line of shear vorticity 300 hPa, yellow: isotachs 300 hPa

 

Potential Vorticity and TA 700

22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: potential vorticity, red: temperature advection 700 hPa

 

Distinction between Wave and Upper Wave according to the key parameters


Most parameters show similar distribution and characteristics, except:

Upper Wave Wave
No separate trough or minimum at surface Distinct short Wave trough or minimum at surface
Sharp pronounced trough at upper levels Trough at upper levels similar to fronts
CA at 700 hPa Distinct WA maximum at 700 hPa
Higher numerical values of curvature vorticity at upper levels (300 hPa) Higher numerical values of curvature vorticity at lower levels (700 hPa)

The set of IR images below show the differences described in the above table. Below left is the case of an Upper Wave, below right the case of a Wave. While the Upper Wave has no separate surface trough superimposed, the Wave case from 4 July 2005 shows a distinct surface low as well as maximum of warm advection superimposed upon the Wave bulge.


22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; red: temperature advection 700 hPa, magenta: height contours 1000 hPa
04 July 2005/12.00 UTC - Meteosat 8 IR 10.8 image; red: temperature advection 700 hPa, magenta: height contours 1000 hPa

 

Typical Appearance In Vertical Cross Sections

  • Isentropes:
    Isentropes show mainly frontal character. No impact from Upper Wave
  • Temperature Advection:
    Maximum at 300 hPa or at higher levels, connected to tropopause folding (related to potential vorticity), cold advection below the Wave bulge at lower levels
  • Vorticity Advection:
    Significant maximum of positive vorticity advection at upper levels connected to the sharpening of the upper level trough at 300 hPa
  • Vertical motion:
    Vertical motion indicates rising air, with the maximum at upper levels, due to a rising upper relative stream
22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated

 

Temperature Advection

22 August 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thin: temperature advection - CA, red thick: temperature advection - WA, orange thin: IR pixel values, orange thick: WV pixel values

 

Vorticity Advection

22 August 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), green thin: vorticity advection - NVA, green thick: vorticity advection - PVA, orange thin: IR pixel values, orange thick: WV pixel values

 

Vertical Motion

22 August 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan: vertical motion (omega) - upward motion, orange thin: IR pixel values, orange thick: WV pixel values

 

Potential Vorticity

22 August 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), dark green thin: potential vorticity <1 unit, dark green thick: potential vorticity >=1 unit, orange thin: IR pixel values, orange thick: WV pixel values

 

Distinction between Wave and Upper Wave according to vertical cross sections


Upper Wave Wave
TA indicates a CF in CA, CA dominates WA dominates at all levels in the area of cloud band
Intense PV sinking only down to 400 hPa, PV=0.8 units below 500 hPa, no significant PV at lower levels PV = 1 down at middle and low levels, interaction with upper levels
Maximum of upward motion at higher levels Strong upward motion throughout the low and mid levels of the troposphere

Below left is the case of an Upper Wave, below right is the case of a Wave (2nd row below left shows vertical cross section over Wave bulge). While the Upper Wave has no separate maximum of PV>=1 unit at low levels, the Wave case from the 4 July 2005/12.00 UTC shows a distinct maximum at low levels, which is usually more intense and can exceed PV>=2 units.

22 August 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), dark green thin: potential vorticity <1 unit, dark green thick: potential vorticity >=1 unit, orange thin: IR pixel values, orange thick: WV pixel values
04 July 2005/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), dark green thin: potential vorticity <1 unit, dark green thick: potential vorticity >=1 unit, orange thin: IR pixel values, orange thick: WV pixel values
04 July 2005/12.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated

 

Weather Events

An Upper Wave is indicative of various processes occuring at upper levels in the troposphere. There are no weather events at the surface associated with the Upper Wave bulge. The example below shows an Upper Wave over England.

Parameter Description
Precipitation
  • No precipitation under the Wave bulge. Precipitation pattern according to a Cold Front
Temperature
  • Changes according to frontal passage
Wind (incl. gusts)
  • Changes according to frontal passage
Other relevant information
  • Multilayered cloudiness
  • High level cloudiness also in form of fibres

22 August 2005/12.00 UTC - Meteosat 8 IR 10.8 image; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm, yellow:fog, black: no precipitation)

 

References

General Meteorology and Basics

  • HOSKINS B. J., MCINTYRE M. E. and ROBERTSON A. L. W. (1985): On the use and significance of isentropic potential vorticity maps; Quart. J. R. Meteor. Soc., Vol. 111, p. 877 - 946
  • KURZ M. (1990): Synoptische Meteorologie - Leitfäden für die Ausbildung im Deutschen Wetterdienst; 2. Auflage, Selbstverlag des Deutschen Wetterdienstes
  • BLUESTEIN, H.B., 1993: Synoptic Dynamic Meteorology in Midlatitudes, Volume I, Oxford Univ. Press

General Satellite Meteorology

  • BADER M. J., FORBES G. S., GRANT J. R., LILLEY R. B. E. and WATERS A. J. (1995): Images in weather forecasting - A practical guide for interpreting satellite and radar imagery; Cambridge University Press, chapter 4.5

Specific Satellite Meteorology

  • BLUESTEIN, H.B., 1993: Synoptic Dynamic Meteorology in Midlatitudes, Volume I, Oxford Univ. Press
  • BROWNING K. A. and PARDOE C. W. (1973): Structure of low-level jet streams ahead of mid-latitude cold fronts; Quart. J. R. Meteor. Soc., Vol. 99, p. 619 - 638
  • HOSKINS, B.J., DRAGHICI, I., DAVIES, H.C., 1978: A New Look at the Omega Equation, Quart. J. R. Meteorol. Soc., pp31
  • MARTIN J. E., LOCATELLI J. D. and HOBBS P. V. (1992): The synoptic evolution of a deep tropospheric frontal circulation and attendant cyclogenesis; 5th Conference on Mesoscale Processes, Atlanta, Georgia, 5 - 10 January 1992
  • SHAPIRO M. A. (1981): Frontogenesis and geostropically forced secondary circulations in the vicinity of jet stream - frontal zone systems; J. Atmos. Sci., Vol. 38, p. 954 - 973
  • UCCELLINI L. W. (1990): Process contributing to the rapid development of extratropical cyclones; in: Extratropical Cyclones, The Erik Palmen Memorial Volume, Ed. Chester Newton and Eero O Holopainen, p. 81 - 105

 

Special Investigation: Can An Upper Wave Develop Into A Wave?

When applying the conceptual model of an CM Upper Wave in operational forecasting, it is important to know if an Upper Wave can develop into a "classical" Wave and even further, to an Occlusion. Out of all cases studied about 25% show some indications of development from an Upper Wave to a Wave. But only very few cases show significant development, let alone development into true cyclogenesis.

From the point of view of key parameters and vertical cross sections, only a few parameters show a complete change into a "classic" Wave. That means that Upper Waves tend to adopt single Wave - like characteristics, e.g. amplification of the Wave bulge, but are most do not completely become "classical" Waves.

In the following example, dated 16 March 2005, there is modification of an Upper Wave when seen from the CM point of view:

 

Appearance in Satellite Data

Satellite images show at least an amplification of the cloud bulge and a retardation of the propagation of the wave along the frontal cloud band (i.e. the Wave becomes "slower"). The character of the cloud bulge is then similar to a "classic" Wave, but there is no further development of the Wave bulge.

In the IR image for loop 16 March 2005 shown below, such a development can be seen over the North Sea, Danmark and the Baltic Sea. At the end of the loop a Wave like feature can be seen over Russia.

Loop: 6 March 2005 16/18.00 - 17/15.00 UTC 3-hourly image loop
16 March 2005/18.00 UTC - Meteosat 8 IR 10.8 image

Though the case of 16 March 2005 shows a distinct amplification of the Wave bulge during its lifecycle, the Upper Wave is still dominated by upper relative streams. The limiting streamline between the Upper Relative Stream (u.r.s.) and the Warm Conveyor Belt (w.c.b.) is situated far in front (see schematic below).

In the IR10.8 image loop, below right, the relative streams on the 312K surface can be observed during the development phase. It can be seen that at this layer, which is situated in the middle of the troposphere, the Warm Conveyor Belt has minimal influence on the circulation over the Wave bulge during the following 18 hours. This pattern fits well with the CM of the Upper Wave. The processes described by relative streams do not support a transformation from an Upper Wave to a "classic" Wave.

Loop: March 2005 16/18.00 - 17/12.00 UTC 6-hourly image loop
16 March 2005/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values
16 March 2005/18.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 312K, yellow: isobars; position of vertical cross section indicated

 

Key Parameters

The most important parameters to distinguish between an Upper Wave and a Wave are the height contours at 1000 hPa and the temperature advection field at 700 hPa. By 17 March 2005 at 12.00 UTC a flat surface trough connected to the Cold Front can be seen over the cloud band. No distinct separate surface low has yet developed in the region of the Wave bulge of the Upper Wave over Estonia, although the Wave bulge has amplified significantly during the life cycle (seen in the IR10.8 loops above). A maximum of temperature advection is observed in the vicinity of the cloud bulge, but the Wave bulge itself is still under cold advection (see schematic and image below).

17 March 2005/12.00 UTC - Meteosat 8 IR 10.8 image; red: temperature advection 700 hPa; magenta: height contours 1000 hPa

 

Typical appearance in vertical cross section

Usually the parameters do not change much during the life cycle of an Upper Wave but, as mentioned before, single parameters show a tendency to develop Wave - like characteristics. If there is an amplification of an Upper Wave, the PV=1 unit, which can only be seen initially at levels higher than 500 hPa, will expand down to lower levels. In most of the cases a single maximum of PV=0.8 units develops at middle levels. This resembles the appearance of potential vorticity associated with a classic Wave, but the PV values are much less than those within a maximum of potential vorticity in a "classic" Wave.

The case of 16 March 2005 shows such a development. In the IR10.8 image loop below the vertical cross sections for each time step (6 hours) are indicated.

Loop: 16 March 2005 16/18.00 - 17/12.00 UTC 6-hourly image loop
16 March 2005/18.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated

The sequence of cross sections (see below) displays the development of a single maximum of potential vorticity at middle levels for the same period of time as in the IR 10.8 loop above (cross sections are indicated in the IR loop above). A tear - off into a secondary maximum of potential vorticity which is typical for this type of development can be seen during the loop. The numerical values of the PV maximum at low and middle levels (about 0.8 PVU) are less than those associated with a "classic" Wave. Usually values much greater than PV = 1 unit can be observed below the bulge of a Wave.

Loop: March 2005 16/18.00 - 17/12.00 UTC 6-hourly image loop
16 March 2005/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), dark green thin: potential vorticity <1 unit, dark green thick: potential vorticity >=1 unit, orange thin: IR pixel values, orange thick: WV pixel values