Validation of the PALM model system 6.0 in a real urban environment: a case study in Dejvice, Prague, the Czech Republic

Abstract

Abstract. In recent years, the PALM 6.0 modelling system has been rapidly developing its capability to simulate physical processes within urban environments. Some examples in this regard are energy-balance solvers for building and land surfaces, a radiative transfer model to account for multiple reflections and shading, a plant-canopy model to consider the effects of plants on flow (thermo)dynamics, and a chemistry transport model to enable simulation of air quality. This study provides a thorough evaluation of modelled meteorological, air chemistry, and ground and wall-surface quantities against dedicated in situ measurements taken in an urban environment in Dejvice, Prague, the Czech Republic. Measurements included monitoring of air quality and meteorology in street canyons, surface temperature scanning with infrared cameras, and monitoring of wall heat fluxes. Large-eddy simulations (LES) using the PALM model driven by boundary conditions obtained from a mesoscale model were performed for multiple days within two summer and three winter episodes characterized by different atmospheric conditions. For the simulated episodes, the resulting temperature, wind speed, and chemical compound concentrations within street canyons show a realistic representation of the observed state, except that the LES did not adequately capture night-time cooling near the surface for certain meteorological conditions. In some situations, insufficient turbulent mixing was modelled, resulting in higher near-surface concentrations. At most of the evaluation points, the simulated surface temperature reproduces the observed surface temperature reasonably well for both absolute and daily amplitude values. However, especially for the winter episodes and for modern buildings with multilayer walls, the heat transfer through walls is not well captured in some cases, leading to discrepancies between the modelled and observed wall-surface temperature. Furthermore, the study corroborates model dependency on the accuracy of the input data. In particular, the temperatures of surfaces affected by nearby trees strongly depend on the spatial distribution of the leaf area density, land surface temperatures at grass surfaces strongly depend on the initial soil moisture, wall-surface temperatures depend on the correct setting of wall material parameters, and concentrations depend on detailed information on spatial distribution of emissions, all of which are often unavailable at sufficient accuracy. The study also points out some current model limitations, particularly the implications of representing topography and complex heterogeneous facades on a discrete Cartesian grid, and glass facades that are not fully represented in terms of radiative processes. Our findings are able to validate the representation of physical processes in PALM while also pointing out specific shortcomings. This will help to build a baseline for future developments of the model and improvements of simulations of physical processes in an urban environment.

Budgets of Second-Order Turbulence Moments over a Real Urban Canopy

“…The PALM model has been widely used to study flows over both idealized (Letzel et al 2008;Park et al 2012;Gronemeier and Sühring 2019;Nazarian et al 2020;Blunn et al 2022) and real urban canopies (Kanda et al 2013;Park et al 2015;Gronemeier et al 2017) and it has been extensively validated (Fröhlich and Matzarakis 2020;Gronemeier et al 2021;Resler et al 2021). Heinze et al (2015) used the PALM model to study secondorder moment budgets in cloud topped boundary layers and found that the PALM model results agree with the results of other LES models except for the TKE dissipation rate.…”

uDALES 1.0: a large-eddy simulation model for urban environments

“…Several possible approaches for studying the urban climate can be found in the literature: e.g. Mirzaei and Haghighat (2010), Mirzaei (2015), Moonen et al (2012), Arnfield (2003), Rizwan et al (2008), Grimmond (2007), and Barlow (2014). A common approach is the use of stand-alone urban energy balance (UEB) models.…”

“…However, the continuous increase in computational resources makes LES models the most promising method to provide sufficiently detailed and accurate information on how the interaction of heat, moisture and momentum exchanges affects the local urban microclimate. Examples of LES models capable of dealing with the urban terrain are PALM-4U (Maronga et al, 2020;Resler et al, 2021) and OpenFoam2. Indeed, PALM-4U has similar goals and many similarities to uDALES.…”

Characterization of Terrain-Induced Turbulence by Large-Eddy Simulation for Air Safety Considerations in Airport Siting

“…With increasing computational capabilities, LES has nowadays become one preferred methodology for the simulation of turbulent flow in the ABL. This includes basic boundary layer studies for convective [12,13] and stable [14][15][16] stratification or terrain induced flows [17,18], as well as a wide range of atmospheric applications, e.g., related to wind energy [19][20][21], or pollutant transport and dispersion in urban environments [22,23]. LES simulations for air safety purposes are, in contrast, still rather sparse and often focus on the potential effects of vegetation and buildings on LLT affecting the runway [24][25][26].…”