In this note verification results are shown of the Hirlam including
the CBR (Cuxart, Bougeault, Redelsperger, 2000) scheme for vertical diffusion.
In the first part of this note wind speed at the two lowest model levels
and 10m is compared with measurements made in a 200m high meteorological
tower. In the second part the results of Hirlam including CBR are compared
with the Hirlam including the Holtslag scheme during the "Danish" 3 december
storm. Finally some plans for the future are presented.
Verification of Hirlam CBR wind speed against tower observations
For the period of one month (July 1996) we compared wind speed at 10m
and the lowest two model levels with observations made in the 200m high
meteorological tower in Cabauw (near the centre of the Netherlands). In
the first figure the typical behaviour, at least for this month, of the
model wind speed at 140 (second lowest model level) and 10m is shown. During
strong wind, near neutral, conditions, the wind speed at 140m is largely
underestimated. See e.g. Fig. 1 (also in postscript)
at hour 528 where the wind speed at 140m is underestimated with 5.5 m/s
. At the same time the 10m wind speed is about correct or even a bit too
Figure 2 shows that this suggests too much vertical mixing of momentum.
In Figure 3 (also in postscript )
the overall results for the complete month are shown. The strong negative
bias in the wind speed at 140m is the result of the underestimation's during
near neutral, strong wind conditions. The 30m wind speed has a positive
bias. With Figure 2 in mind this clearly indicates the too strong momentum
mixing. The almost zero bias of the 10m wind speed can be explained by
the difference in roughness length between the model and the local conditions
in Cabauw (this Representativeness Problem is discussed in De Rooy, 1999).
Cabauw is situated in a relatively open terrain in comparison with the
corresponding average Hirlam grid box area. Hence the actual (direction
dependent) roughness length in Cabauw is significantly smaller than the
roughness length used in the model to transform the wind speed at 30m downwards
to 10m. Here we have a nice illustration of a compensating error and the
fact that the wind speed at 10m should not, or very carefully, be used
for validation purposes.
Comparison between Hirlam with CBR and Hirlam with Holtslag during a storm
As we have seen from the last section, the CBR scheme seems to result in too much vertical mixing during strong wind situations. We therefore decided to investigate the performance during a storm and to compare the results with the output of Hirlam including the Holtslag scheme for vertical diffusion. For the comparison the "Danish" 3 December 1999 storm is chosen. During this storm a station near the coast of Denmark reported 85 kts, just before it crashed. So near neutral conditions in this area can be guaranteed. All verification results in this section refer to 3 december 18:00 GMT when the storm was at its maximum. Further we only present +12h forecasts (other forecast lengths show the same behaviour). At the KNMI, Hirlam version 4.3, thus including the Holtslag scheme for vertical diffusion, is still the operational version. Therefore data produced by the operational model could be used for this comparison study. To obtain the results for the CBR version, Hirlam version 4.6.2 including the CBR scheme, was started at 1 December 0:00 GMT in order to let the model spin up and simulate the normal assimilation cycle. The area on which both model versions run is the same. However to save disk space, the output fields of the CBR version cover a smaller area as can be seen in Figs. 4, 5 and 6.
Figure 4 Pressure at mean sea level
Figure 4, where the pressure at mean sea level is plotted, shows that the depression core is situated at about the same location for both Hirlam versions. However, the depression core is approximately 5 hPa less deep in the CBR version (CBR=965 hPa, Holtslag=960 hPa, observed=958 hPa). This might be explained by the stronger vertical mixing of momentum in the CBR version which will lead to a stronger ageostrophic wind and as a consequence an increased filling up of the depression core.
Fig. 5 Wind speed at 10m
Fig. 5 with the wind speed at 10m shows the larger area with high wind speeds in the CBR version. Looking e.g. at the centre of the Netherlands, the wind speed is 20 m/s in the CBR and 12.5 m/s in the Holtslag version. As mentioned before the wind speed at 10m is not a good validation measure but the Holtslag values are near the observed wind speeds whereas the CBR winds are almost twice as high. Again the results are consistent with the too-much-mixing-hypothesis of the last section (see Fig. 2).
Fig. 6 Wind speed at 925 hPa
Finally the wind speed at 925 hPa is plotted in Fig. 6. This figure clearly reveals the much higher wind speeds at 925 hPa with the Holtslag scheme. This implies the relatively small difference in wind speed near the surface and at higher levels in the CBR scheme. In summary, it can be stated that this storm case confirms the behaviour of the CBR scheme described in the last section, namely too much vertical mixing of momentum during near neutral conditions.
Plans for the future
The above mentioned results were send to Joan Cuxart, who is the "C"
in CBR and also the person who made the CBR scheme available for Hirlam.
Three points in his reply are mentioned here.
So a combination of the last two points might solve the problem and will result in realistic vertical mixing of momentum in the CBR scheme during near neutral conditions. At the moment the necessary modifications are further investigated at the INM.
I would like to thank Geert Lenderink, Sander Tijm, and Pier Siebesma for many helpful discussions and suggestions
Cuxart, J., P. Bougeault, and J.-L. Redelsperger, 2000: A turbulence scheme allowing for mesoscale and large-eddy simulations. Quarterly Journal of the Royal Meteorological Society, Vol. 126 pp. 1-30
De Rooy, W.C., 1999: Hirlam near-surface ouput. Investigation of error sources and possible improvements in the Hirlam 2m-temperature and 10m-wind speed, Hirlam newsletter No. 33