Nights are on average warming rapidly during springtime and cooling fairly quickly during autumn. The span of the model climate (about 4 weeks) means that a wider range of minima is included when the M-Climate is calculated (e.g. minima during the period late April to late May are included in the calculation of the M-Climate for early May). However, during summer or winter there are usually only small seasonal changes and the range of minimum temperatures during the M-Climate calculation period is correspondingly reduced from those around the equinoxes and is rather more representative of the mid-point of the period. There is of course continuing day-to-day variation in minimum temperatures.
The effect during spring and autumn is to extend the tails of the M-Climate (as seen on Cumulative Distribution Function (CDF) diagrams) towards warmer and colder values - i.e. increase the spread beyond what it should be. One investigation showed that the cold tail should change by about 1C per week, but of course the magnitude of this will depend very much on location.
The tails have a disproportionate influence on the Extreme Forecast Index (EFI) and for minima at least the extra, anomalous spread seen in the transition seasons can lead to a reduction in EFI values by, say ~0.2, from what they would be if the M-Climate had been evaluated over a shorter period. Of course the reason we don't evaluate over a shorter period is that the sample size would reduce, so we would be replacing an occasional over-spreading issue with a continuous under-sampling issue, which would make the EFI even less reliable.
Similar arguments apply to EFI for maxima in the transition seasons. However, it seems from case studies that whilst the impact upon CDFs and EFIs for maximum temperatures is also there it is rather less pronounced.
In summary, users should be aware that, in extreme situations during the transition seasons, cold EFIs in particular will typically be damped to be of smaller magnitude than they should be. This is because the M-Climate range is artificially extended in those seasons. Users should expect equivalent adverse impacts on the SOT also, which by definition focuses on the distribution tails where the issue lies.
Fig8.1.4.8.1: EFI forecast for 2m minimum temperature T+24 VT 14 May 2020, DT 00Z 14 May 2020. Minimum temperatures over SE England between 0°C and -2°C observed widely.
Fig8.1.4.8.2: Central England Minimum temperatures (source Met Office) highlighting the changes in minimum temperatures through the year. The black bar annotations highlight what the M-Climate would be aiming to represent, for Central England, if the period over which the graph lines were constructed had mirrored that used for the reforecasts (that are actually used to define the M-Climate).
On average, through the periods used for calculation of M-Climate (approx 4 weeks):
- in spring/autumn minimum temperatures rise/fall by around 4°C,
- in summer and winter minimum temperatures remain relatively constant.
A similar effect may be seen regarding maximum temperatures but changes during the spring/autumn calculation periods are less pronounced than for minimum temperatures.
One must expect differences from this general picture in other regions - e.g. in Siberia, which has a huge annual temperature range, the transition season changes would be much larger.
Amended/Updated 24/03/21 – New Section.
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