Chapter 7. Land surface parametrization

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Table of contents

Chapter 1. Overview

Chapter 2. Radiation

Chapter 3. Turbulent diffusion and interactions with the surface

Chapter 4. Subgrid-scale orographic drag

Chapter 5. Convection

Chapter 6. Clouds and large-scale precipitation

Chapter 7. Land suface parametrization

Chapter 8. Methane oxidation

Chapter 9. Climatological data

REFERENCES

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7.7 Sea/lake ice

Any non-land point (i.e., a grid point with land cover less or equal 0.5) can have two fractions, open water and ice. A surface analysis defines the ice fraction, c_I, and the temperature of the open water fraction; both quantities are kept constant during the forecast. No distinction is made between surface and skin temperature for the open water fraction (see Table 7.2).

The ice fraction is modelled as an ice slab, with open water underneath and a skin temperature for the thermal contact with the atmosphere. The main caveats in the sea ice parameterization are:

(a) Fixed depth of the slab (which can be relaxed once there is a reliable data set to specify its geographic distribution;

(b) Fixed fraction, which is a reasonable assumption for a 10-day forecast period, and avoids the need for the momentum balance of the ice and its complex rheology (see, e.g., Hibler and Flato 1992) and the definition of the ocean currents; and

(c) No snow accumulation on top of the ice (although one of the main effects of snow, i.e., a markedly different surface albedo, is partially emulated by the prescribed seasonal albedo in Table 2.2).

The ice heat transfer is assumed to obey the following Fourier law of diffusion

(7.70)

where

is the volumetric ice heat capacity,

is the ice temperature, and

is the ice thermal conductivity. The boundary condition at the bottom is the temperature of the frozen water, T_fr = T₀ - 1.7 and the top boundary condition is the net heat flux at the surface, obtained from the solution of the ice skin thermal budget.

Eq. (7.70) is solved with the ice disretized in four layers, with the depth of the top three layers as in the soil model and the depth of the bottom layer defined as

(7.71)

and the total depth of the ice slab,

, is prescribed as 1.5 m. In order to ensure a constant ice fraction, the solution of the ice thermal budget is capped to the ice melting temperature, T_ml = T₀ at all levels. The details of the numerical discretization can be found in Section 7.8.

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05.04.2002