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Jan-Peter Schulz 6 Sep. 2010 Introducing sea ice in the COSMO model Jan-Peter Schulz Deutscher Wetterdienst COSMO General Meeting, 6 - 10 Sep. 2010, Moscow
Jan-Peter Schulz 6 Sep. 2010
Introducing sea ice in the COSMO model
Jan-Peter Schulz
Deutscher Wetterdienst
COSMO General Meeting, 6 - 10 Sep. 2010, Moscow
Jan-Peter Schulz 6 Sep. 2010
Motivation
The presence of sea ice on the ocean's surface has a significant impact on the air-sea interactions. Compared to an open water surface the sea ice completely changes the surface characteristics in terms of albedo and roughness, and therefore substantially changes the surface radiative balance and the turbulent exchange of momentum, heat and moisture between air and sea.
In order to deal with these processes the operational GME model includes a sea ice module which was now also implemented in the COSMO model code.
Jan-Peter Schulz 6 Sep. 2010
The sea ice model
It is a one-layer thermodynamic model which basically computes the energy balance at the ice's surface. From this the evolution of the ice surface temperature and the ice thickness are deduced.
In particular, the model allows for very low surface temperatures which can be significantly lower than the water temperature below the ice.
Jan-Peter Schulz 6 Sep. 2010
The sea ice model
Tmelt = -1.7oC
Tice
HiceQI
QA
Sea ice
Ocean
Tice : Sea ice surface
temperature
Hice : Sea ice thickness
Jan-Peter Schulz 6 Sep. 2010
Sea ice distribution
As input the model needs the sea ice distribution which is provided by the operational analysis of sea surface temperature (SST).
The remote sensing based sea ice mask of NCEP is provided by the SST analysis to the DWD global model GME. This GME sea ice mask is then interpolated to the COSMO-EU grid again by its SST analysis.
Jan-Peter Schulz 6 Sep. 2010
Sea ice in COSMO-EU
Currently the GME sea ice surface temperature is used to “emulate” the existence of sea ice in COSMO-EU by providing a realistic temperature at water points which are regarded as being ice covered. Using the threshold value Tmelt = -1.7oC the COSMO-EU water points are distinguished between
open water or ice covered. The albedo and the roughness length are set accordingly.
Now with the sea module implemented in the COSMO model the sea ice can actually be simulated. There are two new prognostic variables, the sea ice surface temperature Tice and the sea ice thickness Hice. They allow for a better
thermodynamically coupled treatment of sea ice in the COSMO model as lower boundary condition for the atmosphere.
Jan-Peter Schulz 6 Sep. 2010
Experiments
The sea ice model together with the modified SST analysis shall be tested in experiments with COSMO-EU for the winter 2009/2010.
Two experiments were started, one with sea ice and one without as reference. The start date is 02 Feb. 2010, 21 UTC, with the assimilation cycle, the sea ice is introduced with the SST analysis on 03 Feb. 2010, 00 UTC.
Jan-Peter Schulz 6 Sep. 2010
Cold start from GME using the SST analysis …
Jan-Peter Schulz 6 Sep. 2010
Jan-Peter Schulz 6 Sep. 2010
Jan-Peter Schulz 6 Sep. 2010
Jan-Peter Schulz 6 Sep. 2010
Jan-Peter Schulz 6 Sep. 2010
GME T_ICE
COSMO-EU T_SO(0) without sea ice
COSMO-EU:
T_SO(0) = 0.5 (T_ICE + Tmelt)
Tmelt = -1.7oC
Jan-Peter Schulz 6 Sep. 2010
GME T_ICE
COSMO-EU T_SO(0) without sea ice
COSMO-EU:
If GME=ice and BSH=ice:
use same formula as in first case.
Jan-Peter Schulz 6 Sep. 2010
GME T_ICE
COSMO-EU T_SO(0) without sea ice
COSMO-EU:
If GME=no ice and BSH=ice:
create new ice point.
Initialize: T_SO(0) = Tmelt – eps
Then: Search in the neighbourhood
for the warmest ice point which
originates from GME. Use this
temperature for T_SO(0).
eps = 0.05oC
Jan-Peter Schulz 6 Sep. 2010
GME T_ICE
COSMO-EU T_SO(0) without sea ice
COSMO-EU:
If GME=ice and BSH=no ice:
remove ice point, create water.
Initialize: T_SO(0) = Tmelt + eps
Jan-Peter Schulz 6 Sep. 2010
GME T_ICE
COSMO-EU T_SO(0) with sea ice
COSMO-EU:
At all sea ice points: T_SO(0) = Tmelt
Water temperature below the ice.
Jan-Peter Schulz 6 Sep. 2010
GME T_ICE
COSMO-EU T_ICE with sea ice
COSMO-EU:
GME values of T_ICE are
interpolated to the COSMO-EU grid.
Jan-Peter Schulz 6 Sep. 2010
GME T_ICE
COSMO-EU T_ICE with sea ice
COSMO-EU:
If GME=no ice and BSH=ice:
create new ice point.
Initialize: T_ICE = Tmelt – eps
Jan-Peter Schulz 6 Sep. 2010
GME H_ICE
COSMO-EU H_ICE with sea ice
COSMO-EU:
GME values of H_ICE are
interpolated to the COSMO-EU grid.
Jan-Peter Schulz 6 Sep. 2010
GME H_ICE
COSMO-EU H_ICE with sea ice
COSMO-EU:
If GME=no ice and BSH=ice:
create new ice point.
Initialize: H_ICE = 0.2 m
Jan-Peter Schulz 6 Sep. 2010
The first forecast run …
Jan-Peter Schulz 6 Sep. 2010
Jan-Peter Schulz 6 Sep. 2010
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Jan-Peter Schulz 6 Sep. 2010
Jan-Peter Schulz 6 Sep. 2010
Conclusions• The GME sea ice module was implemented in the COSMO model code.
• The code of the external SST analysis and the operational shell scripts for running the COSMO model at DWD were adapted to the sea ice model (thanks to Martin Lange and Thomas Hanisch at DWD).
• Experiments with COSMO-EU were started on 02. Feb. 2010. The cold start from GME sea ice fields via the SST analysis works fine. It is planned to simulate basically a full cycle of freezing and thawing of sea ice in the Baltic Sea from February until May 2010.
• The new prognostic variables, sea ice surface temperature and sea ice thickness, immediately started to adapt to the atmospheric conditions in COSMO-EU. Thus, they will provide a much better fully coupled lower boundary condition for COSMO-EU than the interpolated temperature from GME which is kept constant.
• Operational implementation is planned for autumn 2010.
