Proceedings - C-SRNWP Project

with the new surface scheme. The satellite image of that day shows that there is still a lot of
snow lying in the mountains after a snow event a few days earlier. The temperature will
therefore be much closer to the ones in the new snow scheme than in the old scheme, which
clearly has no snow cover anymore. More about the new surface scheme can be found
elsewhere in this volume.
Shallow convection
One area of research in which HIRLAM is involved quite heavily is the parametrization of
shallow convection. The current schemes in HIRLAM, STRACO and Kain-Fritsch, are not
describing shallow convection very well, although the results are improving. Intercomparison
studies and 1D ideal cases have led to the development of a new detrainment parametrization.
In most current shallow convection schemes the detrainment rate in mass flux schemes has a
fixed value, but LES results for e.g. the ARM-case, show that the detrainment is strongly
determined by environmental conditions and the depth of the cloud. In the new
parametrization, the detrainment is a function of the critical fraction averaged over the cloud
depth.
The impact of the new parametrization can be seen very clearly in the mass flux profiles in the
ARM-case, see figure 2. In LES a very clear development from many small clouds and a few
deeper clouds at the onset of the shallow convection (a quick decrease in mass flux with
increasing height) to the later stages of the convection where there is a relatively large mass
flux even at the mid level of the total cloud depth. With the old parametrization, the mass flux
decreases much more slowly and is not depending at all on the environmental conditions.
Figure 2: Normalized mass flux profiles from LES (left) and with a fixed detrainment rate
(right) for the ARM case.
Overall tuning of synoptic scale HIRLAM
A few features of the current HIRLAM system seem to be linked together and may be
resolved by a good tuning of the overall physics. One of these features is the regular
occurrence of fog over the sea, especially when the sea is relatively cold in Spring and
Summer. In these seasons fog can be found quite regularly in the model while this is not
found in the real atmosphere (see figure 3). This may be caused by a too strong evaporation
over the sea.
Another thing that can be seen in HIRLAM too often is a too strong development of smallscale
systems. Small-scale lows often become too deep, give too much precipitation and too
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