Please note that numbering/ordering does not indicate/imply any sort of priority. Recent entries/changes/updates are shown in green.
| Topic / title | Description | Related activities |
|---|---|---|
| 2m Temperature | ||
| 1. 2m temperature in the presence of inversions | In common with all models, 2m temperature forecasts from the IFS tend to have much larger errors, on average, during low level inversion situations, which are particularly common at high latitudes in winter. The basic physical explanation is that a set change in atmospheric energy content has a much larger impact on screen temperature in inversion situations than in unstable situations, because the energy change is commuted through a much smaller depth of the atmosphere (e.g. metres rather than kilometres). The lower the inversion, the larger is the potential error. There is also sensitivity here to the method we use to interpolate between air temperature at the lowest model level (~10m) and skin temperature (2m temperature is a diagnostic, not direct model output). | |
| 2. City temperatures too low | Due to the urban heat island effect not being represented, screen temperatures in large urban areas, particularly cities, are commonly too low compared to observations. The problem can be accentuated in winter by snow cover. | 'Urban tiles' due in land surface scheme in 2016/17 |
| 3. Screen temperatures fall too much near coasts | As a consequence of the radiation grid being larger than the model grid (due to computational constraints) night-time radiative cooling over land near to the coast is often too rapid. This is because cooling progresses according to T4, and at near-coast points T is approximately the average temperature of the land and (warmer) ocean. As a result screen temperatures drop too much - related errors can sometimes exceed 10C. The problem is enhanced (i) when there is snow cover, (ii) at high latitudes, and (iii) where coasts have a convex shape (land-relative). | Improvements due to radiation code 'fixes' were introduced with cycle 41R2 in March 2016. In example cases the impact of these changes has been very positive. More substantial radiation code changes are likely in the longer term. |
| 4. Meteogram temperature issues in complex topography | In addition to the normal problems of representing screen temperatures in complex topography in current-generation global models, the user should be aware that the method by which screen temperatures on Meteograms are generated from model screen temperatures assumes a standard lapse rate (6.5oC drop per km increase in altitude), and so if the difference in height between the site chosen, and the nearest model gridpoint (as shown in the ENSgram title) is large, the scope for large errors/biases increases. This is especially true in winter-time when inversions are more common: by definition an inversion implies a temperature increase with height, not a decrease, so the temperature correction applied could even be in the wrong direction. This issue is compounded by item 1 above. | Resolution upgrade in March 2016 (4R2) has helped. Re-calibration project should help even more. |
| 5. China cold spot | In products that intrinsically display 2m temperature output in some 'anomaly' form - such as monthly forecast anomalies, seasonal forecast anomalies, and in the shorter ranges EFI and SOT - there is a semi-permanent winter-time 'cold spot' over eastern China. It is not real in the sense that temperatures are not always 'below normal' in this area when they are shown to be. The cold spot owes its existence to incompatibilities between the current forecasting system, and ERA-Interim (ERA-I). ERA-I re-analyses are used to start the re-forecasts which form the 'climatology' against which current forecasts are compared. So whilst these re-forecasts are rightly performed with the latest model version, they also inherit, as a starting point, auxiliary data such as snow depth from ERA-I, in fact from the ERA-I 'offline fields'. In turn this offline snow depth inevitably derives, in part, from what the ERA-I model puts on the ground in the way of snowfall, and this model's climatology is such that there is less snowfall in this area, on average, than in the current HRES. In turn, an intrinsic part of the offline run is a re-scaling using GPCP climatology, and the GPCP climate meanwhile is much drier in winter than either the IFS or ERA-I. So HRES and other IFS components are inclined to have a much deeper snow cover in their analyses through the winter than the re-forecasts have in theirs, which encourages the development of 2m temperatures that are similarly much lower - ie the 'cold spot' anomalies. The problem is compounded by a historical dearth of observations in this area which might have helped bring things back on track, and also by a snow analysis scheme 'feature' that excludes all observations above 1500m (the said area is around 4000m). This cut-off helps avoid problems in complex terrain, though could be improved by using instead a measure of sub-grid orography. The limited number of observations that do exist suggest that the winter-time snowfall climatology is rather better in IFS than in ERA-I offline, implying that the IFS' absolute 2m temperature forecasts should be more reliable than our anomaly-format products. | |
| Precipitation | ||
6. Marine convection propagation | In reality shower cells have a finite lifetime, so precipitation associated moves with the showers, as one can see on radar. In the IFS showers are instantaneous (as they are parametrised) and the related precipitation does not propagate. So showers triggered over the sea do not generally move inland in the model as they should. This can lead to under-prediction errors of several mm in inland locations, 10mm or more in extremis. The degree to which the error extends inland depends on the windspeed at the steering level for showers. For stronger winds the errors extend further inland. For snow showers the errors can be worse still, compounded by the relatively slow fall speed of snowflakes (up to say one tenth of that of raindrops). So a snowflake starting its descent at the coast might end up on the ground 100km inland, if winds are strong, whereas a raindrop in equivalent summer conditions might only propagate 20km before reaching the ground. | |
| 7. Underestimation of orographically-enhanced precipitation | As a consequence of topographical barriers being too low, in general (due to resolution), both the orographic enhancement of precipitation and the rain shadow effect tend to be underestimated in the IFS (more so in ENS than HRES, and more so in ENS after 10 days when resolution changes). | Resolution upgrade in March 2016 (41R2) has helped a little, though because the spectral resolution of the model will be unchanged, and because orography is tied more to this, improvements are not that large. |
8. Underestimation of convective precipitation extremes | As a consequence of resolution, and the related parametrisation of convection, localised extreme values in precipitation totals will be systematically "underestimated" in IFS output. Differences equal to about one order of magnitude are possible. However this is not as bad as it seems, because when verified over areas that are the same size as the effective model gridbox size the agreement is generally much better. | Resolution upgrade in Mar 2016 helps a bit. A new project started in Jan 2016, to look at new ways of forecasting, a priori, the degree of sub-grid variability in precipitation totals. |
| 9. Tropical rainfall extremes greatest on day 1 | If one examines the distribution, in forecasts, of daily rainfall totals for locations in the tropics, the (wet) tails tend to be longer for very short lead times (eg T+0 to T+24), implying that ENS and HRES have a greater propensity to generate extreme rainfall in short range forecasts than they do in medium range forecasts. For example the 99th percentile of daily rainfall at some locations at day 1 is twice what it is at day 3. This would appear to be a 'spin down' issue, of sorts, related to the handling of convection. Formulation of the EFI and SOT is such that they intrinsically account for this (though note item 19 below), so the problem arises for the user particularly when referencing the direct model output. | |
10. Extreme rainfall at certain gridpoints ("rain bombs") | At particular gridpoints, that lie in areas of complex topography, IFS forecasts (notably HRES) can occasionally generate extreme localised precipitation totals in a matter of hours, say well over 100mm, when at neighbouring locations the amounts are far less. These extremes are incorrect; the error occurs in convective situations with light winds. Only a small number of gridpoints around the world are affected - mainly these lie in the following areas: southern China, parts of Eastern Africa, Papua New Guinea, along the Andes, southern Mexico. The cause is understood though is too involved to describe in detail here; in short it relates to a weakness in the semi-Lagrangian scheme (part of the model numerics). Tests have shown that the error will go away when the grid structure and model resolution are changed with the next cycle. | This issue was resolved with the introduction of cycle 41r2 in March 2016. |
| Snow | ||
| 11. Snow drift in convective situations | When snow falls through cloud or beneath cloud it drifts with the wind. For large-scale (dynamic) precipitation IFS physics accounts for this. For convective precipitation however it does not; there is no drift, the precipitation arrives at the surface instantaneously once the convection is diagnosed, in the place that it is diagnosed. As a result snow arising from convective processes may be misplaced in the model (too far upwind), and the errors will be larger if winds along the snowflake path are stronger. Errors can be of order 100km. Item 6 relates. The same issues exist for rain, but given the faster fallspeed of raindrops relative to the IFS model resolutions these errors are negligible. Clearly one also has to take account of the melting level. | |
| 12. Snow on the ground takes too long to melt | In both ENS and HRES small amounts of snow on the ground tend to take too long to melt, even if the temperature of the overlying air is well above zero. This is because, for melting purposes, the snow that there is is assumed to be piled up high in one segment of a gridbox. For smaller nominal depths, the pile becomes higher, though at the same time covers a much smaller fraction of the box. The reason this is used is to improve the handling of screen temperature; by confining the snow to gridbox segments the impact on the temperature of that snow is reduced, and on average we find smaller errors and biases in 2m temperature as a result. The main downside is that snow cover pictures can look misleading, particularly at longer leads (when they can not of course be rectified by observational data). The cut-off above which snow is assumed to cover the full grid box is a 10cm depth - this is why a green hue used on standard snow depth charts on the web, which suggests to the eye the presence of some vegetation, disappears at 10cm. | 4-year project to address snow issues ("Earth2Observe") |
| 13. Mixed rain/snow leads to snow accumulation | In marginal snow situations, when precipitation at the surface comprises both rain and snow, the snow component accumulates as lying snow. In the vast majority of cases this is wrong - it should melt instantaneously. This behaviour occurs because small snow depths within the model are assumed to be piled up into a small segment of a gridbox, and as such it is very difficult for them to melt quickly (as in item 12 above). | 4-year project to address snow issues |
| 14. Spurious snowfall in freezing rain situations | In certain winter situations, when snow descends through the atmosphere and melts to rain in a warm layer, before descending again through a cold (sub zero) layer, the model turns the precipitation back to snow far too readily. So surface precipitation in freezing rain situations commonly appears as snow, and that snow also accumulates on the ground. HOWEVER, it seems that where this precipitation is diagnosed as convective, this re-freezing problem does not exist. | Resolved with physics changes implemented in May 2015, though monitoring still required. |
| 15. Multiple snow layers | The model assumes that all snow on the ground has the same density (though that density does vary - e.g. increasing with age). So layers of different density, which arise in the real world, are not catered for. This can impact on several things, such as total snow water content, and upward heat conductivity, which in turn has the potential to adversely affect 2m temperature. In addition, when new snow falls onto old, the change in snow depth is commonly less that it should be, because the density assigned to the fresh snow depends in part on the density of the pre-existing lying snow, and so tends to be greater than it should. The magnitude of the error (in snow depth change) increases when the pre-existing snow is deeper and/or has a greater density. Example: if 10cm of new snow (ratio 12:1) fell onto 10cm of old lying snow (ratio 2.5), snow depth in the model would increase by only 3.5cm. | 4-year project to address snow issues |
| Tropical Cyclones | ||
| 16. Tropical cyclone intensity | Resolution limits our ability to fully capture the core of strong winds around many tropical cyclones; likewise depths can be under-estimated, by over 50hPa in extremis. The problems are larger for smaller systems, with a smaller eye - Haiyan was one such example. Often minimum pressure in HRES will be lower than in all the ENS members, and likewise winds stronger; this is because of the higher resolution of HRES. In such situations HRES guidance will often be better, but not always. | With cycle 41R2 introduced in March 2016 came a resolution upgrade, and a substantial improvement to resolution used in the EDA. Key impacts have been a marked reduction in positive depth bias in ENS analyses and forecasts, and more spread in ENS depth forecasts. |
| 17. Relatively slow-moving TCs can deepen too much in HRES | There is no coupling with the ocean in HRES. So for relatively slow moving TCs, when fluxes and mixing might in reality lead to a reduction in SST, the SST will remain at an elevated level and this can give the TC extra impetus to deepen too much (provided other factors such as shear remain favourable). For fast moving TCs the affected ocean is left behind, and so the problem is less acute or non-existent. Whilst item 16 above generates errors in the opposite sense that might sometimes fortuitously cancel, there are nonetheless recorded cases where TCs have been over-deepened, by as much as 50mb, because of the lack of coupling. | Coupling of HRES with the ocean may commence in 2017. |
| Miscellaneous | ||
| 18. Under-estimation of strong gusts in convective situations | Although there is a helpful convective contribution in the computation of maximum gusts (as used in direct model output and the EFI), experience has shown that extreme gusts are generally under-represented, particularly when vigorous convection is involved, such as one might see with MCSs or squall lines - eg 60kt gusts or more might be observed when 30-40kt gusts are predicted. This relates to (i) an inability, at current model resolution, to represent the 3-d circulation around convective systems, and (ii) the fact that it is impossible to design an adjustment in the gust computation that will work in all cases. | New EFI parameters relating to severe convection were introduced in test mode in summer 2015; these can provide pointers to when errors of this type are likely. |
| 19. Jumpiness in EFI and SOT, especially at short lead times | A consequence of the re-forecast strategy is that extreme events are sometimes not well sampled. Especially at short lead times, say 1 or 2 days, the 11 members that go up to make the re-forecast can be very similar, and so if the re-forecast dates (twice per week since May 2015) happen to be just before certain extreme events there may be some over-sampling, whilst if extreme events fall inbetween the re-forecast dates, there may be some under-sampling. Thus the tails of the model climate (M-Climate) distribution can be jumpy as we move from one lead time to another, and as EFI and SOT depend heavily on these tails, much more than they depend on solutions around the median, they can be jumpy too. | The increase from 500 to 1980 re-forecast realisations effected in May 2015 has reduced the magnitude of this problem; we will continue to monitor and perhaps drop this issue in due course. |
| 20. Sunshine duration | The integrity of this post-processed output parameter is strongly compromised by the radiation timestep in the model (3 hours in ENS, 1 hour in HRES), which because of computational cost is longer than the basic model timestep. This manifests itself in the sunshine duration parameter being (a) an undesirable function of longitude and (b) more generally unreliable. | Radiation code changes were introduced in cycle 41R2 (March 2016) which markedly reduced the dependance on longitude. Reliability also improved, providing a better match to the WMO sunshine definition. However duration will still be overestimated, at times, in relatively cloudy conditions. |
| 21. Sea ice evolution and associated weather | Sea ice cover does not change in any interactive way in the forecasts as we do not have a sea ice model. So none of the following are represented: sea ice formation due to low air temperatures, break up due to wind effects or melting, and advection by currents and winds. In turn this affects weather that relates, such as 2m temperatures over and downwind of, and convection triggered over water but not over ice. Wave model output will naturally also be affected. In the twice daily forecasts to day 15 sea ice cover is fixed. At longer ranges, to capture the seasonal cycle, there is relaxation towards the ice cover of the last five years; for monthly forecasts all ENS members use the average value, for seasonal forecasts members are divided into five sets (of 10) each one of which uses the pattern for one of those five years. | Sea ice model being developed |
| 22. Very poor SST evolution near New York | Due to the lack of resolution in the ocean component of the semi-coupled ENS system we are now running (introduced in Nov 2013 with 40R1), and an associated poor handling of the gulf stream wall, there is a major anomalous upward drift in SSTs over and S and E of the New York Bight (which itself lies just SE of New York city), in the first 10 days of the ENS forecasts. The area affected is about the size of England, and the size of the error that develops in 10 days can exceed 10C. | 0.25 degree ocean model under development. |
| 23. 'Hot spots' near to glaciers | When cycle 41R1 was introduced on 12 May 2015 an error began to appear, over certain glaciated/partly glaciated regions (e.g. Iceland, the fringes of Greenland), on 2m temperature products that represent, directly or indirectly, anomalies. Affected fields include EFI/SOT (large positive values), Meteograms with climate (M-Climate too cold) and monthly forecasts (positive anomalies regularly forecast). This error is not a reflection of the absolute forecast values themselves - those should generally be OK - but is instead indicative of an error that was inadvertently introduced into the re-forecast suite. This error makes the 2m temperature forecasts in those re-forecasts, close to glaciers, much colder than they should be, causing the actual forecasts to look like they are indicating strong positive anomalies. Initially, in May, the error was less or non-existent because it was masked by residual seasonal snow cover. | The error has now been corrected. Affected re-forecasts will not themselves be rerun, although newly created re-forecasts should, from data times in early July 2015 onwards, be correct. This means that the mentioned issues slowly went away between early July and mid August 2015, as the fraction of the re-forecasts that were contaminated steadily reduced. |
| 24. 'Cold ring' around sea ice after day 10 | Since the introduction of 41R1 on 12 May 2015 a ring of cold SST values (= -1.8C), about one gridbox wide, has appeared at the day 10/11 resolution change, along the edges of areas of sea ice. Locally SSTs may suddenly drop by more than 5C. This is a complex issue but relates to a change in the threshold at which sea ice cover is accepted (it is now 2%, it used to be 20%), the fact that we have to interpolate SST values onto a different grid when resolution changes, and the fact that SST is now set to -1.8C in gridboxes that include some sea ice.. A related complication is the fact that the two-way coupled ocean model runs on a different grid (1 degree). The ice cover threshold change was made to improve the handling of ocean waves in ice-margin zones, and to pave the way for introduction of a full sea ice model in the future. These advantages are considered to outweigh the 'cold ring' disadvantage. The day 10-15 EFI 2m temperature field can also be affected (showing a band of low values in the ice margin zones). | In March 2016 (cycle 41R2) the resolution change was moved to be at day 15, and so this issue now only affects the monthly forecast. |
| 25. Missing islands | A new land-sea mask was introduced with cycle 41R1 on 12 May 2015, in order to improve representativeness. Unfortunately there were some deficiencies in the source dataset, which has meant that a few islands, that should really be there, are no longer present in the IFS. The problems are mainly in HRES. Samoa is the island group where the greatest impact is seen. These issues will clearly reduce the utility of some IFS output, such as meteograms for some islands. For more details, including illustrations, go here. Note that the ocean wave model is not affected. | Corrected in March 2016, with the introduction of cycle 41R2. |
| 26. Seasonal lakes | In some locations lakes can undergo large changes in areal coverage, seasonally and with further modulation due to anomalous weather types. In the IFS lake areas are fixed, and are configured to match an ESA "GlobCover" dataset, which itself was derived by blending satellite images. So there are two potential sources of lake-cover-related errors in the IFS; one is inaccuracies in GlobCover, the other is temporal variations in lake size. In parts of Australia lake cover issues have had an adverse impact on forecasts of dew point, temperature, cloud cover and convection, not only in the immediate vicinity but also, via advection, in regions well beyond. Similar problems may also exist in other areas. | Through acquisition of more accurate lake-cover datasets - e.g. from forecast users - it may be possible to alleviate some of these problems. |
| 27. Visibility biases | Visibility is a very difficult parameter to predict with a global model, a problem exacerbated somewhat by there being no aerosol emissions/transport. Since introduction of this diagnostic variable in May 2015 characteristics have been investigated, over Europe. Impressions of overall performance were positive, though the following general issues were noted a) visibility in radiation fog tends to drop a bit too low, on average (e.g. 50m when 100m would be better) b) radiation fog formation tend to occur bit too late, and fog clearance a bit too early (e.g. 1-3 h typical bias in each case) c) background visibility (when no fog or precipitation) seems to be a bit too high overall d) hill fog seems to be under-represented (though this may relate to model orography limitations) e) visibility tends to drop a bit too low during precipitation |