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- ERA5 uncertainty: although small values of ensemble spread correctly mark more confident estimates than large values, numerical values are over confident. The spread does give an indication of the relative, random uncertainty in space and time.
ERA5 suffers from an overly strong equatorial mesospheric jet, particularly in the transition seasons.
From 2000 to 2006, ERA5 has a poor fit to radiosonde temperatures in the stratosphere, with a cold bias in the lower stratosphere. In addition, a warm bias higher up persists for much of the period from 1979. The lower stratospheric cold bias was rectified in a re-run for the years 2000 to 2006, called ERA5.1, see "Resolved issues" below.
- Discontinuities in ERA5Discontinuities in ERA5: ERA5 is produced by several parallel experiments, each for a different period, which are then appended together to create the final product. This can create discontinuities at the transition points.
- The analysed "2 metre temperature" can be larger than the forecast "Maximum temperature at 2 metres since previous post-processing".
- The analysed 10 metre wind speed (derived from the 10 metre wind components) can be larger than the forecast "10 metre wind gust since previous post-processing".
- ERA5 diurnal cycle for winds: the hourly data reveals a mismatch in the analysed near surface wind speed between the end of one assimilation cycle and the beginning of the next (which occurs at 9:00 and 21:00 UTC). This problem mostly occurs in low latitude oceanic regions, though it can also be seen over Europe and the USA. We cannot rectify this problem in the analyses. The forecast near surface winds show much better agreement between the assimilation cycles, at least on average, so if this mismatch is problematic for a particular application, our advice would be to use the forecast winds. The forecast near surface winds are available from MARS, see the above section, Data organisation and how to download ERA5.
- ERA5: large 10m windsERA5: large 10m winds: up to a few times per year, the analysed low level winds, eg 10m winds, become very large in a particular location, which varies amongst a few apparently preferred locations. The largest values seen so far are about 300 ms-1.
- ERA5 rain bombs: from time to time, the rainfall (precipitation) can become extremely large in small areas.
- Large values of CAPE: occasionally, the Convective available potential energy in ERA5 is unrealistically large.
- Ship tracks in the SST: prior to September 2007, in the period when HadISST2 was used, ship tracks can be visible in the SST.
- Prior to 2014, the SST was not used over the Great Lakes to nudge the lake model. Consequently, the 2 metre temperature has an annual cycle that is too strong, with temperatures being too cold in winter and too warm in summer.
- Wind values are far too low on pressure levels at the poles in the Climate Data Store (CDS)Wind values are far too low on pressure levels at the poles in the Climate Data Store (CDS)
- The Potential Evaporation field (pev, parameter Id 228251) is largely underestimated over deserts and high-forested areas. This is due to a bug in the code that does not allow transpiration in case to occur in the situation where there is no low vegetation type is present.
- Wave parameters (Table 7 above) for the three swell partitions: these parameters have been calculated incorrectly. The problem is most evident in the swell partition parameters involving the mean wave period: Mean wave period of first swell partition, Mean wave period of second swell partition and Mean wave period of third swell partition, where the periods are far too long.
- ERA5 surface photosynthetically available radiation (PAR) is too low, so surface PAR and clear sky surface PAR have not been published. ERA5 is produced by the ECMWF Integrated Forecasting System (IFS), which we suspect has a bug in the calculation of surface PAR in that it looks like it is taken from the wrong parts of the spectrum. We have shortwave bands that include 0.442-0.625 micron, 0.625-0.778 micron and 0.778-1.24 micron. PAR is coded as if it was intending to sum all of the radiation in the first of these and 0.42 of the second (to account for the fact that PAR is normally defined to stop at 0.7 microns. However, PAR is in fact calculated from the sum of the second band plus 0.42 of the third. We will try to fix this in a future cycle, but it is not possible to correct previously released data.
Expand title The instantaneous turbulent surface stress components (eastward and northward) and friction velocity tend to be too small ERA5 has output analysis values for the instantaneous surface stress components and friction velocity.
There is however an issue with those analysis values over the oceans.
The analysis for those type of surface parameters is obtained by running the surface module to connect the surface with the model level analysed variables.
However, at that stage, the surface aero-dynamical roughness length scale (z0) over the oceans is not initialised from its actual value but a constant value of 0.0001 is used instead.
This initial value of z0 is needed to determine the initial value of u* and the surface stress based on solving for a simple logarithmic wind profile between the surface and the lowest model wind. This initial u* is in turn used to determine the value of the exchange coefficients needed to determine the output 10m winds (normal and neutral) and u* (see (3.91) to (3.94) with (3.26) in the IFS documentation). The surface stress is output as initialised.
This initial value for z0 is generally too low ( by one order of magnitude or more):
Over the oceans, for winds above few m/s, z0 is modelled using the Charnock relation:
z0 ~ (alpha/g) u*^2
where alpha is the Charnock parameter, g is gravity, and u* is the friction velocity
with typical values of
alpha ~ 0.018
g=9.81
u*^2 = Cd U10^2
where Cd is the drag coefficient
Cd ~ 0.008 + 0.0008 U10
for U10=10m/s => z0 ~ 0.003
As a consequence, the instantaneous surface stress components and friction velocity will tend to be too low
For forecast data, the same problem affects step 0, however, it will not affect the accumulated surface stress parameters (recall the accumulated parameters are produced by running short range forecasts),
because, the accumulation starts from the first time step (i.e. at time step 0 all accumulated variables are initialised to 0.).
Note that this problem can easily be fixed, by using the initial value of Charnock that is available at initial time.
- ERA5 back extension 1950-1978 (Preliminary version): tropical cyclones are too intense
- ERA5 back extension 1950-1978 (Preliminary version): large bias in surface analysis over Australia prior to 1970
- ERA5 back extension 1950-1978 (Preliminary version): the deep soil moisture tends to be too dry
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