Notes

This page shows examples of the 'diagnostics in action' to give an idea of what you may expect to see in periods of peak AEW activity. All of the images shown come from our 2005 season archive, no post-processing has taken place. Below each example a brief explaination is given.

This example shows the main 4-panel image for 2nd Sept 2005. 3 AEWs can be seen: (i) A relatively small AEW moving into the Atlantic basin near 20W (ii) A 'Mature' AEW near the Greenwich meridian, which possesses many of the typical AEW traits (e.g. twin low-level vortices either side of the AEJ) and (iii) A incipient AEW forming over western Chad/easter Nigeria near 15E.

This example is chosen as it illustrates a common trait often associated with mature AEWs. Here, we can see two distinct vorticity centres associated with the AEW near the west African coast (near 10W). The nothernmost of the two (centred on approx 17N, 13W) is poleward of the AEJ axis and is colocated with some of the highest 925hPa potential temperature values observed over tropical north Africa. The southern most vorticity centre (centred near 8N, 10W) is equatorward of the AEJ axis and is colocated with active deep convection (not shown). This structure conforms with the composite structures shown in the literature and the conceptual model put forward by Berry and Thorncroft, 2005.

The image selected (for 25th September 2005) illustrates the impact of AEWs on the low-level monsoon flow. Note behind the trough located near 5W, high equivalent potential temperature values are surging northwards through eastern Mali, consistent with the southerly monsoon flow behind the AEW trough. Ahead of this trough in the highest theta-e values, strong localized ascent can be seen, suggesting the presence of deep convection.

The model CAPE and CIN allow further insight into the impact of AEWs on convection that is suggested in the theta-e/vertical motion field shown previously. In the example time shown above (2nd October 2005) it can clearly be seen that the trough line near 3W seperates regions of high CAPE at relatively low latitudes (ahead of the trough) from regions of lower CAPE that spread further northwards (behind the trough). An area of persistently high CIN can be observed along the west African coast north of 15N, which results from an inversion between the relativel cool marine boundary layer and the elevated Saharan mixed layer.

The example shows pronounced contrasts in the factors that contribute to the maintainence of deep convection ahead of and behind the AEW trough. Note to the west of the trough located near 8E there is strong vertical wind shear in the 925-700hPa layer with significant moisture flux convergence. At the trough axis there is an abrupt reduction in the magnitude of the vertical wind shear and the moisture flux convergence, suggesting that convection in this case is favoured ahead of the trough axis.

This example shows an active scene from the 2005 season. In the Gulf of Mexico hurricane Katrina (which can be tracked earlier as being associated with an AEW) can be seen. In the mid-Atlantic two AEWs can be seen (near 55W and 40W), both these system have an African history and both exhibit PV maxima associated with deep convection. These systems ultimately are absorbed into the extratropics via interation with an extratropical PV tail that can be observed moving equatorwards near 30N between 20 and 40W. Another AEW can be seen moving off the West African coast - this system exhibits two troughs which have arisen from convective bursts (and the diabatic generation of PV) at different locations within the AEW. Over the ocean, the PV centres merge and result in a single trough.

This images shows a mature oceanic AEW near 40W, which is interacting with an extratropical PV tail (located north of 20N). The IR imagery also suggest an interaction, with cold cloud streaming northwards on the eastern periphery of the extratropical feature. This Particular AEW is completely absorbed into the extratropics a few days later by this PV tail. A second weaker AEW can be observed near 70W extending from Puerto Rico to Columbia, where it is colocated with a region of deep convection. Over the next few days this AEW moved northwestwards along the central american mainland and may have associated with Hurrican Fernanda in the east Pacific basin.