The effects of horizontal grid spacing on simulated daytime boundary layer depths in an area of complex terrain in Utah
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The effects of horizontal grid spacing on simulated daytime boundary layer depths in an area of complex terrain in Utah Gert-Jan Duine1
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Stephan F. J. De Wekker1
Received: 1 February 2017 / Accepted: 8 September 2017 Ó Springer Science+Business Media B.V. 2017
Abstract The influence of grid spacing on daytime planetary boundary layer (PBL) depths is investigated using 2 years of hourly output from a weather forecast system run operationally for an area of complex terrain in Utah. The model domain includes Dugway Proving Ground, the target area for the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program. Differences in PBL depths between a coarse(10 km horizontal grid spacing) and a fine-grid (3.3 km) domain are expressed as a function of several parameters that describe the terrain variability, including the standard deviation and the Laplacian of the terrain elevation. PBL depths on fair weather days are larger in the coarse domain than in the fine-grid domain by more than 200 m over areas with unresolved ridges in the coarse-grid domain. Absolute differences are an order of magnitude larger in summer than in winter while relative differences are similar and on the order of 10%. The PBL depth differences can only be partly removed after correcting for the fine-grid terrain elevation in the coarse domain, indicating that unresolved atmospheric processes in the coarse-grid domain also account for the PBL depth differences. The results also demonstrate the importance of distinguishing between PBL depths and PBL heights when evaluating the performance of coarse-grid numerical models. Keywords PBL depth and height Terrain smoothing Subgrid orography variability Complex terrain Horizontal grid spacing WRF
& Gert-Jan Duine [email protected] Stephan F. J. De Wekker [email protected] 1
Department of Environmental Sciences, University of Virginia, Charlottesille, VA, USA
123
Environ Fluid Mech
1 Introduction The planetary boundary layer (PBL) controls the exchange of heat, moisture and trace gases between the Earth’s surface and the atmosphere. The PBL deepens during the day due to surface heating and entrainment processes combined with vigorous turbulent mixing. Typical PBL depths in mid-latitudes are on the order of 1–2 km [40]. For nearground emissions and a clean overlying free atmosphere, trace gas and aerosol concentrations in the PBL are inversely proportional to the PBL depth [11]. Model biases in PBL depths may therefore lead to errors in the calculation of surface gas concentrations [31] and of greenhouse gas fluxes in carbon transport models [10, 23, 26]. It is therefore important that PBL depths are properly simulated in mesoscale and global models. This can be challenging, especially over mountainous terrain where PBLs can vary considerably in space and time [5]. The validity of using simulated PBL depths from models with coarsegrid spacings ( 10–100 km) over mountainous terrain can therefore be questionable and needs to be investigated. The evaluation of simula
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