Soil physics in COSMO numerical meteorological model Grzegorz Duniec and Andrzej Mazur Department for Numerical Weather Forecasts, Institute of Meteorology and Water Management – National Research Institute. 61 Podleśna str., PL-01-673 Warsaw, Poland; http://www.imgw.pl; http://www.pogodynka.pl;
[email protected]
Introduction Soil and atmosphere boundary layer (ABL) interact with each other and influence on physical processes happen in soil and atmosphere. Current parameterization of soil physics processes in TERRA_ML (multilayer soil module of COSMO model) was prepared 40 years ago and does not give satisfactorily forecast results. New parameterizations should take into consideration physical processes in soil (microphysics processes in soil, fluid dynamics in porous media, soil dynamics etc.), water cycle in soil and soil–plant–water relation. The aim of the project was an improvement of present parameterizations of the TERRA_ML in COSMO model. Results of first part of work, connected to the parameterization of physical processes of bare soil evaporation, vertical and horizontal soil water transport and a runoff from soil layers, are presented in this paper. Theoretical description of vertical soil water transport is improved with temperature dependency of hydraulic diffusivity for different sort of soil.
Methods The TERRA_ML parameterization in COSMO model accounts for the five regular soil types: sand, sandy loam, loam, loamly clay and clay, together with three special soil types: peat, ice and rock. For last two hydrological processes in a ground are not considered. Potential evaporation is assumed to occur over ice but Soil Water Content (SWC) remains unchanged. The soil model consists of two parts. In the first part the computation of bare soil evaporation and plant transpiration is performed, in the second part – heat conduction and Richards equations are solved. Also, melting of snow is computed here. In TERRA_ML six layers are introduced for water cycle and seven – for thermal processes (fig. 1, left chart). In this parameterization hydrological (evapotranspiration, interception reservoir, infiltration of rain etc. – center chart) and thermal (temperature of snow-free and snow-covered soil, snow albedo, melting and thawing etc. – right chart) processes are considered.
Fig. 1. Level distribution for water and energy processes in soil model (left chart). Hydrological processes and soil thermodynamics (center and right chart, respectively). Thermal processes are described by the heat conduction equation and SWC at the surface and in the soil is predicted by the governing equations for water mass budgets: ∂ θ l ,k ∂θ i = α Pr + Ei − I perc − Rint er surface: ρ w soil: ρ w = δ 1,k Eb + I snow + I perc + (1 − α ) Pr − Rinf il + Fk ,k + 1 − (1 − δ 1,k ) Fk − 1,k + Trk − Rk + S k ∂t ∂t ∂ θ ice ,k ∂ θ snow ρw = Psnow + Esnow − I snow − Rsnow ρw = − Sk ∂t ∂t Dickinson’s parameterization (formula [1] below) describing water flux through a soil, in current experiments was replaced by Darcy equation [2] with temperature dependency added ([3] and [4]).
[
5 K 0 5 , 5 + 0 , 8 B 1 + 0 , 1 ( B − 4 ) log B − 3 ,7 + K R Dmin B + 2 st B 1,02 Dmax su Fm = ρ m 1 + 1550 Dmax B+ 5 su
st zu zt
]
Fm = − D (θ )∇ θ
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[1] For results see Fig. 2 b – d for “a” equal to -1, 0.5 or 1 (formula 3) and Fig. 3 b, d for exponential form (formula 4). Results and discussion
)
T ⋅ exp( )∇ θ T0
[4]
Fig. 2. Results for winter season, dew point temperature. a) Dickinson’s (reference) parameterization – absolute values, b) difference between (a) and results for modified Darcy equation with exponent “a” equal to 2, c) difference between (a) and results for Darcy equation with exponent “a” equal to 1, d) difference between (a) and results for Darcy equation with exponent “a” equal to -1
Dew point temperature, 2m agl.
Dew point temperature, 2m agl.
Air temperature, 2m agl.
Air temperature, 2m agl.
Fig. 3. Results for winter season, dew point and air temperature, observations – forecasts. (a) and (c) – Dickinson’s (reference) parameterization, (b) and (d) Darcy equation with exponential temperature dependency. White areas mark regions where measured values (observations) are close or equal to values of forecast Windspeed at 10m agl. (16.05.2013)
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Fig.4 Examples of results for summer season for wind speed, air temperature and dew point temperature. Comparison between measurements, results of reference (Dickinson) parameterization and of new parameterization of soil moisture flux – Darcy equation with: temperature dependency (series exp(T)) and with time-depending correction factor (series α(t)exp(T)) added. The analysis proved that replacement of Dickinson’s parameterization with temperature-dependent Darcy equation improved forecast of dew point temperature (Fig. 2, b – d) and relative humidity. Further experiments with the value of exponent “a” showed that parametrization of bare soil evaporation with Darcy equation (as a replacement for Dickinson’s parameterization) could be linked with distinction of soil types (eg. for clay a=-1, for sandy soil a=– 0.5, for loam soil a=1). In turn, exponential temperature dependency in Darcy equation (Fig. 3, b, d) induced changes in results only in northeastern part of Poland (with vast majority of sandy soil). Additional forecast improvement was induced by introduction of time-depending correction factor (see Fig. 4, series “α(t)exp(T)”). Literature 1. Doms G., Forstner J, Heise E, Herzog H.–J, Mironov D, Raschendorfer M, Reinhardt T, Ritter B, Schrodin R. Schulz, J.–P, Vogel G, 2011, A Description of the Nonhydrostatic Regional Model LM, Part II: Physical Parameterization, DWD. 2. Dickinson, R. E., 1984. “Modeling evapotranspiration for three-dimensional global climate models” Climate Processes and Climate Sensitivity. Geophysical Monograph 29, Maurice Ewing Volume 5, 5, pp. 58–72. 3. Duniec G., Mazur A., 2013a, “New Approach to Parameterization of Physical Processes in Soil in COSMO Model – Preliminary Results”, COSMO Newsletter 13, pp.50-55 4. Duniec G., Mazur A., 2013b, “The Soil Physics In Meteorological Model COSMO-LM – TILE- And MOSAIC Parameterization”, (in Polish: Fizyka gleby w meteorologicznym modelu COSMO-LM – parametryzacja TILE I MOSAIC), Przegląd Geofizyczny, 3-4, pp. 21-41 5. Duniec G., Mazur A., 2014, Experiments in soil physics – case study, COSMO Newsletter 14, pp. 43-53.
