Performance Assessment of Dynamic Downscaling of WRF to Simulate Convective Conditions during Sagebrush Phase 1 Tracer Experiments

Sudheer Bhimireddy,Kiran Bhaganagar

doi:10.3390/atmos9120505

Sudheer Bhimireddy, Kiran Bhaganagar

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https://doi.org/10.3390/atmos9120505

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Journal: Atmosphere	Publication Date: Dec 19, 2018
Citations: 10	License type: CC BY 4.0

Affiliation: The University of Texas at San Antonio

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Large-Eddy Simulations (LES) corresponding to four convective intensive observation periods of Sagebrush Phase 1 tracer experiment were conducted with realistic boundary conditions using Weather Research and Forecast model (WRF). Multiple nested domains were used to dynamically downscale the conditions from domain with grid size of 24 km to local scales with grid size of 150 m. Sensitivity analysis of mesoscale model was conducted using three boundary layer, three surface layer and two micro-physics schemes. Model performance was evaluated by comparing the surface meteorological variables and boundary layer height from the mesoscale runs and observed values during tracer experiment. Output from mesoscale simulations was used to drive the LES domains. Effect of vertical resolution and sub-grid scale parameterizations were studied by comparing the wind speed and direction profiles along with turbulent kinetic energy at two different heights. Atmospheric stability estimated using the Richardson number and shear exponent evaluated between 8- and 60-m levels was found to vary between weakly unstable to unstable. Comparing the wind direction standard deviations coupled with the wind speeds showed that the WRF-LES underestimated the wind direction fluctuations for wind speeds smaller than 3-ms − 1 . Based on the strengths of convection and shear, WRF-LES was able to simulate horizontal convection roll and convective cell type features.

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With increased solar forcing on the earth’s surface in the afternoon, air inside the lower part of atmosphere gets heated due to surface heating resulting in the formation of a Convective BoundaryLayer (CBL)
We evaluated the model performance subjected to different parameterizations used in mesoscale runs and sub-grid scale (SGS) models used in Large-Eddy Simulations (LES) simulations
YSU scheme was used with Revised MM5 Monin–Obukhov scheme (RMO) and MYJ scheme was used with Monin–Obukhov–Janjic scheme (MOJ)

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Introduction

With increased solar forcing on the earth’s surface in the afternoon, air inside the lower part of atmosphere gets heated due to surface heating resulting in the formation of a Convective BoundaryLayer (CBL). Turbulent processes inside CBL range across various scales, from microscale (O(10 m)). Understanding microscale atmospheric boundary layer (ABL) processes is crucial for applications such as air quality modeling, transport and dispersion of airborne agents or hazardous material. Pioneering studies [1,2,3,4] have used LES to reproduce atmospheric turbulence inside ABL using multiple grid points in horizontal and vertical directions. Most LES studies related to CBL were limited to idealized conditions, i.e., using periodic boundary conditions, homogenous surface properties and user-given surface fluxes.

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