Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Validation of Boundary Layer Parameters and Extension of Boundary
Conditions of the Climate Model REMO – Snow Cover
Michael Woldt, Eberhard Reimer
Institut für Meteorologie, Freie Universität Berlin, Carl-Heinrich-Becker-Weg 6-8, 12165 Berlin, Germany.
woldt@zedat.fu-berlin.de
1. Introduction
The observations of snow cover in the BALTEX region is of
special interest in the REMO-model evaluation because of
its strong affects on energy balances and dynamics in the
boundary layer.
For the attempt of spatial field interpolations of the snow
cover and depth, the three-hourly standard measurements of
hundreds of synoptic stations within the BALTEX region
have been used. The data are provided by the national
weather services and are received by the Meteorological
Institute at the FU Berlin from the Deutsche Wetterdienst.
The resulting interpolations have been compared with the
output-parameter “snow water equivalent” of the REMOmodel
for the period 1999-2001.
2. Data and Methods
The field interpolation of the snow cover and snow depth is
based on the standard measurements of about one thousand
meteorological synoptic stations. Not all station were available
for the whole period, but at leased 700 station could
always be used.
First analyses showed numerous and different errors in the
data, resulting from observation and coding mistakes, disturbances
during data transmissions and processing errors.
To achieve reliable data, multiple error detection methods
have been developed to sort out erroneous data.
Synoptic stations only report snow depths but not that there
is no snow. So, to distinguish between stations with no snow
cover and missing observations, analysis schemes have been
developed, using other synoptic parameters, like observed
weather, state of ground, surface and air temperature.
Additionally it was tried to determine probabilities for each
observation time for snow cover or no snow cover.
The snow measurements have then been interpolated for the
three-hourly synoptic observation times, using weighted
interpolation, if possible.
So 12 years (1990-2001) of snow cover time series were
determined for each available synoptic station in the BAL-
TEX area.
In a first step, these data were used to interpolate daily snow
covers and snow depths on a grid of 1/60° to 1/120° (about 1
km to 1 km).
During the interpolation process for each box of the grid, the
distance to the synoptic stations and their differences in
height above sea level were taken into account.
Up to eight synoptic stations nearest to the boxes (within a
given range) are used for interpolation. Stations with too
large differences in height to the boxes were ignored. Only
if no stations could be found within the first range, it was
extended in several steps, up to a given maximum.
There were only a few areas where, at certain times, no
interpolation could be calculated due to the lack of synoptic
stations.
The resulting interpolated fields were then used for testing
the REMO-model for the period 1999-2001.
To compare the interpolated snow depth data with REMO,
they first had to be converted to the 1/6° grid used by
REMO.
Like most models REMO gives no snow depth, but the
snow water equivalent. That is the thickness of the water
layer that would result, if all snow is melted. It depends on
the density of the snow, which is variable.
Snow water equivalent
Snow density = ----------------------------------
Snow depth
The snow density can range from about 0.05 for new snow
at low temperatures to about 0.5 for old snow after
gravitational settling, wind packing, melting and
recrystallization (NARC).
Since both parameters can not be compared directly, snow
densities were calculated by using the snow water equivalent
from the REMO-model and the snow depth from the
interpolation.
If both are in a comparable range, a sensible snow density
will result. (Correct results are, of course, also determined,
if both show no snow).
3. Results
The daily snow densities have been added and analyzed
for months, seasons and years.
Generally they show good compliances between the
REMO-model output and the interpolated snow cover for
larger snow depths and long lasting snow covers.
References
NARC, What is Snow Water Equivalent?,
http://www.or.nrcs.usda.gov/snow/about/swe.html