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112
Impact of surface waves in a Regional Climate Model
Anna Rutgersson 1 , Björn Carlsson 1 , Alvaro Semedo 1 and Øyvind Sætra 2
1) Department of Earth Sciences, Uppsala University, Sweden, Anna.rutgersson@met.uu.se
2) Norwegian Meteorological Institute, Oslo, Norway
1. Introduction
When modelling the atmosphere it is of crucial importance
to correctly describe the boundary conditions. The
atmospheric-ocean boundary is an important source of
turbulence and there is a significant exchange of momentum,
heat and moisture. The marine atmospheric boundary layer
(MABL) has a considerable impact on global climate
atmospheric models since 70 % of the global surface is
covered with water. The turbulence in the atmosphere as
well as the surface fluxes is different over the ocean since
the roughness of the surface (the waves) changes as a
response to the atmospheric forcing. Surface waves can be
divided into growing sea (young sea) and decaying sea
(swell) with very different impact on the atmosphere.
2. Model
The atmosphere model RCA3 (Rossby Centre atmospheric
regional climate model) is developed at SMHI (Swedish
Meteorological and Hydrological institute) and its domain
covers Europe. It is a hydrostatic model, with terrainfollowing
coordinates and the calculations are semi-
Lagrangian, semi-implicit and with 30-min time step. The
horizontal resolution is c. 44 km and resolved in the vertical
by 24 levels between 90 m above the surface and 10 hPa.
The model is forced at the lateral boundaries by ERA40 data
and from below by sea surface temperature (and ice) and
phase speed of the dominant waves from the same data set.
For more details, see e.g. Jones et al. (2004).
Figure 2. Drag coefficient for growing sea (dashed
and dashed-dotted lines show different expressions)
and swell (solid lines). Figure from Carlsson et al
(2009).
The altered wind profiles with a low level wind maximum
can also be reproduced by an analytical model Figure 3
(Semedo et al, 2009).
The atmosphere is coupled to the WAM wave model. The
WAM model is a state-of-the-art third generation wave
model (Komen et al, 1994). RCA and WAM are coupled
with a two-way coupling each time-step (see Figure 1).
Δ t
Δ t
RCA
time
u 10
u 10
α
u 10
α
WA
time
Figure 1. Sketch of the RCA-WAM coupled system.
In addition wave-data from the ERA40 re-analysis are used
as wave-input to the RCA model for one part of the
investigation.
3. Theory
The situation with decaying sea (swell) has in several
experimental investigations been shown to give significantly
lower friction at the surface as well as altered wind profiles
and atmospheric turbulence (Smedman et al, 1999;
Rutgersson et al, 2001). The lower friction at the surface can
be expressed by a reduced drag coefficient (Figure 2).
Figure 3. Measured and modeled wind profile for
three different cases (left). Measured wave spectra
for the corresponding situations (right).