Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Regional Climate Modelling over Estonia: Some Preliminary Results with
the RegCM3
Oliver Tomingas 1 , Piia Post 2 and Jaak Jaagus 1
1 Department of Geography, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia, olivertm@ut.ee
2 Department of Environmental Physics, University of Tartu, Tähe 4, 51014 Tartu, Estonia
1. Introduction
The climate of Estonia is characterized by the influence of
the Baltic Sea from the west and continental areas from the
east. Cyclonic activity of the Atlantic ocean makes the
regional climate very variable and therefore difficult to
predict by climate models. These models have to account for
the large scale circulation over the Baltic Sea and Estonia, as
well as for sub-grid scale forcings (topographical features
and land cover inhomogeneity). Regional climate model
RegCM3 is used to address the following questions:
• What are the main physical mechanisms behind
extreme events and anomalous periods in Estonian
climate?
• How sensitive is the climate of Estonia to changes
in land surface conditions (e.g. land use, snow
cover extent)?
• Which are the most reliable physical
•
parameterization schemes for the region?
Does the different horizontal resolution have an
affect on the model results in the long-term mean?
2. Model
The RegCM3 is a 3-dimensional, σ-coordinate, primitive
equation regional climate model. It was originally developed
at the National Center for Atmospheric Research (NCAR)
and has been applied to studies of regional climate and
seasonal predictability around the world (Giorgi, 1990;
Giorgi and Mearns, 1999). The model is based on the
concept of one-way nesting, that has been used for many
years for regional climate studies. The basic idea of this
technique is that large-scale meteorological fields from
general circulation model runs (or analysis of observations)
provide initial and time-dependent lateral boundary
conditions for high-resolution limited area model
simulations. The dynamic equations and numerical
discretizations of the model are described by Grell et al.
(1994).
3. Physical parameterization
RegCM3 uses the radiation scheme of the NCAR CCM3.
The solar component, which accounts for the effect of O3, H2O, CO2, O2, follows the δ-Eddington approximation of
Kiehl et al. (1996). The surface physics are performed using
BATS1E (Biosphere-Atmosphere Transfer Scheme), which
is described in detail by Dickinson et al. (1993). The
planetary boundary layer scheme, developed by Holtslag et
al. (1990), is based on a nonlocal diffusion concept, that
takes into account countergradient fluxes resulting from
large-scale eddies in an unstable, well mixed atmosphere.
Convective precipitation is computed using Grell scheme
(Grell, 1993) with the closure assumption of Fritsch and
Chapell (1980). Subgrid Explicit Moisture Scheme
(SUBEX) is used to handle nonconvective clouds and
precipitation resolved by the model. SUBEX accounts for
the subgrid variability in clouds by linking the average grid
cell relative humidity to the cloud fraction and cloud water
(Pal et al. 2000).
3. Experiment design
Two months were selected for the first short-term climate
simulations. July of 1994 in Estonia is characterized by
very warm and dry conditions. In contrast, July 1998 was
significantly colder than average and the precipitation
sums over the region were higher than normal. The
purpose of the runs was to test the ability of the RegCM to
simulate local climate conditions during the selected time
periods. The domain that was used for this experiment is
centered over the Baltic Sea (58°N; 17°E), covering 60
points in the east-west and 55 points in the north-south
direction. Horizontal resolution of the model is 60 km.
The model has 18 vertical levels. USGS GTOPO30
orography and GLCC Land Cover Characterization 10
min datasets were used to create the terrain files. Sixhourly
NCEP/NCAR Reanalysis 2 data provided initial
and boundary conditions for the one-month model run.
Figure 1. Total precipitation (mm) of July 1994,
simulated with the RegCM.
4. Results
Preliminary results of two month-long simulations for July
1994 and July 1998 are presented here. The validation of
the model was performed by using observational data from
15 meteorological stations in Estonia. The general pattern
of monthly mean surface temperature in Estonia simulated
by the RegCM3 was in a good agreement with local
station data. Absolute values of monthly means were
considerably underestimated, but this does not necessarily
imply poor model performance (Giorgi et al. 1993).
Patterns of total precipition were simulated realistically in
both cases. The total precipitation of July 1994 (Figure 1),
that is less than normal for July in Estonia, can be
associated with local scale convective processes. It
appeared that RegCM3 could capture the regional
precipitation gradient, that decreased from the Eastern
Estonia towards the Western Estonian Archipelago.
Higher than normal precipitation over the area in July
1998 (Figure 2) can be accounted for the large scale
circulation anomalies and it is modeled less accurately