850-1000 hPa: This is useful for defining the temperature structure in the lowest 1500 m or so of the atmosphere, and can therefore be used in such things as rain/snow prediction, maximum temperature forecasting etc.
700-1000 hPa: Similar to 500-1000 hPa but focussed rather more on the lowest 3 km of the atmosphere and therefore an attempt to combine the broader measure of the 500-1000 hPa and the finer details obtained by layers nearer the earth's surface.
500-700 hPa/700-850 hPa: Used in studies of differential thermal advection, particularly when considering possible convection, degrees of instability etc.
[ Q ] What is the historical relevance of 'gridding'?
[ A ] Before the advent of super-fast main-frame computers, and the better understanding of the character and physics of the upper air, upper wind forecasts up to 24 hours ahead depended upon a process known as 'gridding' - the arithmetical manipulation of layer thicknesses. A surface/msl pressure chart would be analysed, then the isobaric pattern would be converted to equivalent 1000 hPa height contours, taking into account the temperature if this deviated significantly from the 'standard'; the thickness pattern would be drawn, using thermal wind relationships and known patterns associated with frontal systems, then the 1000 hPa and thickness patterns overlaid, and at the intersection of the 'grid' of such contours, the resultant 500 hPa (or other height, depending upon the thickness used) could be achieved, using the relationship that h(500) = h(1000)+h'(thickness).
This gave better results than just using the poor network of actual 500 hPa heights/winds, and by using thicknesses (i.e. differences), the systematic errors of differing radio-sondes could be effectively ignored .
More importantly, to produce a forecast upper air chart, first the forecast surface pattern was produced using known empirical relationships/rules of thumb etc., then the thickness pattern adjusted to fit around this forecast pattern, keeping the correct relationship found both in analysis and conceptual models in mind, then again by gridding, or graphical addition of the 1000 hPa contours and the thickness pattern, the upper contour pattern could be produced. It was essentially this process that was used to forecast all upper winds until the work could be put on a more rigorous/mathematical basis by the solving of complex equations using the big number- crunching machines from the late 1960's. (Incidentally, you can tell the vintage of someone working in meteorology by whether they refer to the parameter 500-1000 hPa thickness as the Total thickness ... a hangover from these days of gridding charts.)
Further reading: For a useful summary of this method of upper wind forecasting, see Ref
2) and for a historical perspective, see Ref
4) "Bomber Command upper air unit" - RAS Ratcliffe.
[ Q ] Does gridding have any relevance today?
[ A ] The reverse of gridding (graphical addition of thickness values to a lower surface height), is de-gridding, and can be a useful technique to master, both to achieve a surface pattern from a 500 hPa/Total Thickness chart, and to attain a simplified conceptual idea of development due to upper air processes.
From the definition of Total Thickness (h') where: h' = h(500) - h(1000) ..... Eq(d)
where h' = total thickness
h(500) = height of the 500 hPa barometric surface
h(1000) = height of the 1000 hPa barometric surface
re-arranging Eq(d), we have h(1000) = h(500) - h' ... Eq(e)
At levels near msl, the
approximate relationship [
see cautionary note below # ] holds that a difference of 6 dam = a difference of 8 hPa. ... Eq(f)
Thus, from Eq(e), if at a certain point on a chart, a 500 hPa contour of 540 dam, is crossed by a thickness isopleth of 528 dam, the value of h(1000) = 540-528 = +12 dam. From Eq(f), this relates to an isobaric value of (12/6)*8 = +16 hPa (or 1000+16 = 1016 hPa) All other intersections of the 540 hPa/500 contour, and the 528 thickness isopleth yield the same value. Consideration of other intersections will build up a pattern of 1000 mb (and hence msl) values, and the surface pattern can then be inferred from these two fields.
We can go further, and see that it is clear from Eq(e) that by either
decreasing the 500 hPa contour value (trough approaching), or
increasing the thickness values (warm advection), the height of the 1000 hPa surface will lower, and because 1000 hPa and mslp are linked, mslp will lower===> development. Where
both terms are strong (omega development, whereby an upper short- wave trough engages an area of strong warm advection), then explosive cyclogenesis is possible, all other factors being suitable. This is of course a very simplistic explanation of development, but it is a useful conceptual idea to keep in mind when trying to interpret upper air charts.
[ # The relationship 6 dam=8 mbar is a
very approximate one, and was accepted at the time because upper air charts were drawn to 6 dam intervals, surface charts to 8 mbar intervals (or multiples thereof), and accepting the inferred 0.75 dam per millibar (i.e. 6/8) relationship kept things simple. In reality, using the hydrostatic equation (Eq(b) above), for average mean sea level pressure of 1013mbar, average surface temperature of 15degC and taking a narrow 'slice' of the air around mslp, then the true value is
0.83 dam/mbar: other values are 0.80 dam/mbar for mean temperature 0degC and 1000mbar, and at 20degC and 1000mbar, it would be 0.86]
[ Q ] How are thickness isopleths shown on synoptic charts?
[ A ] Total Thickness (500-1000 hPa) isopleths are conventionally drawn as long-dash lines, with the values either thus [540] or white numerals on a black/solid rectangle. Certain isopleths are considered 'standard', mainly for historical reasons, and are coloured (in UK Met.O use) according to the following convention):
474 -
red
492 -
purple
510 -
brown
528 -
blue
546 -
green
564 -
red
582 -
purple
Operational charts usually show isopleths at 6 dam intervals but some international forecast output will only have the standard isopleths as above: for example the Bracknell 2-5 day charts.
[ Q ] What about advection? How can I use it?
[ A ] Advection is simply the meteorologists word for movement of air in bulk. When we talk about warm advection, we mean that warm air replaces colder air, and vice-versa. These bulk movements of air of differing temperatures can be seen very well on thickness charts, and differential advection, important in studies of stabilisation/de-stabilisation, can also be inferred by considering advection of partial thicknesses.
Example:
If you pull up thickness charts from the web, it it useful to highlight the isopleths of thickness, and work out, from either the mslp pattern, or the 500 hPa pattern, (or indeed any wind in the layer) whether cold or warm advection is taking place. It should be possible with practice to find warm and cold fronts (tight thickness pattern), and areas where the thermal gradient (spacing of thickness lines) is changing - note particularly areas where developments tend to decrease spacing of thickness lines -->> increased potential for atmospheric development. See also the section relating to
thermal winds
Below, is an example of a mslp and thickness chart drawn from the NWS site. Red (warm) and blue (cold) arrows show some areas of significant advection.
[ Q ] What about the various patterns of thickness isopleths?
[ A ] The most obvious patterns that the thickness isopleths can take up look very like those you would see on a surface/isobaric chart: highs, lows, troughs and ridges. A closed high-value contour, usually labelled 'W' is referred to as a warm dome; a closed low-value contour (or series of same), labelled 'K' is a cold pool. The 'W' and 'K' denote WARM and KALT respectively - possibly from the Norwegian/German roots of much of the research into upper air patterns. Cold pools are especially important, being often the first indications that the potential for small/mesoscale deep and vigorous convective activity exists, given a suitable trigger action and sufficient moisture. ( They are sometimes not resolved by NWP suites adequately either, particular those with long grid lengths. ) The horizontal spacing of thickness isopleths is also a useful indicator of the potential for development. Close spacing shows that cold air lies adjacent to warm air -- no doubt with an attendant frontal surface. More importantly, the fact that such a baroclinic zone (surfaces of temperature and pressure intersecting at an angle) exists, means that the
potential for development is strong -- a slight displacement, i.e. forcing by a high level jet streak, will lead to substantial falls of pressure and consequent 'weather'.
(Examples of all these features, with further notes, can be found by clicking here.)
Meteorologists will always play close attention to zones of 'tight' thickness contouring for this reason. Troughs and ridges denote tongues of cold and warm air respectively, and, from work by
RC Sutcliffe (& others) during and after the Second World War, they can also be used to infer the magnitude and sign of development on the surface. The details of the mathematics are beyond this FAQ, but it is only necessary to recognize the patterns as below:
[ Q ] Can the 500-1000 hPa patterns be used to infer the snow risk?
[ A ] Until the advent of NWP suites capable of using model variables to routinely predict the 850-1000 hPa partial thickness (the one now commonly used in the UK to assess likelihood of snow - see below), the 500-1000 hPa total thickness was used much more extensively than now. In the 1950's and 1960's, some studies were published which attempted to refine the TTHK association with snow/rain prediction ... the results are presented below BUT REMEMBER, better predictors are available and should be used where possible.
(1) Lamb: Q Jnl R.Met.Soc. 1955 Critical value for equal probability of rain and snow(NW Europe)
Average: 527 dam (extremes: 521 to 546)
Established snowfields: 536 dam
Windward edge of snowfields: 528 dam
Seas with SST around 10degC and over windward coasts: 523 dam
Lamb's analysis was undertaken for the whole of NW Europe, including places like Riga and Stockholm, which may explain the difference in the average figure from that in the following section. However, Lamb did make a more detailed analysis for inland stations in the British Isles, where he found that when there
was snow lying, the critical values were 5305-5335 gpm, and with
no snow lying 5225 gpm, which accords more with Murray below)
(2) Murray: 1959 Using an analysis for the
UK only, found the following:
Rain and snow are equally likely when the 500-1000 hPa thickness is about 5225 gpm (or 522 dam)
Rain is rare when the 500-1000 hPa thickness is less than 5190 gpm
Snow is extremely rare when the 500-1000 hPa thickness is greater than 5395 gpm; it is rather uncommon when the value is greater than 5305 gpm.
A personal view here, but I have always regarded the 522 dam isopleth as a better 'first guess' at snow vs rain than the 528 value for frontal precipitation. Indeed, I joined the Met.Office at a time when TTHK were still being extensively used for this purpose, and 522 was regarded as the 'snow-line'
(3) Murray: 1959 Also carried out a more detailed investigation using a combination of predictors, the 500-1000 hPa TTHK, the surface (screen) temperature and the height of the freezing level. (I have only shown the TTHK/screen temp relationship as this is the most useful one for those with access to WWW met.products.) He presented the results in graphical form, but for ease of use, I have converted them to tabular format, using the 'standard' TTHK values. The use of a graph implies greater precision than is usual anyway.
(a) percentage probability (P) of type of precipitation in relation to surface temperature and 500-1000 hPa thickness.
| [SIZE=-1]SCREEN TEMPERATURE (degC) >> |
