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MOLOCH : ‘MOdello LOCale’ on ‘H’ coordinates. Model description

Piero Malguzzi:. MOLOCH : ‘MOdello LOCale’ on ‘H’ coordinates. Model description. ISTITUTO DI SCIENZE DELL'ATMOSFERA E DEL CLIMA , ISAC-CNR. Objectives Develop a tool for very high resolution-short range operational weather forecast;

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MOLOCH : ‘MOdello LOCale’ on ‘H’ coordinates. Model description

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  1. Piero Malguzzi: MOLOCH : ‘MOdello LOCale’on ‘H’ coordinates. Model description ISTITUTO DI SCIENZE DELL'ATMOSFERA E DEL CLIMA , ISAC-CNR

  2. Objectives • Develop a tool for very high resolution-short range operational weather forecast; • Resolve explicitly atmospheric convection (without parameterisation); • Develop a tool for research purposes (cloud model, flows over complex orography) Piero Malguzzi: • Model dynamics • non hydrostatic, fully compressible • Arakawa C grid; terrain-following coordinate • time split, implicit for vertically propagating sound waves, FB for horizontally propagating waves • advection: FBAS (Malguzzi & Tartaglione, 1999); also Weighted Average Flux WAF (Toro 1989; Hubbard & Nikiforakis, 2001) • nested in BOLAM runs • Model physics • radiation, vertical diffusion, surface turbulent fluxes, soil water and energy balance provisionally similar to BOLAM • new cloud microphysics (partly based on Drofa, 2003) • no dry and moist convection

  3. H-Coordinate Piero Malguzzi: Terrain following vertical coordinate smoothly relaxing to horizontal surfaces above the orography h(x,y): The vertical scale H is given by the density scale height

  4. Piero Malguzzi: The non hydrostatic model MOLOCH:parameterisation of microphysical processes qV qCI qCW

  5. Piero Malguzzi: Accretion of cloud ice by snow Accretion of cloud water by rain “Riming” Accretion of cloud ice by freezing rain Accretion of freezing rain by snow Accretion of cloud water by melting snow qPW qPI Accretion of rain by melting snow

  6. Microphysical hypothesis Piero Malguzzi: Precipitating particles Cloud particles Liquid and solid precipitation: Marshall-Palmer distribution Liquid and solid cloud particles: gamma-distribution (Levi distribution) for the number of particles per unit volume and unit radius D: N0 = 8·106 (m-4) for precipitating drops N0 function of crystal shape for precipitating ice N0 =106 -107 (m-4-) and =6 for cloud drops N0 = 107-108 (m-4-) and =3 for cloud crystal  and  are determined from the condition: where m is the mass of a particle of diameter D: and where a=/6·W , b=3 for cloud and precipitating water; a=100, b=2.5 for cloud ice; a and b function of crystal shape (temperature) for precipitating ice. The result is:

  7. The rate of change of the specific concentration q due to a particular microphysical process is given by: Piero Malguzzi: where is the rate of change of the mass of a single particle Condensation-sublimation where F is the ventilation coefficient, equal to 0.8 for cloud particles. For precipitating particles the following expression is implemented: where dif is the dynamical molecular viscosity of air: U, the terminal velocity of the particle; and Sc, the Shmidt number (= 0.6). The suffix k can be W or I, indicating liquid water or ice, respectively. LWV and LIV are the condensation and sublimation latent heat; , the coefficient of molecular diffusion of vapour into air; Ka , the thermal conductivity of air; and qV , the specific humidity.

  8. Melting and freezing Piero Malguzzi: where LIW is the latent heat due to ice melting. Accretion of cloud particles by rain/snow/graupel In this process it is assumed that all cloud particles have the same probability to be collected by a precipitating particle Accretion of cloud water by melting snow where D and u are diameter and terminal velocity of accreting particles (solid or liquid precipitation), qk is specific mass of the accreted particles (cloud water or cloud ice), E is the accretion coefficient (0.6 for the interaction rain–cloud water: 0.1 for the interaction snow/graupel–cloud ice; 1.0 for the interaction rain-cloud ice and snow/graupel-cloud water[riming]). Accretion of rain by iced precipitating particles Accretion of rain by meltig snow where U is the precipitation terminal velocity and E=1. The rate of change of the specific concentration of precipitating ice is the given by:

  9. Autoconversion of precipitation Piero Malguzzi: The parameterisation of this process is based on the following formulas: for rain for snow/graupel is the autoconversion coefficient, in the range from 1 to 3 10-3 s-1 qth is the threshold value of cloud specific mass: qcwth=5·10-4 kg/kg, qcith=10-3 kg/kg.

  10. Piero Malguzzi: Fall of precipitating species The terminal velocity of one precipitating particle is: where n=0.8 and k=842 m1-ns-1 for rain n and k function of the crystal shape for snow/graupel Average terminal velocity: The fall is computed with the backward upstream scheme (unconditionally stable and dispersive).

  11. Conservation of entropy during microphysical processes Piero Malguzzi: Piero Malguzzi: Piero Malguzzi: Specific heat at constant pressure partial pressure of water vapor Saturation formulas (Pressman) with respect to water and ice

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