Fourth Study Conference on BALTEX Scala Cinema Gudhjem

During the experiment the water was generally warmer than
the air, which is a very typical feature for the Baltic Sea
during the late summer, autumn and early winter. This
results in the generation of a convectively driven boundary
layer over the water.
The soundings were performed daily at noon with an ascent
velocity of about 1-3 m s -1 . During the days from 31 October
to 2 November the radiosounding programme was
intensified to 4 to 6 soundings per day. The depth of the
boundary layer was subjectively estimated from the
soundings, based mainly on the profile of the potential
temperature, and taken as the height where the potential
temperature starts to increase, simultaneously considering
the humidity profile.
The analysis is solely based on measurements from the
period 30 October to 3 November where the wind directions
(west-north-east) result in a water fetch from the closest
upwind coast to Christiansø of the order of 100 km or more.
In the beginning of the period the wind speed is fairly high,
followed by moderate values at the end. Gryning and
Batchvarova (2002) give details.
Both of the above presented bulk Richardson-number
methods to extract the boundary-layer height from the
output of Numerical Weather Prediction (NWP) models
were applied to the hourly output from the HIRLAM model.
The result from the analysis using the bulk Richardson
numbers suggested by Sørensen (1998) and by Vogelezang
and Holtslag (1996) are - as expected from the foregoing
discussion - that the generally recommended value of the
critical Richardson number of 0.25 resulted in overprediction
of the height of the marine atmospheric boundary
layer (Gryning and Batchvarova, 2002).
The critical Richardson number that subjectively gave the
overall best fit to the measurements was found to be 0.03 for
the method suggested by Sørensen (1998) and 0.05 for
Vogelezang and Holtslag (1996). This is illustrated in Figure
2 where it also is evident that the Vogelezang and Holtslag
(1996) method for this limited set of measurements gives a
slightly better overall fit than Sørensen (1998)’s method.
MARINE ATMOSPHERIC
BOUNDARY-LAYER HEIGHT (m)
1200
800
400
0
30-Oct 31-Oct 1-Nov 2-Nov 3-Nov 4-Nov
Figure 2. Height of the marine atmospheric boundary layer.
The dashed line illustrates the boundary-layer height
predicted by the method of Sørensen (1998) when applying
a critical Richardson number of 0.03, the full line shows the
predictions of the Vogelezang and Holtslag (1996) with a
critical Richardson number of 0.05. Bullets show
measurements.
- 47 -
4. Conclusions
Over water, owing to the small roughness length, the wind
speed is typically high. Considering the formulation of the
bulk Richardson numbers put forward by by Sørensen
(1998) and Vogelezang and Holtslag (1996), this suggests
a dependence between the critical Richardson number and
the surface roughness. The dependence is not necessarily
the same for the two bulk Richardson-numbers because
Eq. (1) by Sørensen (1998) is formulated with the use of
the wind velocity and Eq. (2) by Vogelezang and Holtslag
(1996) with the wind profile. In an experimental campaign
in the Baltic Sea, it was found that the predicted marine
boundary-layer height in general is too large which
suggest that the marine critical Richardson number is
smaller than 0.25 – we found critical Richardson numbers
around 0.03 to 0.05 to perform best over the sea. It was
observed that the sensitivity to the value of the critical
Richardson number is much less for the Richardson
number suggested by Vogelezang and Holtslag (1996)
than it is for the Richardson number in Sørensen (1998).
Recently it has been suggested that also for stable
conditions the critical value of the Richardson number is
not constant. In conclusion, the traditionally used value
0.25 for the critical Richardson number is a simplification.
It seems that the critical Richardson is not constant but a
function of both surface roughness and stability.
5. Acknowledgements
It is a pleasure to acknowledge fruitful co-operation with
Anna Rutgersson and Ann-Sofi Smedman. We thank the
Swedish Meteorological and Hydrological Institute for the
data from the HIRLAM simulations. The measurements
were carried out as a part of a Pilot study on Evaporation
and Precipitation over the Baltic Sea (PEP-in-BALTEX)
supported by the European Union (ENVC4-CT97-0484).
Finally, the analysis was supported by NATO Linkage
Grant (EST-CLG-979863).
References
Batchvarova E and Gryning S.E., 1991: Applied model for
the growth of the daytime mixed layer. Boundary-
Layer Meteorol. 56, 261-274.
Gryning S.E. and E. Batchvarova, Marine boundary layer
and turbulent fluxes over the Baltic Sea:
measurements and modelling. Boundary-Layer
Meteorol., 103, 29-47, 2002.
Sørensen J. H., Sensitivity of the DERMA Long-Range
Gaussian Dispersion Model to Meteorological Input
and Diffusion Parameters. Atmos. Environ., 24, 4195-
4206, 1998.
Vogelezang, D. H. P. and A. A. M. Holtslag, Evaluation
and Model Impacts of Alternative Boundary-Layer
Height Formulations. Boundary-Layer Meteorol., 81,
245-269, 1996.