Article by Michael/doctormog continued
Making sense of 1000-500hPa thicknesses - 528dam and all that
As I mentioned on the previous post, don’t automatically think blue is cold or orange is warm on the 500hPa charts, as although this can be the case it is not always so. The GFS has its own 1000-500hPa charts but as far as I’m aware it is the only main model to have this readily available so here’s a quick guide of how to calculate the 1000-500hPa thicknesses from a chart like the one shown in the previous post.
Using the chart in Part 1 (and the scale on the right hand side note the 528dam 500hPa height (turquoise)). This essentially means you'd have to go up to a height of 528dam (5280 metres) before the pressure fell to 500hPa.
However, the surface pressure varies from 995hPa to circa 1020Pa (or millibars) in the 528dam turquoise on this chart.
In order to calculate the 1000-500 thickness the following formula should be used
For example in the above chart, looking at NE Iceland where the turquoise 528dam thicknesses are shown, to calculate the 1000-500hPa thickness the following would be used:
(1000-1005hPa)*0.8 + 528dam
i.e. -4 +528dam = 524dam 1000-500hPa thickness.
This value is low enough to indicate (when taken in the context of other factors including the t850hPa, see below) a reasonably high probability of any precipitation falling as snow to low levels.
For general purposes although as ever there are many exceptions to the following, these values give an indication of the likelihood of snow.
The much talked about 528dam tends to be the recognised threshold which marks the realistic chance of seeing any snow at low levels in the UK however for an increasing chance of seeing snowfall look for values of 522 and lower. 520dam air which is very unusual will almost guarantee precipitation falling as snow (but not quite!)
546dam is often associated with mild and wet or zonal conditions in winter or cool to average in the summer.
If you see the 564dam air and above (in summer) you can expect warm or even hot conditions – the higher the hotter (usually!)
t850hPa is the term used to refer to the temperature at the pressure value of 850hPa (usually about 5000ft above sea level). These charts like the one shown in this link: http://www.wetterzentrale.de/pics/Rtavn122.html (again from the GFS model) show the t850hPa values. These values can be useful to determine the surface temperatures but like all things in model discussion there are exceptions to the rule and so the t850 values should never be used on their own to work out surface temperatures.
Under cyclonic conditions (i.e. low pressure) you can often get an indication of the surface temperature by adding 8-10 degrees on to the t850hPa, this value will increase slightly under certain conditions during the summer. However the main thing to look out for when using these charts is when there is high pressure in control of the weather during the late autumn, winter and early spring due to the effect known as the temperature inversion. In these situations the temperature will actually increase with height as opposed to the norm where the opposite is true. In these situations low lying areas can be much colder than surrounding mountain tops and therefore the t850s should not be used as an estimate of surface temperatures.
Following on from the above post on t850s it is worth mentioning at this stage the much talked about ensembles. Again rather than reinvent the wheel I quote an excellent explanation of what the model ensembles are. This information is from the very useful FAQ section of the uk.sci.weather newsgroup:
“Ensemble: a collection of NWP runs (typically in excess of 15, many having 50 or more) from the same start time (t=0) and using the same model physics, but each run (or 'member') having a slightly perturbed (altered) set of initial conditions from the 'control' run (q.v). The alterations are constrained within limits which are calculated in various ways - one example being that of performing a separate short-range model run and identifying the errors that would grow most over a 48 hr period. These errors are then applied in varying amounts to the initial conditions before performing the operational ensemble run. Another technique is to use (known) errors from a previous run and apply these in small amounts to the initial conditions of the new run. [NB: these output are in addition to (and run some time after) the 'operational' model output, i.e. the deterministic run which is the set of charts most often seen on web sites: it should not be assumed that the operational run (OP/OPER: q.v.) is close to the ensemble mean (q.v.) or mode - significant deviations can and do occur at longer lead times. Also note that a particular centre's operational model is often run at a higher spatial resolution than that used for the ensemble generation - the control.]”
I realise that all sounds a bit complicated but the main messages to take from the above and to bear in mind when looking at the ensembles are:
- The tighter the clustering on the ensembles i.e. the closer all the lines are together then the more agreement and therefore likelihood there is that the shown setup will occur. Conversely when there is a large spread of lines the model output for that particular model should be taken with some reservation as there is a low level of certainty in the particular outcome shown
- Like all model output the higher the +t hours (i.e. the further the time from the initial starting point) the lower the reliability of the data, although ensemble data at say +240hrs should in theory still be more reliable than that solely from a model’s operational run (which is the one plotted on the charts)
The most commonly discussed ensemble charts are the t850hPa charts although the ECMWF ensemble data is sometimes publicly available. The following is an example of the GFS t850hPa ensemble charts from Aberdeen:
The thick red line indicates the 30 year t850hPa for that date, the thick white line indicates the mean value of all ensemble members and the thick blue line is the control member from that particular run. The thick green line is plotted from the data of the higher resolution operational run (the one in all the main WZ GFS charts). The other lines indicate the various “perturbed” members from that particular model run.
In addition to the above the 500hPa/SLP and t850hPa data from individual GFS ensemble members is now also available to view.
The following link: http://www.wetterzentrale.de/topkarten/fsenseur.html will give you access to all the GFS ensemble data. The time beside where it says "500hPa Geopot. - Alle Member " will show you the individual ensemble members SLP/500hPa charts. ("850 hPa Temp. - Alle Member " for the t850 panels of individual ensemble members).
More recently ensemble data have become available from a range of sources for the main weather models from both www.meteociel.com and www.wetterzentrale.de/topkarten. The same principles as above can be used for interpreting these charts although the presentation these charts may change depending on which site they are viewed from.
Edited by moderator
10 December 2010 07:51:42
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