The Microscale Obstacle Resolving Meteorological Model MITRAS: Model Theory

Abstract

Abstract. This paper describes the developing theory and the underlying processes of the microscale obstacle resolving model MITRAS Version 2. MITRAS calculates wind, temperature, humidity and precipitation fields as well as transport within the obstacles layer using Reynolds averaging. It explicitly resolves obstacles, including buildings and overhanging obstacles, to consider their aerodynamic and thermodynamic effects. Buildings are represented by impermeable grid cells at the building positions so that the wind speed vanishes and zero turbulent kinetic energy is assumed in these grid cells. Wall functions are 15 used to calculate appropriate turbulent fluxes. Most exchange processes at the obstacle surfaces are considered in MITRAS, including convective and radiative processes in order to obtain an accurate surface temperature. MITRAS is also able to simulate the effect of wind turbines. They are parameterized using the actuator-disk concept to account for the reduction in wind speed. The turbulence generation in the wake of a wind turbine is parameterized by adding an additional part to the turbulence mechanical production term in the turbulent kinetic energy equation. Effects of trees are considered explicitly, 20 including the wind speed reduction, turbulence production, and dissipation due to drag forces from plant foliage elements as well as the radiation absorption and shading. The paper is providing not only a documentation of the model dynamics and numerical framework but also a solid foundation for future microscale model extensions.

Including trees in the numerical simulations of the wind flow in urban areas: Should we care?

“…Validations of MITRAS for this approach can be found in Salim et al (2014b), Schlünzen et al (2011b), and Schlüter (2006).…”

“…To parameterize the additional mechanical production of turbulence due to the trees, an additional source term (Q veg,E ) is added to the turbulent kinetic energy (E) equation in the model equations (Salim et al, 2014a). According to Liu et al (1996) this additional term reads…”

“…MITRAS is a 3-dimensional, non-hydrostatic, prognostic, numerical model for wind, temperature, humidity and concentrations within the obstacle layer (Salim et al, 2014a). MITRAS solves the three-dimensional Reynolds-Averaged Navier Stokes equations (URANS) with the k-ε model as turbulence closure.…”

“…MITRAS includes Coriolis force effects, however, these are neglected in the current investigation. Model validations have been performed according to the Association of German Engineers (VDI) guidelines (VDI, 2005) for obstacle resolving micro-scale models (Grawe et al, 2013;Salim et al, 2014b).…”

Evaluation of thermal indices for their applicability in obstacle-resolving meteorology models

“…There are several reviews on the assessment of the thermal environment (Cheng et al 2012 ; Djongyang et al 2010 ; Goshayeshi et al 2013 ; Walgama et al 2006 ), including the reviews by Monteiro ( 2005 ) and Coccolo et al ( 2016 ) that focus on the outdoor environment. However, no paper reviews the application of thermal indices in combination with obstacle-resolving numerical models of the atmosphere (from here on shorted ORM) such as ENVI-met (Bruse and Team 2015 ; Bruse and Fleer 1998 ) or MITRAS (Salim et al 2018 ; Schlünzen et al 2003 ), although those are increasingly used for urban planning (Goldberg et al 2013 ) to assess different design strategies (e.g., implementation of vegetation and building orientation) as suggested by the German Engineering guideline (VDI 2008a ).…”