WRF Hindcasts of Cold Front Passages over the ARM Eastern North Atlantic Site

Naud, C. M., Columbia University

Cloud Processes

Warm Boundary Layer Processes, High Latitude Processes

Lamraoui F, J Booth, and C Naud. 2018. "WRF hindcasts of cold front passages over the ARM Eastern North Atlantic Site: a sensitivity study." Monthly Weather Review, 146(8), 10.1175/MWR-D-17-0281.1.


Two-dimensional distributions of the precipitable water as a cold front passed the ENA site (white dot) from (a) ERA-Interim and WRF hindcasts using: (b) a 3000x3000 km domain, and (c) a 6000x6000 km domain, centered on the site on 2015-12-25, 00:00 UT. The white boxes in panels (a) and (c) indicate the full region shown in panel (b)


Two-dimensional distributions of the precipitable water as a cold front passed the ENA site (white dot) from (a) ERA-Interim and WRF hindcasts using: (b) a 3000x3000 km domain, and (c) a 6000x6000 km domain, centered on the site on 2015-12-25, 00:00 UT. The white boxes in panels (a) and (c) indicate the full region shown in panel (b)

Science

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 offers a great opportunity to address this problem. In order to isolate and compare the cloud-scale physics involved in low-level clouds with observations from the site, it is useful to use a regional model. The first step in such an analysis is the subject of this paper: ensuring the modeled atmospheric dynamics behind the cold fronts are correct. Our study finds that modeling the dynamics correctly depends on a balance between placing the lateral boundaries close enough to the ARM-ENA site to guide the synoptic-scale physics, and also allowing the model to have an adequately large domain so that it can generate the interactions between frontal and synoptic scale features (see figure).

Impact

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, and utilizing a domain as large as computationally possible in order to increase the data available from the model for sampling purposes. All 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.

Summary

We explore 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 (ENA) observation site on the Azores Islands. The sensitivity analysis indicates that in order 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 our sensitivity test, we generated a framework to provide guidance on an optimal configuration to reduce boundary-layer and near-surface biases.