Cold Fronts Depend on the Details
Scientists work out the details of high-resolution regional model simulations of extratropical cold fronts over the ARM Eastern North Atlantic (ENA) site.
Low-level clouds behind extratropical cyclone cold fronts are a significant source of biases in General Circulation Models (GCMs), and the ARM East North Atlantic (ARM ENA) site is well-situated to provide observations to address this problem. ASR-funded scientists plan to use a regional, high-resolution model to explore the physical parameterizations that are important for accurately representing cloud properties in these synoptic conditions. However, before the modeled clouds can be examined, the scientists need to ensure that the model gets the atmospheric dynamics driving the cold fronts correct. Since regional model results can be sensitive to the domain configuration, this study analyzes three specific cases of cold front passages over the ARM ENA site to identify the optimal model configuration.
Scientists find that simulating the large-scale atmospheric dynamics over the ARM ENA site in a regional model depends on a balance between placing the lateral boundaries close enough to the site to guide the synoptic-scale physics, and having an adequately large domain so that the model can generate the interactions between frontal and synoptic scale features. The analysis provides a framework to optimize regional climate model configurations for the sake of both generating a realistic “hindcast” of dynamics over the ARM ENA site during a specific day, and utilizing a domain as large as computationally possible to increase the data available from the model for sampling purposes. Future numerical model experiments that intend to study boundary-layer and cloud-related processes at the ARM ENA site can use this study as a guide to configure their models.
This study explores the ability of the Weather Research and Forecasting (WRF) Model to accurately reproduce the passage of extratropical cold fronts at the ARM Eastern North Atlantic observation site on the Azores Islands. The sensitivity analysis indicates that to accurately reproduce cold fronts with WRF over the ENA site one needs to either: (a) position the site of interest at 1000 km up to 1500 km at most from the boundary of dominant inbound flow, or (b) apply a spectral nudging above the boundary layer for prolonged durations. The model demonstrated biases in the upper-level circulation that affect near-surface circulation. These challenging biases may be site-dependent, but they suggest a need for greater attention to the modeled coupling between upper-level circulation and surface fronts. Based on the sensitivity tests, this study generated a framework to provide guidance on an optimal configuration to reduce boundary-layer and near-surface biases.
City University of New York
The research is funded by the U. S. Department of Energy Office of Science Office of Biological and Environmental Research grant number DE-SC0016344 as part of the Atmospheric System Research program. The ENA datasets were obtained from the ARM archive: http://www.archive.arm.gov/discovery.
Lamraoui, F., J. Booth, and C. Naud. “WRF hindcasts of cold front passages over the ARM Eastern North Atlantic Site: a sensitivity study.” Monthly Weather Review, 146, 2417-2432 (2018). DOI:10.1175/MWR-D-17-0281.1