Saturday, February 27, 2021
Home Weather All I want for Christmas is some PV maps…

All I want for Christmas is some PV maps…

By: Ben Harvey

For a few years, the division webpages have hosted real-time plots of large-scale atmospheric situations primarily based on operational ECMWF evaluation information. Over the previous few months we’ve been revamping them with a brand new webpage and higher-resolution photos. I thought I’d describe some of the brand new plots right here, in case you discovered your self with time in your palms over the Christmas break!

First up, right here’s the brand new internet hyperlink: www.met.studying.ac.uk/~ben/current_weather.

The first set of pages (‘Dynamical Tropopause Maps’) present a set of variables at tropopause degree. More exactly, they present variables interpolated to the peak the place the potential vorticity (PV) reaches a sure worth. This is a very good proxy for the tropopause as a result of the PV, which mixes details about the stratification and rotation of air parcels, tends to be low within the troposphere and excessive within the stratosphere (see http://rammb.cira.colostate.edu/wmovl/vrl/tutorials/satmanu-eumetsat/satmanu/basic/parameters/pv.htm  for a quick intro to PV).

But why is this handy? Jet streams and synoptic-scale climate programs are intimately linked to the form of the tropopause. Figure 1 exhibits an instance from final weekend. On Sunday 13 Dec there was a powerful however wavy jet stream crossing proper throughout the North Atlantic (left panel) – a typical incidence this time of yr. The tropopause was over 10 km excessive to the south of the jet however solely 5 km excessive to its north, with the peak dropping very quickly throughout the jet core (proper panel). There’s usually unimaginable construction within the tropopause top subject; strive clicking by means of the plots on the webpage for the previous few days to discover the vary of patterns that happen.

Figure 1: Wind pace (left) and geopotential top (proper) on the tropopause from 18Z on 13 December 2020. The annotation highlights the place of the jet stream.

To assist visualise what’s happening, Figure 2 exhibits a cross part by means of the North Atlantic jet stream taken from an plane subject marketing campaign again in Autumn 2016. You can see how the tropopause top drops throughout the jet stream and the way the utmost wind speeds are inclined to lie on the tropopause itself. Figure 1 successfully follows the black tropopause line in Figure 2, and subsequently tends to select the strongest wind speeds at every location.

Figure 2: A North-south cross part by means of the jet stream (daring contours) additionally exhibiting potential vorticity (shading; models: PVU), potential temperature (skinny contours) and the tropopause (black line). This instance is from an NWP mannequin forecast. The dashed line exhibits the monitor of a analysis flight aimed toward measuring the construction of the tropopause within the core of the jet. (Adapted kind Harvey et al., 2020)

Perhaps probably the most hanging options in Figure 1 are the massive north-south meanders of the jet stream. These are Rossby waves. They happen as a result of the rotation of the Earth, mixed with its curvature, inhibits the north/south movement of air parcels on massive scales. Instead, they have a tendency to curve again to their unique latitude. Figure 3 (left panel) exhibits the north/south part of the wind (once more at tropopause degree) and the alternating green-purple sample highlights these Rossby waves and their evolution.

Figure 3: Meridional wind on the tropopause (left) and low-level equal potential temperature (proper) at 18Z on 13 December 2020. The proper panel additionally exhibits floor stress (gray contours) and a few contours of potential temperature on the tropopause (blue and black). The annotations spotlight the jet place from Figure 1.

The subsequent set of pages (‘Lower-troposphere Maps’) present floor situations, together with sea-level stress, lower-tropospheric windspeed, and equal potential temperature. These could be in comparison with the tropopause maps to grasp how the large-scale tropopause constructions relate to the floor climate we expertise, and vice versa. In our instance there are two extratropical cyclones growing beneath the meanders of the jet stream (Figure 3, proper panel). You might recall, the deeper cyclone situated simply to the west of the UK dumped fairly a little bit of rainfall over Reading on Sunday evening (13 Dec).

Finally, the ‘Isentropic PV Maps’ present the PV itself, interpolated to potential temperature surfaces. To visualise this, observe one of many potential temperature contours in Figure 2 (e.g. the one at 6 km altitude on the left fringe of the plot). Moving northwards, the floor begins within the troposphere however rises and crosses the tropopause into the decrease stratosphere and far larger values of PV. Isentropic PV maps present deep perception into the evolution of the environment as a result of, to a very good approximation, each PV and potential temperature are conserved following air parcels. This means PV options on these maps are merely advected by the winds on every floor (see Hoskins et al., 1985, for additional particulars). Any adjustments in PV following the winds could be pinned all the way down to the presence of both diabatic heating (e.g. part adjustments of water in clouds and radiative heating/cooling) or to frictional results. As such, the non-conservation of PV seems to even be very helpful for understanding the affect of those processes on climate programs and local weather extra usually. Have a click on by means of the plots from the final 2 weeks and see which PV options you possibly can monitor by means of time, and which of them are created or destroyed.

These webpages are nonetheless in growth and might be added to (when time permits!). Please ship any recommendations for enhancements/additions to b.j.harvey@studying.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 technology of unfavorable potential vorticity and its affect on the North Atlantic jet stream. Quarterly Journal of the Royal Meteorological Society146(728), pp.1477-1497.

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