All I want for Christmas is some PV maps…

By: Ben Harvey

For many years, the department webpages have hosted real-time plots of large-scale atmospheric conditions based on operational ECMWF analysis data. Over the last few months we’ve been revamping them with a new webpage and higher-resolution images. I thought I’d describe some of the new plots here, in case you found yourself with time on your hands over the Christmas break!

First up, here’s the new web link: www.met.reading.ac.uk/~ben/current_weather.

The first set of pages (‘Dynamical Tropopause Maps’) show a suite of variables at tropopause level. More precisely, they show variables interpolated to the height where the potential vorticity (PV) reaches a certain value. This is a good proxy for the tropopause because the PV, which combines information about the stratification and rotation of air parcels, tends to be low in the troposphere and high in the stratosphere (see http://rammb.cira.colostate.edu/wmovl/vrl/tutorials/satmanu-eumetsat/satmanu/basic/parameters/pv.htm  for a brief intro to PV).

But why is this useful? Jet streams and synoptic-scale weather systems are intimately linked to the shape of the tropopause. Figure 1 shows an example from last weekend. On Sunday 13 Dec there was a strong but wavy jet stream crossing right across the North Atlantic (left panel) – a common occurrence this time of year. The tropopause was over 10 km high to the south of the jet but only 5 km high to its north, with the height dropping very rapidly across the jet core (right panel). There’s often incredible structure in the tropopause height field; try clicking through the plots on the webpage for the past few days to explore the range of patterns that occur.

Figure 1: Wind speed (left) and geopotential height (right) at the tropopause from 18Z on 13 December 2020. The annotation highlights the position of the jet stream.

To help visualise what’s going on, Figure 2 shows a cross section through the North Atlantic jet stream taken from an aircraft field campaign back in Autumn 2016. You can see how the tropopause height drops across the jet stream and how the maximum wind speeds tend to lie on the tropopause itself. Figure 1 effectively follows the black tropopause line in Figure 2, and therefore tends to pick out the strongest wind speeds at each location.

Figure 2: A North-south cross section through the jet stream (bold contours) also showing potential vorticity (shading; units: PVU), potential temperature (thin contours) and the tropopause (black line). This example is from an NWP model forecast. The dashed line shows the track of a research flight aimed at measuring the structure of the tropopause in the core of the jet. (Adapted form Harvey et al., 2020)

Perhaps the most striking features in Figure 1 are the large north-south meanders of the jet stream. These are Rossby waves. They occur because the rotation of the Earth, combined with its curvature, inhibits the north/south motion of air parcels on large scales. Instead, they tend to curve back to their original latitude. Figure 3 (left panel) shows the north/south component of the wind (again at tropopause level) and the alternating green-purple pattern highlights these Rossby waves and their evolution.

Figure 3: Meridional wind at the tropopause (left) and low-level equivalent potential temperature (right) at 18Z on 13 December 2020. The right panel also shows surface pressure (grey contours) and a couple of contours of potential temperature on the tropopause (blue and black). The annotations highlight the jet position from Figure 1.

The next set of pages (‘Lower-troposphere Maps’) show surface conditions, including sea-level pressure, lower-tropospheric windspeed, and equivalent potential temperature. These can be compared to the tropopause maps to understand how the large-scale tropopause structures relate to the surface weather we experience, and vice versa. In our example there are two extratropical cyclones developing beneath the meanders of the jet stream (Figure 3, right panel). You may recall, the deeper cyclone located just to the west of the UK dumped quite a bit of rainfall over Reading on Sunday night (13 Dec).

Finally, the ‘Isentropic PV Maps’ show the PV itself, interpolated to potential temperature surfaces. To visualise this, follow one of the potential temperature contours in Figure 2 (e.g. the one at 6 km altitude at the left edge of the plot). Moving northwards, the surface starts in the troposphere but rises and crosses the tropopause into the lower stratosphere and much higher values of PV. Isentropic PV maps provide deep insight into the evolution of the atmosphere because, to a good approximation, both PV and potential temperature are conserved following air parcels. This means PV features on these maps are simply advected by the winds on each surface (see Hoskins et al., 1985, for further details). Any changes in PV following the winds can be pinned down to the presence of either diabatic heating (e.g. phase changes of water in clouds and radiative heating/cooling) or to frictional effects. As such, the non-conservation of PV turns out to also be very useful for understanding the impact of these processes on weather systems and climate more generally. Have a click through the plots from the last 2 weeks and see which PV features you can track through time, and which ones are created or destroyed.

These webpages are still in development and will be added to (when time allows!). Please send any suggestions for improvements/additions to b.j.harvey@reading.ac.uk.

Hoskins, B.J., McIntyre, M.E. and Robertson, A.W., 1985. On the use and significance of isentropic potential vorticity maps. Quarterly Journal of the Royal Meteorological Society111(470), pp.877-946.

Harvey, B., Methven, J., Sanchez, C. and Schäfler, A., 2020. Diabatic generation of negative potential vorticity and its impact on the North Atlantic jet stream. Quarterly Journal of the Royal Meteorological Society146(728), pp.1477-1497.

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