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Appearance in Satellite Data

13 January 2008/12.00 UTC, Meteosat 9 IR10.8 image

The image above shows a large system over the Atlantic and the western and northwestern Europe, consisting of Occlusion, Warm Front and Cold Front. South of Ireland there is a Wave on the front and behind it a large area, containing differently shaped cellular cold air clouds, can be observed. The cloud tops, white in the IR image, are the tops of the clouds which are the topic of this chapter - EC or Enhanced Cumulus. They are associated not only with the enhanced instability but also with some other effects causing increased upward motion, which will be discussed in the following chapters (see Meteorological physical background and Key parameters).

For EC (Enhanced Cumulus) areas the following satellite image features exist:

  • The satellite images show a meso-scale area of cellular, sharp edged cloud clusters, with some variety in shape and size. Usually these cloud clusters are surrounded by cloud cells with warmer tops, i.e. typical cellular Cold Air Cloudiness.
  • The enhanced cells of EC are usually white in the IR, VIS and WV images indicating thick, multi-level convective clouds.
  • There are types of ECs which are white in VIS but only light grey to white in IR and grey in WV indicating that the tops do not reach to high levels
  • In well-developed situations, the cirrus shields of single Cb cells merge, leading to a very cold smooth cloud shield in the IR but a transparent appearance in the VIS image.
  • There are different developments associated with ECs:
    • Under certain circumstances Cold Air Cloudiness (CAC) grows into EC
    • EC can grow further into a Comma, and
    • A Comma can grow into Cold Air Development (CAD).

The diagrams below show different kinds of convective cloud as well as the difference between Cb Clusters and ECs.

13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image (zoom)
13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image
13 January 2008/12.00 UTC - Meteosat 9 HRVIS image (zoom)
13 January 2008/12.00 UTC - Meteosat 9 HRVIS image
13 January 2008/12.00 UTC - Meteosat 9 WV 6.2 image (zoom)
13 January 2008/12.00 UTC - Meteosat 9 WV 6.2 image

In the images above, one can take a closer look at the cloud structures introduced at the beginning of the chapter. ECs usually show up behind the Cold Front. They can be recognized as the accumulation of cells larger than the cold air cloudiness around, brighter than the surrounding cells in the IR (which means colder) and in the VIS (HRVIS) image (which means thicker). The cells are here well developed and easily detectable also in the WV channel, which is not often the case.

High spatial resolution of 1km in SSP (Sub-Satellite-Point), provided by Meteosat HRVIS images, makes small-scale cloud structures more distinct. In the above images the meso-scale area with cellular cloud clusters is nicely visible.

Typical Developments: From Cb Cluster to EC

The case of 02 April 2003, below, shows a development over the North Sea immediately east of England over a period of 3 hours. At the beginning (left image, 02.00 UTC) there are only single cells while at 05.00 UTC a more compact area of cells has developed which has all the features of an EC.

02 April 2003/02.00 UTC - Meteosat IR image
02 April 2003/05.00 UTC - Meteosat IR image

The following loop shows with half-hourly time steps the development from a Cb Cluster to an EC.

02 April 2003/02.00 - 05.00 UTC - Meteosat enhanced IR image; half-hourly image loop

Typical Developments: From EC to Comma

The following 3-hourly loop shows the transition of an EC configuration to a Comma feature.

22 November 2002/21.00 UTC - Meteosat IR image; EC and Comma indicated; 22 November 21.00 - 24 November 00.00 UTC 3-hourly image loop

First there is a rather rapid increase in the extent of the EC from which a Comma spiral then develops over a longer period of time. Looking only at the satellite images the EC moves rather quickly from an area within the Cold Air Cloudiness, which is far away from the front across the cold air region to an area much closer to the front. During this movement the cloud area develops a spiral structure and finally becomes a very well developed Comma.

Appearance in Meteosat RGB composite imagery

To take into account and combine different types of information retrieved from SEVIRI imagery, such as optical thickness of clouds, particle size and phase, upper and mid level moisture and cloud top temperature, combinations of channels are constructed.

As the name itself says, the Dust RGB is more suited for the detection of dust in the atmosphere but it can also be used for convection monitoring. The icing in the clouds makes the cells appear red, so when the clouds change their colours from orange to the reddish ones we can conclude that there is vertical motion going on which is building up cumuli-like clouds.

13 January 2008/12.00 UTC -Meteosat 9 Dust RGB image (IR12.0-IR10.8, IR10.8-IR8.7, IR10.8) (zoom)
EC as it appears in Dust RGB

One of the RGBs frequently used for convection detection and monitoring is so called Convective Storms RGB (WV6.2 -WV7.3, NIR3.9 -IR10.8 and NIR1.6 -VIS0.6). In this type of composite cold Cb tops with small ice particles appear in yellowish colours and well developed Cbs with large ice particles appear red.

In the Convective Storms RGB shown below ECs appear in different shades of pink. The most striking feature of clouds over the Atlantic is again their cellular shape.

13 January 2008/12.00 UTC -Meteosat 9 Convective Storms RGB image (WV6.2 -WV7.3, NIR3.9 -IR10.8 and NIR1.6 -VIS0.6) (zoom)
EC as it appears in Convective Storms RGB

The Airmass RGB (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i) is very useful for detection of the Enhanced Cumuli clouds for several reasons. ECs are expected to be present in the cold air behind the front, so they will be usually seen in the dark bluish areas. Moreover, reddish bands behind the front locate the regions favorable for convection because they point out the areas where the dry stratospheric air, which enhances the instability, is intruded. Although EC is thick and reaches high levels and as such would appear white, it is in fact pinkish due to the cold environment with high PV values in which it is most frequently found.

13 January 2008/12.00 UTC -Meteosat 9 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i) (zoom)
EC as it appears in Airmass RGB

Appearance in AVHRR imagery

  • In the observed areas to the east and southeast of Iceland, AVHRR channel imagery show more detailed structures than Meteosat imagery.
  • Comma clouds to the NNE and N. of Iceland (see below) have developed from the Enhanced Cumulus (EC). Note: these areas are too far to the north to be seen by Meteosat.
  • RGB-combination of channels (below left, second row) provides a quick overview of high and middle level cloudiness.
  • Channel manipulation (third row, right) highlights cloud patterns and physical features.

17 February 2000/04.35 UTC - NOAA RGB image (channel 3, 4 and 5)
17 February 2000/04.35 UTC - NOAA CH5 image; EC in Atlantic: SE and E. of Iceland (approx. 63N/15W)

In the images above, cold air is flowing from Greenland to the Norwegian Sea. Cold Air Cloudiness develops downstream, and there are Enhanced Cumulus areas southeast of Iceland (northeast of Iceland a "Comma" can also be seen). In the EC area the white spots are associated with high cold cloudiness (clustering Cb). At the edges of these white spots thin high and middle level clouds can be recognized.

17 February 2000/04.35 UTC - NOAA RGB image (channel 3, 4 and 5)
17 February 2000/04.35 UTC - NOAA RGB image (channel 1, 2 and 4); EC in Atlantic: SE and E. of Iceland (approx. 64N/10W)

In the images above, cold air is flowing to the south of Iceland from Greenland to the Norwegian Sea. Enhanced Cumulus areas east of Iceland are located near the left exit of a jet streak. At the east side of this cloudiness, large shadows can be seen since sun elevation is low and Scandinavia already dark. (At this latitude (+/- 65N) Meteosat is not very useful any more because of the low resolution of a geostationairy satellite at that latitude.)

17 February 2000/04.35 UTC - NOAA CH2 image
17 February 2000/04.35 UTC - NOAA CH1 minus CH3B- image; EC in Atlantic: SE and E. of Iceland (approx. 64N/10W)

In the images above, the EC cloud patterns east of Iceland are easily identified, with a high probability of precipitation. The thin, lower level cloud, seen in the image above left (and also in the second row of images), is filtered out of the above right image.

Meteorological Physical Background

Enhanced Cumulus areas (ECs) are mesoscale cloud phenomena, which develop within the cold air mass behind a Cold Front in the area of an upper level trough. They do not develop at the boundary of two air masses like Cold or Warm Fronts. In some cases ECs are embedded within cellular cloudiness with tops reaching only up to the lower levels of the troposphere (see Cloud structure in satellite image). Although ECs develop in cold air, temperature advection is relatively small. The two main conditions for development of EC are:

  1. Increased vertical instability in the troposphere;
  2. PVA maxima
    • either immediately in front of the upper level trough, where the air parcels are under the influence of strong cyclonic rotation and PVA indicates a moving trough
    • or in the left exit region of a jet streak where PVA indicates the advection of more cyclonic shear

enhanced_cumulus

Instability is an important requirement for the formation of any type of Cold Air Cloudiness. EC can only develop with additional dynamical forcing.

Instability

Instability can be generated in several different ways. Two possibilities are mentioned here:

Cold air streaming over warm sea water

This type mostly occurs in autumn and winter. The cold air streaming over the sea surfaces is heated from below, due to the relatively warm water temperatures in comparison to the air (diabatic heating). As a consequence of this heating the atmosphere becomes unstable and lifting, with additional condensation, takes place. This additional condensation leads to the release of latent heat. According to the omega equation, latent heat will force vertical motion. Therefore more condensation will take place, because of the cooling of the air by upward vertical motion.

On the other hand, the upper levels of the troposphere are characterised by sinking dry air, which may even originate from the stratosphere. Looking at the situation on isentropic surfaces, this sinking motion is caused by the relative stream of the dry intrusion. Therefore the upward motion in the convective cells is restricted from above and after some time an equilibrium between upward motion (represented by the cloud cells) and downward motion (indicated by the cloud-free areas in between) develops, resulting in large areas filled up with the so-called cellular Cold Air Cloudiness (see Cloud structure in satellite image).

Cold air streaming over warm land surface

This type mostly occurs in spring and summer, when cold air flows over a warmed land surface.

For the development of ECs within a cold air mass, additional dynamical forcings plays a significant role.

Vorticity Advection

Curvature Vorticity and its advection

ECs usually appear immediately in front of an upper level trough, but they can also appear immediately in front of the main upper level trough as well as behind it if there is a secondary small trough.

At the downstream side of the trough axis positive vorticity advection appears. In the area of the maxima of positive vorticity advection the development of Cold Air Cloudiness into EC is likely to occur. Usually those mesoscale PVA maxima are connected to secondary troughs within the synoptic scale upper level trough.

EC in the area of maximum positive vorticity advection at the leading edge of a small trough preceding a pronounced upper level trough
EC in the area of maximum positive vorticity advection in front of a pronounced upper level trough
EC in the area of maximum positive vorticity advection at the leading edge of a small trough behind the main upper level trough
13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; cyan: height contours 500 hPa, green: vorticity advection 500 hPa
24 April 2008/12.00 UTC - Meteosat 9 IR10.8 image; cyan: height contours 500 hPa, green: vorticity advection 500 hPa

In both examples there is a huge synoptic scale upper level trough with PVA maxima indicating areas where cyclonic rotation is greater than in the surrounding region. In the example of the 13th of January it is more evident than in the one of the 24th of April. The correspondence of the ECs with the PVA maxima is evident.

Shear Vorticity

EC and Cb clusters often develop in the left exit of a jet streak. The left exit of the jet is on the cyclonic side of the jet. Upstream from the left exit there is a maximum of positive or cyclonic shear vorticity. As a consequence of the movement of the jet, a vorticity advection maximum will be found in the left exit of the jet. This maximum will lead to divergence in the upper levels of the troposphere, resulting in upward motion at middle levels (see Front Intensification by Jet Crossing ).

24 April 2008/12.00 UTC - Meteosat 9 IR10.8 image; cyan: height contours 500 hPa
24 April 2008/12.00 UTC - Meteosat 9 IR10.8 image; yellow: isotachs 500 hPa, green: Positive vorticity advection 500 hPa
24 April 2008/12.00 UTC - Meteosat 9 IR10.8 image; yellow: isotach 300 hPa, red: positive vorticity advection 300 hPa
13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; cyan: height contours 500 hPa
13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; yellow: isotachs 500 hPa, green: vorticity advection 500 hPa
13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; yellow: isotach 300 hPa, red: Positive vorticity advection 300 hPa

For shear vorticity the example of 24th of April is more appropriate because there is strong jet which is more visible at 300 hPa level. The ECs over Great Britain are placed very near to the left exit region of a jet streak but the other ones are also located in the region where the shear exists although there is no PVA maximum in the fields.

Looking at PVA maxima as additional driving factors for the development of EC cloud, the increase of PVA with height is the most important factor in the development of ECs. Most cases show a PVA maximum between 700 hPa and 300 hPa, and because of this it more usually seen on 500 hPa than on 300 hPa. This differential vorticity advection causes divergence in the upper levels resulting in upward motion at mid levels of the troposphere. Most cases show a maximum vertical motion(omega) of -18 hPa/h. The average level of maximum upward motion is around 600 hPa.

Mean vorticity advection of 30 investigated cases. P5 = vorticity advection at 500 hPa, P3 = vorticity advection at 300 hPa.

enhanced_cumulus

Key Parameters

  • Equivalent thickness and temperature advection at 700 hPa:
    In the area of EC the equivalent thickness is marked by a trough or a minimum, indicating cold air. This is an area of relatively low potential wet bulb temperature (ThetaW) values at 850 hPa. Usually this is also an area of synoptic scale CA; but there is no distinct relation between an EC and a CA maximum; ECs can also be in areas of smaller scale WA (for instance in connection with a following Warm Front system)
  • Height contours at 500 hPa:
    EC and Cb Cluster features are connected to upper level troughs; many of them develop ahead of embedded smaller, rapidly moving upper level troughs. This happens usually at the leading edge of the trough. If ECs are at the rear side of troughs they are in an area of high shear and are connected to jet streaks.
  • Isotachs:
    ECs and Cb Clusters often develop at the left exit region of a strong jet streak. This is often close to the rear of the upper level trough
  • Positive Vorticity Advection (PVA) at 500 and 300 hPa:
    Vorticity advection shows maxima at 500 and 300 hPa; vorticity advection generally increases in the layer between 500 and 300 hPa.
  • Instability indices:
    EC and Cb cluster appear in areas of instability characterised by the Boyden Index (>95) or the Showalter index (<3); but the values are typically less unstable than in the warm air ahead of fronts.

Equivalent thickness and temperature advection at 700 hPa

13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; green: equivalent thickness 500/850 hPa, blue: thermal front parameter 500/850 hPa, red: temperature advection 700 hPa
24 April 2008/12.00 UTC - Meteosat 9 IR10.8 image; green: equivalent thickness 500/850 hPa, blue: thermal front parameter 500/850 hPa, red: temperature advection 700 hPa

The example of 24th of April is better since the most of ECs are located within the cold air.

Height contours and Vorticity Advection at 500 and/or 300 hPa

13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; cyan: height contours 500 hPa, green: vorticity advection 500 hPa
24 April 2008/12.00 UTC - Meteosat 9 IR10.8 image; cyan: height contours 500 hPa, green: vorticity advection 500 hPa

Isotachs and PVA at 500 and/or 300 hPa

24 April 2008/12.00 UTC Meteosat 9 IR10.8 image; yellow: isotachs 500 hPa, green: vorticity advection 500 hPa
24 April 2008/12.00 UTC - Meteosat 9 IR10.8 image; yellow: isotachs 300 hPa, red: voriticity advection 300 hPa
13 January 2008/12.00 UTC Meteosat 9 IR10.8 image; yellow: isotachs 500 hPa, green: voriticity advection 500 hPa
13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; yellow: isotachs 300 hPa, red: voriticity advection 300 hPa

In both cases the PVA at 500 hPa is evident. The PVA at 300 hPa is not seen in this two examples. One of the reason is that this cases are winter cases when the tropopase is much lower then in the summer time. The PVA shows its maximum usually bethween 700 and 300 hPa, so it is more notable at 500 hPa then at 300 hPa.

Instability

24 April 2008/12.00 UTC - Meteosat 9 IR10.8 image; green: equivalent thickness 500/850 hPa, yellow: Showalter index
13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image; green: equivalent thickness 500/850 hPa, yellow: Showalter index

Typical Appearance In Vertical Cross Sections

  • Isentropes:
    ECs and Cb clusters exist within an unstable airmass. This can be observed in the cross section by decreasing values of isentropes with height within lower and mid-levels of the troposphere (usually between surface and 700 hPa).
  • Relative humidity:
    There are high values of relative humidity within the lower and mid-levels of the troposphere. In the layer between 1000 - 700 hPa, a humidity maximum can often be observed. The relative humidity isolines show a maximum (90-100%) near the surface, but also relative maxima higher in the atmosphere in the vicinity of the clusters.
  • Divergence:
    The field of divergence shows convergence within lower and mid-levels in the area of the EC and divergence above.
  • Vorticity advection:
    The field of vorticity advection is characterised by maxima generally at higher levels of the atmosphere. A maximum can often be observed between 500 and 300 hPa.
  • Vertical motion:
    A maximum of vertical velocity is found in the layer between 700 and 600 hPa.

24 April 2008/12.00 UTC - Meteosat 9 IR10.8 image; position of vertical cross section indicated
13 January 2008/18.00 UTC - Meteosat 9 IR10.8 image; position of vertical cross section indicated

Isentropes

24 April 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE)
13 January 2008/18.00 UTC - Vertical cross section; black: isentropes (ThetaE)

Relative Humidity

24 April 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity
13 January 2008/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity

Divergence

24 April 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thin: divergence, magenta thick: convergence
13 January 2008/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thin: divergence, magenta thick: convergence

Vorticity Advection

24 April 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), green: vorticity advection
13 January 2008/18 UTC - Vertical cross section; black: isentropes (ThetaE), green: vorticity advection

Vertical Motion

24 April 2008/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion
13 January 2008/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), cyan thick: vertical motion (omega) - upward motion, cyan thin: vertical motion (omega) - downward motion

Weather Events

ECs are convective cells often with heavy precipitation.

Parameter Description
Precipitation
  • Moderate to heavy rain or snow showers
  • Hail and thunderstorms are likely.
Temperature
  • Surface temperature and sea surface temperature are forcing features of convection.
Wind (incl. gusts)
  • Strong gusts are common
Other relevant information
  • Risk of moderate to severe icing and turbulence
  • Sometimes waterspouts or small tornadoes are observed
  • Poor visibility during heavy snowfall

04 October 1999/15.00 UTC - NOAA IR image
04 October 1999/15.00 UTC - KNMI Radar image
04 October 1999/15.00 UTC - IR image; weather reports

References

General Meteorology and Basics

  • BLUESTEIN H.B. Synoptic Dynamic Meteorology in Midlatitudes - Volume I Principles of Kinematics and Dynamics; Oxford University Press.
  • BLUESTEIN H.B. Synoptic Dynamic Meteorology in Midlatitudes - VolumeII Obeservations and Theory of Weather Systems; Oxford University Press.
  • BROWNING K. A. (1986): Conceptual models of precipitation systems; Weather & Forecasting, Vol. 1, p. 23 - 41
  • CONWAY B. J., GERARD L., LABROUSSE J., LILJAS E., SENESI S., SUNDE J. and ZWATZ-MEISE V. (1996): COST78 - Meteorology - Nowcasting, a survey of current knowledge, techniques and practice - Phase 1 report; Office for official publications of the European Communities

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
  • ZWATZ-MEISE V. (1987): Satellitenmeteorologie; Springer Verlag, Berlin - Heidelberg - New York - London - Paris - Tokyo

Specific Satellite Meteorology

  • DELDEN A. van (1997): The synoptic setting of a thundery low and associated prefrontal squall line in western Europe, Meteorol. Atmos. Phys. 65, 113-131 (1998)
  • HONGNIAN D. A (1993): Synoptic climatology of convective weather in the Netherlands, KNMI, Scientific Reports; WR 93_04